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This is to certify that the

dissertation entitled

CLUSTERS AND EXTENDED ARRAYS WITH METAL
IONS AND NITROGEN DONOR LIGANDS

presented by

Cristian Saul Campos Fernandez

has been accepted towards fulfillment
of the requirements for

Ph . D . degree in Chemis t ry

 

 

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CLUSTERS AND EXTENDED ARRAYS WITH METAL
IONS AND NITROGEN DONOR LIGANDS

By

Cristian Saul Campos Fernandez

AN ABSTRACT OF A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSPHY

Department of Chemistry

2001

Professor Kim R. Dunbar

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ABSTRACT

CLUSTERS AND EXTENDED ARRAYS WITH METAL IONS AND
NITROGEN DONOR LIGANDS

By

Cristian Saul Campos Fernandez

The study of nitrogen heterocyclic ligands in coordination chemistry has
experienced a steady increase in the last thirty years. The increasing versatility of
synthetic methods used in the preparation of ligands has permitted increasingly more
complex molecules to be assembled. The work reported in this thesis involves the
systematic study of the chemistry of five diimine ligands that have not been investigated
thoroughly to date. These are 2-2—pyridyl-l,8-naphthyridine (pynp), 3,6-bis-pyridyl-
tetrazine (bptz), 2,2’-bis-bipyridyl-azo (abpy), 1,3,6-trispyridyl-triazine (tptaz), and
2,3,5,6 tetrapyridyl pyrazine (tppz).

The chemistry of the bptz ligand was explored with a variety of first row
transition metals, with the result being the high yield syntheses of cyclic coordination
compounds (metallocyclophanes). The formation of these cyclic entities rather than
polymeric materials or dinuclear complexes is attributed, in great degree, to the choice of
the anion which functions as a template in the self-assembly of the cationic
metallocyclophanes. Anions that are similar in size and geometry, e.g. [BF4]' and [C104]',
yield molecular squares, whereas the larger anion, [SbF6]', leads to the assembly of a

molecular pentagon. The existence of these species in solution was demonstrated by l9F

 

 

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NMR studies conducted on the diamagnetic metallocyclophane,
[Zn4(bptz)4(CH3CN)g][BF4]3), the results of which indicate that the encapsulated anion is
present in the solution form of the compound. Electrospray mass spectrometry revealed
that the formation of these metallocyclophanes with other transition metals such as Mn",
Fe", Coll and Cu" is feasible, but that they are not as stable as the Ni" and Zn11 analogs.

The chemistry of tppz has languished for almost two decades, most likely due to
reports that coordination of more than one metal center was not possible. We embarked
on the synthesis and characterization of a series of mononuclear tppz compounds
[M(tppz)2]n+ that can potentially be used as building blocks for the synthesis of higher
nuclearity species. In addition, a new application for this ligand was discovered in our
laboratory, namely as a linker for metal-metal compounds to give molecular rectangles.
In this vein, the compound [Rh4(02CCH3)2(tppz)2(CH3OH)4][PF6]6 was prepared which
is the first of its kind in the M-M bond field.

Naphthyridine based ligands have been used relatively infrequently compared to
phosphine, halide or carboxylate derivatives in metal-metal bond chemistry. The presence
of two pynp ligands in the dimetal units [M2]“ (M = M0", Ru" and Rh") allows for a rich
electrochemistry that is not observed for most metal-metal compounds with more
electronically “innocent” ligands. In the case of two of the new compounds in this study,
four reversible one electron ligand based reductions, are observed. The compound
[Ru2(02CCH3)2(pynp)2][PR]; shows Class HI Robin-Day behavior with all four electrons
being fully delocalized throughout the molecule, whereas in the [M02]4+ derivative, Class

II behavior is found.

 

In memory of my grandmother

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ACKNOWLEDGEMENTS

This work would not have been possible without the support of my advisor Dr.
Kim Dunbar, whose constant support, understanding and patience allowed me to
overcome several obstacles during my graduate student years. I also wish to thank all the
staff members and Dunbar students at Michigan State University as well as Texas A&M
University who taught me laboratory skills and the use of many techniques. I would also
like to extend special thanks to Lisa Thompson and John Koomen who greatly helped me
with theoretical and mass spectrometry studies at the latter stages of my career at Texas
A&M. In addition, I will like to extend special thanks to the members of my research
group, Rodolphe Clerac and Jose Ramon Galan Mascaros for their invaluable help with
magnetic measurements and Brad Smucker, Kemal Catalan and Elizabeth Lozada for the
good moments that we spent in the lab. In general, I would like to thank to all of the
Dunbar group members who I met throughout the last six years.

I would also like to acknowledge a very special person in my life, someone who
influenced my life greatly and taught me the importance of education; my grandmother.
She cannot be with me at this important moment of my life, but all the times that we
talked about the importance of education really encouraged me to pursue post graduate
studies in chemistry.

Finally, I will like to give thanks to my family who supported me even though
they never quite understand what I was doing here. Without their support and

encouragement I would have never had the will power to finish this difficult task.

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TABLE OF CONTENTS

LIST OF TABLES ................................................................................. X
LIST OF FIGURES .............................................................................. XIII
LIST OF SYMBOLS AND ABBREVIATIONS ........................................................ XXIII
Chapter I ............................................................................................................................ 1
Introduction ......................................................................................................................... l
I. Background ..................................................................................................................... 2
Nitrogen Heterocylic Ligand Chemistry ............................................................................. 2
Mixed Valency in Ligand-Bridged Metal Assemblies ......................................................... 4
II. New Applications of Bipyridine and Terpyridine Chemistry ........................................ 5
Specific Supramolecular Interactions: Anions as Templates ............................................ 11
III. Naphthyridine Ligands in Metal-Metal Bonded Compounds ...................................... 19
References ......................................................................................................................... 25
Chapter II ......................................................................................................................... 30
I. Introduction ................................................................................................................... 31
11. Experimental Section .................................................................................................... 33
Methods and Starting Materials. ....................................................................................... 33
Physical Measurements. .................................................................................................... 34
[Ni4(bptz)4(CH3CN)3][BF4]g (l). ...................................................................................... 34
[Zn4(bptz)4(CH3CN)g][BF4]3 (2) ....................................................................................... 35
[Ni4(bpt1)4(CH3CN)g][ClO4Is (3) ...................................................................................... 35
[Zn4(bptz)4(CH3CN)g][C1043 (4). .................................................................................... 36
[Ni5(bptz)5(CH3CN)lo][SbFG]lo (6). .................................................................................. 36
{[Mn(bptz)2(CH3CN)2][BF4]2}... (7). ................................................................................ 37
2,2‘ Azo-Bipyridine (abpy). .............................................................................................. 37
[Ni2(abpy)(CH3CN)2][N03]4 (12). .................................................................................... 38
[Cu2(abpy)(CH3CN)g[BF4]4 (13) ....................................................................................... 38
Data Collection and Refinement. ...................................................................................... 39
[Ni4(bptz)4(CH3CN)g][BF4]3'4CH3CN (l). ....................................................................... 39
[Zn4(bptz)4(CH3CN)g][BF4]3'4CH3CN (2) ........................................................................ 43
[Nl4(prZ)4(CH3CN)3][C104]8'2CH3CN'C4H30 (3). ......................................................... 43
[Zn4(bptz)4(CH3CN)g][C104]3-3CH3CN (4). ..................................................................... 44
[Ni2(bptz)(CH3CN)g][C104]4 (5). ...................................................................................... 44
[Ni5(bptz)5(CH3CN);o][SbF6]10'2CH3CN (6) .................................................................... 45
{[Mn(bptz)2(CH3CN)2][BF4]2}.° (7). ................................................................................ 45
[Ni4(bptz)4(CH3CN)g] [BF4] [PF6]3[SbF6]4 (8). .................................................................. 46
[Ni4(bptz)4(CH3CN)3][C104][104]7-2CH3CN (9). ............................................................. 47

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[Cu(bopd)2(HzO)2][BF4]2 (l 1). ......................................................................................... 48
[Ni2(abpy)(CH3CN)2] [NO3]4 (12). .................................................................................... 48
[Cu2(abpy)(CH3CN)g][BF4]3 (l3) ..................................................................................... 49
{[Cu(abpy)2][BF4] L... (14). ................................................................................................ 49
[Ni4(bptz)4(CH3CN)g][I][SbF6]-, (15). ............................................................................... 49
III. Results and Discussion. ............................................................................................... 50
A. Self-Assembly of Molecular Squares ........................................................................... 50
A1 Syntheses of [M4(bptz)4(CH3CN)g][X]g (M = Ni2+ and Zn”) Compounds .............. 50
MI!) metallocyclophanes .................................................................................................. 5 1
Zn(II) metallocyclophanes ................................................................................................. 51
A2. Spectroscopic and Cyclic Voltammetric Studies of [M4(bptz)4(CH3CN)g][X]g M =
Ni(II), Znal); X= [BF4]', [CIOJ ....................................................................................... 52
A3. X-ray Crystallographic Results .................................................................................. 53
Ni(II) molecular squares: [Ni4( bptz)4( CH 3CN)3][X )3; X = [BF4]', [C104]' and
[Ni2( bptz)( CH3CN)3][ C104 14 ............................................................................................ 53
Zn(II) molecular squares: [Zn4(bptz)4(CH3CN)3][X]3, X =[ BF41', [C104l' .................... 64
AA. NMR Studies ............................................................................................................. 68
B. A New Generation of Metallocyclophanes: Molecular Pentagons ............................... 71
B.1. Synthesis of [Ni5(bptz)5(CH3CN).o][SbF6]10 ............................................................. 71
3.2. X-ray Crystallographic Results. ................................................................................. 72
B.3. Spectroscopic and Cyclic Voltammetric Studies. ...................................................... 77
C. Chemistry of Bptz with Other 3d Transition Metals .................................................... 78
X- ray Crystallographic Studies of Bptz Products with Mn“, Coz”, Cu”. ....................... 79
D. Electrospray-MS Studies of Bptz Reactions with Mn“, Fe“, Co“, Ni“,
Cu2+ and Zn“. ................................................................................................................... 83
E. Interconversion of the [Nis]10+ and [Ni4]8+ ................................................................... 92
E.S-MS Studies of Interconversion Reactions .................................................................. 97
G. 22’ A20 Bipyridine Chemistry .................................................................................... 99
G1 Synthesis of [Ni2(abpy)(CH3CN)2(N03)4], [Cuz(abpy)(CH3CN)3][BF4]4 and
{ [Cuz(abpy)] [BF4]2 }.. Compounds .................................................................................... 99
6.2. Spectroscopic and Electrochemical Studies. ........................................................... 100
0.3. X-ray Crystallographic Studies ................................................................................ 101
IV. Future Work .............................................................................................................. 107
Reactions of the metallocyclophanes ............................................................................. 107
V. Conclusions ................................................................................................................ l l 1
VII. References ................................................................................................................ 1 15
Chapter III ..................................................................................................................... 119
I. Introduction: ................................................................................................................. 120
11. Experimental Section .................................................................................................. 123
Preparation of Compounds. Methods and Materials. ...................................................... 123
Physical Measurements. .................................................................................................. 123
Synthesis .......................................................................................................................... 124
2,3,5 ,6-tetrapyridylpyrazine (tppz). ................................................................................. 124

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2,4,6-tris(2-pyridyl)-l,3,5-triazine (tptaz). ...................................................................... 124

[Ni(tppz)2][N03]2 (16). ................................................................................................... 125
[Co(tppz)2] [P136]; (17). .................................................................................................... 125
[Fe(tppz)2][ClO4]2 (18). .................................................................................................. 125
[Mn(tppz)2] [C104]; (19). ................................................................................................. 126
[Ni(tppz)2][ClO4]2 (20). .................................................................................................. 126
[Ni2(tppz)(CH3CN)6][BF4]4 (21) ..................................................................................... 127
[Rh(tppz)2][BF4]3 (23) ..................................................................................................... 127
[Rh2(02CCH3)2(tppz)2(CH30H)4][PF6]6'CH30H (24). .................................................. 128
[Coz(tppz’)2][CoZCl7][PF6] (25). ..................................................................................... 128
[Ni(tptaz)(CH3CN)2(H20)][BE]; (27) ............................................................................ 128
[Fe(tptaz)2] [C104]; (28) ................................................................................................... 129
[Mn(tpta2)(phen)(H20)][O3SCF3]2 (29) .......................................................................... 129
X-ray Data Collection and Refinement ........................................................................... 130
[Nl(tpp2)2] [NO3]2 (16). ................................................................................................... 130
[C0(tppZ)2] [PF5]2 (l7). .................................................................................................... 135
[Fe(tppz)2][ClO4]2 (l8). ..................................................................................................................... 135
[Mn(tppz)2][ClO4]2 (19). ................................................................................................. 136
[Ni(tppz)2][ClO4]2 (20). .................................................................................................. 136
[Ni2(tppz)(CH3CN)6][BF4]4-CH3CN (21). ...................................................................... 137
{ [Coz(tppz)(CH3CN)2Cl4] [PF6] [BF4] }.., (22). ................................................................. 137
[Rh(tppz)2][BF4]3 (23) ..................................................................................................... 138
[Rh4(02CCH3)2(tppz)2(CH30H)4][PF6]6-CH3CN (24) .................................................... 138
[C02(tppz’)2]PF6] [C02Cl7] (25). ...................................................................................... 139
[Co(tp)’)2Cl2] [313.12 (26). ................................................................................................ 139
[Ni(tptaz)(CH3CN)(H20)][BF4]2 (27). ............................................................................ 140
[Fe(tptaz)2] [C104]; (28) ................................................................................................... 140
[Mn(tptaZ)(phen)][Mn(tptaz)(phen)(H20)][CF3S03]4 (29). ........................................... 140
II. Results and Discussion ............................................................................................... 141
A. [M(tppz)2]2+ Series (M = Ni“, Co“, Fe“, Mn“): ..................................................... 141
A1. Synthesis. .................................................................................................................. 141
A2. Spectroscopic and Cyclic Voltammetric Studies. .................................................... 142
A3. X-ray Crystallographic Results ................................................................................. 146
B. Synthesis of the Dimer [Ni2(tppz)(CH3CN)6][BE]. ................................................. 157
B1. Synthesis ................................................................................................................... 157
BZ. Spectroscopic and Cyclic Voltammetric Studies ...................................................... 158
B3. X-ray Crystallographic Studies. ................................................................................ 158
c. Reactivity Studies of [M(tppz)2]2+ (M = Niz“, Fe”, Co“, Mn“) and
[Ni2(tppz)(CH3CN)6]“. ................................................................................................... 160
D. Reactions of CoClz with tppz and tpy Ligands. ........................................................ 161
DJ. Synthesis. ................................................................................................................. 161
D2. Spectroscopic and Cyclic Voltammetric Studies .................................................... 164
D3. X-ray Crystallographic Studies ................................................................................ 164
E. Reactions of cis-[Rh2(02CCH3)2(CH3CN)6][BF4]; and [Rh2(CH3CN).o][BF4]4 with
tppz. ................................................................................................................................. 17 1

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E.1. Synthesis .................................................................................................................. 171

E2. Spectroscopic Studies. ............................................................................................. 172
E3. X-ray Crystallographic Studies. ............................................................................... 172
F. A Hybrid Ligand of 2,2’ Bypiridine-azo and Tetrapyridyl pyrazine: 2,4,6-Tris(2-

pyridyl)-l,3,5-triazine (tptaz) .......................................................................................... 178
El. Synthesis .................................................................................................................. 178
F2. Spectroscopic and Cyclic Voltammetric Studies ..................................................... 180
F3. X-ray Crystallographic Studies. ............................................................................... 183
IV. Conclusions ............................................................................................................... 191
V. References .................................................................................................................. 193
Chapter IV ..................................................................................................................... 195
I. Introduction .................................................................................................................. 196
II. Experimental Section .................................................................................................. 197
Physical Measurements. .................................................................................................. 197
Theoretical Details .......................................................................................................... 198
Synthesis .......................................................................................................................... 198
2-(2-pyridyl)-l,8-Naphthyridine (pynp) Synthesis. ......................................................... 198
Z-aminonicotinaldehyde. ................................................................................................. 198
Preparation of 2-(2-pyridyl)-1,8-Naphthyridine (pynp). ................................................. 199
[Moz(02CCH3)2(pynp)2][BF4]2 (30) ................................................................................ 199
[Ruz(02CCH3)2(pynp)2] [PF6]2 (31). ................................................................................ 200
[Rh2(02CCH3)2(pynp)2] [31:4]; (32). ............................................................................... 200
X-ray Data Collection and Refinement. .......................................................................... 201
[Moz(02CCH3)2(pynp)2] [BF4]2‘3CH3CN (30). ............................................................... 201
[Ruz(OzCCH3)2(pynp)2] [PF6]2'CH3OH (31) ................................................................... 203
[Rh2(OzCCH3)2(pynp)2] [BF4]2'C7H3 (32) ....................................................................... 203
[Rh2(02CCH3)2(pynp)2(CH3CN)2] [BF4] [P136] '2CH3CN (33). ........................................ 203
III. Results and Discusion ................................................................................................ 204
Homologous Series of Redox-Active Dinuclear Compounds ......................................... 204
A.l. Synthesis of Compounds [M2(02CCH3)2(pynp)]2+, M = Mo“, Ru“, Rh“. .......... 204
A2. X-ray Crystallographic Studies ................................................................................ 205
A3. Theoretical Calculations. ......................................................................................... 214
A.4. UV-Visible and NMR Spectroscopic Studies. ........................................................ 214
A5. Cyclic Voltammetric Studies ................................................................................... 216
A.6. Magnetic Studies. .................................................................................................... 220
III. Conclusions ............................................................................................................... 221
IV. References ................................................................................................................. 223
Appendix I. .................................................................................................................... 225

 

LIST OF TABLES

Page
1. Crystallographic Data for Compounds 1-15 .......................................... 4O
2. Selected bond distances (A) and angles (°) for
[Ni4(bptz)4(CH3CN)3][BF4]3'4CH3CN (1) 55
3. Selected bond distances (A) and angles (°) for
[Ni4(bptz)4(CH3CN)8][ClO4Js'2CH3CN'C4I-130 (3) 55
4. Distortions of the bptz ligand in the molecular squares ......................................... 57
5. Selected bond distances (A) and angles (°) for
[N12(bptZ)2(CH3CN)3] [C104]4 (5) ........................................................ 63
6. Selected bond distances (A) and angles (°) for
[Zn4(bptz)4(CH3CN)3][BF4]3‘4CH3CN (2) ............................................. 65
7. Selected bond distances (A) and angles (°) for
[Zn4(bpt2)4(CH3CN)3][C104]3'3CH3CN (4) ............................................ 65
8. Selected bond distances (A) and angles (°) for
[Ni5(bptz)5(CH3CN)5][Sng].o-2CH3CN (6) .......................................... 72
9. Selected bond distances (A) and angles (°) for
{[Mn(bptz)2(CH3CN)2][BF4]2}.. (7) .................................................. 80
10. Selected bond distances (A) and angles (°) for
[Ni4(bpt2)4(CH3CN)g] [BF4][PF6]3[SbF5]4CH3CN (8). . . . . . . . . . . . . . . . . .. 94

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11. Selected bond distances (A) and angles (°) for
[Ni4(bptz)4(CH3CN)g][C104][IO4]7'2CH3CN (9) ................................. 94

12. Selected bond distances (A) and angles (°) in
[Ni2(abpy)(CH3CN)2(NO3)4]'2CH3CN (12) ....................................... 101

13. Selected bond distances (A) and angles (°) for
[Cu2(abpy)(CH3CN)g][BF4]2 (13) .................................................. 102

14. Selected bond distances (A) and angles (°) in
{ [Cu(abpy)2][BF4]2 }.. (14) ............................................................ 103

15. Selected bonds (A) and angles (°) for

[Ni4(bptz)4(CH3CN)g][I][SbF6]7 (15) ................................................ 108
16. Crystallographic Data for Compounds 16-29 ....................................... 131
17. UV—Visible data for compounds (16)-(20) .......................................... 143

18. Values of the distortions exhibited by the tppz
ligand by coordination to Co(II) (17), Ni(II) (l6),
Mn(II) (19) and Fe(II) (18) ............................................................. 147

19. Selected bond distances (A) and angles (°) for
[Ni(tppz)2][N03]2 (16) ............................................................. 149

20. Selected bond distances (A) and angles (°) for
[Co(tppz)2][PF5]2'CH3CN (l7) ...................................................... 149

21. Selected bond distances (A) and angles (°) for
[Fe(tppz)2][ClO4]2 (18) ............................................................... 152

22. Selected bond distances (A) and angles (°) for
[Mn(tppz)2] [C104]2 (19) ............................................................... 152

xi

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23. Selected bond distances (A) and angles (°) for
[N i(tppz)2][ClO4]2. (20) ................................................................. 155

24. Selected bonds (A) and angles (°) for
[Ni2(tppz)(CH3CN)6][BF4]4-CH3CN (21) .......................................... 160

25. Selected bond distances (A) and angles (°) for
[C02C12(tPY)2(CH3CN)2][BF4]2. (26) ................................................ 165

26. Selected bond distances (A) and angles (°) for
{[Coz(tppz)(CH3CN)2Cl2][BF4][PF6] - CHzClz }.. (22) .............................. 167

27. Selected bond distances (A) and angles (°) for
{[Coz(tppz’)2][PF6][C02Cl7] (25) .................................................... 169

28. Selected bonds distances (A) and angles (°) for
[Rh4(02CCH3)2(tppZ)2(CH3OH)4] [PF6]6'CH3OH (24) ......................... 173

29. Selected bond distances (A) and angles (°) for
[Rh(tppz)2][BF4]3-2CH3CN'C4H30 (23) ........................................... 176

30. UV-Visible spectroscopic data for compounds (27)-(29) .......................... 181

31. Selected bond distances (A) and angles (°) for
[Ni(tptaz)(CH3CN)2(H20)][BF4]2 (27) ................................................. 184

32. Selected bond distances (A) and angles (°) for
[Fe(tptaz)2][C104]2-2CH3CN (28). .................................................. 186

33. Selected bond distances (A) and angles (°) for
{[Mn(tPtaZ)(phen)(H20)][CF330312 }

{[Mn(tptaz)(phen)(H20)][CF3803]; }°H20 (29) ................................... 188
34- Crystallographic Data for Compounds 30-33 ........................................ 202
xii

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35. Selected bond distances (A) and angles (°) for
[Moz(OzCCH3)2(pynp)2] [BF412'3CH3CN (30) .................................... 207

36. Selected bond distances (A) and angles (°) for
[Ruz(02CCH3)2(pynp)2][PF6]2°2CH3OH (31) ..................................... 209

37. Selected bond distances (A) and angles (°) for
[Rh2(02CCH3)2(pynp)2][BF4]2-C7Hg (32) ............................................ 21 l

38. Selected bond distances (A) and angles (°) for
[Rh2(OzCCH3)2(pynp)2(CH3CN )2] [BF4[PF6]'2CH3CN (33) ..................... 21 1

39. UV-Visible data for compounds (30)-(32) .......................................... 214

40. Ratio of Metal to Ligand (MIL) and Ligand to Metal (UM)
character of the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO) as calculated
at the B3LYP level of theory for the dication [M2(OZCCH3)2(pynp)2]2+
(M = M0“, Ru2+ and Rh“) of (30), (31), and (32) ................................... 230

41. Orbital Energies (hartrees) and Ratios for Important Molecular
Orbitals in singlet (S = 0) dinuclear complex Mo(II) (30)
and dinuclear complex Rh(II) (32), dication, neutral and dianion species ....... 231

42. Molecular Orbital Energies for the Pertinent Molecular Orbitals
in triplet (s = 1) [RU2(02CCH3)2(PYHP)2]2+ (31).
doublet (s = 1/2) (2)<'*"‘> and singlet (s = 0) 31m" .................................... 232

xiii

 

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LIST OF FIGURES

Page

1. Schematic drawing of the ligands (a) 2,2’-bipyridine,

(b) l, lO-phenanthroline, (c) 2,2’:6’,2”-terpy1idine and

(d) l,3,5-tris[4’-2,2’:6,2”-terpyridinyl]benzene ......................................................... 3
2. Schematic drawing of the ligands (a) 2-2-pyridyl-1,8 naphthyridine (pynp),

(b) 3,5-bis-pyridyl- 1,2,4,6-tetrazine (bptz), (c)

2-azo-bis pyridyl (abpy), (d) l,3,6-trispyridyl-triazine (tptaz)

and (e) 2,3,5,6 tetrapyridyl pyrazine (tppz) ................................................................ 6
3. Some examples of metallodendrimers ....................................................................... 9
4. Supramolecular chemistry hierarchy ....................................................................... 10
5. Schematic drawing of the molecule [(tpy)Ru(tppz)Ru(tpy)]4+ ............................... l3
6. Schematic drawings of metal tppz building blocks ................................................. 15

7. Schematic drawing of the molecule [Rh4(OzCCH3)2(tppz)2(CH3OH)4][PF6]6 ,,,,,,,,,,,, 16

8. Different functionalities in the ligands (a) l,3,6-t1ispyfidyl-triazine

(tptaz) and (b) 2-azo-bis pyridyl (abpy) .................................................................. 17

9. Compounds with the radical form of the ligand abpy .............................................. 19

10. Molecular orbitals for the M-M species ................................................................ 20
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15.

l6.

17.

18.

Schematic drawing of the series [M2(02CCH3)2(pynp)2]2+ (M = Mo(II),

Ru(H) and Rh(II)) .................................................................................................. 21
Possible outcome of linking four cis-[M2(OzCCH3)2(pynp)2]2+ units ................... 23
Schematic drawing of (a) 3,6 bis(2—pyridyl)tetrazine (bptz) and

(b) 2,2’ bispyridyl azo (abpy) ................................................................................ 31
Schematic drawing of the distortions experienced by the ligand bptz in the
tetrametallocyclophanes ........................................................................................ 56
(3) Thermal ellipsoid representation of the cation in

[Ni4(thZ)4(CH3CN)8] [BF418'4CH3CN -
(b) Assymetric unit with 50% probability ellipsoids.

Hydrogen atoms have omitted for the sake of clarity .......................................... 58

Thermal ellipsoid representation of the cation in
[Ni4(bpt2)4(CH3CN)s][C104ls'2CH3CN'C4HsO-(3)
(b) Assymetric unit with 50% probability ellipsoids.

Hydrogen atoms have been omitted for the sake of clarity ................................ . 59

Space filling diagrams of the cations in

(a) [Ni4(bptz)4(CH3CN)g][BF4]3 4CH3CN and

(b) [Ni4(bptl)4(CH3CN)s][C104]s.. 2CH3CN C4HsO

with the encapsulated anion"

Packing diagram of [Ni4(bptz)4(CH3CN)g][BF4]8~4CH3CN .......................

19. Thermal ellipsoid representation of the cation at the

20.

50% probability level in [Ni2(bptz)(CH3CN)g][C104]2.
Hydrogen atoms have been omitted for the sake of

(a)Thermal ellipsoid diagram of the cation in
[Zn4(bptz)4(CH3CN)g][BF4]3-4CH3CN. (b) Asymmetric unit in
[Zn4(bptz)4(CH3CN)g][BF4]3-4CH3CN at the 50% probability level.

Hydrogen atoms have been omitted for the sake of clanty . . . . . . . . . . ..

XV

...60

61

62

66

21. (a) Thermal ellipsoid di gram of the cation in
[Zn4(bptz)4(CH3CN)s][C104]8°CH3CN.
(b) Asymmetric unit in [Zn4(bptz)4(CH3CN)g][C104]3°CH3CN.
Thermal ellipsoids are drawn at 50% probability level, and
hydrogen atoms have been omitted for the sake of clarity ................................. 67

22. Variable temperature "’13 NMR spectra for
[Zn4(bptz)4(CH3CN)g][BF4]3 in CD3CN 69

23. (a) Thermal ellipsoid drawing of the molecular cation [Ni5(bptz)5(CH3CN)lo]'°+.
Thermal ellipsoids are drawn at the 50%
probability level and (b) space-filling diagram of the
molecular cation with the encapsulated [SbF6]' ..................................................... 73

24. Thermal Ellipsoid drawing at the 50% level of
the asymmetric unit of [Ni5(bptz)5(CH3CN)m]10+ in
[Nis(bpt2)s(CH3CN)lo][SbFo]lo'ZCHaCN (6).
Hydrogen atoms were omitted for the sake of clarity ...................................... .74

25. A skeletal view of the Ni5(bptz)5 pentagon superimposed
on an ideal pentagon to emphasize the distortion of the ligands ........................... 75

26. Packing diagram of [Ni5(bptz)5(CH3CN)lo][SbFo] 10' .................................. 76

27. Possible outcomes for the reaction between of bptz with an octahedral-metal
precursor ................................................................................................................ 78

28. (a) Thermal ellipsoid diagram view of a portion of the polymer
{[Mn(bptz)2(CH3CN)2]2+}.. at the 50% probability level, (b) asymmetric unit.
Hydrogen atoms were omitted for the sake of
clarity ................................... , ........................................................................... 81

29. Thermal ellipsoid drawing of the mononuclear complexes (a)
[C0(b0pd)2(CH30H)2][PFo]2 and (b) [CU(b0pd)2flizOz)][BF4]2
Thermal ellipsoids are drawn at the 50 % probability level.
Hydrogen atoms were omitted for the sake of clarity ........................................ 82

xvi

 

30. ESI-MS spectra of the 1:1 reaction between [Mn(HzO)6][ClO4]2
and bptz taken after 10 minutes .................................................................. 85

31. ESI- MS spectra of the 1: 1 reaction between [Co(HzO)6][ClO4]2
andbptztaken after lOminutes... .. ... . 86

32. ESI- MS spectra of the 1:1 reaction between [Fe(HZO)6][ClO4]2
andbptztakenafterlOminutes.... 87

33. ESI— MS spectra of the 1:1 reaction between [Cu(HzO)6][ClO4]2
andbptztaken after lOminutes... ...... . 88

34. ESI-MS spectra in acetonitrile of the metallocyclophanes
(a) [Ni4(bpt2)4(CH3CN)sl[BF4]s. (b) [Ni4(bpt2)4(CH3CN)a][(310413
and (c) [Zn4(bptz)4(CH3CN)g][C104]3 ................................................................... 89

35. ESI-MS spectra of the pentagon compound
[N i 5(bptz)5(CH3CN)lo][SbF6]lo dissolved in acetonitrile ...................................... 90

36. ESI-MS spectra of [Ni4(bptz)4(CH3CN)g][C104]3 in different solvents.
The peak being depicted is the parent ................................................................... 91

37. (a) Thermal ellipsoid drawing of the [N 14(bptz)4(CH3CN)g][BFs][SbF(,]6+
unit in [Ni4(bptz)4(CH3CN)g][BF4][PF6]3[SbF6]4 with 50%
probability ellipsoids, (b) space filling diagram and (c) asymmetric unit.
Hydrogen atoms have been omitted for the sake of clarity..............................95

38. (a) Thermal ellipsoid drawing of the [Ni4(bptz)4(CH3CN)g][C104][104]6+ unit in
[Ni4(bptz)4(CH3CN)g] [C104] [104]7 with 50% probability ellipsoids,
(b) space filling diagram that emphasizes the molecular volume of
the perchlorate versus the periodate and (c) asymmetric unit.
Hydrogen atoms have been omitted for the sake of clarity .................................. 96

xvii

 

 

 

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39.

40.

41.

42.

43.

45.

46.

47.

48.

Conversion of the square into the pentagon by addition of an excess of TBASbF6
to a refluxing solution of [Ni4(bptz)4(CH3CN)g][C104]3.

(a) represents the initial sample of the square and

(b) represents the final solution ........................................................................... 98

Thermal ellipsoid plot of [Ni2(abpy)(CH3CN)2(NO3)4].2CH3CN (12)
at the 50% probability level.
Hydrogen atoms have been omitted for the sake of

Thermal ellipsoid plot of the cation in
[Cuz(abpy)(CH3CN)g][BF4]4 (13) at 50% probability.
Hydrogen atoms have been omitted for the sake of clarity ................................. 105

(a) Thermal ellipsoid plot of the basic unit in the polymer
{[Cu(abpy)2][BF4] L... (b) an extended portion of the polymeric chain ................. 106

Schematic drawing of the molecular square and pentagon
with indicated sites for substitution ................................................................... 109

. (a) Thermal ellipsoid plots of the cation in

[Ni4(bptz)4(CH3CN)3][I][SbF6]7 (15) at the 50% probability ellipsoid level,
(b) space filling diagram with the iodine anion inside the
metallocyclophane and (c) asymmetric unit.

Hydrogen atoms have been omitted for the sake of

Schematic drawing of the ligand 2,3,5,6-tetrapyridylpyrazine (tppz) ................. 120
The three coordination sites in Tptaz ................................................................... 121

Cyclic voltammogram for [Fe(tppz)2][ClO4]2 in
acetonitrile with 0.1 M [n—BusN][PF6] ................................................................ 144

Different types of distortion experienced by tppz upon coordination .................. 148

xviii

 

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49. Thermal ellipsoid representation of the [Ni(tppz)2][NO3]2-CH3CN (16)
molecular cation with 50% probability ellipsoids.
Hydrogen atoms have been omitted for the sake of

50. Thermal ellipsoid representation of cation in
[Co(tppz)2][PF6]2-CH3CN (17) with 50% ellipsoids.
Hydrogen atoms have been omitted for the sake of clarity ................................ 151

51. Thermal ellipsoid representation of cation in
[Fe(tppz)2][ClO4]2 (18) with 50% ellipsoids.
Hydrogen atoms have been omitted for the sake of

52. Thermal ellipsoid representation of cation in
[Mn(tppz)2][ClO4]2-CH3CN (19) with 50% ellipsoids.
Hydrogen atoms were omitted for the sake of clarity ....................................... 154

53. Thermal ellipsoid representation of cation in
[Ni(tppz)2][ClO4]2-CH3CN (20) with 50% ellipsoids.
Hydrogen atoms have been omitted for the sake of clarity ................................. 156

54. Thermal ellipsoid diagram of the molecular cation in
[N12(tppz)(CH3CN )6][BF4]4 CH3CN (21) with 50% probability ellipsoids.
Hydrogen atoms are omitted for the sake of clarity... . 159

55. Schematic drawing of the proposed synthesis of [C03(tpy)2(tppz)2]6+
from the reaction of [Co(tppz)2][PF5]2 with two equivalents
of [Co(tpy)(CH3CN)3][BF4]2 .............................................................................. 162

56. Rearrangement of the tppz ligand in the presence of CoClz .............................. 163

57. Drawing of the cation in [C02Clz(tpy)2(CH3CN)2][BF4]2 (26)
with 50% probability ellipsoids.
Hydrogen atoms were omitted for the sake of

xix

 

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59.

60.

61.

62.

63.

65.

(a) Thermal ellipsoid representation of the 1-D polymer
{[C02C12(tppz)2(CH3CN)2][BF4][PF6]}... (22) with 50% probability ellipsoids.
(b) Asymmetric unit diagram with 50% probability ellipsoids ........................... 168

Thermal ellipsoid diagram of [Coz(tppz’)2][C02Cl7][PF6] (25)

(a) ligand unit, (b) and (c) metallocyclophane, (d) asymmetric unit

and (e) packing diagram with 50% probability ellipsoids.

Hydrogen atoms have been omitted for clarity 170

Thermal ellipsoid diagram of the asymmetric unit in
[Rh4(02CCH3)2(tPPZ)2(CH30H)4][PFolo'CH30H (24)
at the 50% probability level ................................................................................ 174

(a) Thermal ellipsoid representation of the full molecular cation in
[Rh4(02CCH3)2(tPPZ)2(CH30H)4][PFoIo'CflsOH (24)

at the 50% probability level, (b) space filling diagram of the cation.

Hydrogen atoms have been omitted for the sake of clarity 175

Representation of the cation in [Rh(tppz)2][BF4]3-C4HgO-2CH3CN (23)
with 50% probability thermal ellipsoids.
Hydrogen atoms have been omitted for the sake of clarity ................................ 177

Cyclic voltammogram of [Fe(tptaz)2][ClO4]2 in 0.1M TBAPF6
acetonitrile at a Pt disk electrode versus Ag/AgCl ............................................ 182

Thermal ellipsoid plot of the cation in
[Ni(tptaz)(CH3CN)2(HzO)][BF4]2 (27) at the 50% probability level.
Hydrogen atoms have been omitted for the sake of clarity........... . . . . . ............l85

Thermal ellipsoid plot of the cation [Fe(tptaz)2]2+
at the 50% probability level.
Hydrogen atoms have been omitted for the sake of

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67.

68.

69.

70.

71.

72.

73.

Thermal ellipsoid plot of the six coordinate Mn" ion in
1[Mn(tptaZ)(phen)(H20)l[CF380312 l 1[Mn(tpta2)(phen)(H20)][CF380312 }'H20 (29)
at the 50% probability level.

Hydrogen atoms have been omitted for the sake of

Thermal ellipsoid plot of the heptacoordinate Mn" ion in
1[MDOPtaZXphenXHzOH[CF380312 } { [Mn(tPtaZ)(Phen)(H20)l [CF330312 l'HzO (29)
at the 50% probability level.

Hydrogen atoms have been omitted for the sake of

Schematic drawings of the bpnp and pynp ligands ............................................ 196

Thermal ellipsoid plot at the 50% level of the cation in
[M02(02CCH3)2(P)’HP)2][BF412'CH3CN (30)-
Hydrogen atoms have been omitted for the sake of

Thermal ellipsoid plot of the cation in
[Ruz(OzCCH3)2(pynp)2][PF6]2-CH3OH (31) at the 50% probability level.
Hydrogen atoms have been omitted for the sake of clarity ......................... 210

Thermal ellipsoid plot of the cation in
[Rh2(02CCH3)2(pynp)2][BF4]2-C7H,3 (32) at the 50% probability level.
Hydrogen atoms have been omitted for the sake of clarity 212

Thermal ellipsoid plot of the cation in
[Rh2(02CCH3)2(pynp)2(CH3CN)2][BF4][PF6]2~CH3CN (32)
at the 50% probability level.

Hydrogen atoms have been omitted for the sake of

Cyclic voltammograms for compounds (30)-(32) in acetonitrile with 0.1 M
[n-BuaN][PF6] at 3 Pt disk electrode versus Ag/AgCl ....................................... 218

xxi

 

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74.

75.

76.

Diferential pulse voltammogram of [Rh2(OzCCH3)2(pynp)2][BF4]2 in
0.1 M TBAPF6 acetonitrile at a Pt disk electrode versus Ag/AgCl ................... 219

Plot of xT versus T (red) and 1 versus T (blue)
for [RU2(02CCH3)2(P)’HP)2][PFoiz at an applied
field of 1000 G .............................................................................................. 220

B3LYP calculated 0.05 isodensity surface of the HOMO and LUMO for
[M2(02CCH3)2(pynp)2]2+, where AE(B3LYP) is the calculated difference between the
energy of the HOMO and LUMO, and AE(EXP) is the experimentally determined
excitation energy ........................................................................................... 234

xxii

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ESI

LIST OF SYMBOLS AND ABBREVIATIONS

xxiii

Angstrom

2-azo-bis-pyn'dyl

Silver-silver chloride reference electrode
Boltzmann constant
Tetrakis(3,5-bis(triflouromethyl)phenyl)
borate
2,5-(2-pyridyl)—1,3,4-oxadiazole
3,5-bis-py1idyl— 1,2,4,6-tetrazine

2-7 bis-(2-pyridyl)-l,8 naphthyridine
2,2’-bipyridine

2,2’ bipyrimidine

broad

centimeter

cyclic voltammetry

degree centi grade

doublet (NMR)

parts per million (ppm)

half-wave potential

anodic peak potential

cathodic peak potential
electromagnetic unit

electrospray ionization

'0‘:

’4’}!

4553‘“

v us

‘Cfl

51

‘Ju_l

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- 410'

HOMO

(M)Hz

mg

min

mL

mmol

nm

“P

xxiv

molar extinction coefficcient
Lande factor

Gauss

hour

Hamiltonian

Highest Occupied Molecular Orbital
(mega)hertz

infrared

Exchange parameter

Kelvin

Lowest Unoccupied Molecular Orbital
medium

moles per liter

milligram

minute

metal-to-ligand charge transfer
milliliter

millimole

mass-to-charge ratio

bridging ligand

Avodagro’s constant
nanometer

naphthyridine

‘a
I“\

.\.‘

1n

.5.

-D;F

, b

mllw but
a s. L hD
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sh
SQUID
TBABF4

TBAPF6

TBAI

TBAIO4

Tptaz

Tppz

tP)’

VS

XXV

nuclear magnetic resonance
trifluoromethanesulfonate
(triflate, CF3S03')

phenanthroline

parts per million

2-2’-pyridyl-1,8 naphthyridine
singlet (NMR), strong (IR)
shoulder

Super Quantum Interference Device
tetra-n-butylammonium tetraflouroborate
tetra—n—butylammonium
hexafluorophosphate
tetra-n-butylammonium iodide
tetra-n-butylammonium periodate
tetrah ydrofuran
1,3,6-trispyridyl-triazine

2,3,5,6 tetrapyridyl pyrazine
2,2’;6’,2”-terpyridine

ultraviolet

Volt

frequency

VCI‘SUS, very strong

 

ll

w weak

ZFS Zero-field splitting

xxvi

Chapter I

Introduction

 

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I. Background

Nitrogen Heterocyclic Ligand Chemistry

The use of nitrogen heterocyclic ligands in coordination chemistry has witnessed
a steady increase in the last four decades. The versatility of synthetic methods used to
prepare these molecules has allowed chemists to design increasingly complex derivatives
with different applications in mind. Among the ligands that have been investigated are
the common diimine ligands such as 2,2’-bipyridyl (Figure 1a), 1,10-phenanthroline
(Figure 1b), 2,2’:6’,2”-terpyridine (Figure 1c) as well as more elaborate ones such as
1,3,5-[tris[4’-(2,2’:6’,2”-terpyridinyl)] benzene (Figure 1d).1

One of the most simple and well studied coordination compounds based on
diimine ligands is the tris-chelate complex [Ru(bpy)3]2‘i.2 In general, coordination
compounds of this type based on the 2,2 bipyridine motif exhibit strong luminescence in
solution at room temperature, and have a powerful photosensitization capacity for
electron and energy-transfer processes.3 The photoluminescent excited state is a strong
reductant as well as an excellent oxidant due to the presence of an electron deficient d5
metal center and an excess electron located in the ligand network in the excited state.
Modifications to the 2,2’ bipyridine unit permits one to tune of the redox potentials of the

ligands over a significantly wide range.4 Such strategies have allowed for the synthesis of

Ru(H) complexes that are either good oxidants or reductants in the excited state.2

 

“We 1.

(bl l
(d) l

 

Figure 1. Drawing of the ligands (a) 2,2-bipyridine,

(b) 1,10-phenanthroline,(c) 2,2':6',2"-terpyridine and
(d) 1,3,5-tris[4'-2,2':6,2"-terpyridinyl]benzene.

.Uit‘t‘d i 1;;

Mix:

One important application of metal polypyridyl chemistry is in solar energy
conversion processes.:"5 For example, a major use for the photosensitiser [Ru(bpy)3]22+ is
the photodissociation of water by visible light, but a major drawback is that it can only
transfer a single electron to a substrate.3 Consequently, much effort has been expended
over the last ten years to design polymetallic systems that incorporate numerous

[M(bpy)2]2+ moieties within the same molecule.6

Mixed Valency in Ligand-Bridged Metal Assemblies

Mixed-valence materials that contain several redox sites in more than one
oxidation state have attracted the interest of chemists for many years. Robin and Day7
defined three broad classes of mixed-valence materials that are referred to as Class I, II
and HI compounds. Class I mixed-valence species are those in which the interaction
between redox centers bridged by a ligand is so weak that the mixed-valence material
exhibits the properties observed for isolated mononuclear species. For Class III
compounds, the opposite is encountered, namely the interaction between the two centers
is so strong that the properties of individual redox centers are not observed, but rather
new properties characteristic of the coupled redox units are exhibited, i.e., the system is
delocalized. Intermediate between these two categories are Class II materials that exhibit
redox characteristics which are slightly altered from the properties of the isolated units;
these are referred to as partially delocalized systems.

Cyclic voltammetry is an invaluable tool for establishing the placement of any
compound within the Robin-Day classification scheme. Based on the precedence of

Taube’s work, it is possible to define a comproportionation equilibrium constant, K, for

 

 

 

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the equilibrium between two valence states; the separation between two successive

reversible redox potentials allows for its calculation as shown in equation 1.8

 

 

[xwxl + lxwxlz' zlxwxl-,K.

(Eq- 1)
where anc = eprEl/a (in mV)
25.69

If there is no communication between the redox centers (Class I), the Kc constant is S
102. In the case of complete delocalization between the redox centers, KC is very large, 2
106. The intermediate situation, in which there is electronic coupling but not full

delocalization, yield Kc values in the range 102< K¢<105.
II. New Applications of Bipyridine and Terpyridine Chemistry

One of the current challenges in chemistry is the manipulation of noncovalent
interactions in the design of new materials in a manner akin to Nature. Chemists are
striving to design building blocks, often called tectons, that self-assemble into larger
conglomerates with the ability to store information in the form of electronic, magnetic or
redox properties.9 Much of the supramolecular chemistry in recent years has involved the
self—assembly of metal containing molecules with the use of polydentate nitrogen ligands.
The work reported in this dissertation involves the application of nitrogen heterocyclic
chemistry in areas where they have not been traditionally employed. Specifically, the
research involves the coordination chemistry of the five ligands in Figure 2. These

include (a) 2-2-pyridyl-1,8-naphthyridine (pynp), (b) 3,6-bis-pyridyl-l,2,4,6-tetrazine

\/ \_/ \/

bptz

 

 

Figure 2. Schematic drawing of the ligands (a) 2-2-pyridyl-1,8 naphthyridine (pynp),
(b) 3,6-bis-pyridy1- 1,2,4,6-tetrazine (bptz), (c) 2-azo-bis pyridyl (abpy),
(d) l,3,6-trispyridyl-triazine (tptaz) and (e) 2,3,5,6 tetrapyridyl pyrazine (tppz).

 

 

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(bptz), (c) azo-2,2’-bipyridine (abpy), (d) 1,3,6-trispyridyl-triazine (tptaz), and (e)
2,3,5,6 tetrapyridyl-pyrazine (tppz). In the past, such ligands have been used mainly in
reactions with Ru(II) and Os(II) centers,l0 but in the present work, they are used to
assemble arrays of paramagnetic first row metal ions and low valent second row

transition metal ions that form metal-metal bonds.

 

O Q
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Scheme 1.

The ligands have been chosen on the basis of their demonstrated ability to produce
compounds with interesting electronic and redox properties.ll Ligand a (pynp) bridges
dinuclear units through the naphthyridine unit, thereby allowing for the formation of
molecules with the dimetal core "[M2(pynp)2]2+". Ligands b-e possess two or more

coordinating domains that permit the synthesis of polymetallic arrays in the form of

1U'9

 

 

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cyclic units or polymers. Ligands b (bptz) and c (abpy) share some common features,
i.e., both can be envisioned as bis-pyridyl units joined by either one or two azo (tetrazine)
groups (scheme 1).ll The azo group possesses low lying 112* orbitals (LUMO) that are
suitable for promoting good electronic communication between metal centers.H

In the case of ligands d (tptaz) and e (tppz), the triazine and pyrazine rings are
known to provide a good it pathway for electronic or magnetic coupling. Both ligands
possess terpyridine coordination sites.‘2

Each chapter in this dissertation describes the chemistry of a specific ligand and
its application in the design of tectons (building blocks) and their intended use in building
larger molecules. The dissertation is organized into four chapters including the

introduction.

A. Bipyridine-based ligands in Paramagnetic Arrays

Supramolecular Chemistry ofpolytopic bpy ligands

The design of arrays with the goal of exploring electronic or magnetic coupling
between metal centers has been under intense investigation in the last three decades.'3
The synthesis of large heterocyclic receptors (ligands) with multiple coordination
domains (polytopic), has enabled chemists to design large molecules that exhibit new
properties as a result of cooperative effects between the constituent building blocks. An
excellent example of this approach can be found in the family of materials known as
metallodendrimers (Figure 3).” Progress in this field requires the availability of

molecular components (building blocks) with well-defined structures and properties.

 

Figure 3. Some examples of metallodendrimers.12

This approach has major drawbacks, namely multistep procedures similar to a typical
organic covalent approach, which results in low yields for the synthesis of large
molecules. The self-assembly of large organic molecules in living organisms takes place
thorough collective weak forces including ion-pairing,'5 hydrophobic or hydrophilic
interactions,’6 hydrogen-bonding,l7 host—guest interactions,l8 rt-stacking,l9 and donor-
acceptor interactions.20 By manipulating these supramolecular forces, the linkage of small

units (molecules) with useful characteristics to form large entities (supramolecular

 

Macroscopic conglomerate

 

Supramolecular assembly

 

 

Supramolecular , ,
array = new tecton .
Increasrng
Superstructural
Complexity

Supramolecular assembly

 

Building Block
(T ecton) = Primary structure

 

Figure 4. Supramolecular chemistry hierarchy18

10

 

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species) can be more efficiently achieved. Supramolecular chemistry is described as the
chemistry of the “intermolecular bond”, as defined by J. M Lehn.”

The driving force in supramolecular chemistry is the self-assembly of larger
entities from specifically engineered smaller building blocks. Once these smaller entities
have been assembled, they may perform complex functions such as light harvesting,22
conversion of light into chemical23 or electrical energy“, or function as memory
devices.” Supramolecular systems are often categorized according to different levels of
hierarchy. Primary structure corresponds to small building blocks that are referred to as
m. In purely organic supramolecular structures, tectons aggregate via noncovalent
bonds to generate polymolecular aggregates called superrnolecules. These

superrnolecules can, in turn, associate with one another to render gigantic macroscopic

conglomerates (Figure 4).263

Specific Supramolecular Interactions: Anions as Templates

As described in the previous section, non-covalent interactions (hydrogen
bonding, 7t stacking, van der Waals interactions, cation or anion interactions) play an
essential role in the assembly of supramolecular structures. Of particular relevance to this
dissertation is the use of anions as templates in the assembly of metallocyclophanes.
Templating is one of the more efficient procedures for self-assembly at the disposal of the
chemist. It involves the use of temporary or permanent ‘helper” species, of organic or
inorganic nature, to assist in the process of assembly. The vast majority of the template
effect cases reported in the literature are cation related.27 Among these, crown ether
chemistry is the most prominent. By comparison, the study of anion templates is much

less common, and there are relatively few cases reported in the literature to date.28

11

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Chapter 11 describes self-assembly processes in which the divergent ligand, 3,6-bis
pyridyl-tetrazine (bptz) permits the self-assembly of metallocyCIOphanes of different
nuclearities. The bptz ligand is a bis-bipyridine chelate that is capable of assembling
metal cations into cyclic olygomers or polymers. The overwhelming factor in dictating
the outcome of the reaction was found to be the size and shape of the anion. The results
presented in Chapter II constitute an important contribution to the area of
metallocyclophane chemistry and underscore the role of the anion choice in forming a
specific nuclearity for cationic assemblies. It is important to point out that previous
research in the formation of cationic molecular square has not address the role of the

anion (scheme 2).

I Divergent Bridging
Ligand
With two trans
Divergent Metal Precursor binding sites

with six labile positions

Scheme 2.

B. Temyridine ligands in Paramagnetic Arrays

Despite their outstanding photochemical properties, [M(bpy)3]2+ complexes
present two major drawbacks as building blocks for polymetallic arrays, namely stereo

and/or geometric isomerism. The use of ligands with the tpy (tpy = terpyridine)

12

coordination unit are advantageous from the geometric point of view, namely [M(tpy)2]“+
complexes are achiral.29 Synthetic tailoring of tpy based ligands has allowed chemists to
design ligands with multiple tpy binding sites. For example the tris-tpy ligand depicted in
Figure 1d forms large polymetallic arrays based solely on coordination bonds.3'4”5'6 The
main goal of this chemistry is the construction of nanosized components by a “bottom-
up” approach, i.e., beginning with molecular components. The molecules are highly
branched tree-like species commonly called dendrimers and are designed with specific
properties such as the capability to absorb visible light, to luminesce, or to undergo
reversible multielectron redox processes. Extensive research has been carried out in this
area30 with Ru(II) and 05(11):“ being incorporated into building blocks called diads (two
metal centers) and triads (three metal centers) based on [M(tpy)2]2+ type units that can

function as photosensitizers.32

4+

  

/\/\\/
/\ N/
/

\
\ _\\/

/

 

 

F /\

Figure 5. Schematic drawing of the molecule [(tpy)Ru(tppz)Ru(tpy)]4+.

One of the more important characteristics of metallo-tpy arrays is that they exhibit

mixed-valence behavior.33 For example, the complex [(tpy)Ru(tppz)Ru(tpy)]4+ with two

13

 

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equivalent Ru(II) centers (Figure 5)17 exhibits two oxidation couples corresponding to
Ru(H)/Ru(III) and Ru(HI)/Ru(III) species. The large AEV, value for the oxidation
processes is evidence for strong delocalization through the tppz ligand (Figure 2e) which
stabilizes the mixed valence state Ru(H)/Ru(III).

In addition to homometallic systems, multibranched ligands can be used to
synthesize heterometallic systems, e.g., [(tpy)Ru(tppz)erl3]2+.l7 In this case the
ruthenium center is the light absorbing site whereas the Ir(III) center is a catalytic center.
These types of mixed-metal systems present the possibility for developing
supramolecular complexes with widely varying functions due to the different metal
centers. In this manner, compounds with new properties that arise from cooperative
effects are being discovered.

As previously mentioned, polymetallic arrays often undergo multiple electron
redox processes. This versatility in redox chemistry can be envisioned as providing a
switching control, namely the possibility of turning a particular intermolecular interaction
"on" or "off". Conceivably one could affect the strength of different binding sites by
means of redox chemistry, thereby allowing for control over variables such as nuclearity
or binding specificity towards a specific analyte (i.e., a sensor).34 Constable and
coworkers offer an excellent example of this approach wherein the nuclearity of a
complex depends upon the redox state of the chelated metal ion.35

One important aspect of supramolecular compounds based on coordination bonds
is that the presence of metal atoms in the building blocks generates new types of
molecular interactions, which are characteristic of inorganic systems. The self-assembly

approach for the design of metal-based supramolecules offers an alternative to the

14

 

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classical organic route, in which one building block is added in each step of the synthesis.
This process takes advantage of metal-ligand interactions that are fairly kinetically labile.
The resulting supramolecular entities are often obtained in high yields and require fewer
steps than equivalent covalent syntheses. The reversibility of the coordination bonds
present in the assembly contribute to a defect-free product because the intermediates are
in equilibrium with each other as the final, more thermodynamically stable compound is

being formed36

Tridentate

I | — Tridentate
coordination N/ \ *—

coordination

 

 

 

site site
/ \ / \ I”
H N CCH3 .
Iabile 3C0 \ )1 LabrleI .
acetonitrile H3CCN—M—N / \ N—M—NCCH3 acetonrtnle

molecules N/ \ _ / \ molecules
H30C I \ NCCH3
_ \ / J

Figure 6. Schematic drawings of metal tppz building blocks.

Ligands based on the terpyridine binding motif have not been used extensively to
connect first row transition metals. Chapter III is devoted to the self-assembly of

paramagnetic metal ions of the 3d elements with the ligand tppz (Figure 2c). The general

 

 

“hummer.

it mm

 

goal is to probe the magnetic behavior of metal-tpy arrays. The tppz molecule, reported in
1959 by Lion and coworkers,37 possesses two tpy coordination domains that allow for the
preparation of dimetallic or higher order metal-arrays. Three precursors of the type
[M(tppz)2][X]2 (where M = Coz", Fe“, Ni2+ and x = [N03]‘, [c104]; [Pm and [31:41)
were synthesized (Figure 6). They exhibit different degrees of electronic coupling
between the two tppz units ranging from complete delocalization (Class HI) for

[Fe(tppz)2][ClO4]2 to no electronic interaction at all in the case of [Ni(tppz)2][NOg]2.

 

 

— -_i 6+
A N
O O
i .~‘\ ,e“
HsCHO—Rh Rl one... NVT ......
N
\r NVN N'\
l l»
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l/ i N N
/
H3CHO Rh IRh—OHCHa o
\‘ \‘
N\\ i N\\\ i
O O -02CCH3
V O
__ __.i

 

Figure 7. Schematic drawing of the molecule [Rh4(OzCCH3)2(tppz)2(CH3OH)4][PF6]6.

The mononuclear building blocks [M(tppz)2]"+ posses two dangling tpy binding
sites for further coordination to other metal ions. In addition to the electrochemical

behavior, some of the molecules exhibit interesting magnetic properties, for example

16

 

.... ...;i
p.31. :

{f flM/l

 

 

[Co(tppz)2][PF6]2 exhibits spin crossover behavior. Besides the mononuclear compounds,
a dinuclear [Ni2(tppz)(CH3CN)6][BF4]2 and a polymer of the form
[Coz(tppz)(CH3CN)2C12][BF4][PF6]... were also synthesized and structurally
characterized.

An unexpected application for tppz is reported in chapter 111, namely as a bridge
between two metal-metal bonded units (M-M). Earlier reports by Pruchnick and
coworkers38 indicated that two tpy ligands can bind to one dirhodium unit, viz, in the
compound [Rh2(OzCCH3)(tpy)2]Clz. This result prompted us to attempt the use of tppz to
link two [ha]4+ units, which we believed could occur to give and open structure. Instead
the linkage of two [Rh2(OzCCH3)]3+ units led to the formation of a molecular rectangle

composed of short Rh-Rh bonded sides and long Rh-tppz-Rh sides (Figure 7).

C. Dual Bipyridine/Temyridine ligands

Terpyridine domain

 

 

  
 

Azo group
Bipyridine domain

 

Figure 8. Different functionalities in the ligands (a) 1,3,6-trispyridyl-triazine
(tptaz) and (b) 2-azo-bispyridyl (abpy).

17

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Chapter IH also contains a brief account of the coordination chemistry of the
ligand 2,4,6-tris-pyridyl triazine (tptaz) with first row transition metals. The ligand can be
envisioned as being a combination of two different coordination motifs, namely bpy and
tpy (Figure 8a).

In this part of the chapter, we reported the syntheses of three mononuclear
compounds in which the metal is bound to the tpy domain and the bpy site is free to be
used to coordinate to other metal ions. Cyclic voltammetric studies reveal a rich
electrochemistry for these mononuclear complexes, the most interesting example being
the [Fe(tptaz)2]2+ compound which displays four reversible one-electron reductions, and

"'. The electrons added to the molecule are

an oxidation corresponding to Fe"—>Fe
partially delocalized throughout the tptaz molecules as determined from the
electrochemical data. A few dinuclear compounds with this ligand have been reported,
but minimal characterization was reported.39 The inductive effect on the triazine ring with
binding of the first metal center decreases the basicity of the nitrogen atom of the open

coordination site, but the possibility of accessing reduced species will increase the

probability of coordinating a second metal. This will be explored in future studies.
D. Azopyridine based ligands in Paramagnetic Arrays

Chapter 11 includes a brief description of the synthesis and characterization of
dinuclear compounds with the ligand azo-2,2’-bipyridine (abpy) (Figure 8b) which
possesses low lying 1t* orbitals based on high electron density located at the coordinating
N centers.40 This characteristic allows the abpy ligand to function as a molecular bridge

between metal centers.“ Previous studies with metals such as Ru(II) and Os(II) have

18

 

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revealed that there is strong electronic coupling (a large comproportionation constant, Kc)
indicating good stability for the mixed-valence species with these metals.

Dinuclear complexes with the abpy spacer have a short distance between metal
centers as compared to ligands with similar coordination capabilities such as bptz
(chapter 11), bpym and bppz. Another noteworthy aspect to consider is that the abpy can
be reduced to a stable radical spacer in coordination compounds e.g. in the compounds
[Cu2(u-n‘-abpy--)(Ph2P(CH2)6PPh2)1tam“ and toaabpy'-)(Br)(c0><PPh3)21” (Figure
10). Three different compounds were synthesized with this ligand, namely the dinuclear

compounds [Cu2(abpy)(CI-I3CN)8][BF4]4 and [Ni2(abpy)(CH3CN)2][N03]4 and the

polymeric material [Cu(abpy)2]°°.

 

 

 

 

A _ -
F P\C/P + \ Ph3
I I e u ~ ~ ‘ ' / N00,”, ,.\\\\\\\C0
N . N o
/ \ N// \N / N\° 8\
s ‘ ‘ ’ I \ Br
— C U!
/ \ P PPh3
\
P P = Ph2P(CH2)6PPh2 _ —
39.40

Figure 9. Compounds with the radical form of the ligand abpy.

III. Naphthyridine Ligands in Metal-Metal Bonded

Compounds

The realization of the existence of a quadruple bond in [ReClg]2' by F. A Cotton

and coworkers“ marked a major event in the history of transition metal chemistry. Before

19

0* Ajfid“
= .3 $3610“ 232$“
‘\\\\ \\\\
——8* v8 (g
__ 5 fi“\\\ \\\\
x2_yz ’ 8
xz,yz
$\\*\\
— o
2

Figure 10. Molecular orbitals for M-M species.

20

 

 

 

“gun ll

From :
*1 fitters)“;

iii-ill: and i

 

this fundamental discovery, a debate regarding the existence of M-M bonding had been
going on for nearly ten years. Originally, the determination of the X-ray structure of
[Re2(CO)m] by Dahl, Ishishi and Rundle in 1957 45 provided evidence of the existence of
the M-M bond with bridging groups between the metal centers, but the elucidation of the
X-ray structure of [ReZClgf' led to the realization that low valent metals can form very
short, unbridged contacts in their compounds. This fact has been born out in thousands of
other examples of M-M bonded compounds that have been prepared since the 1960's.46 A

simple model of the atomic orbital overlap to form molecular orbitals in M-M

compounds is shown in Figure 10.

 

 

 

 

 

Figure 11. Schematic drawing of the series [M2(02CCH3)2(pynp)2]2+ (M = Mo(II),
Ru(II) and Rh(II)).

From the early 1960's, and continuing to the present day, there has been a great
deal of interest in M-M bond chemistry, much of it being conducted with carboxylate,

phosphine and halide ligands. We noted that there has been comparatively little M-M

21

a- \

. '9'
0' ..D'
\ vb
.-

5“ ‘
.1; 1
u. a, U1

£\:~'\1"‘p
if“ \

will“. W.

. 'u

Krlf \LNln‘

. _ J\\L r'l
T5556 0

IV" 'I ...l.’

't‘r‘hmlifwiaf

7‘

Wine

 

 

 

chemistry with non-innocent ligands such as polypyridyl (bpy, terpy etc.,) and related
ligands.47 Even less research has been performed with ligands such as naphthyridine and
its derivatives, presumably because they are not commercially available and their
syntheses are not trivial. As part of the dissertation, the ligand pynp 2-(2-pyridyl)-1,8-
naphthyridine (pynp) was synthesized and use to prepare three dinuclear compounds that
comprise a homologous series, namely [M2(02CCH3)2(pynp)2]2+ (Figure 11).

Chapter IV of this dissertation describes the syntheses and characterization of
metal-metal bonded compounds of pynp that show unusual redox behavior. These may
be envisioned as building blocks that could be taken to the next hierarchical level, e. g., a
cyclic oligomer. The electrochemical behavior of the [M2(02CR2)2(pynp)2]2+ series is
quite different than that exhibited by their carboxylate counterparts [M2(02CR)4]2+
(where M = M0“, Ru“, Rh“) which do not display a rich redox chemistry. This series
exemplifies the three different cases of mixed-valence behavior on the Robin-Day scale.l7
[Ruz(OAc)2(pynp)2][PF6]2 displays Class III behavior with the electrons being completely
delocalized over the two pynp units; [Moz(OAc)2(pynp)z][BF4]2 displays class H
behavior, with the four electrons being partially delocalized, and finally
[Rh2(OAc)2(pynp)2][BF4]2 is a fully localized system (class I).

These compounds represent unprecedented building blocks for the construction of
supramolecular metal-metal compounds through substitution of the equatorial
carboxylate ligands (Figure 12)48 with the appropriate linker. A macrocycle that

incorporates redox active M-M units is expected to exhibit unusual electronic properties

22

 

 

ligur

 

   

Substitution of acetates by a bifunctional
carboxylate

 

 

 

 

 

 

 

Figure 12. Possible outcome of linking four cis-[M2(02CCH3)2(pynp)2]2+ units.

23

not observed in metal-metal compounds with ligands that do not promote delocalization

of electrons between the metals and the ligands.

24

 

 

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27

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28

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29

Chapter II

Coordination Chemistry of Nitrogen Heterocycles with

Bis-Bypyridine Functionalities.

30

I. Introduction

The self-assembly of metal cations with nitrogen heterocyclic bridging ligands is
a central theme in supramolecular chemistry aimed at developing light-harvesting

1'2'3'4 Applications of metal complexes

assemblies of electronically coupled metal centers.
with delocalized Tt-ligand systems include their use as electron “propagating”
components for inner-sphere electron-transfer reactions between metals,5 as building
blocks for magnetic materials,6 models for metalloenzymes,7 and precursors to low-
dimensional conducting polymers.8 One such ligand that bears promise for such
applications is 3,6 bis(2-pyridyl)tetrazine (bptz), (Figure 13a).9 It has been noted that

metal-coordinated bptz can be reduced to form stable anion radicals and that the metals

are strongly coupled through the both the neutral and reduced forms of the ligand.'0

N_
(a) (b)

Figure 13. Schematic drawing of (a) 3,6 bis(2—pyridyl)tetrazine (bptz) and
(b) 2,2’ bispyridyl azo (abpy).
Studies in our laboratories,H have revealed another application for the bptz
m016cule, namely as a cis-directing, bis-bipyridine chelate that promotes the self-
asseInbly of cyclic oligomers with metal ions.ll The high interest in cyclic molecules or

metallocyclophanes as they are often called, stems from their promise in host-guest

31

 

 

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yu§i "- .

“,4"

x21“ u.\\ b J

' .. 0 ~
, _ . . v
3:" "P.\ 'k
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.515 , at,
"RU allklui

~ I
..0‘3.‘ ‘Q’ ...”
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nu. llrubugw

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is.“ iUnS’ 0n

 

123.b 12c,d

chemistry, photochemistry and catalysism A number of successful methods have

been used to prepare cyclic molecules based on transition metals, most notably the use of
late transition metal, square-planar metal complexes to prepare molecular squares.” '4

One obvious region of the periodic table for expanding molecular square
chemistry is in the octahedral cations of the first-row transition series.15 Our goals are to
explore this possibility and to understand how self-assembly between these relatively
labile metal cations and multidentate ligands is controlled. A main parameter that
influences the outcome of these self-assembly reactions is the rigidity of the linkers, as
well illustrated by the synthesis of molecular grids.l6 In some cases, however, two or
more products ensue from a single metal/ligand stoichiometry. The structure of the
product can be difficult to predict because it depends on subtle factors17 such as

a

interligand stacking interactions,” or the participation of non-convalently bonded

8

cations1 or anions'9 as templates in the assembly process. In the present chemistry, the

formation of molecular squares is evidently controlled to some extent by the anions
which act as templates.‘9

This chapter describes a new family of molecular squares of general formula
[M(bptz).,(CH3CN)8]8+ (M = M“ or Zn“) along with the pentagon
[Ni5(bptz)5(CH3CN)m]'°+. In addition to extensive characterization by mass spectroscopy,
single crystal X-ray data were used to verify structures for Ni” and Zn2+ compounds.
Among the new compounds prepared are unprecedented, partially solvated squares and
pentagons. The difference between obtaining one product versus another was shown to be
entirely on the choice of counterion. It was also demonstrated that, under certain

conditions, one can obtain dimers and polymers from the same reactions that lead to the

32

.4 '3' Th;

x';h). r.\

1

F;' 1"'n‘ '1'

“knit ‘U*‘) r‘
In 3.x

are . “

as!» 3350 C

1;

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squares. These results are incontrovertible evidence that the self-assembly reactions of
metal ions with bptz are controlled by subtle factors.

In addition to bptz chemistry, reactions of the related ligand abpy (Figure 13b)
were also explored. The azo group permits good electronic coupling between metal
centers,20 and, in light of the presence of low lying 7t* orbitals, a facile reduction of azo
compounds is favored. The possibility of ligand reduction has prompted researchers to
use abpy as a radical linker in coordination compounds which is advantageous for
improving magnetic superexchange.2| Three compounds were isolated and characterized
in this work, namely two dinuclear compounds of Ni(II) and Cu(II), and a polymer Cu(I).

Characterization of the new compounds in this chapter by X—ray crystallography,
cyclic voltammetry, spectroscopic techniques, and magnetic measurements is presented.
In some cases, electrospray mass spectroscopy was used in lieu of a crystal structure to

characterize the products.

11. Experimental Section

Methods and Starting Materials.

All operations were carried out under a nitrogen atmosphere using standard
Schlenk-line techniques. All solvents were pre-dried over 4 A molecular sieves with the
exception of acetonitrile, which was pre-dried over 3 A molecular sieves. Diethyl ether
and toluene were freshly distilled over Na/K, and methylene chloride was distilled over
P205. The starting materials [Ni(HZO)6] [C104]; [Ni(HZO)6][NO3]2 and [Zn(HzO)6][C104]2
were purchased from Aldrich and used as received. The precursors [Ni(CH3CN)6][BF4]2,
[Mn(CH3CN)4][BF4]2. [CU(CH3CN)4][BF4]2. [Ni(CH3CN)sl[SbFs]2. and

[Zn(CH3CN)4][BF4]2 were synthesized by published methods.22 The bptz ligand was

33

 

 

“refil'at‘
,.~.I»‘-"
.

a Ia-h“.
i-__
lUilspfar ‘0
filiegdmns

imam.

N‘bplzht‘c

prepared from a literature method9 and recrystallized from benzene. The abpy ligand was
synthesized as reported with minor modifications.23

Physical Measurements.

The 1H, HB, and l9F NMR spectra were recorded on either a XL-300 or XL-
400 Varian NMR spectrometer. IR spectra were measured on a Nicolet 740 FT—IR
spectrometer as Nujol mulls on KBr plates. Magnetic susceptibility measurements were
obtained with the use of a Quantum Design SQUID magnetometer MPMS-XL (housed in
the Department of Chemistry at Texas A&M University). Measurements were performed
on a finely ground polycrystalline sample. Data were corrected for the sample holder and
diamagnetic contributions were calculated from Pascal constants.24 Positive ion
electrospray mass spectra were acquired using a Sciex Qstar Pulsar with a Protana
Nanospray ion source. Data were acquired with TOFMA 2.0RC3 and analyzed with
BioiMultiView 1.5RC3. Acetonitrile solutions of the metal complexes were diluted to
approximately 10 11M, and a 7.0 111 was loaded into a Au/Pd coated, silica spray needle.
Ionspray voltage was set between 900 and 1200 volts. Theoretical isotope ratio
calculations were performed using ISOPR03.0 (Senko, M. Sunnyvale, CA,
http://members.aol.com/msmssoftl)
[Ni4(bpt2)4(CH3CN)s][BF4]s (1)-

The salt [Ni(CH3CN)6][BF4]2 (100 mg, 0.209 mmol) was dissolved in 20 mL of
acetonitrile and treated with solid bptz (50 mg, 0.209 mmol) under nitrogen. Over the
course of 1 hour, the solution changed from a pale blue to a dark brown-green color. The
solution was stirred overnight and then layered with toluene to yield crystals of

[Ni4(bptz)4(CH3CN)3][BF4]3'4CH3CN within a three-day period; yield, 105 mg (84%). IR

34

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(KBr mull) cm": 3457 (s), 2321 (w), 2320 (w), 1605 (w), 1458 (vs), 1407 (s), 1376 (m),
1263 (w), 1068 (vs), 791 (w), 736 (w), 721 (w), 611 (w). ”P NMR 52.73 ppm (br,s), ”B
0.18 ppm (s). ES-MS: m/z =1780 ([Nia(bptz)4][BF4]7)+, 878 ([Ni4(bptz)4][BF4]6)2+.
[Zn4(thZ)4(CHsCN)s] [BF 418 (2).

The reagent [Zn(CH3CN)6][BF4]2 (150 mg, 0.309 mmol) was dissolved in 20 mL
of acetonitrile, and bptz (73 mg, 0.309 mmol) solid was added under nitrogen. During a 1
hour stirring period, the solution changed from a colorless to an orange color. The
solution was stirred for an additional 12 hours without further change, and then layered
with CHzClz to yield crystals of [Zn4(bptz)4(CH3CN)3][BF4]3-4CH3CN within a three-day
period; yield; 125 mg, (67%). IR (KBr mull) cm": 3400 (w,br), 1607 (w), 1504 (m, br),
1460 (w), 1404 (w),1377 (w), 1054 (s, b), 721(w). 1H NMR 9.25 ppm ((1), 8.55 ppm (t),
8.30 ppm (t), 8.10 ppm ((1), 2.67 ppm (s) ”B NMR -025 ppm (weak) (s), -l.60 ppm
(intense) (s), 19F NMR. -151.2 ppm (weak) (s), -151.2 ppm (intense) (s). ES-M.S: m/z =
1868; ([Zn2(bptz)][BF4]3)+, 621.9; ([Zn(bptz)][BF4])+, 386.01
[Ni4(thZ)4(CH3CN)8][CIO4]8 (3)-

The salt [Ni(HzO)6][ClO4]2 (100 mg, 0.309 mmol) was dissolved in 20 mL of
acetonitrile and treated with solid bptz (73 mg, 0.309 mmol) under anaerobic conditions.
Over the course of 1 hour, the solution changed from a light green to a dark brown-green
color. The solution was stirred overnight. Crystals of
[Ni4(bptz)4(CH3CN)g][C104]g-3CH3CN-C4HgO can be isolated by either layering the
reaction solution with toluene or by slow diffusion of diethyl ether or THF into
acetonitrile; yield, 120 mg (62 %). IR (KBr mull) cm": 2320 (w), 2318 (w), 2072 (w),

1603 (w), 1460 (s), 1377 (m), 1405 (w), 1261 (w), 1098 (br,s), 798 (w), 722 (w). ES-

35

 

 

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MS: m/z = 1868; ([Nia(bptz)4][C104]7)+, 1376; ([Ni3(bptz)3][ClOa]5)+, 1140;
(1N13(thZ)2][C104]5)+s 384-9; ([Ni2(thZ)2][C104l3)+. 629; ([Ni(bptz)2][C104])+.
[Zm(thZ)4(CH3CN)s][Cloak (4)-

The salt [Zn(HzO)6][ClOa]2 (100 mg, 0.269 mmol) was dissolved in 20 mL of
acetonitrile and treated with bptz (64 mg, 0.269 mmol) under anaerobic conditions to
yield an orange solution within 1 h. The solution was stirred overnight. Crystals of
[Zn4(bptz)4(CH3CN)3][ClOa]3-3CH3CN were obtained within 2 days by slow vapor
diffusion of diethyl ether into an acetonitrile solution of the compound; yield, 60 mg
(36%). IR (KBr mull) cm": 3342 (w,br), 1604 (w), 1402 (w), 1377 (w), 1260 (w) 1098
(8, br), 959 (w), 790 (w), 721(w). 1H NMR 9.26 ppm ((1), 8.56 ppm (t), 8.35 ppm (t), 8.16
ppm ((1), 2.65 ppm (s). ES-MS: m/z = 1893; ([Zn,(bptz),][c104]7)*. 1656;
(lZn4(thZ)3][CIO4]7)+. 1161.8; ([Zn4(bpt2)4][C104lv)+. 1630.8; ([Zn3(bptz)4][C104ls)+.
1531.9; ([Zn3(bptz)4][C104]4)+, 1394.7; ([Zn3(bptz)3][CIO.,]5)+, 1295;
([Zn3(bptl)3][C104]4)+. 1097.9; ([Zn3(bptz)3][CIO4]2)+. 1132-9; ([an(bptl)3][C104]s)+.
896.9; ([Zn2(thZ)2] [C10413)+. 660; ([Zn2(bpt2)][C104]3)+. 635; ([Zn(bptZ)2][C104]) +-
[Nis(thZ)5(CH3CN)1o][SbFrs]10 (6)-

The salt [Ni(CH3CN)6][SbF6]2 (250 mg, 0.322 mmol) was dissolved in 40 mL of
acetonitrile and treated with bptz (77 mg, 0.322 mmol) under anaerobic conditions. Over
the course of 1 hour, the solution changed from a pale blue to a brown-green color. The
solution was stirred overnight and then layered with toluene to yield crystals of
[Nis(bptz)5(CH3CN)lo][SbFa](o-2CH3CN (6) within a three day period; yield, 240 mg
(82%). IR (KBr mull) cm": 2320 (w), 2319 (w), 1605 (w), 1303 (w), 1263 (w), 1153 (w),

1082 (w), 1022 (w), 970 (w), 789 (w). ES-MS: m/z =3584; ([Nis(bptz)5][SbF6]9)+,

36

 

 

 

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1674.6; ([Nis(bptz)5][SbF6]g)+, 1528.7; ([le(bptl)5][SbF6]g)2+, 1528.8;
([Ni2(thZ)3][SbF613)+. 1292.7; ([Ni2(thZ)21[SbF6]3)+. 1057.8; ([Ni2(bptZ)2][SbF6]2)+.
l[MMthZXCHsCle][BF4]2}.. (7)-

[Mn(CH3CN)4][BF4]2 (290 mg, 0.74 mmol) was dissolved in 40 mL of acetonitrile
and treated with bptz (174 mg, 0.61 mmol) under anaerobic conditions. Over the course
of l h, a pale red precipitate began to form. The solution was stirred overnight, and a

reddish powder of {[Mn(bptz)2(CH3CN)2][BF4]}m was collected by vacuum filtration

under anaerobic conditions; yield, 123 mg (36%). IR (KBr mull) cm": 2724 (w), 1605
(w), 1401 (w), 1376 (s), 1259 (w), 1062 (s, br), 722 (w).
2,2’ Azo-Bipyridine (abpy).

The abpy ligand was synthesized as reported with some modifications.23 The
reagent 2-aminopyridine (5.0 g, 0.027 mol) was dissolved in 200 mL of water and the
solution was cooled by placing it in an ice bath for one hour. This cold solution was
slowly added to 600 mL of chilled NaOCl 5% aqueous. Once the two solutions are
mixed, the reaction mixture developed a bright orange color within 5 minutes. The
solution was stirred for one hour, “after which time the bright orange solution was
extracted with copious amounts of diethyl ether. The diethyl ether solution was dried over
MgSOa, and the solvent was removed on a rotary evaporator. The orange solid was
redissolved in hot hexanes and allowed to cool slowly. Orange crystals were obtained;
yield 1.20 g (24 %). UV-Vis (acetonitrile, nm, c = 2.72 x 104M): A = 450, e = 4.8 ><102

L(mol-em)"; A = 361, e = 1.47 x 104 L(moi-em)"; x = 203, e = 5.29 x 102 L(mol-cm)".

37

   

 

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[Niz(abPY)(CH3CN)2][N0314 (12)-

A sample of [Ni(HzO)6][NO3]2 (100 mg, 0.34 mmol) was dissolved in 30 mL of
acetonitrile in a beaker. The pale green solution was stirred until the entire solid had
dissolved, after which time a quantity of abpy (32 mg, 0.17 mmol) was added. The
solution, which immediately changed to an intense green color, was stirred for 4
additional hours and then concentrated to 10 mL. Slow vapor diffusion of diethyl ether
into the reaction solution afforded dark green crystals within 6 days; yield, 80 mg (75%).
IR (KBr mull) cm": 2724 (w), 1620 (w), 1147 (w), 1039 (w), 1022 (w), 801 (w), 722 (w).
UV-Vis (acetonitrile, nm, e = 9.8 x 10'6M): A = 344, e = 1.8 x104 L(moI-cm)".
[Cu2(abpy)(CH3CN)5][BF 414 (13)-

The salt [Cu(CH3CN)4][BF4]2 (100 mg, 0.25 mmol) was dissolved in 20 mL of
acetonitrile and treated with abpy (22 mg, 0.12 mmol) which led to an instantaneous
color change from a blue to a pale green color. The reaction mixture was stirred
overnight, after which time the solution was concentrated and treated with diethyl ether to
the point of saturation. The solution was then placed in the freezer, and within 5 days
green crystals were obtained; yield, 82 mg (80%). IR (KBr mull) cm": 2724 (w), 2296
(w), 2323 (w), 1589 (w), 1261 (w), 1229 (w), 1027 (br,s), 805 (w), 722 (w). UV-Vis

(acetonitrile, nm, c = 8.0 x 10'6M): x = 351, s = 2.7 x104 L(mor-em)"; 1. = 230, e = 1.98

x 104 L(mol-cm)".

38

 

 

0812 C1

Data Collection and Refinement.

The X-ray data were collected on a SMART 1K area detector diffractometer
equipped with graphite monochromated Mo K01 radiation (ha = 0.71073 A). The frames
were integrated in the Siemens SAINT25 software package and the data were corrected
for absorption using the SADABS program.26 The structures were solved using the
direct-methods program SI-IELXS-97.27 Crystal parameters and basic information
pertaining to data collection and refinement are summarized in Table 1
[Ni4(bpt2)4(CH3CN)s][BF 413°4CH3CN (1)-

Crystals of [Nia(bptz)4(CH3CN)g][BF4]3-4CH3CN were grown by slow diffusion
of toluene into an acetonitrile solution of the title compound. A dark green prism of
dimensions 0.05 x 0.12 x 0.25 mm3 was secured on the tip of a glass fiber with Dow
Corning silicone grease and cooled to 173(1) K in a cold N2(g) stream. A total of 24786
reflections was collected, 16456 of which were unique. Five out of the eight [BF4]' ions
were disordered, therefore restraints for chemically equivalent distances (B-F = 1.37A)
were applied. Displacement parameters of related fluorine positions were equated by
means of constraints in order to minimize variables since the [BF4]' anions exhibit
positional/rotational disorder patterns. The extensive disorder accounts for the slightly
higher than usual R factors. The final full-matrix refinement was based on 9444 observed
reflections with Fo>40(Fo) that were used to fit 1554 parameters to give R1 = 0.0831 and

wR2 = 0.1957. The goodness-of-fit index was 1.026 and the highest peak in the final

difference map was 1.182 e'/A'3.

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42

[Zn4(thZ)4(CH3CN)s][BF413'4CH3CN (2)-

Single crystals of [Zn4(bptz)4(CH3CN)g][BF4]3'4CH3CN were obtained by slow
diffusion of dichloromethane into an acetonitrile solution of the title compound. A light
orange platelet of dimensions 0.11 x 0.09 x 0.24 mm3 was mounted on the tip of a glass
fiber with Dow Corning silicone grease and cooled to 110(1) K in a cold N2(g) stream. A
total of 63108 unique reflections was collected of which 24635 were unique. Two of
eight of the [BET ions were disordered, therefore restraints on chemically equivalent B-F
(1.37 A) bonds were applied. Displacement parameters of related fluorine atoms were
constrained to minimize parameters. The final refinement was based on 13131 reflections
with Fo>40(Fo) that were used to fit 1394 parameters to give R1 = 0.0695 and wR2 =
0.2017. The goodness-of-fit index was 1.035 with the highest peak being 1.681 e'lA’3.
[Ni4(bptz)4(CH3CN)g][C104]3-2CH3CN°C4H30 (3).

Crystals of [Ni4(bptz)4(CH3CN)g][C104]3°2CH3CN-C4HgO were grown by slow
vapor diffusion of THF into an acetonitrile solution of the compound. A dark green prism
of dimensions 0.11 x 0.08 x 0.02 mm3 was secured on the tip of a glass fiber with Dow
Corning silicone grease and placed in a cold N2(g) stream at 110(1) K. Of the 79465
reflections that were collected, 25404 were unique. The final refinement was based on
9335 reflections with Fo>40(Fo) that were used to fit 1384 parameters to give R1 =
0°06“ and wR2 = 0.1584. Restraints for the chemically equivalent distances were
applied to the one disordered [C104]' (Cl-O = 1.44 A). The displacement parameters of
related oxygen positions were equated by means of constraints in order to minimize
Variable$.The goodness-of-fit index was 0.9 and the highest peak in the final difference

map was 1.541 e'lA'3.

43

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[Zn4(bptZ)4(CH3CN)3][Cl04]3°3CH3CN (4)-
Light orange, single crystals of [Zn4(bptz)4(CH3CN)3][C104]g'3CH3CN were

grown by slow vapor diffusion of diethyl ether into a solution of the compound in
acetonitrile. A platelet crystal of dimensions 0.1 x 0.25 x 0.03 mm3 was placed on the tip
of a glass fiber with Dow Corning silicone grease and placed in a cold stream at 110(1)
K. A total of 44125 reflections was collected of which 7997 were unique. Atoms C(19),
C(20), C(33) and C(34) were heavily disordered, and were, therefore, refined
isotropically. In addition, four of the eight [C1041 ions were disordered, and restraints on
their distances were applied (Cl-O 1.44 A). The final refinement was based on 4776
reflections with Fo>46(Fo) that were used to fit 731 parameters to give R1 = 0.0764 and
wR2 = 0.2218. The goodness-of-fit index was 1.024 and the highest peak in the final

difference map was 1.410 e'/A‘3. The extensive disorder accounts for the higher than

usual R factors.
[Ni2(bptz)(CH3CN)g][C104]4 (5).

Single crystals of [Ni2(bptz)(CH3CN)3][C104]4 were obtained by slow evaporation
of a filtrate obtained after harvesting a bulk sample of [Ni4(bptz)4(CH3CN)3][ClO4]g. A
pale green needle of dimensions 0.25 x 0.03 x 0.05 mm3 was mounted on the tip of a
glass fiber with Dow Corning silicone grease and placed in a cold N2(g) stream at 110(1)
K. The data set included 4433 reflections of which 3371 were unique. The final
refinement was based on 1968 reflections with Fo>40(Fo) that were USCd to fit 313

Parameters to give R1 = 0.0648 and wR2 = 0.1538. The goodness-of-fit index was 0.957

With the highest peak being 0.733 e'lA'3.

[Nis(bptz)s(CH3CN)lo][SbF6]lO'ZCHSCN (6).

Crystals of (6) were grown by slow diffusion of a acetonitrile solution of
[Ni5(bptz)5(CH3CN).0][SbF(,]lo into dichloromethane. A green prism of approximate
dimensions 0.13 x 0.2 x 0.05 mm3 was secured on the tip of glass fiber with Dow
Corning silicone grease and placed in a cold N2(g) stream at 110(1) K. A total of 36580
reflections was collected of which 12327 were unique. Four of the ten [SbF6]' ions were
disordered and therefore required modeling in several orientations. Constraints on the Sb-
F distances were applied (1.88 A). Furthermore, the displacement parameters of related
fluorine positions were equated by means of constraints in order to reduce parameters.
The atoms Sb(1), F(l), F(2), F(3), F(4), F(5) and F(6) were refined isotropically. The
final refinement cycle was based on 8016 reflections with F0>40(Fo) that were used to fit
891 parameters which led to R1 = 0.1160 and wR2 = 0.3645 and a goodness-of-fit of
1.069. The highest peak in the final difference map is 4.551 e'lA3 and is associated with a
disordered [SbF6]' anion. The extensive disorder accounts for the higher than usual R
factors.

{[Mn(bpt1)2(CH3CN)2][BF412l-o (7)-

Crystals of {[Mn(bptz)2(CH3CN)2][BE];}.. were grown by slow diffusion of a
acetonitrile solution of [Mn(CH3CN)4][BF4]2 into a dichloromethane solution of bptz. A
liglib-orange platelet of approximate dimensions 0.2 x 0.1 x 0.02 mm3 was secured on the
tip Of a glass fiber with Dow Corning silicone grease and placed in a cold N2(g) stream at
l 10 (UK. A total of 12882 reflections was collected of which 4480 were unique. The
final least-squares refinement of 382 parameters based on 2339 reflections with

F">46(Fo) resulted in residuals of R1 = 0.0666 and wR2 = 0.1744 and a goodness-of-fit

45

index of 0.951. The final difference Fourier map revealed the highest peak to be 1.711 e'
#13.
[Ni4(bpt2)4(CH3CN)s][BF4][PF a]3[SbF 614 (8)-

Single crystals of [Ni4(bptz)4(CH3CN)3][BF4][PF6]3[SbF6]4 were grown by slow
diffusion of a solution of [Ni5(bptz)5(CH3CN).o][SbF6].0 in acetonitrile into a solution of
toluene that contained a mixture of [TBA][BF4] and [TBA]PF6. Single crystals grew over
a period of two weeks in a sealed glass tube of 5mm diameter. A crystal of dimensions
0.15 x 0.8 X 0.05 mm3 was secured on the tip of a glass fiber with Dow Corning Silicone
grease and placed in a cold N2(g) stream at 100(1) K. A total of 5030 reflections was
collected of which 3459 were unique. The highest peak in the final difference map was
1.561 e’lA'3. All of the anions present in the structure are disordered, thus restraints on
chemically equivalent distances (B-F 1.40911, Sb-F 1.820A, P-F 1.622 A) were applied.
The tetraflouroborate anion displays a two-way positional/rotational disorder for the
fluorine atoms. In addition, the B(l) atom is disordered over two different positions.
Fluorine atoms F(7), F(8), F(9) coordinated to Sb(2) are involved in a two-way positional
disorder; therefore, the displacement parameters of the opposite fluorine positions were
ecluated by means of constraints in order to minimize variables. In the case of the [PF6]'
anion, the displacement parameters of related fluorine atoms were also constrained. The
final refinement cycle was based on 3459 reflections with Fo>40(Fo) that were used to fit
419 Parameters which led to R1 = 0.0765 and wR2 = 0.1075. The goodness-of-fit index
Was 1.030, and the highest peak in the final difference map was 1.25 e'lA3. Extensive

disorder accounts for the slightly higher than usual R factors.

46

 

 

 

[Ni4(bptz)4(CH3CN)3][C104][IO4]-,-2CH3CN (9)-
Crystals of the product were grown by slow diffusion of a acetonitrile solution of
[Ni4(bptz)4(CH3CN)g][C10413 into a toluene solution containing a large excess of

[Bu4N][IO4]. Crystals grew at the interface of the two solvents within two weeks. A pale-

3

brown rectangular crystal of dimensions 0.3 x 0.1 x 0.06 mm was mounted on the tip of

a glass fiber with Dow Corning silicone grease and placed in a cold N2(g) stream at
100(1) K. A total of 35238 reflections was collected of which 8434 were unique. Atoms
C(34), C(32), 0(6A) and 0(8A) were heavily disordered, and were therefore refined
isotropically. In addition, four of the [104]' anions are involved in a two-way
positional/rotational disorder. Restraints on I-0 distances were applied (1.710 A). The
displacement parameters of related oxygen atoms were constrained to be equivalent in
order to minimize variables. The final refinement cycle was based on 3996 reflections
with Fo>40Fo that were used to fit 727 parameters; this produced residuals of R1 =
0.1063 and wR2 = 0.2895. The goodness-of-fit index was 1.161, and the highest peak in
the final difference map was 1.25 e'/A3. The extensive disorder accounts for the higher
than usual R factors.

[C°(b0Pd)2(H20)2][PF612 (10).

Crystals of [Co(bopd)2(H20)2][PF6]2 grew as a side-product from the 1:1 reaction
between [Co(CH3CN)6][BF4]2 and bptz. Slow diffusion of the reaction mixture into a
tOluene solution saturated With [TBA][PF6] afforded crystals of (10) within one week. A
brow" Platelet of dimensions 0.06 x 0.1 x 0.09 mm3 was secured on the tip of glass fiber

with Dow Corning silicone grease and placed in a cold N2(g) stream at 110(1) K. Full-

matrix least-squares refinement of 2950 reflections with Fo>40(Fo) observed reflections

47

.1“?

an: -
s

.7 "9‘ -

L.“\¢l:\.\\‘

s

{V

1"“ 7 -

“.....lg 51
b

3?. . ‘
Ni 9"
“ORR:

using 727 parameters produced residuals of R1 = 0.0561 and wR2 = 0.1275. The

goodness-of-fit index was 1.020, and the highest peak in the final difference map was

0.548 e'/A3.
[CU(b0pd)2(H20)2] [BF 412 (11).

Crystals of [Cu(bopd)2(H20)2][BF4]2 were obtained by slow diffusion of a 1:1
ratio of [Cu(CH3CN)4][BF4]2 and bptz in acetonitrile with toluene. A single green platelet
of dimensions 0.15 x 0.13 x 0.08 mm3 was mounted on the tip of glass fiber with Dow
Corning silicone grease and placed in a cold N2(g) stream at 110(1) K. The final
refinement cycle was based on 2607 reflections with Fo>46(Fo) that were used to fit 727
parameters. The final R values are R1 = 0.0582 and wR2 = 0.1586. The goodness-of-fit
index was 1.092, and the highest peak in the final difference map was 0.595 e'/A3.
[Ni2(3bPY)(CH3CN)2][N0314 (12)-

Crystals of the product were grown by slow vapor diffusion of diethyl ether into
an acetonitrile solution of the title compound. A dark brown prism of dimensions 0.2 x
0.32 x O. 13 mm3 was secured on the tip of a glass fiber with Dow Corning silicone grease
and placed in a cold N2(g) stream at 110(1) K. A total of 8160 reflections was collected
Of which 3252 were unique. The final refinement cycle was based on 2815 reflections
With Fo>40(Fo) that were used to fit 199 parameters which led to R1 = 0.0416 and wR2 =
0.1054. The goodness-of-fit index was 1.087, and the highest peak in the final difference
map was 1.337 e'lA3.

[Cuzabpyxcnecmsl[Brill (13>
I«ight green platelet crystals of [Cu2(abpy)(CH3CN)g][BF4]4 were obtained by

placing an acetonitrile solution of the compound in the freezer at —4 °C for a week. A

48

 

i.
(’0')

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9'
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light green platelet was secured on the tip of a glass fiber with Dow Corning silicone
grease and placed in a N2(g) stream at 110(1) K. The data collection involved a total of
7358 reflections of which 3484 were unique. The final refinement cycle was based on
2907 reflections with Fo>40(Fo) that were used to fit 298 parameters which led to R1 =
0.0750 and wR2 = 0.2053. The goodness-of—fit index was 1.058, and the highest peak in
the final difference map was 1.121 e'/A3.
{[Cu(abpy)2][BF4]}e (14).

Crystals of {[Cu(abpy)2][BF4]}.. grew over the period of three weeks from a
solution of (13) layered with toluene. The crystals formed as dark green needles on the
edges of the glass tube. A green needle of dimensions 0.05 x 0.03 x 0.12 mm3 was
covered with Paratone oil, secured on the tip of a glass fiber with Dow Corning silicone
grease, and placed under a N2(g) stream at 110 (1)K. A total of 11151 reflections was
collected of which 2040 were unique. The final refinement cycle was based on 1096
r(iflections with Fo>40(Fo) that were used to fit 181 parameters which led to R1 = 0.0861
and wR2 = 0.2055. The goodness-of—fit index was 0.987, and the highest peak in the final
di fference map was 1.361 e'/A3.

[Ni4(bpt2)4(CH3CN)s][1][SbFalv (15).

Single crystals of [Ni4(bptz)4(CH3CN)3][I][SBF6]7 grew over the period of one
m()nth from a solution of [Ni5(bptz)5(CH3CN).o][SBF6]lo in acetonitrile that had been
layered with a toluene solution saturated with [Bu4N]I. A light brown prism of

approximate dimensions 0.1 x 0.08 x 0.2 mm3 was mounted on the tip of a glass fiber
With Dow Corning silicone grease and placed in a cold N2(g) stream at 110 (1)K. A total

of 19922 reflections was collected, of which 5202 were unique. The final refinement

49

cycle was based on 4618 data points with Fo>40(Fo) that were used to fit 396 parameters
which led to R1 = 0.0741 and wR2 = 0.2283. The goodness-of—fit index was 1.092, and

the highest peak in the final difference map was 2.094 e‘lA3.

III. Results and Discussion.

A. Self-Assembly of Molecular Squares.

A.l. Syntheses of [M4(bptz)4(CH3CN)s][X]s (M = Ni2+ and Zn“) Compounds.

acetonitrile
[M(CH3CN)6][X]2 + bptz > [M4(bptz)4(CH3CN)s][X]s (Eq. 2)

4-8 hours

 

M = Ni2+ ; x = [BF4]' : 70-80%
M = N12+ ; x = [C104]' : 60-65%
M = Zn2+ ; X = [BF4]' : 60-67%
M = Zn2+ ; x = [C1041 : 36%

A major focus of this study was to prepare paramagnetic complexes with nitrogen
heterocyclic ligands with the bptz ligand (Figure 13a). Previous studies indicated that
bptz showed promise for allowing electronic communication between metal centers of
mass III in the Robin-Day scale.28 No reports of magnetic exchange using bptz have been
rePorted however. With this goal in mind, we embarked on the study of self-assembly
reactions of first row transition metals with bptz. Equation (2) summarizes the main

reactions under investigation.

50

A1,

a.“

4 5T!

53
"‘Mr

N(II) metallocyclophanes.

Studies began with 1:1 reactions of [Ni(CH3CN)6][BF4]2 in acetonitrile under
anaerobic conditions (Eq. 2). The reaction is essentially instantaneous as judged by a
color change from a blue solution of [Ni(CH3CN)6][BF4]2 to a brown color upon addition
of bptz. The Ni(II) products are stable under a variety of conditions. If the reaction is
refluxed for a week, the product is obtained, unchanged, in quantitative yields.
Furthermore, a 10:1 ratio of bptz to [Ni(CH3CN)6][X]2 (X = [BF4]' or [C104]') still yields
the tetrametallocyclophane as indicated by mass spectrometry and X-ray crystallography.

Identical reactions performed in acetone or nitromethane also yield the tetramers
(vide infra) as corroborated by mass spectroscopy, but alcohols and water appear to limit

the formation of metallocyclophanes; instead lower nuclearity species (monomers and
dimers) or even polymers are obtained as evidenced again by mass spectrometric studies
(Vide infra) and also by insoluble solids.

Zn(II) metallocyclophanes.

Analogous results are obtained when Zn(II) is used in place of Ni(II) in these
reactions. These metallocyclophanes are synthesized from the 1:1 reaction of bptz with
[Zn(CH3CN)4][BF4]2 or [Zn(H20)6[ClO4]2 in acetonitrile (Eq. 2). In both cases, the
reaction solution changes from colorless to orange of addition of bptz. X-ray studies
revealed that these species exist with either a (BET or a [C104]' anion in the cavity of the
molecular cation.

A main difference in the Ni(II) and Zn(II) chemistry with bptz was noted. There

are drastic differences in the stability of these species in solution. Solutions of

[Zn4(bptz)4(CH3CN)g]8+ that are allowed to stand for more than three days begin to

51

decompose with deposition of insoluble precipitates (polymers). A possible explanation

is the greater lability of the Zn-N bonds, which allows for reversible coordination which

could eventually lead to more insoluble oligomers/polymers with high concentrations.

Moreover, unlike their Ni(II) analogues, the Zn(II) squares are sensitive to reaction

conditions such as temperature, concentration and solvents. Dilute solutions favor the

formation of the tetrametallocyclophane whereas highly concentrated solutions lead to
insoluble materials presumably which are presumed to be polymers. Mass spectrometric
studies (vide infra) indicate that the [Zna]8+ product is not being formed in solvents other
than acetonitrile. For example, reactions performed in nitromethane and acetone led to
only low nuclearity species by mass spectrometry. It is worth pointing out that if the
solutions of [Zn4(bptz)4(CH3CN)g]8+ are stored at low temperature (0°C), the
decomposition process is considerable delayed.
A-Z. Spectroscopic and cyclic voltammetric studies of [M4(bptz)4(CI-I3CN)3][X]3 M =
Ni, zn; x= [1317413100.]:

The [Ni4(bptz)4(CH3CN)3]8+ species exhibit two characteristic electronic

transitions in their UV-Visible spectra. The first one occurs at 512 nm (8 = 530 M"°cm")

a"Cl is similar to a transition in the free bptz ligand, suggesting that this is a n—m’“ ligand
baSled transition. The second transition at 680 nm (8 = 540 M"-cm'l) is characteristic of a
I«aporte forbidden d-d transition for Ni(II) compounds. The UV-Visible spectra for the
[Zn4(bptz)4(CH3CN)g]8+ salts, (2) and (4), exhibit only one transition at 540 nm with e =

270 M"-cm'l which is assigned to a bptz 1t—->7t* transition.
The [Ni4(bptz)4(CH3CN)g][BF4]3 compound exhibits two weak stretches in the

infrared spectrum at 2321 and 2325 cm" which correspond to the v(C_=.N) modes from

52

..V

Klu-

.4L

..\.

..ka
D»

the bound acetonitriles. In addition, a broad and strong stretch at 1069 cm'1 is observed,
which corresponds to the v(B-F) mode of the tetrafluoroborates. The
[Zn4(bptz)4(CH3CN)g][X]g compounds display a characteristic v(B-F) stretch at 1054 cm'
I for (2) and 1098 cm'l for a v(Cl-O) stretch in the case of (4). The expected v(CEN)
stretches were not observed, however, possibly because prolonged pumping led to the
loss of the ligands in the solid state, due to the fact that they maybe weak to be observed.
Although previous work with dinuclear complexes of bptz28 revealed good
electronic communication between metal centers, (Class II or Class 111 Robin Day
behavior),29 cyclic voltammetric studies of the [Nis]8+ species do not indicate significant
electronic coupling between metal centers. The compounds [Ni4(bptz)4(CH3CN)3][X]8
(X = [BF4]' or [C104]') exhibit a quasi-reversible ligand based reduction at Em = +0.4 V
and irreversible waves at EN = -1.95 V and EN = -1.45 V and Ep,a = -0.62 V. In a similar
fashion, the Zn(II) squares do not exhibit communication between metal centers;
Compounds (2) and (4) display a quasireversible ligand based reduction at Ev. = 0.37 V
and irreversible waves at EN; = -0.85 V and EN = -1.4 V followed by decomposition. It is
Possible to rationalize the irreversible behavior by the fact that the tetrazine ring is not
Planar which interrupts the 1: pathway for electronic communication.
AC3). X-ray crystallographic results.
Nim) molecular squares: [Ni4(bptz)4(CH3CN)8][X]8; X = [BF4]’, [01041' and
I N i2( thZX CHJCNM I C104] 4.
The molecular cation in compound (1) is presented in Figure 15 whereas
Compound (3) is depicted in Figure 16. [Ni4(bptz)4(CH3CN)3][BF4]3-4CH3CN (2)

crystallizes in the triclinic space group P1 and

53

1"

‘1’.-

+2-”,
llg)‘“

3112:.

[Ni4(bptz)4(CH3CN)g][C104]3-2CH3CN-C4H30 (4) crystallizes in the monoclinic space
group P21/n. As expected, X-ray studies reveal that each Ni(II) center is in a distorted
octahedral geometry with two bptz units coordinated in a cis fashion and two acetonitrile
molecules completing the coordination environment.

Typical angles in compound (1) are [Ni(5C)-Nil-N(1C)] = 78.0 (2)° for the bptz
binding, and [N(5)-Ni(1)-N(6)] = 89.4 (5)° for the acetonitrile interactions (Figure 15b).
In terms of the Ni-N distances the Ni-N(tetrazine) is the longest bond, followed by the
Ni-N(pyridyl) and finally Ni-N(acetonitrile); these distances are Ni(1)-N(5C), 2.098 (5)
A, Ni(1)-N(1C) 2.084 (6) A, and Ni(l)-N(5) 2.035 (6) A respectively (Figure 15b). The
dimensions of the square are 6.87 A on the edge and 9.69 A along the diagonal (Figure
1 5a). The average vertex angle (0) for the compound
[N i4(bptz)4(CH3CN)3][BF4]3'4CH3CN (1) is 923° which represents a deviation of 23°
from the ideal angle for a square (Figure 14). It is worth mentioning that each bptz unit is
distorted in order to satisfy the coordination requirements. In compound (1), for example,
the tetrazine ring shows distortions ranging from 186° to 11.70° ((1)) (Figure 14). The
tetirazine ring and the pyridyl are distorted from planarity as well, with angles ranging
from the almost negligible 0.38o (almost coplanar) to 761° ((1)) (Figure 14).

One of the most important aspects of the structure of (1) is the presence of a
tetrafluoroborate anion inside the cavity. This anion has the correct surface area and
Volume to fit inside the cavity, which suggests its possible role as a template in the

formation of this cyclic tetramer (Figure 15a and Figure 17a).'9 A similar anion in size
and shape such as [C1041 renders similar results, namely the assembly of a molecular

square with an encapsulated anion (Figure 17).

54

 

 

 

 

 

Table 2. bond distances (A) and angles (°) for
[Ni4(bpt2)4(CH3CN)3][BF413'4CH3CN (1)-
Ni 1-N5 2.035(6) Ni2-N4 2.038(7)
Ni 1-N6 2.045(6) N i2-N3 2.052(6)
Nil-N 1 B 2.067(6) N i2-N2C 2.057(6)
Ni 1-N1 C 2.084(6) N i2-N2D 2.069(6)
Ni 1-N5B 2.094(5) N 12-N 3C 2.089(5)
Ni 1-N5C 2.098(5) Ni2-N6D 2.107(6)
N 5-Ni l-N6 89.4(2) NIB-Ni 1-N5B 77.6(2)
N5-Nil-N1B 96.4(2) NlC-Ni l-NSB 88.6(2)
N6-Ni1-N1B 92.5(2) N5-Ni 1-N5C 86.1(2)
N5-Ni1-N1C 97.5(2) N6-Ni1-N5C 172.6(2)
N6-Nil-N1C 96.9(2) N lB-Ni l -N5C 93.8(2)
N lB-Nil-NlC 163.2(2) N1C-Ni1-N5C 78.0(2)
N5-Ni1-N5B 173.8(2) N5B-Ni1-N5C 95.8(2)
N6-Ni1-NSB 89.3(2)
Table 3. bond distances (A) and angles (°) for
[N i4(bptz)4(CH3CN)g][C104]g'2CH3CN'C4HgO (3).
N 4B-N i 1 2.083(5) N2A-Ni2 2.089(5)
N l4-Nil 2.040(6) N2B-Ni2 2.082(5)
N6-Ni1 2.077(5) N12-Ni2 2.040(6)
N 5B-N i 1 2.083(5) N6A-N 12 2.057(5)
N13-Nil 2.029(6) N6B-Ni2 2.073(5)
N2-Ni1 2.078(5) N1 1-N12 2.037(6)
N 13-Nil-N14 90.5(2) N3B-N4B-Ni1 125.4(4)
N 13-Ni 1-N6 97 .2(2) N6-Ni1-N4B 89.4(2)
N 14-Ni1—N6 96.1(2) N2-Ni1-N4B 92.7(2)
N13-Ni1-N2 174.5(2) N13-Ni1-NSB 91 .5(2)
N14-Ni 1-N2 87.2(2) N14-Ni l-NSB 96.3(2)
N6-Ni1-N2 78.1(2) N6-Ni l-N SB 164.7(2)
N13-Ni1-N4B 90.1(2) N2-Ni1-N5B 93.7(2)
N14-Ni l—N4B 174.4(2)

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Table 5. Selected bond distances (A) and angles (°) for [Ni2(bptz)2(CH3CN)3][C104]4 (5).

 

Nil-N4A 2.044 (7) Nil-N4D 2.072 (7)
Nil-N1 2.069 (7) Nil-N2 2.073 (7)
Nil-N4C 2.073 (7) Ni 1-N4B 2.079 (7)
N4A-Ni1-N1 92.2 (3) N4C-NiI-N2 95.0 (3)
N4A-Ni1—N4C 92.0 (3) N4D-Nil-N2 90.4 (2)
Nl-Nil-N4C 171.8 (3) N4A-Nil-N4B 90.3 (3)
N4A-Nil-N4D 177.9 (3) Nl-Ni 1-N4B 92.1 (3)
N1-Nil-N4D 89.8 (2) N4C-Ni1-N4B 94.8 (3)
N4C-Ni1-N4D 86.1 (3) N4D-Nil-N4B 88.9 (3)
N4A-Nil-N2 90.8 (3) N2-Nil-N4B 170.1 (3)
N1-Ni1-N2 78.0 (3)

The cation in compound (3) exhibits an octahedral geometry about each Ni(II)
center, with two bptz units and two acetonitrile molecules coordinated in a cis fashion
(Figure 16). The presence of an encapsulated perchlorate anion (47 A3)'9" (Figure 17b)
instead of a tetraflouroborate anion (38 A3) results in larger distortions of the ligand units

as compared to the previous Ni(II) tetramer (1). Complex (3) exhibits an average vertex
aF‘Igle (0) of 94.05°, which is a 405° deviation from the ideal value for a square; this is
Compared to the 92.3° found in (1). The tetrazine ring also shows a larger distortion from
Planarity [12.66° (m)], compared to the [BF4]° salt which is 11.67° (Figure 14). The
di hedral angle between the planes of the pyridyl and tetrazine ring increases to 686° ((1)).
The Ni-N distances follow the same trend as the tetraflouroborate.
It was possible to isolate crystals of a minor product, namely
[Ni2(bptz)(CH3CN)3][ClO4]4 (5) (Figure 19) from a filtrate after isolation of (3). In this
Case, each Ni(II) center exhibits a distorted octahedral geometry with the bptz unit

co()rdinated in bidentate fashion and four acetonitrile molecules. The angle for the bptz

63

binding is 78.0 (3)° [N(1)-Ni(1)-N(2)], and the average angle between the acetonitrile
ligands is 90.4 [3]°. As in the case of (1) and (2), the tetrazine moiety and the pyridyl ring
do not lie in the same plane. In this case there is an angle of 526° between planes of the
two rings. The Ni-N distances follow the same trend as before with the longest Ni-N
distance being the tetrazine ring Ni(l)-N(2) 2.073 (7) A followed by the pyridyl nitrogen
Ni(1)-N(l) 2.069 (7) A; the shortest distance is to an acetonitrile ligand Ni(1)-N(4A)
2.044 (7) A (Figure 19).
Zn(II) molecular squares: [Zn4(bptz)4(CH3CN)g][X]8, X = [BF4]', [C104]'.
Both of the Zn(II) molecular squares exhibit a distorted octahedral geometry.
Crystals of [Zn4(bptz)4(CH3CN)3][BF4]g-4CH3CN (2) are triclinic, P1 and those of
[Zn4(bptz)4(CH3CN)3][C104]3-2CH3CN (4) are monoclinic, C2/c. The metric parameters
Within (2) exhibit the same bonding trends as its Ni(II) counterpart, namely the tetrazine
Nitrogen exhibits the longest bond followed by the pyridyl and acetonitrile; the distances
are; Zn(1)-N(1A) = 2.266(5)A, Zn(1)-(5A) = 2.091(4) A, and Zn(1)-N(10) = 2.077 A
(Pi gure 20b) respectively. The corresponding distances in (4) are Zn(l)-N(3) = 2.234
(4)/A, Zn(1)-N(1) = 2.144 (5) A, and Zn(1)-N(11) = 2.045 (5)A (Figure 21b). There is an
inCrease in the M-N distances as compared to the Ni(II) species, as expected due to the
131‘ ger radius of Zn(II). Both [Zn4]8+ cations contain an encapsulated anion. The distances
b'E=t\Jl/een the Zn(II) centers in the two squares are 7.2 A for (2) and (4) respectively
(Fi gure 20 and Figure 21). The tetrazine rings in (2) exhibit a twist angle as large as 4.74°

((1)) and a torsion angle of 535° ((1)) between the tetrazine and pyridyl rings (Figure 14).

In (4), the tetrazine exhibits a torsion angle of 11.92° ((0) and an angle between the planes

of the tetrazine and pyridyl ring of 10.140 ((1)), which is the largest distortion among the

four structurally characterized squares (Figure 14).

 

 

 

 

 

Table 6. Selected bond distances (A) and angles (°) for
[Zn4(thZ)4(CH3CN)81[BF413’4CH3CN (2)-

an-N9 2.070(5) Zn2-N12 2.072(4)
an-NIO 2.077(5) Zn2-Nl 1 2.082(4)
an-NSA 2.092(5) Zn2-N6A 2.094(4)
an-N6 2.131(5) Zn2-N4B 2.293(4)
an-N4 2.248(4) Zn2-N5B 2.116(4)
an-NlA 2.262(4) Zn2-N3A 2.255(4)
N9-Zn1-N10 92.95(18) N10-Zn1-N4 172.5309)
N9-Zn1-N5A 98.2908) N5A-an-N4 90.82(17)
NIO-an-NSA 96.58(19) N6-an-N4 74.64(16)
N9-an-N6 102.6708) N9-Zn1-N1A 172.9208)
N10-an-N6 98.91(18) N10-Zn1-N1A 89.1606)
N5A-an-N6 153.1306) N6-an-N1A 83.6506)
N9-an-N4 84.9506) N4-an-N1A 93.7905)

Table 7. Selected bond distances (A) and angles (°) for

[Zn4(bptl)4(CH3CN)sl[C10413'3CH3CN (4)-
Zn 1 -N11 1.957(8) Zn2-N10 2.01100)
Zn 1 -N13 2.088(9) Zn2-N9 2.06000)
Zn 1 -le 2.125(8) Zn2-N6 2.094(10)
Zn 1 -N1 2.139(8) Zn2-N8 2.107(9)
Zn 1 -N3 2.237(8) Zn2-N7 2.246(8)
Zn 1 -N14 2.240(8) Zn2-N2 2.29900)
N l l-an-N13 88.8(4) N12-an-N3 92.6(3)
l l-an-N12 98.0(3) Nl-an-N3 74.4(3)
N 1 3-Zn1—N12 94.7(3) N1 l-an-Nl4 90.2(3)
N 1 l-an-Nl 97.1(3) Nl3-Zn1-N14 169.5(3)
N 1 3-an-N1 102.7(3) N12-Zn1-N14 75.1(3)
N 1 2-Zn1-Nl 157.1(3) N1-Zn1-Nl4 87.8(3)
1 l-an-N3 168.2(3) N3-Zn1-N14 97.5(3)
1 3-an-N3 85.2(3)

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67

Compound (2) displays an average vertex angle (0) of 93.80 (Figure 14). In the
perchlorate salt (4), this angle is 974°, which is the largest deviation in the

tetrametallocyclophane series, namely 7.4°.

A.4. NMR studies

Thus far, support for an anion template effect in this chemistry is based solely in
X-ray crystallography. The question that comes to mind is do these species exist in
solution with an anion inside the cavity? The diamagnetic molecular square
[Zn4(bptz)4(CH3CN)3][BF4]3 is an ideal species for studying the solution behavior of
these metallocyclophanes.

In order to check for the presence of encapsulated anions in solution, '9F and HB
NMR studies were first performed on [Ni4(bptz)4(CH3CN)3][BF4]8, but it exhibited only
one broad peak in both the HB and 19F NMR spectra due to the paramagnetism of the
compound. On the other hand, the [Zn4(bptz)4(CH3CN)3][BF4]3 derivative, being a
diamagnetic species, is an ideal candidate for probing the behavior of these entities in
solution by 19P NMR and HB NMR techniques.

If the anion resides in the cavity of the cation in solution, one would expect to
observe two peaks for the two distinctive type of [BET anions: a major one, that
corresponds to the 7 [BF4]' anions outside the cavity and a minor one, that corresponds to
the encapsulated one. The llB NMR spectrum of [Zn4(bptz)4(CH3CN)g][BF4]3 contains
resonances at —l.60 ppm and —0.20 ppm. Similarly, the I9F NMR spectrum displays two

resonances, a major one at -151.2 ppm and the minor one at -151.0 ppm. The relative

68

1
ll 60 °C

 

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l
N 0 °C
2‘

 

A"... -40 °C

1 l
l 1 $

 

-150 -151 -152

Figure 22. Variable temperature l9F NMR spectra for [Zn.,(bptz),,(CH:,CN),,][BF,,]8 in
CD3CN.

69

(I)

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integration of the peaks corresponding to the encapsulated and the free [BF4]' anions in
the ”B and I9F NMR spectra are not exactly in the expected 7:1 ratio, but differences in
relaxation times can account for this phenomenon.

A variable temperature ”P NMR experiment was performed from —44.0 up to
80.0 °C. (Figure 22). As the spectra in Figure 22 show, there are two resonances in the
temperature of ~0 °C-60 °C. Broadening occurs at higher temperature due to faster
exchange of the [BET inside the cavity and the “outer-sphere” anions. At lower
temperature the broadening is attributed to solubility problems. Coalescence was not
reached due to limitations of the solvent temperature range. The chemical shifts that
occur over the temperature range are most likely due to differences in ion-pairing
interactions with temperature.

The aforementioned NMR experiments provide evidence that the anions are
retained in the cavity of the molecular squares in solution. With this evidence in hand, we
conclude that the anion plays a role in the assembly of the molecular squares. If so, we
argued, it should be possible to tune the size and shape of the metallocyclophane by the
choice of the anion. In this vein, numerous anions with different sizes and shapes were
attempted in this chemistry; among these are [CF3803]', [PF6]', [AlCl4]', [104]} [Barfl',
X' (X = Cl', Br', 1’). None of the previously mentioned anions led to a stable
metallocyclophane as corroborated by ES-MS and the lack of crystalline materials
suitable for X-ray analysis. By using the anion [SbF6]', however, our goal of tuning the

ring size of the oligomer was accomplished.

7O

 

B. A New Generation of Metallocyclophanes: Molecular Pentagons.

Both the [C104]' and [BF4]' anions promote the formation of molecular squares,
presumably because of similarities in their sizes and shapes. Polygons of different
nuclearities should be possible to obtain if the appropriate anion is used. As previously
mentioned, different anions were used without success in terms of isolating a tractable
product. The larger anion [SbF6]', however, allowed for the assembly of a larger polygon,
namely an unprecedented pentagon.

8.1. Synthesis of [Ni5(bptz)5(CH3CN)m][Snglm

[Ni(CH3CN)6][SbF6]2 was treated with bptz in a 1:1 ratio in acetonitrile which led
to a color change from pale blue to dark orange within minutes. After two hours, a
brownish green color persisted (Eq. 3). X-ray studies revealed the compound to be the
unprecedented, partially solvated, molecular pentagon

[N i5(bptZ)5(CH3CN)10][SbF6] 10'2CH3CN (6)-

acetonitrile ,
[Ni(CH3CN)6][SbF6]2 + bptz > [le(bptz)s(CH3CN)io][SbFs]io (Eq. 3)

4 hours 80-85% yield

 

As in the case of the [N14]8+ compounds, the [Nis]10+ pentagon is stable in solution
and, in fact, the compound is obtained in good yields even after one week of refluxing
conditions. Different ratios of bptz to [Ni(CH3CN)6][SbF6]2 other than 1:1 do not affect
the identity of the product, but they do lead to lower yields. The results were confirmed

by mass spectrometry (viole Win) and X-ray crystallography.

7l

 

 

Table 8. bond distances (A) (°) for
[Ni5(thZ)5(CH3CN)5][SbF6]10' 2CH3CN (6)-

NiI-N16 2.035 (12) Ni2-Nl9 2.057 (14)
Nil-N17 2.039 (14) Ni2-Nl8 2.054 (11)
Nil-N3 2.058 (12) Ni2-N6 2.060 (11)
Nil-N4 2.062 (10) Ni2-N13 2.056 (11)
Nil-N7 2.068 (10) Ni2-N1 2.077 (10)
Nil-N11 2.093 (12) Ni2-N14 2.066 (12)
N16-Ni1—N17 91.8 (5) Nl9-Ni2-Nl3 176.2(5)
N 16-Ni1-N3 96.5 (5) Nl8-N12-N13 87.0 (4)
N17—Ni1-N3 92.4 (5) N6-Ni2—N 13 89.0 (4)
N16-Nil-N4 174.9 (5) N19-Ni2-N1 89.8 (4)
N17-Ni1-N4 89.5 (5) N18-N12-N1 176.1 (4)
N3-Ni1-N4 78.5 (4) N6-Ni2-Nl 78.2 (4)
N16—Ni1-N7 86.7 (4) N 18-Ni2—N 14 93.5 (4)
Nl7-Ni1-N7 177.8 (5) N6-Ni2-N14 162.4 (4)
N3-Ni1-N7 89.5 (4) N13-Ni2-N14 78.5 (4)
N4-Ni1-N7 92.1 (4) Nl-Ni2-N14 89.7 (4)
N16-Ni1-N11 93.9 (5) N20#-Ni3-N20 90.8 (10)
N17-Ni1-N11 99.4 (5) N20#1-Ni3—N9 176.5 (6)
N3-Ni1-N11 164.0 (4) N 20#1-Ni3-N9#1 88.9 (6)
N4-Ni1-N11 90.7 (4) N20#1-Ni3-N9 176.5 (6)
N7-Ni1-N1 1 79.0 (4) N9-Ni3-N9#1 91.6 (6)
Nl9-Ni2-N18 92.1 (5)

Symmetry transformations used to generate equivalent atoms: #1 -x+l,y,-z+1/2,#2 -
x+2,y,-z+1/2
B.2. X-ray crystallographic results.

As in the case of the tetranuclear metallocyclophanes, the pentagon has an anion
residing in the central cavity, in this case [Sde' (Figure 23b), the presence of which is
essential for the formation of the pentagonal unit. The six-coordinate Ni“ ions are
coordinated to two cis bptz ligands and two CH3CN molecules in a distorted octahedral
geometry. The M-N distances follow the same trend observed in the Ni(II) tetramers. The

coordinated nitrogen atom of the tetrazine ring exhibits the longest Ni-N distance Ni(l)-

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76

Figure 26. Packing diagram of [Ni5(bptz)(CH3CN),0][SbF6]10.1

 

N(4) = 2.062 (10)A followed by the pyridyl distance of 2.058 (12% and the Ni-NCCH3
distance of 2.039 (14)A (Figure 24).

It is worth mentioning that, although an ideal pentagon requires 108° vertices, the
angles subtended by N-Ni-Ni edges are much smaller e.g. N(4)-Ni1)-N(7) = 92.1(4)°
(Figure 25). In order to solve the problem created by using 90° disposed L-M-L building
blocks to form a pentagon, the flexible bptz edges adopt 8.4° dihedral angles between the
pyridyl and the tetrazine rings. In this way the overall angles between the Ni vertices of
921° + (2 x 8.4°) = 108.9° which is nearly ideal for a five-membered ring (Figure 25).
B.3. Spectroscopic and cyclic voltammetric studies.

The UV-Visible spectra exhibit two electronic transitions; the first one located at
510 nm (8 = 270 M"-cm") is attributed to a 1t—-> 1t* transition and a second at 690 nm (e
= 260 M"-cm") is a forbidden d-d transition for Ni“. Infrared spectroscopy revealed two
weak stretches at 2350 and 2354 cm'I which correspond to the v(CEN) modes for the
bound acetonitrile, and weak v(C-H) stretches below 900 cm'1 from the aromatic pyridyl
hydrogens.

Cyclic voltammetric studies in acetonitrile revealed a quasireversible ligand-based
reduction at E”: 0.28 V and an irreversible reduction at Ev,c = -l.26 V. The lack of
reversibility is attributed to the distortion from planarity of the tetrazine ring. There is an

average distortion of 251° out of planarity for the tetrazine ring.

77

C. Chemistry of bptz with other 3d transition metals.

 

 

 

F 4 ‘ 2+
\ \ I // + M
I, x
* DIVERGENT
- .4 BRIDGING
DIVERGENT METAL PRECURSOR LIGAND

 

 

 

11+

 

 

 

 

 

Figure 27. Possible outcomes for the reaction between bptz with an octahedral-metal
precursor.

78

 

In addition to Ni(II) and Zn(II), reactions of bptz with other first row transition
metals were investigated. In contrast to the previously discussed metals (Ni2+ and Zn“),
Mn(II), Fe(II), Co(II) and Cu(II) reactions tend to favor lower nuclearity species
(monomers, dimers) as judged by mass spectroscopy or insoluble materials, although the
tetranuclear product is formed to some degree as evidenced by mass spectroscopy.

For example Mn(II) forms a zig-zig polymer as determined by X-ray
crystallography as well as a lower nuclearity species according to mass spectroscopy
studies (vide infra). The ES-MS data show evidence of the presence of the
“[Mn4(bptz)4]8+” unit in the first hour of the reaction, but, with time, an insoluble product
begins to be deposited at the bottom of the reaction flask. Furthermore, the ES-MS data
show evidence for higher concentration of dimers and monomers in marked contrast to
the solutions of Ni(II) and Zn(II), in which the molecular squares are the main species
even days after the reaction has been performed.

X- ray crystallographic studies of bptz products with Mn(II), Co(II), Cu(II).

Reactions of Mn(II) with bptz led to the isolation of the zig-zag polymeric
material { [Mn(bptz)2(CH3CN)2][BF4]2}°° (7) (Figure 28). In this compound, each Mn(II)
center exhibits a distorted octahedral geometry with two bptz units coordinated in a cis
fashion along with two acetonitrile ligands. As noted earlier in the case of the cyclic
oligomers, the tetrazine and the pyridyl rings are not coplanar (dihedral angle = 532°)
(Figure 28a) and the M-N distances follow the same trend as the metallocyclophanes with
the shortest bond being to the acetonitrile ligand Mn(l)-N(4B) = 2.143 (6) A, followed
by the pyridyl interaction Mn(l)-N(l) = 2.230 (5) A and finally the tetrazine interaction

Mn(l)-N(2) = 2.320 (5)/31. The formation of the an+ polymer (7) points out the fact that

79

both Open and closed structures are formed in the reaction and that subtle factors such as
identity of the metal, solvent and anion choice can promote one or the other of the two

structures (Figure 27).

 

Table 9. Selected bond distances (3.) and angles (°) for {[Mn(bptz)2(CH3CN)2][BF4]2}....

 

(7)-

Mnl-N4B 2.142 (6) Mnl-Nl 2.230(5)
Mnl-N4A 2.167 (6) Mnl-N2A 2.309 (5)
Mnl-NlA 2.229 (6) Mnl-N2 2.320 (5)
N4B-Mnl-N4A 93.7 (2) N1-Mn1-N2A 88.19 (18)
N4B-Mnl-N1A 108.7 (2) N4B-Mnl-N2 168.3 (2)
N4A-Mnl-N1A 101.4 (2) N4A-Mnl-N2 86.0 (2)
N4B-Mnl-Nl 97.2 (2) N1A-Mnl-N2 82.77 (19)
N4A—Mnl—Nl 99.2 (2) N1-Mn1-N2 71.36 (19)
NlA-Mnl-Nl 145.5(2) N2A-Mn1-N2 94.77 (18)
N4B-Mnl-N2A 87.1 (2) N1A-Mnl-N2A 172.4(2)
N4A- Mnl-NZA 71.24 (19)

It is worth mentioning another factor that should be taken into consideration in
this chemistry, namely the tendency for bptz to undergo rearrangements in the presence
of metal ions. For example, reactions of [Cu(CH3CN)4][BF4]2 and [Co(CHgCN)6][PF6]2
with bptz yield mononuclear complexes that contain the transformed ligand 2,5-(2—
pyridyl)-l,3,4-oxadiazole (bopd) (Figure 29). The combination of an active metal and
traces of water apparently promotes this rearrangement. Crystals of the compounds
[Cu(bopd)2(H20)2]2+ and [Co(bopd)2(CH30H)2]2+ were obtained from layering the
reaction mixtures in acetonitrile with toluene. The thermal ellipsoids plots for these

compounds are depicted in Figure 29.

80

  
    

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N(lA)

(b)

Figure 28. (a) Thermal ellipsoid diagram of a portion of the polymer
{[Mn(bptz)2(CH3CN)2]2+}.. at the 50% probability level, (b) asymmetric
unit. Hydrogen atoms were omitted for the sake of clarity.

81

 

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[C0(b0pd)2(CH3OH)2][PF6]2 and [Cu(bopd)2(H20)][BF4]2 (b). Thermal
ellipsoids are drawn at the 50 % probability level. Hydrogen atoms were
omitted for the sake of clarity.

82

D. Electrospray-MS studies of bptz reactions with Mn“, Fe“, Co“, Ni“,

Cu2+ and Zn“.

The metal/bptz products for Mn(II), Fe(II), Co(II) and Cu(II) were studied by the
Electrospray mass spectroscopy technique. The following reactions were studied (Eq. 4).

acetonitrile .
[M(CH3CN)6][CIO4]2 + bptz > [M4(bptl)4(CH3CN)3][(310413 (EQ- 4)

 

(M = Mn2+, Co“, Cu”, Fe“)

As mentioned earlier, the polymeric material {[Mn(bptz)2(CH3CN)2][BF4]2}..
(Figure 28) was isolated in low yield (30%), but we were not able to isolate a cyclic
oligomer (Figure 24). Mass-spectral studies of aged solutions of Mn(II) ions and bptz do
not indicate the presence of tetranuclear products, but instead reveal the presence of
monomers and dimmers. If MS data are collected immediately after the reaction has been
performed, however there is evidence for the presence of [Mn4(bptz)4][ClO4]77+ (m/z =
1856.66) (Figure 30). One hour later, the same solution does not exhibit any signs of the
molecular square. The yield of the insoluble materials (polymer) is low (30 %), after
several hours, but afterseveral days, the yield increases to 50 %. These observations point
to a lack of stability of the square [Mn4(bptz)4(CH3CN)g]8+ which evidently decomposes
into the polymeric form.

The Co(II), Fe(II) and Cu(II) analogues follow the same trend as Mn(II), as noted
in Figures 31, 32 and 33. There is an indication of the formation of [M4(bptz)4][ClO4]7+
(M = C0“, Fez“, Cu“) species, immediately after the reaction has been performed
(Figure 31, 32, 33). An hour later, the signal for the parent ion has disappeared and only

lower nuclearity species are observed.

83

The solution data are compelling evidence that a major factor in controlling the
outcome of these reactions is the metal identity. One could attribute this behavior to ion
size. For example a large cation such as Mn(II) may not form a stable cyclic entity
because the cavity inside the molecular square is too large for the [BET or [C104]' to
have the same stabilizing effect as they have with Ni(II) and Zn(II). In the case of Cu(II),
the Jahn-Teller distortion could play a predominant role in the instability of the
metallocyclophanes because of the weak interactions along the axial sites of the metal
center. Finally, cations such as Co(II), Fe(II) may exhibit lower lability to their M-N
bonds than Ni(II) or Zn(II); consequently, the self-healing mechanism that takes place in
the high-yield assembly of the cyclic species may not be occurring. In these cases the
kinetic product (polymeric form) would be favored instead.

Electrospray mass spectroscopy studies performed on solutions of
[Nl4(bp[Z)4(CH3CN)3][X]3 and [Zn4(bptz)4(CH3CN)g][X]g, where X = [BF4]' or [C104]',
in acetonitrile revealed the parent ion [l\/I4(bptz)4(CH3CN)g][X]7+ (M = Ni2+ or Zn”) and
several fragmentation peaks (Figure 34). These species were detected in a solution that
had been standing for more than a week, which is good evidence for the stability of these
metallocyclophnes in solution.

The MS data in Figure 34 indicate that the products of the perchlorate salts are
more stable in the gas phase than the [BET salts in that less fragmentation is observed for
the perchlorate derivatives (3 and 4) (Figure 34 b and c), than for the tetrafluoroborate
compounds (Figure 31a). Compound (2) is not stable enough in the gas phase to allow for

the observation of the parent peak ([Zn4(bptz)4][BF4]7+).

84

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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minutes

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1000 1400 1800 2000

Figure 32. ESI-MS spectra of the 1:1 reaction between [Fe(H20)6] [C104]; and bptz taken
afier 10 minutes.

87

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
 

 

    

 

 

 

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1000 1200 1400 1600 1800 2000

Figure 33. ES] spectra of the reaction between [Cu(H20)6][ClO4]2 and bptz taken after
10 minutes.

88

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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M H" “m" Mass (mlz) Mass (mlz)
600 800 1000 1300 1400 1600 1800
Mass (mlz)

Figure 34. ESI-MS spectra in acetonitrile of the metallocyclophanes

(a) [Ni4(bptz)4(CH3CN)8][BF4]8: (b) [Ni4(bptz)4(CH3CN)8][ClO4]Sa

and (C) [Zm(thZ)4(CH3CN)s][BF4]8.

89

 

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91

Solutions of [Ni5(bptz)5(CH3CN).o][SbF6]to (6) clearly show the signal for the parent ion
[Ni5(bptz)5][SbF6]9+ at 3584 mass units (Figure 35). In addition, there are numerous
fragmentation peaks corresponding to [Nia], [Ni3] and [Niz] species, as it can be observed
in Figure 35.

All X-ray structures of the metallocyclophanes were performed on crystals grown
in acetonitrile. A question that comes to mind is whether they exist in other solvents
besides acetonitrile. Electrospray mass spectroscopy is an excellent technique for probing
this question. Since the perchlorate salts were already known to be more stable in the gas
phase (as noted earlier), ES-MS studies of these salts in different solvents were
performed. Reactions of [Ni(HzO)6][ClO4]2 with bptz in a 1:1 ratio were performed in
aprotic solvents such as nitromethane and acetone and evidence for the formation of the
tetramers was obtained by mass-spectrometry (Figure 36). On the other hand, protic
solvents such as alcohols or water do not promote the formation of the

metallocyclophanes, instead only lower nuclearities were observed.
F. Interconversion of the [Nis]10+ and [Ni4]°”.

With all of the above mentioned data in hand, the evidence for the predominant
role of the anion in the assembly of these metallocyclophanes was compelling. Similar
anions, in terms of size and geometry such as [BF4]‘ and [ClOJ yield tetramers whereas
the larger anion ([SbF6]') produces the pentamer (6). Armed with this knowledge, we
asked ourselves if it was possible to transform the pentamer into the tetramer by treating
a pure sample of [Nis]10+ with an excess of tetrafluoroborate or perchlorate in order to
exchange the [Sng]', or to transform the [Nia]8+ molecule into the [Ni5]'°+ compound by

reacting it with an excess of [Sde'. In order to test this hypothesis, two different

92

 

approaches were taken. In one case, a large excess of tetraflouroborate was added to a
solution of [Ni5(bptz)5(CH3CN)lo][SbF6]10 and allowed to stir for several days. In order to
obtain crystallographic evidence for the outcome, layering experiments were performed
and crystals were obtained. The results showed that the transformation to the tetramer
[Ni4(bptz)4(CH3CN)g][BF4]3-4CH3CN (1) had occurred. With the same idea in mind,
experiments were performed with a mixture of [TBA][BF4] and [TBA][PF6] which lead
to the isolation of the mixed anion salt [Ni4(bptz)(CH3CN)3][BF4][[PF6]3[SbF6]4-CH3CN
(8) (Figure 37). In this salt the only [BF4]’ anion that is present is the one inside the cavity
of the tetramer. Presumably, the [Sde' and [PF6]' anions are too large to occupy the
cavity reside and, therefore, in the interstices between the squares.

It was found that the transformation of the molecular squares into the pentagon is
not as facile as the reverse reaction. A layering of [Ni4(bptz)4(CH3CN)g][BF4]3 or
[Ni4(bptz)4(CH3CN)g][ClOdg in acetonitrile with a saturated solution of [BU4NHSbF6]
afforded only crystals of the tetramers. The lack of a transformation was corroborated by
mass spectrometry. In order to attempt to force this transformation, a large excess of
[Bu4N][SbF6] was added to a solution of [Ni4(bptz)4(CH3CN)g][ClOdg in acetonitrile (>
50:1 ratio) and the solution was refluxed for two days. Mass spectrometric studies
revealed the transformation of the tetramer into the pentamer was successful in this case
(Figure 39). The fact that the transformation of the pentagon into the square takes place
under less forcing conditions is a good indication that the square nuclearity is more stable

than its pentagon counterpart.

93

 

 

 

 

Table 10. Selected bond distances (A) and angles (°) for
[Ni4(thZ)4(CH3CN)8][BF4][PF6]3[SbFol4'CH3CN (8)-

Nil-N6 199(3) Nil-N3 210(2)
Ni l-N5 208(2) Ni 1-N4 2.10(2)
Ni l-N2 2.09(2) Ni l-Nl 210(2)
N2-Ni1-Nl 76.9(9) N3-Ni1-N4 77.6(8)
N2-Ni1-N3 88.7(10) N4-Ni1-Nl 95.1(8)
N2-N i 1 -N4 92.6(9) N5-Ni l -N 1 91 .0(9)
N3-Ni1-N1 163.7(11)

Table 1 1. Selected bond distances (A) and angles (°) for
[N i4(thZ)4(CH3CN)8] [C104] [10417'2CH3CN (9)-

Nil-N14 2.045(14) Ni2-N 12 204(2)
Nil-N13 2.043(13) Ni2-N7 2.061(17)
Ni l-Nl 2.065(13) Ni2-N6 2.075(16)
Ni 1-N8 2.068(15) Ni2-N9 2.073(14)
Nil-N10 2.088(15) Ni2-N11 2.084(16)
Ni l-N2 2.096(13) Ni2-N5 2.085(14)
N14-Ni 1-N13 86.2(6) Nl2—Ni2-N7 96.5(6)
N14-Nil-N1 93.5(6) N12-Ni2-N6 92.6(6)
N13-Ni l-Nl 93.6(5) N7-Ni2-N6 162.9(5)
N14-Ni1-N8 94.1(6) N12-Ni2-N9 173.2(6)
N13-Ni1-N8 92.5(5) N7-Ni2-N9 77.3(6)
N1-Nil-N8 170.6(5) N6-Ni2-N9 94. 1(5)
Nl- Ni 1 -N10 171.9(5) N12-Ni2-N1 l 90.0(6)
N l - Ni l-NlO 92.6(6) N7-Ni2-N1 l 99.9(6)
Nl-Ni 1-N10 94.6(5) N6-Ni2-Nl 1 945(6)
N8-Ni1-N10 7 8.0(5) N9-Ni2-Nll 88.3(6)
Nl4-Ni 1 -N2 87.7(5) N12-Ni2-N5 91.1(6)
N13-Ni1-N2 170.0(6) N7-Ni2-N5 87.7(5)
N l-Ni l-N2 78.9(5) N6-Ni2-N5 77.7(5)
N8-Ni l-N2 95.8(5) N9-Ni2-N5 91.5(5)
N 10-Nil-N2 94.6(5) N1 l-Ni2-N5 172.1(7)

A good example of

demonstrated by the

the predominant role of the anions in the self-assembly is

addition of 50 equivalents of [Bu4N][IO4] to a solution of

94

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Figure 37. (a) Thermal ellipsoid drawing of the [Ni4(bptz).4(CH3CN)3][BF4][SbF6]6+ unit
in [Ni4(bptz)4(CH3CN)g][BF4][PF6]3[SbF6]4 with 50% probability ellipsoids,

(b) space filling diagram and (c) asymmetric unit. Hydrogen atoms have been
omitted for the sake of clarity.

95

 

 

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Figure 38. (a) Thermal ellipsoid drawing of the [Ni4(bptz)4(CH3CN)g] [C104] [I04]6+ unit
in [Ni4(bptz)4(CH3CN)g][C104][104]7 with 50% probability ellipsoids, (b)
space filling diagram that emphasizes the molecular volume of the
perchlorate versus the periodate and (c) asymmetric unit. Hydrogen atoms
were omitted for the sake of clarity.

96

 

[Ni4(bptz)4(CH3CN)3][C104]3. The initial goal of the experiment was to obtain
metallocyclophanes of nuclearities other than four or five. The outcome of the reaction,
however, was the molecular square [Ni4(bptz)4(CH3CN)3][C104][104]7 (9) (Figure 38).
Although the solution was saturated with [104]' the compound retains a [ClO4]' inside the
cavity and exchanges only the outer anions.

E.S-MS studies of interconversion reactions.

Another aspect of this chemistry investigated by mass spectrometry is the
conversion of [Ni5(bptz)5(CH3CN)1o][SbF6]10 the pentagon into the molecular square in
the presence of an excess of either [BF4]‘ or [ClO4]'. This conversion was accomplished
easily as previously mentioned and confirmed by X-ray crystallography, but the reverse
transformation from the square into the pentagon did not take place under similar mild
conditions.

A reaction that involved addition of a large excess of [Bu4N][SbF6] to a solution
of [Ni4(bptz)4(CH3CN)3][C104]g with two days of reflux was followed by ESI-MS. As
indicated by the data in Figure 39, the solution contains only [Ni4(bptz)4(CH3CN)g]8+ in
the earlier stages of the reaction (Figure 39a). After two days (under constant refluxing),
there are signs of [Ni5,(bptz)5(CH3CN).o]10+ (Figure 39b), but the conversion is not
complete. These data suggest that the molecular square is more stable that the molecular

pentagon.

97

 

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98

G. 2,2’ azo bipyridine Chemistry.
G.1. Synthesis of [Ni2(abpy)(CH3CN)2(NO3)4], [Cu2(abpy)(CH3CN)3][BF4]4 and
{[Cu2(abpy)][BF4]2}.. Compounds.

The compounds [Ni2(abpy)(CH3CN)2(N03)4] (12) and
[Cuz(abpy)(CH3CN)g][BF4]4 (13) were synthesized in acetonitrile. The reactions proceed
virtually instaneously upon the contact of both reagents. In marked contrast to previous
reports, in which is stated that there is a large activation barrier for the coordination of a
second metal center after the first metal is bound, the formation of the dinuclear species is
relatively facile in this case.30 Specifically [Ni2(abpy)(CH3CN)2(N03)4] was synthesized
by reaction of [Ni(H20)6][N03]2 with abpy in a 2:1 ratio in a beaker (Eq. 5). The reaction
takes place rapidly as evidenced by the color change from pale blue to green within
minutes. Crystals of the product were obtained by slow vapor diffusion of diethyl ether

into the reaction mixture.

 

 

acetonitrile ,
2[Ni(H20)6][N03]2 + abpy » [N12(abpy)(CH3CN)2][N03]. (Eq. 5)
75% yield
acetonitrile
2[Cu(CH3CN)4][BF4]2 + abpy , [CU2(abpy)(CH3CN)3][BF414 (Eq. 6)
80% yield

The compound [Cu2(abpy)(CH3CN)3][BF4]4 (13) was synthesized under
anaerobic conditions by dissolving two equivalents of [Cu(CH3CN)4][BF4]2 in
acetonitrile, and adding one equivalent of abpy (Eq. 6). The reaction is fast, leading to a

color change from pale blue to light green within minutes. Crystals of

99

 

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[Cuz(abpy)(CH3CN)3][BF4]4 (13) were obtained by saturating the solution with diethyl
ether and placing it in the freezer. The compound [Cu2(abpy)(CH3CN)g][BF4]4 (13) is not
indefinitely stable and eventually decomposes to the polymer {[Cuz(abpy)][BF4]2}... In
this process, Cu(II) is reduced to Cu(I) over the period of several months when the title
compound in acetonitrile, is layered with toluene as judged by the isolation of
{[CU(abP)’)2][BF4] }~ 0") crystals»

G.2. Spectroscopic and Electrochemical studies.

The abpy ligand possesses low lying 1t* orbitals, which renders it an excellent
electron acceptor. Early reports indicate that these compounds exhibit rich
electrochemistry and interesting electronic properties.”'33 The ligand itself exhibits three
electronic transitions in the UV-Visible region at Arm = 450, 361 and 203 nm with 8
values of 4.8 x 102, 1.5 x 104 and 5.3 x 102 L(mol-cm)", respectively. These transitions
correspond to 1: —> 1t* transitions.

The compound [Ni2(abpy)(CH3CN)2(NO3)4] (12) displays only one electronic
transition in the UV-Visible range, namely at Am” = 344 nm which is assigned to an
MLCT band given the e value of 1.8 x 104 L(mol°cm)". Its counterpart
[Cuz(abpy)(CH3CN)3][C104]4 (l3) exhibits a similar transition at 351 nm with e = 1.5 x
104 L(mol-cm)"along with a second electronic transition at higher energy, km,“ = 230 nm,
e = 2.0 x 104 L(mol'cm)".

The cyclic voltammogram of [Ni2(abpy)(CH3CN)2(N03)]4 (12) in acetonitrile
exhibits two oxidation features at Ep,a = +0.62V and Ep,a = +0.4V with no associated
return waves. In addition there are three reversible reductions located at EV, = -0.10V, Ev,

= -0.4V and E9, = -0.9V.

100

The dinuclear Cu(II) (13) compound also exhibits a rich electrochemistry with an
irreversible oxidation at Ep,a = +1.10 V, reversible reductions at E), = 0.65V, En, =
+0.40V and a quasireversible reduction at Er), = 0.0V.

G.3. X-ray Crystallographic studies.

The molecular structure of [Ni2(abpy)(CH3CN)2(N03)4] (12) is based on a
distorted octahedral geometry around the metal centers (Figure 40). Each nickel(II) center
is coordinated to one bidentate and one monodentate nitrate, an acetonitrile molecule, and

an abpy ligand.

 

Table 12. Selected bond distances (A) and angles (°) in
[Ni2(abPY)(CH3CN)2(N03)4]'2CH3CN (12).

 

Ni 1-06 1.990(2) Ni 1-03 2.106(2)
Ni l-Nl 2.037(2) Ni 1-N2 2.121(2)
Ni l-N5 2.044(2) Ni 1-02 2.125(2)
N5-Ni1-O3 8669(9) N5-Ni l-N2 170.06(8)
O6-N i l-N 2 9556(8) O3-Ni l-N 2 101.99(8)
N1-Ni1-N2 7645(9) N 1 -Ni 1-03 l62.44(8)
O6-Ni l-Nl 105.01(8) O6-Ni 1-02 153.92(7)
O6-Nil-N5 8882(9) N1-Ni1-02 101.0l(8)
Nl-N i l-N 5 9380(9) O3-Ni 1-02 61.43(8)
O6-Ni 1-03 9255(8) N 2-Ni 1-02 8874(8)
N5-Ni1-02 91 .24(9)

The Ni-N distance to the acetonitrile ligand is 2.044 (2) A which is essentially the
same as the distance in [Ni2(bptz)(CH3CN)3][C104]4; (Ni(l)-N(5), 2.044 (7)A) (Figure
19). The azo nitrogen distance to Ni(II) is the longest of the three Ni-N interactions at
Ni(1)-N(2) = 2.121 (2)A. Compared to a similar type of interaction in
[Ni2(bptz)(CH3CN)g][C104]4 in which there is a tetrazine ring instead of an azo linkage,

this interaction is 0.04 A longer.

101

The angles within the coordination sphere also provide some helpful insight into
the distortion of the ligand environment of this compound. The binding angle for the
chelating nitrate O(3)-Ni(1)-O(l) is 61.43 (8)°. The angle between the pyridyl and the
acetonitrile ligand is close to the ideal value for an octahedron, (N(5)-Ni(1)—N(1) = 93.80
(2)°). The binding angle of abpy, N(1)-Ni(l)-N(2) of 76.45 (9)°, is very close to angles of
coordinated bptz in [Ni2(bptz)(CH3CN)3][ClO4]4 78.10 (3)°. The two Ni(II) atoms are
4.942 A apart, which is much shorter than the corresponding distance in
[Ni2(bptz)(CH3CN)g][ClO4]4, (6.904A). An important feature to note in the structure of
(12) is the elongation of the N=N bond by 0.025%. with (N(2)-N(2#) 1.271 A), compared
to the corresponding value in free abpy, (1.246 A)“ This phenomenon indicates n-back
donation from the Ni(II) centers into the 1r* (abpy) orbital. This effect has been

previously noted for coordination complexes of abpy with Cu(I).32

 

Table 13. Selected bond distances (A) and angles (°) for [Cu2(abpy)(CH3CN)g][BF4]2

 

(13).

Cul-N4 2.004(5) Cul-N6 2.318(5)
Cul—N 3 2.007(5) Cul-N2 2.403(5)
Cu 1 -N5 2.029(6) Cu 1 -N1 2.032(5)
N6-Cu1-N 2 166.00(18) N3-Cul-N6 94.4(2)
N4-Cul-N3 l74.83(l9) N5-Cul-N6 87.0309)
N4—Cu1-N 5 91 .6(2) N l-Cu 1 -N6 94.59(19)
N3-Cu1-N5 86.9(2) N4-Cul-N2 86.35(18)
N4-Cu1-Nl 89.5(2) N3-Cul-N2 89.35(19)
N3-Cul-Nl 91 .8(2) N5-Cul-N2 106.66(18)
NS-Cul-Nl 178.01(l9) Nl-Cul-N2 71.79(18)
N4-Cul-N 6 90.4 1(19)

[Cuz(aszy)(CH3CN)g][BF]4 (13) exhibits typical parameters for a Cu(II) metal

center in an distorted octahedral environment. It displays a Jahn-Teller distortion which

102

corresponds to the Cu-azo nitrogen interaction, Cu(I)-N(Z) 2.403 (5)A and the Cu-N
acetonitrile interaction Cu(I)-N(6) 2.318 (5)A along the z axis (Figure 41). The other
four coordination sites are filled by three acetonitrile molecules, which exhibit an average
Cu(I)-N distance of 2.013 [5] A, and the pyridyl ring, Cu(l)-N( 1) 2.032 (5) A. The L-Cu-
L binding angles are distorted from the ideal 90 or 180 ° expected for an octahedron. The
angles N(2)—Cu(1)-N(5) = 106.6 (2)° and N(2)-Cu(1)—N(1) = 71.8 (2)° are typical values.
The N-N distance in the azo group N(2)-N(2A), 1.277 (1) A is indicative of it back-
donation from the Cu(II) center into the 71* (abpy) orbital. This distance is 0.03 A larger
than the one found in free abpy, which is 1.246 (2)A.3 ' The Cu(II) centers are separated
by a distance of 5.408A, which is 0.46 A longer that the corresponding separation in the

Ni(II) case, as expected for a larger metal radius.

 

Table 14. Selected bond distances (A) and angles (°) in {[Cu(abpy)2][BF4]2}.o (14).

 

Cul-N4 1.934(9) Cul-Nl 2.060(9)
Cul-N3 2.009(9) N3-N3A 1.300(16)
Cul-N2 2.009(9) N3-C9 1.396(13)
N4-Cul-N3 146.5(4) N2-Cu1-N1 135.6(4)
N4-Cul-N2 117.1(4) N3-N3A-Cu1 115.7(9)
N3-Cu1-N2 79.5(3) C9-N3-Cu1 129.0(7)
N4-Cu1-N1 77.8(4) Cl-Nl-Cul 131.2(7)
N3-Cu1-Nl 111.9(3) CS-Nl-Cul 113.0(7)

Finally, {[Cu(abpy)2][BF4]2}m (14) is a 1-D polymer with Cu(I) ions instead of
Cu(II). As expected for Cu(I), the metal center exhibits a distorted tetrahedral geometry
(Figure 42). This is illustrated by the angles observed between the pyridyl-azo, N(3)-
Cu(1)-N(1) = 111.9 (3)°, and the two pyridyl groups N(1)-Cu(1)-N(2) = 135.6 (4)°,

which are far from the ideal 109.5° for a tetrahedron. The distance N(3)-N(3#) = 1.300 A

103

 

 

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106

to the azo group is evidently involved in appreciable 7t back-donation from the Cu(I), as
expected for an electron rich (1‘0 center. The N=N distance is 0.054 A, which is nearly

twice the lengthening observed for the Ni(II) and Cu(II) complexes.

IV. Future work

Reactions of the metallocyclophanes.

In our search for supramolecular compounds with interesting properties, an
interesting family of metallocyclophanes was isolated. A close look at the structures
reveal the presence of two acetonitrile ligands on each metal center as emphasized in
Figure 43. Substitution of these molecules by organic linkers may allow the formation of
higher nuclearity species (Figure 43). With this goal in mind, several bridging ligands
were reacted; these include 1,4’ phenyldiamine, pyrazine, cyanide, 4,4’-azo-bipyridine,
2,2’-bipyrimidine, 4,4’-bipyridine, 1,4’-benzonitrile and thiocyanide. Reactions of
[Ni4(bptz)4(CH3CN)3][BF4]3 with a large excess of either [Bu4N][CN1 or bpym led to the
isolation of the compounds [Bu4N]2[Ni(CN)4] and [Ni(bpym)3][BF4]2 as verified by X-
ray crystallography.

Halides (Cl', Br' and I') react to yield insoluble species in acetonitrile. The
reaction is effectively instaneous, and IR/spectral analyses of the precipitates reveal the
disappearance of the v(CEN) and v(B-F) modes that are normally observed for
[Ni4(bptz)4(CH3CN)3][BF4]3. It is worth mentioning at this point, that reactions of the
pentagon with halides are much slower, with eventual precipitation of an insoluble
precipitate that lacks CEN stretches. Moreover, in the case of reactions of the pentagon
[Ni5(bptz)5(CH3CN)lo][SbFo]lo with I', there is more than one product. After the initial

insoluble solid is removed, the remaining light green solution affords crystals when

107

layered with toluene. The crystals are the unexpected compound
[Ni4(bptz)4(CH3CN)3][I][SbF6]7 (Figure 44) in which the central cavity is occupied by an
iodide atom. The iodide ion is another anion which has the correct size to occupy the
cavity of the molecular square. Attempts to synthesize this compound by using NiIz as
the source of Ni(II), led to only an insoluble, material which is either a polymer or a
neutral square.

Other organic linkers that appear to react with the [Ni4]8+ square are isocyanide
and oxalate. In both cases, there is a disappearance of the characteristic v(CEN) mode
with precipitation of insoluble products. This can be attributed to the formation of either
neutral species that are insoluble in acetonitrile or molecular grids, which would not be
soluble. Unfortunately, attempts to obtain crystals by layering experiments or by slow

vapor diffusion were unsuccessful.

 

Table 15. Selected bonds (A) and angles (°) for [Ni4(bptz)4(CH3CN)g][I][SbF6]7 (15).

 

Ni 1-N6 2.026(10) Ni 1-N4 2.098(10)
Nil-N3 2.052(11) Nil-N1 2.100(11)
Nil-N2 2.066(9) Nil-N7 2.130(17)
N 6-Ni l-N3 96.7(4) N2-Ni1-N l 78.1(4)
N6-N i 1-N2 174.2(4) N4-Nil-N1 94.6(4)
N 3-Ni l-N 2 89. 1(4) N6-Ni1-N7 90.4(5)
N6-Ni l-N4 91 .9(4) N3-Ni1-N7 94.3(4)
N3-Ni1-N4 79.0(4) N 2-Ni 1 -N7 88.4(4)
N 2-Ni 1-N4 89.9(4) N4-Ni l-N7 173.1(4)
N6-Ni l-Nl 96.2(4) N 1 -Ni l-N7 91 .5(4)
N3-Ni1-Nl 165.8(4)

Symmetry transformations used to generate equivalent atoms: #1 -x+2,-y,z ; #2 x,-y,-z
#3 -x+1,-y,z; #4 -x+l,y,-z; #5 -x+1,y,-z+1

Other organic linkers that were used in pursuit of linked metallocyclophanes are

1,4 disubstitued benzenes, such as 1,4 benzonitrile and 1,4 phenyldiamine. These ligands

108

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A=BorCl

X=ForO'

M = Ni2+ or
an+

Each comer of the
square has two
acetonitrile molecule
that can be
substituted by an
organic linker

  

 

Each comer of
the pentagon
has two
acetonitrile
molecules that
can be
substituted by
an organic
linker

 

 

Figure 43. Schematic drawing of the molecular square and pentagon with indicated sites

for substitution.

109

 

N(6)1\",
C(15) p ’1

C(14) \k‘. \\,
I

 

 

5-
81888

8:588
81'

\
/

  
 

§\\ ‘\\\. :5 \\ \
3‘. \\\;\\‘ ‘ ‘\\\
.5
§

8}
83358
5,)
88%,1
5-

V

1%
t: N")

Figure 44. (a) Thermal ellipsoid plot of the cation in [Ni4(bptz)4(CH3CN)3][I][SbF6]7
(15) at the 50% probability ellipsoid level, (b) space filling diagram with the
iodine anion inside the metallocyclophane and (c) asymmetric unit.
Hydrogen atoms have been omitted for the sake of clarity.

110

did not react swiftly as was the case for reactions of halides or cyanide, but, under
refluxing conditions for 4 hours, there was a noticeable color change. Layering
experiments and slow vapor diffusion have thus far been unsuccessful in leading to single
crystals growth. IR data of the products reveal an absence of nitrile stretches, indicating
that the substitution of the acetonitrile molecules has occurred.

Finally, the use of neutral ligands such as pyrazine, 4,4’-azo-bipyridine and 4,4’-
bipyridine do not appear to react with [Ni4(bptz)4(CH3CN)3][BF4]3 in acetonitrile, even
with a large excess of the ligands under refluxing conditions. Reaction solutions afforded

crystals of only the starting materials [Ni4(bptz)4(CH3CN)3][BF4]3 and the linking ligand.

V. Conclusions

The metallocyclophanes presented in this work are outstanding examples of how
subtle factors such as anion choice, solvent and metal identity can lead to differences in
self-assembly reactions. Molecular squares ([Ni4]8+ or [Zn4]8+) and the pentagons
([Ni5]‘°+) are obtained in good yields when the precursor salts that contain
tetraflouroborate, perchlorate or hexaflouroantimonate anions are reacted in 1:1 ratio with
bptz. In addition to acetonitrile, other solvents such as nitromethane and acetone allow
for the formation of these metallocyclophanes as evidenced by ESI-MS (Figure 36).
These results are attributed to the match between the size of the cavity formed by the
[M4] or [M5] cyclic unit and the anion.19 The tetramers [Ni4(bptz)4(CH3CN)3]8+ and
[Zn4(bptz)4(CH3CN)8]8+ have a cavity size that is well-matched with a [BF4]', [CIOJ and
[I]'.19a The use of a much larger anion, [Sde' (71 A3),'9a led to the pentagon
metallacyclophane, [Ni5(bptz)5(CH3CN)lo][SbFo]10'2CH3CN (6). On the contrary, when

anions such as, [CF3803]', B{C6H3(CF3)2}4', [PF6]’, [AlCl4]‘, [N03]' and [SO4]2‘ were

111

used, the formation of [M4]8+ and [M5]'0+ entities is precluded; instead lower nuclearity
or polymers are assembled.

Molecular pentagons versus squares can be favored, in solution and in the solid-
state simply by altering the anion present in solution. Transformations between molecular
square [Ni4(bptz)4(CH3CN)g]8+ and the molecular pentagon [Ni5(bptz)5(CH3CN)m]l0+ can
be achieved by adding an excess of the anion needed for the assembly; [Sde' for the
first case and [BF4]’ or [ClO4]' for the second. The fact that the nuclearity of these
macrocyclos can be interchanged is excellent evidence of the predominant role of anion
as a template. The conversion of the pentagon into the square takes place under mild
conditions whereas the opposite (square —> pentagon) requires refluxing for two days
(Figure 39). These results indicate that the squares are more stable than the pentagon.

The diamagnetic tetramer [Zn4(bptz)4(CH3CN)g][BF4]3 allows for the study of
these entities in solution by means of l9F NMR and 1'B NMR spectroscopy. Both NMR
experiments show two peaks, one corresponding to the encapsulated and the other to the
exterior [BF4]' anions, thereby proving the existence of these entities in solution.
Coalescence could not achieved in a variable l9F NMR experiment, presumably due to
the formation of aggregates between the metallocyclophanes in solution when the
temperature is varied (Figure 22).

Mass spectrometric studies of the products of different Ni(II) salts
([Ni(CH3CN)6][X]2 X = anions) with bptz show that only those reactions that involve the
[BF4]', [C104]', [SbF6]' anions yield tetramers or pentamers respectively; all others appear
to be leading only to lower nuclearity species or polymer formation.

Tetrametallocyclophanes with Co(II), Fe(II), Cu(II) and Mn(II) are formed initially, as

112

verified by ES-MS (Figures 30 through 33), but they are not stable in solution.
Decomposition takes within a few minutes to one hour, with entities of lower nuclearity
or polymers being formed.

It was noted that the bptz ligand is prone to side-reactions in the presence of water
and Co(II) or Cu(II) which apparently assist in the tetrazine ring-opening and formation
of a five-membered ring (2,5-di(2-pyridyl)-l,3,4-oxadiazole). This is evidenced by the
isolation of the mononuclear compound [Cu(bopd)2(H20)2][BF4]2 and
[Co(bopd)2(CH3OH)2] [BF4]2 (Figure 29).

The new [M.;]8+ and [M5]l0+ molecules are of an unprecedented type of partially
solvated molecular metallocyclophanes. The presence of labile CH3CN ligands on the
periphery of these molecules allows for their use as building blocks for larger molecules
for the elaboration of 2-D square grids (Figure 43).

First-row transition metal coordination compounds with abpy do not exhibit as
good electronic coupling as their counterparts in the second and third row transition
metals. 20 A good approach to improve the capabilities of the abpy as a molecular bridge
for first-row transition metals would be the use of the radical form of abpy instead.
Previous studies with Cu(II) and Fe(II) have demonstrated that this approach affords
mixed-valence species.”33

Dinuclear compounds of Ru(II) and Os(II) with abpy are difficult to obtain and
require tedious and lengthy synthesis, but the mononuclear species were obtained in high
yields. The large activation barrier to the formation of abpy-bridge dimers from

monomeric precursors has previously been attributed to steric interactions between the 6

and 6’ protons of the bridge and the coordinating nitrogen atoms of adjacent ligands.33 In

113

the case of the dimers of Ni(II) and Cu(II), however, they are obtained in large yields just

by stirring for a few hours followed by crystallization without any forcing conditions.

114

VI. References

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J.; Romero-Salguero, F. J .; Lehn, J-M.; Baum, G.; Fenske, D. Eur. J. Inorg. Chem, 1999,
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17. (a) Hosseine, W.; Ruppert, R.; Schaeffer, P.; Cian, A.; Kyritsakas, N.; Fisher, J.; J.
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Lynch, V. Angew. Chem. Int. Ed. Engl., 1996, 35, 2782. (d) Hosseine, W.; Ruppert, R.;
Schaeffer, P.; Cian, A.; Kyritsakas, N.; Fisher, J.; J. Chem. Soc. Chem. Commun., 1994,
2135. (f) Saalfrank R. W.; Low, N.; Hampel, F.; Stachel, H. D. Angew. Chem. Int. Ed.
Engl., 1996, 108. 2353.

18. (a) Jones P. L; Byrom; K. J.; Jeffery, J. C.; McCleferty; J. A.;. Ward, M. D. Chem.
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Boumediene, N.; Dupont-Gervais, A.; Dorsselaer, A. V.; Kneisel, B.; Fenske, D. J. Am.
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20. (a) Kausikisankar, P.; Shivakumar, M.; Ghosh, P.; Chakravorty, A. Inorg. Chem,
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117

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24. Theory and Applications of Molecular Paramagnetism, Boudreaux, E.A.; Mulay,
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25. SAINT, Program for area detector absorption correction, Siemens Analytical X-Ray
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26. Program in the SI-[ELXTL software.

27. SHELXL-97 — g.m. Sheldrick, SHELXL — 97, Program for refining crystal structures,
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118

Chapter III

Reactivity of the Ligands 2,3,5,6 tetrapyridyl pyrazine (tppz)
and 2,4,6-Tris(2-pyridyl)-1,3,5-triazine (tptaz) with

Paramagnetic First Row Metal Ions

119

I. Introduction

In the last thirty years, the scientific community has witnessed a growth of
research in the area of supramolecular chemistry. One of the main approaches in this field
is to use predesigned “building blocks” to assemble larger structures. In order to build
more elaborate molecules, the building blocks must possess reactive sites at which further
chemistry can take place. One of the simple ligands that has been used with these goals in
mind is 2,3,5,6-tetrapyridylpyrazine (tppz) (Figure 45). The tppz ligand was synthesized
in 1959 by Goodwin and Lions, who reported a series of monodentate compounds of the
type [M(tppz)2]2+ (M = first row transition metals) with little characterization to verify
their nature.I At the time, it was believed that once the ligand tppz was coordinated to one
metal, binding to the second site would not occur. This prediction was based on the belief
that the four pendant 2-pyn'dyl rings would not be stable in a coplanar arrangement with

the central diazine ring.

 

Figure 45. Schematic drawing of the ligand 2,3,5,6-tetrapyridylpyrazine (tppz).

120

For almost thirty years the tppz ligand was largely ignored due to these early
predictions, and it was not until 1988 that Thummel and coworkers started using the
ligand to prepare compounds such as (tpy)Ru(tppz)Ru(tpy)]4+, work which demonstrated
that coordination of a second metal was indeed possible.2 A series of compounds with
Ru(II), Os(II), Ir(II), Rh(II) and tppz were synthesized and characterized during the next
15 years, with the ultimate aim being to use them as photosentizers.3 In addition to
displaying photoluminescence, some of these species exhibit strong electronic coupling,
which is an excellent indication that this bridging ligand permits delocalization of
electronic density between metal centers. Within this category, the compound

[(NH3)3Ru(tppz)Ru(NH3)]4+ is the best example, with a comproportionation constant Kc

Tridentate coordination site

A
r ‘

/ N N/
\ N\ \

\N / N
Monodentate

coordination site Brdentate coordination srte

/ [N
\

Figure 46. The three coordination sites in Tptaz.

121

of 2.8 x 108 based on cyclic voltammetric studies. This qualifies the compound as class
III (completely delocalized) on the Robin-Day scale.4

Concomitant with the research on the reaction of the second and third row
transition metal with tppz, Stoeckli-Evans and co-workers undertook reactivity studies
with first row metals.5 “Monomers” and “dimers” such as [M2(tppz)2(L)(,]4+, [M(tppz)]2+
(M: Ni“, Cu“; L = H20) were prepared and found to exhibit significant
antiferromagnetic coupling.5 There were no reported attempts, however, to use these
compounds as building blocks for higher nuclearity species.

Work in our laboratories on the tppz ligand focuses on two main areas: (a) the
synthesis of paramagnetic building blocks with first row transition metals and (b) the
self-assembly of molecular rectangles with metal-metal bonded cores, a novel application
for this ligand. In related chemistry, exploratory chemistry with the ligand tptaz (2,4,6-
Tris(2-pyridyl)—l,3,5-triazine) is also being conducted. The tptaz ligand possesses three
coordination sites (tridentate, bidentate and monodentate) according to the number of
donor nitrogen atoms (three, two and one, respectively) as shown in Figure 46. Given the
multiple coordination sites on tptaz, it can act simultaneously as a tridentate and a
bidentate ligand. Dinuclear compounds of this type have been synthesized with Co(II),6
Hg(I),7 and Ru(II)8 ions. It is worth mentioning, however, that the coordination of the
second metal to this ligand is not particularly common due to the deactivation of the
triazine ring by the inductive effect of the first metal, and also because of steric
interactions between the hydrogen atoms and the metal ion.9

In the chapter, the syntheses of precursors to be used as building blocks for the

preparation of multinuclear metal complexes are reported. Three compounds were

122

synthesized in quantitative yields, namely [Ni(tptaz)(CH3CN)2(HzO)][BF4]2 (12),
[Fe(tptaz)2][ClO4]2 (13) and [Mn(tptaz)(phen)][CF3SO3]2 (14). A detailed
characterization of these compounds including magnetic measurements, cyclic
voltammetry, UV-Visible spectroscopy and X-ray crystallography is presented. A
description of our attempts to form larger supramolecular entities will be presented along

with a rationale for the outcome and possible future approaches to succeed in this goal.

11. Experimental Section

Preparation of Compounds. Methods and Materials.

All operations were performed under a nitrogen atmosphere using standard
Schlenk line techniques unless otherwise indicated. All solvents were pre-dried over 4 A
molecular sieves with the exception of acetonitrile, which was pre-dried over 3 A
molecular sieves. Diethyl ether and toluene were freshly distilled over Na/K, acetonitrile
was distilled over 3 A molecular sieves and methylene chloride was distilled over P205.
The starting materials [Ni(HzO)(,[ClO4]2, [Mn(HzO)6[ClO4]2 and [Fe(HzO)6[ClO4]2 were
purchased from Aldrich and used without further purification. The solvated compounds
[Ni(CH3CN)6][BF4]2 and [Co(CH3CN)6][PF6]2 were synthesized by published methods.10
The tppz ligand was prepared from a literature method and recrystallized from pyridine.2
The tptaz molecule was synthesized by a published method with minor modifications.ll
Physical Measurements.

IR spectra were performed on a Nicolet 740 FT -IR spectrometer using KBr plates
and samples were suspended in Nujol mulls. The magnetic susceptibility measurements
were obtained with a Quatum Design SQUID MPMS-XL magnetometer. Measurements

were performed on finely ground polycrystalline samples. Data were corrected for the

123

sample holder, and the experimental diamagnetic contribution was calculated from the
Pascal constants.12 Electrochemical measurements were carried out by using an EG&G
Princeton Applied Research Model 362 scanning potentiostat in conjunction with a BAS
Model RXY recorder. Cyclic voltammetry was performed in CH3CN containing 0.1 M
tetra-n-butylammonium hexaflourophosphate (TBAPFo) as the supporting electrolyte.
The working electrode was a BAS Pt disk electrode. The ferrocene couple occurs at 0.52
V vs. Ag/AgCl under the same experimental conditions.

Synthesis

2,3,5,6otetrapyridylpyrazine (tppz).

In a 500 mL round-bottomed flask, a 90 g sample of NILOZCCH3 was thoroughly
mixed with 20 g of a-pyrodoin (ClzNzOsz). This mixture was slowly heated to 180 °C
in an oil bath for two hours. After the first hour of refluxing, yellow crystals began to
appear in the brown melt. The thick brown solution was filtered to give a yellow solid
which was washed with a large quantity of diethyl ether. The solid was recrystallized
from pyridine; yield 4.8g (27 %) mp 284°C. 'H NMR (CDC13): three multiplets centered
at 7.30, 7,85 and 8.45 ppm. UV-Visible (acetonitrile, nm, c = 2.47 x 10°5M): Am, = 320,
e = 2.62 x 104 L(mol°cm)"; Am, = 274, e = 2.6 x 10‘ L(mol-cm)". IR (KBr mull) cm",
1680 (w), 1288 (w), 1138 (w), 1061 (w), 992 (w), 748 (m), 789 (w), 724 (w).
2,4,6-tris(2-pyridyl)-l,3,5-triazine (tptaz).

The tptaz ligand was synthesized from a previously published method with some
modifications.'1 The reagent 2-cyano pyridine (6.0 g, 0.058 mol) was loaded in a round-
bottomed flask and mixed thoroughly with NaH (0.1g, 0.004 mol) under nitrogen. This

mixture was heated at 165 °C until the entire solid had melted and then refluxed at 170 °C

124

for one hour. At this point the melt had turned black. The solid was extracted with hot

benzene and recrystallized by slow evaporation; yield 3.0g (50%).

[Ni(tPPZ)2JIN03]2 (16)-

The salt [Ni(HzO)6][NO3]2 (100 mg, 0.344 mmol) was dissolved in 20 mL of
acetonitrile, and two equivalents of tppz were added (270 mg, 0.69 mmol). Within a few
minutes a drastic color change from blue to yellow ensued, and a precipitate appeared at
the bottom of the vessel. The reaction mixture was stirred overnight after which time the
precipitate was removed by suction filtration. The filtrate was layered with toluene to
afford crystals of [Ni(tppz)2][NOg]2 within one week; yield, 190 mg (58%). IR (KBr
mull) cm", 2723 (w), 1598 (w), 1250 (w), 1204 (w), 1149 (w), 1096 (w), 1015 (w), 743
(w), 722 (w). UV—Vis (acetonitrile, nm, c = 1.9 x 10'5 M): Am, = 355, e = 4.3x 104
L(mol-cm)"; hm, = 298 e = 4.0 x 104 L(mol-cm)").

[C0(tPPZ)2][PF6]2°2CH3CN (17)-

The precursor [Co(CH3CN)6][PF6]2 (100 mg, 0.2 mmol) was dissolved in 20 mL

of acetonitrile in a 50 mL round-bottomed flask, and the resulting pink solution was
treated with tppz (155 mg, 0.4 mmol). The solution immediately changed to a red color.
It was stirred under nitrogen overnight, after which time it was layered with toluene to
afford red rectangular crystals; yield, 213 mg (90%). IR (KBr mull) cm", 2726 (w), 1317
(W), 1250 (w), 1152 (w), 840 (s,b), 722 (m), 687 (w). UV-Vis (acetonitrile, nm, c = 1.5 x
104M): Am = 520, e = 2.3 x 103 L(mol-cm)"; Am, = 484, e = 2.3 x 103 L(mol-cm)".
[Fe(tPPZhJIClodz (18)-

The salt [Fe(HzO)6][ClO4]2 100 mg (0.276 mmol) was dissolved in 30 mL of

acetonitrile in a beaker. To this clear solution was added 215 mg (0.553 mmol) of tppz

125

which led to an instantaneous color change from a clear solution to a dark blue. The blue
solution was stirred for 4h, and the volume was decreased by half. The reaction mixture
was layered with diethyl ether and blue crystals were obtained within a week; yield, 230
mg (81%). IR (KBr mull) cm", 2723 (w), 1320 (w), 1220 (w), 1078 (s,b), 722 (m), 619
(w). UV—Vis (acetonitrile, nm, c = 7.4 x 10’6M): Amax = 568, e = 3.7 ><103 L(mol-cm)"; A.
= 341, e = 4.0 x 104 L(mol.cm)"; Am, = 242.5, 8 = 4.4 x 104 L(mol-cm)".
[Mn(tPchHClOdz (19)-

A sample of [Mn(HzO)6][ClO4]2 (100 mg, 0.277 mmol) was placed in a beaker
and dissolved in 30 mL of acetonitrile. To this clear solution was added 215 mg (0.55
mmol) of tppz. The solution turned a pale yellow color within a couple of minutes, was
stirred for two hours, and finally concentrated to one-half of its original volume. The
solution was layered with diethyl ether and a crop of pale yellow crystals was harvested
in two days; yield, 215 mg (75%). IR (KBr mull) cm", 2723 (w), 1303 (w), 1155 (w),
1099 (s,b), 1012 (m), 722 (w). UV-Vis (acetonitrile, nm, c = 2.9 x 10M): 2...... = 351, e

= 4.1 x 104 L(mol-cm)"; Am, = 290, e = 3.5 x 104 mmol-cm)".

[Ni(tPchJIClodz (20).

A quantity of [Ni(HzO]6[ClO4]2 (100 mg, 0.275 mmol) was dissolved in 30 mL of
acetonitrile in a beaker and treated with two equivalents of tppz (215 mg, 0.549 mmol).
The pale blue solution immediately turned yellow. The solution was stirred for four
hours, reduced in volume to 15 mL, and layered with toluene to obtain yellow needles;
yield, 275 mg (83%). IR (KBr mull) cm", 2723 (w), 1568 (w), 1301 (w), 1246 (w), 1148
(w). 1080 (s,b), 788 (w), 769 (m), 723 (w). UV-Vis (acetonitrile, nm, c = 1.4 x 10‘5M):

Am... = 355, e = 4.3x 104 L(mol-cm)"; 2...... = 298 e = 4.0 x 104 L(mol-cm)".

126

[Ni2(tPPZ)(CH3CN)o][BF414 (21).

The salt [Ni(CH3CN)6][BF4]2 (100 mg, 0.2 mmol) was placed in a Schlenk flask
under nitrogen and dissolved in 20 mL of freshly distilled acetonitrile. The stirring pale
blue solution was treated with tppz (39 mg, 0.1 mmol) which led to a color change to
brown-yellow. After stirring overnight, the solution was concentrated, diethyl ether was
added, and the solution was chilled to -10 °C. Crystals of [Ni2(tppz)2(CH3CN)6][BF4]4
grew over a period of two weeks; yield, 145 mg (66%). IR (KBr mull) cm", 3450 (b,m),
2725 (w), 2316 (m), 1600 (w), 1311 (w), 1301 (w), 1260 (w), 1209 (w), 1018 (s,b), 787
(w), 750 (w), 722 (m). UV-Vis (acetonitrile, nm, c = 1.3 x 10'5M): km = 354, e = 2.5 x
10“ L(mol'cm)"; Am, = 292 e = 2.4 x 104 mmol-cm)“; A = 241, e = 4.2 x104 L(mol-cm)’
I.

[Rh(tpPZ)2][BF4]3 (23)-

A sample of [Rh2(CH3CN)10][BF4]3 100 mg (0.12 mmol) was loaded into a round-
bottomed flask under nitrogen and dissolved in 20 mL of acetonitrile. The orange
solution was stirred until the entire solid had dissolved. After which time, 93 mg (0.24
mmol) of tppz were added. This produced a drastic change of color from orange to a dark
brown within minutes. The reaction mixture was stirred overnight, and the solution was
concentrated and treated with diethyl ether to the point of saturation. The vessel was
placed in the freezer, and a microcrystalline compound was collected within five days;
yield, 75 mg (48%). IR (KBr mull) cm], 1653 (s), 1565 (s), 1155 (w), 1014 (w), 839 (w),
722 (w). UV-Vis (acetonitrile, mm, c = 1.8 x lO'SM): Am, = 749, 8 = 2.9 X 103 L(mol'cm)'
1

mm, = 517 e = 1.6 x 104 L(mol°cm)"; Am, = 437, e = 1.4 x104 L(mol-cm)", Am, = 405,

8 = 1.4 x104 L(mol-cm)".

127

IRh2(02CCH3)2(tPPZ)2(CH30H)4][PFsk'CnsoH (24)-

A quantity of [Rh2(OZCCH3)2(CH3CN)6][BF4]2 (100 mg, 0.13 mmol) was
dissolved in 20 mL of CH3OH, and the purple solution was stirred until the entire solid
had dissolved. After this time, tppz (52 mg, 0.13 mmol) was added, and the solution was
heated to 60 °C to dissolve the tppz. The solution changed to a dark green color within 20
minutes, after which time the heating was ceased and diethyl ether was added to the
saturation point. A microcrystalline powder was collected after two days; yield, 65 mg
(48%). IR (KBr mull) cm", 2724 (w), 1170 (w), 1034 (w), 1846 (s), 769 (w), 722 (w).
[C02(tpp2’)2][C02C1711PF61 (25)-

To a sample of CoClz (100 mg, 0.78 mmol) dissolved in 30 ml of acetonitrile was
added tppz (301 mg, 0.78 mmol). An immediate color change from intense blue to a dark
green color ensued, and after ten minutes an orange insoluble solid began to deposit on
the bottom of flask. The solution was stirred overnight under nitrogen, and the solution
was separated from the solid by suction filtration. At this point, the solution was
concentrated, and green crystals of the product were obtained by slow vapor diffusion of
diethyl ether into acetonitrile within three days; yield, 280 mg (52%). IR (KBr mull) cm'
I, 2722 (w), 1585 (w), 1298 (w), 1251 (w), 1143 (w), 1060 (w), 1034 (w), 795 (w), 746
(w). UV-Vis (acetonitrile, nm, 0 = 2.74 x 10'4M): Am = 679.5, 8 = 3.77 x 102 L(mol-cm)'
‘; km, = 588.0 a = 3.74 x 102 L(mol'cm)".

[Ni(tptaZ)(CH3CN)2(H20)][BF412 (27).

A sample of [Ni(CHgCN)6][BF4]2 (100 mg, 0.21 mmol) was dissolved in 20 mL

0f acetonitrile, and tptaz (65 mg, 0.2 mmol) was added to the stirring solution which

affected a color change from bright blue to yellow-green. The reaction mixture was

128

stirred overnight under nitrogen and layered with toluene. (Note that the acetonitrile
should not be anhydrous, otherwise crystals do not grow.) Crystals grew over the period
of a week; yield, 80 mg (62%). IR (KBr mull) cm", 3483 (b,m), 2325 (w), 2298 (w),
1377 (s), 1258 (w), 1060 (w), 1066 (b,s), 773 (w), 722 (w). UV-Vis (acetonitrile, nm, C =
4.96x 10'5M): Am, = 293, e = 5.9 x104 L(mol-cm)", Am, = 253, e = 2.5 x102 L(m01'cm)'
1

[Fe(tptaz)2][ClO4]2 (28)-

The salt [Fe(HzO6)][ClO4]2 (100 mg, 0.28 mmol) was dissolved in 30 mL of
acetonitrile in a beaker and treated with tptaz (175 mg, 0.56 mmol). The solution, which
immediately developed an intense blue color, was stirred for five hours and then
concentrated to 15 mL. Blue platelet crystals were obtained by slow diffusion of diethyl
ether into the reaction mixture in acetonitrile; yield, 215 mg (82%). IR (KBr mull) cm",
1573 (w), 1356 (w), 1258 (w), 1083 (b,s), 768 (w), 622 (w). UV-Vis (acetonitrile, nm, 0
= 4.6 x lO'SM): Am, = 568, e = 6.1 x102 L(mol'cm)", Am, = 341, e = 6.5 x 103
L(mol-cm)", hm, = 243, 8 = 4.3 x 103 L(mol-cm)".

[Mn(tptaz)(phen)(H20)][O3SCF312 (29)-

A sample of Mn(phen)2(O3SCF3)2 (100 mg, 0.14 mmol) was loaded into a round-
bottomed Schlenk flask under nitrogen and dissolved in 20 mL of freshly distilled
acetonitrile. The colorless solution was stirred for a few minutes until the solid had
dissolved, and tptaz (44 mg, 0.14 mmol) was added, giving a pale green solution. The
reaction mixture was stirred overnight and concentrated. Crystals of the product were
Obtained by slow vapor diffusion of diethyl ether into the acetonitrile solution; yield, 110

mg (91%). IR (KBr mull) cm", 1533 (w), 1274 (w), 1222 (w), 1151 (w), 1028 (w), 635

129

(w). UV-Vis (acetonitrile, nm, c = 6.3 x 10‘6M): Am, = 292, e = 5.0 x 10“ L(mol-cm)'l,
Am, = 272, e = 7.0 x104 L(mol-cm)", Am, = 227, a = 6.7 x 104 L(mol-cm)".
X-ray data collection and Refinement.

X-ray structural studies were performed on crystals of (16)-(29) on a SMART 1K
area detector diffractometer equipped with graphite monochromated Mo K01 radiation (ha
= 0.71073 A). The frames were integrated in the Siemens SAINT software package,'3 and
the data were solved using the direct-methods program SHELXS-97.M Crystal data are
listed for compounds (16)-(29) in Table 16.

[Ni(tPPZ)2][N03]2 (16)-

Single crystals of [Ni(tppz)2][N03]2 (16) were grown by slow diffusion of

dichloromethane into an acetonitrile solution of the title compound. A light yellow

3 was secured on the tip of a glass

rectangular crystal of dimensions 0.01 x 0.04 x 0.2 mm
fiber with silicone grease and transferred to the Bruker Smart CCD system. The crystal
was cooled to 110 i 2 K during the data collection via a cold stream of nitrogen gas. A
total of 22900 reflections was collected of
which 4086 were unique. The two [NO3]' ions were heavily disordered, therefore
restraints for chemically equivalent distances (N-O 1.203 A) were applied. The final full-
matrix, least-squares refinement was based on 3344 observed reflections with Fo>40(Fo)
which were used to fit 379 parameters to give R1 = 0.0895 and wR2 = 0.2484. The

goodness-of-fit index was 0.999 and the highest peak in the final difference map was

1.450 e’IA'3. The extensive disorder accounts for the slightly higher than usual R factors.

130

 

 

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134

[Co(tPPZhJIP F 612 (17)-
X-ray quality crystals of [Co(tppz)z][PF6]2 were grown by slow diffusion of
toluene into an acetonitrile solution of [Co(tppz)2][PF6]2 in a thin pyrex tube. A red

3

platelet with dimensions 0.15 x 0.2 x 0.1 mm was mounted on a glass fiber using Dow-

Coming silicone grease and cooled down to 173 i 2 K during data collection via a cold
stream of nitrogen gas. A total of 11742 reflections was collected, of which 4176 were
unique. The final full-matrix least-squares refinement was based on 2916 observed
reflections with Fo>40(Fo) that were used to fit 356 parameters to give R1 = 0.0605 and
wR2 = 0.1014. The goodness-of—fit index was 1.083 and the highest peak in the final
difference map was 0.396 e'/A'3.

[F “11113321100412 (13)-

Single crystals of [Fe(tppz)2][ClO4]2 were obtained by slow vapor diffusion of
diethyl ether into an acetonitrile solution of [Fe(tppz)2] [C104]2. Data were collected on a
dark-blue, rectangle shaped crystal with dimensions of 0.3 x 0.05 x 0.25 mm3 which was
mounted on the tip of a glass fiber, secured with silicone grease and placed under a
stream of N2(g) at 110 i 2 K. A total of 25468 reflections was collected, of which 8599
were unique. Both [C104]' anions exhibit a 2-fold rotational disorder. In order to solve
this problem, restraints for chemically equivalent distances (Cl-O 1.415 A) were applied.
The final full-matrix least-squares refinement was based on 3312 observed reflections
with Fo>40(Fo) which were used to fit 647 parameters to give R1 = 0.0766 and wR2 =
0.1925. The goodness-of-fit index was 0.922 and the highest peak in the final difference

map was 1.322 e'lA'3. The extensive disorder accounts for the slightly high R factor.

135

[Mn(tPPZhIIClOdz (19).

Crystals of the title compound were obtained by slow diffusion of toluene into an
acetonitrile solution of the title compound. A pale yellow crystal of dimensions 0.2 x 0.4
x 0.25 mm3 was mounted on the tip of a glass fiber with silicone grease and placed in a
cold N2(g) stream at 110 i 2K. A total of 13232 reflections were collected, of which 2261
were unique. The final full-matrix refinement was based on 2033 observed reflections
with Fo>40(Fo) which were used to fit 163 parameters to give R1 = 0.1352 and wR2 =
0.3510. The goodness-of-fit index was 1.29 and the highest peak in the final difference
map was 0.898 67.313.

[Ni(tPPZ)2][CIO4]2 (20)-

X—rays quality crystals of [Ni(tppz)2][ClO4]2 were grown by slow diffusion of
toluene into an acetonitrile solution of [Ni(tppz)2] [C104]; in a thin pyrex tube (diameter =
3 mm). A pale yellow needle of dimensions 0.05 x 0.2 x 0.06 mm3 was placed on the tip
of a glass fiber, secured with silicone grease, and placed in a N2(g) stream at 298 :l:2 K.
A total of 44378 reflections was collected, of which 8270 were unique. One of the
[ClOJ anions exhibits a two-fold rotational disorder. In order to solve this problem,
restraints for chemically equivalent distances (Cl-O 1.529 A) were adopted. The final
full-matrix, least-squares refinement was based on 3735 observed reflections with
Fo>40(Fo) which were used to fit 675 parameters to give R1 = 0.0749 and wR2 = 0.1979.
The goodness-of-fit index was 0.983 and the highest peak in the final difference map was

0.939 e'/A'3. As the previous cases, this disorder accounts for the slightly higher R factor.

136

[Ni2(tPPZ)(CH3CN)s][BF414'CH3CN (21)-

Single crystals of (21) were grown by treating a saturated solution of
[Niz(tppz)(CH3CN)6][BF4]4 in acetonitrile with diethyl ether and placing it in the freezer
for two weeks at —5 °C. Crystals formed at the bottom of the flask. A dark yellow platelet
with dimensions 0.1 x 0.24 x 0.07 mm3 was covered in paratone oil and quickly mounted
on the tip of glass fiber due to the rapid solvent loss. It was secured with Dow Corning
silicone grease and placed under a N2(g) stream at 173 i 2K. A total of 21891 reflections
was collected, of which 6042 were unique. The final full-matrix least-squares refinement
was based on 4706 observed reflections with Fo>40(Fo) which were used to fit 343
parameters to give R1 = 0.0531 and wR2 = 0.1123. The goodness-of-fit index was 1.074
and the highest peak in the final difference map was 0.622 e'IA'3.
{[C02(tpp2)(CH3CN)2Cl4][PFol[BF 4] 14 (22).

Good quality crystals of (22) were obtained by slow diffusion of an acetonitrile
solution of the title compound into dichloromethane. A red platelet of dimensions 0.2 x
0.15 x 0.08 mm3 was mounted on the tip of a glass fiber and and placed in a N2(g) cold
stream at 110 :l:2 K. A total of 34759 reflections was collected, of which 6762 were
unique. The [PF6]' anions are involved in a two-fold rotational disorder, therefore
restraints on the chemically equivalent distances (P-F 1.580 A) were applied. The final
full-matrix refinement was based on 4538 observed reflections with Fo>40(Fo) which
were used to fit 563 parameters to give R1 = 0.0807 and wR2 = 0.245. The goodness-of-
fit index was 1.049 and the highest peak in the final difference map was 0.837 e'/A'3.

This disorder accounts for the high R factor of the refinement of the structure.

137

[Rh(tPPZ)2][BF4]3 (23).

Single crystals of [Rh(tppz)2][BF4]3 were obtained by slow vapor diffusion of
diethyl ether into an acetonitrile solution of the title compound. A brown platelet of
dimensions 0.02 x 0.01 x 0.05 mm3 was secured on the tip of a glass fiber with silicone
grease and placed under a N2(g) stream at 110 i 2 K. At the end of the data collection, a
total of 39610 reflections had been collected, of which 14580 were unique. The final full-
matrix refinement was based on 7715 observed reflections with Fo>40(Fo) which were
used to fit 799 parameters to give R1 = 0.0725 and wR2 = 0.1861. The goodness-of-fit
index was 0.942 and the highest peak in the final difference map was 1.169 e'/A'3.
IRh(ozCCH3)2(tPPZ)2(CH30H)4][PF616 (24)-

Single crystals of [Rh4(OzCCH3)2(tppz)2(CH3OH)4][PF6]6 were obtained by slow
diffusion of a toluene solution saturated with [Bu4N][PF6] into a solution of the title
compound in methanol in a thin pyrex tube (diameter = 3 mm). Crystals grew over a
period of two months. A green rectangular platelet of dimensions 0.06 x 0.03 x 0.1 mm3
was mounted on the tip of a glass fiber with silicone grease and placed in a N2(g) cold
stream at 173 3: 2K. At the end of the data collection, a total of 32269 reflections was
collected, of which 6137 were unique. Two [PF6]' anions are involved in a three-fold
rotational disorder. In order to solve this problem, restraints on chemically equivalent
distances (P-F 1.671 A) were applied, and the atoms were refined isotropically. The
carbon atoms C(16) and 0(4) were also refined isotropically. The final full-matrix, least-
squares refinement was based on 4599 observed reflections with Fo>40(Fo) which were
used to fit 493 parameters to give R1 = 0.0786 and wR2 = 0.1906. The goodness—of-fit

index was 1.086, and the highest peak in the final difference map was 2.067 e'/A'3, which

138

is not located near any other atoms. The high degree of disorder accounts for the slightly
high R factor present in the final refinement.
[C02(tpp2’)2]PFs][COzCl7] (25)-

X-ray quality crystals of [Coz(tppz’)2][PF6][C02C17] were isolated by slow
diffusion of toluene into an acetonitrile solution of the title compound in a thin pyrex tube
(diameter = 3 mm). A dark green platelet of dimensions 0.09 x 0.04 x 0.2 mm3 was
mounted on a tip of glass fiber, secured with silicone grease, and placed in a N2(g) cold
stream at 110 i 2K. A total of 21108 reflections were collected, of which 3754 were
unique. The final full-matrix, least-squares refinement was based on 2310 observed
reflections with F0>40(F0) which were used to fit 194 parameters to give R1 = 0.0519
and wR2 = 0.1584. The goodness-of—fit index was 1.052 and the highest peak in the final
difference map was 0.735 e‘/A°3.

[C0(tpy)2Clz][BF4]2 (26).

X-ray quality crystals were obtained by slow diffusion of toluene into an
acetonitrile solution of (26). A pale-red prism of dimensions 0.08 x 0.23 x 0.075 mm3
was mounted on tip of a glass fiber, secured with silicone grease, and placed in a N2(g)
cold stream at 110 i 2 K. During the data collection process, a total of 13051 reflections
was collected, of which 4485 were unique. The final full-matrix, least-squares refinement
was based on 2999 observed reflections with Fo>40(Fo) which were used to fit 253
parameters to give R1 = 0.0475 and wR2 = 0.1119. The goodness-of-fit index was 0.979

and the highest peak in the final difference map was 0.939 e'lA'3.

139

 

 

[Ni(tptaz)(CH3CN)(H20)][BF4]2 (27)-

Crystals suitable for X-ray crystallography analysis were obtained by slow
diffusion of toluene into an acetonitrile solution of the title compound. A light-green
prism of dimensions 0.33 x 0.22 x 0.4 mm3 was secured on a tip of a glass fiber with
silicone grease and placed in a stream of cold N2(g) at 110 3: 2K. During the data
collection a total of 16965 reflections was collected, of which 6596 were unique. The
final full-matrix, least-squares refinement was based on 4144 observed reflections with
F0>40(Fo) that were used to fit 379 parameters to give R1 = 0.0532 and wR2 = 0.1389.
The goodness-of-fit index was 0.995 and the highest peak in the final difference map was
0.948 e'lA’3.

[Fe(tptaz)2][ClO4]2 (28)-

In order to obtain crystals suitable for X-ray crystallography, diethyl ether was
gently layered over a solution of the title compound in acetonitrile. Crystals grew over a
period of 5 days. An intense blue colored needle of dimensions 0.5 x 0.21 x 0.15 mm3
was mounted on a tip of a glass fiber, secured with silicone grease and placed in a N2(g)
cold stream at 110 j: 2 K. A total of 25468 reflections was collected, of which 8599 were
unique. The final full-matrix, least-squares refinement was based on 7393 observed
reflections with Fo>40(Fo) that were used to fit 647 parameters to give R1 = 0.0766 and
wR2 = 0.1925. The goodness-of-fit index was 0.891 and the highest peak in the final
difference map was 1.322 e'/A’3.
[Mn(tptaZXPhenXHzO][Mn(tptaZ)(Phen)(H20)][CF330314 (29).

Suitable quality single crystals were obtained by slow vapor diffusion of diethyl

ether into an acetonitrile solution of the title compound. A light yellow platelet of

140

 

dimensions 0.3 x 0.11 x 0.35 mm3 was mounted on the tip of a glass fiber with silicone
grease and placed in a N2(g) cold stream at 110 i K. A total of 16569 reflections was
collected, of which 8412 were unique. The final full-matrix, least-squares refinement was
based on 8412 observed reflections with F0>40(Fo) that were used to fit 991 parameters
to give R1 = 0.056 and wR2 = 0.1237. The goodness-of—fit index was 0.983 and the

highest peak in the final difference map was 1.427 e'/A'3.

II. Results and discussion

A. [M(tppz)2]2+ Series (M = Ni“, Co”, Fe“, an”):

A.1. Synthesis.

A series of compounds of general formula [M(tppz)2]2+ was synthesized by
reacting the salts [M(HZO)6][ClO4]2 (M = Ni“, Mn“, Fe“), [Ni(H20)6][N03]2 and
[Co(CH3CN)6][PF6]2 with two equivalents of tppz in acetonitrile (Eqs. 7 and 8). The
reactions were performed in air with the exception of the Co(II) reaction, which was
performed under nitrogen. All reactions proceed with a dramatic color change that occurs

within minutes of mixing the reagents.

CH3CN

 

 

[M(H20)6][X]2 + 2 t13131 p [M(tPPZ)2][X]2 (Eq. 7)
M = Ni“. Co“. Fe“. Mn“ [Ni(tpp2)2] [N0312 [Mn(tppz)2][ClO4]2

x = [ N031, [(31041 yield: 58% yield: 75%
[Fe(tppz)2][ClO4]2 [Ni(tPPZ)2][C|0412

yield: 81% yield: 83%

CH CN
[C0(CH3CN)611PF612 + 2 tppz 3 4. [C0(tPPZ)21[PFe]2 (Eq. 8)
yield: 90%

141

 

Experimental conditions such as the anion appear to play a major role in the
chemistry as judged by the Ni(II) reaction with tppz. The mononuclear complex
[Ni(tppz)2]2+ is formed in yields that exceed 80% when perchlorate is used (Eq. 7), but,
when [N03]' is used, the yield is only about 58% (Eq. 7) and a significant amount of
insoluble, presumably polymeric material is formed. Interesting, when the anion is [BF4]',
the dinuclear complex [Niz(tppz)(CH3CN)6][BF4]4 is the main product of the reaction. In
a deliberate attempt to prepare the dinuclear compound [Ni2(tppz)(CH3CN)6][C104]4,
different ratios of Ni(II): tppz were used, but all attempts were unsuccessful; regardless of
changes in temperature or concentration the sole product was still [Ni(tppz)2][ClO4]2. It is
apparent that the anion is playing a dominant role in determining whether the nuclearity
of the product is monuclear, dinuclear or polymeric.

Solvent is another variable that is important in this chemistry. The compound
lCo(tppz)2][PF6]2 was obtained in yields greater than 75% (Eq. 8) in acetonitrile, but, if
an alcohol or acetone is used instead, the yield drops to less than 50% and insoluble
solids are obtained. The same situation is encountered for Fe(II) and Ni(II) as well. It
should be emphasized that attempts to synthesize the dinuclear species
[M2(tppz)(CH3CN)6][X]4 (M = C0“, Fe”, Mn“; X = [PF6]' , [C104]') were unsuccessful
regardless of the ratio of reagents or solvent utilized.

A.2. Spectroscopic and cyclic voltammetric studies.

As indicated from the data in Table 17, compounds (16) through (20) exhibit
characteristic MLCT transitions as expected for polypyridine coordination compounds.
The tppz moiety with its low lying 11* orbitals are easily accessible for electron

promotion from the metal (1 electrons. The Ni(II) compounds (16) and (20) exhibit two

142

 

 

/,—.) /. 7

transitions at 355 nm and 298 run that are assigned to MLCT transitions; the transitions
exhibit 8 values of ~ 4.3 x 104 mol(L-cm)". The cobalt derivative (17) displays two
MLCT transitions as well located at 520 and 484 nm with 8 values of 2.3 x 103
mol(L-cm)". In the case of [Fe(tppz)2][ClOa]2 (18), there are three MLCT transitions
located at 568, 341 and 242 nm; their respective 8 values are 3.7 x 103, 4.0 x 104 and 4.4
x 104 mol(L-cm)". Finally, [Mn(tppz)2][ClOa]2 (19) exhibits two MLCT transitions
located at 352 and 290 nm with e values of 4.1 x 104 and 3.5 x 104 mol(L-cm)"

respectively.

 

Table 17. UV-Visible data for compounds (16)-(20).

 

 

 

 

 

 

Mnm) e mol(L°cm)"

[Ni(tPPZ)2][X12
(16) x = [N031 298 4.3 x 104
(20) x = [c1041 355 4.3 x 10‘
(17) [Co(tppz)2][PFs]2 520 2.3 x 103
434 2.3 x 103
568 3.7 x 103
(18) [Fe(tPPZ)ZJ[ClO4]2 341 4.0 x 10‘
243 4.4 x 104
(19) [Mn(tppz)2][ClO.t]2 352 4.1 x 104
290 3.5 x 10‘

 

 

 

Cyclic voltammetry confirmed the accessibility of the 7t* orbitals in compounds
(16)-(19). shows a very rich electrochemistry in the range 0 to —1.8 volts. Compound

[Fe(tppz)2][ClO.1]2 (18) exhibits the most impressive electrochemical properties with four

143

 

 

reversible reduction couples and a reversible oxidation. The oxidation couple occurs at
E11, = +1.37 V and corresponds to the oxidation of Fe(II) to Fe(III). The ligand-based
reductions occur in the range —0.4 to —1.8 V at EU)” = -0.77, Ema = -1.04, Ema, = -1.50

and E“’.,, = -l.67 V (Figure 47). These cyclic voltammetry measurements allow for the

 

5.0x 10 -6 l I l

3.0x 10 ‘6

1.0x 10 ‘6

 

0.0

 

 

 

-20 x 10 '° 1 1 1
04 -O.8 -1.2 -1.6 -2

V(volts)

Figure 47. Cyclic voltammogram for [Fe(tppz)2][ClO4]2 in acetonitrile with 0.1 M
[n-BlelPFel.

calculation of the comproportionation constant (K) for the stability of the mixed-valence
intermediates as each ligand is reduced.4 The comproportionation constant for the first
two couples is 3.7 x 104, whereas the second set of reductions gives a value of 7.5 x 102
(scheme 3). These equilibrium constants correspond to class H and class I borderline
behavior for the stability of the mixed valence species, respectively. In the first case, the
electrons are partially delocalized throughout the system, therefore the mixed valence
intermediate is moderately stable. In the latter case, the mixed valence species is rather

unstable and corresponds to nearly insulating behavior.

144

 

 

 

 

 

 

 

 

 

[Feappzlzl2+ ‘ . L [13841313421+
-e
Kc = 3.7 x 104
[Fettppzlzr 1 +°_ > [Fe(tPPZ)2]
-e
[Fe(tppz)2] ‘ *8 s1 [Fe(tppz)2]'
'9
Kc = 7.5 x 102
[Fe(IPPZ)2l' 1 f: - [Fe(tppzlzlz‘
Scheme 3.

The Ni(II) derivative, [Ni(tppz)2][C104]2 (20), exhibits three redox events, namely
a reversible reduction at E1), = -0.79 V, follow by a irreversible cathodic wave at 13p,c = -
1.2 V and finally a quasireversible reduction at E1, = -1.61V with subsequent
decomposition of the reduced species. These data represent an electrochemical, chemical,
electrochemical (ECE) series of processes.15

[Mn(tppz)2][ClO4]2 displays an irreversible couple at E = +1.22 V attributed to the
oxidation of Mn(II) to Mn(IH) with the subsequent decomposition of the main species.
Two reversible ligand based reductions were observed, the first one being at E1, = -l.1 V
which involves one electron, and the second one located at Ev, = -1.7 V that involves two
electrons.

Finally, [Co(tppz)2][PF6]2 (17) exhibits two reversible ligand based reductions and

a reversible oxidation. The oxidation couple occurs at EV, = +0.41 V and corresponds to

145

 

 

an oxidation of Co(II) to Co(III). The reduction, at Ema, = -0.28 v and Ema, = -l.18V
each correspond to a two electron process.
A.3. X-ray crystallographic results.

In the mononuclear compounds, the metal centers reside in highly distorted
octahedral environments, as indicated by the angle between the pyridyl moiety and the
pyrazine unit with the metal center. This value averages ~80° as compared to the
expected 90° for an ideal octahedron; the specific angles for the Mn(II), Fe(II), Co(II) and
Ni(II) compounds are 723° (2), 81.4° (3), 792° (8) and 78.3°(2) respectively (Figures
49-53). The average M-N distances to the pyrazine moiety for the different compounds
are 1.870 [3], 1.892 [4], 1.986 [3] and 2.228 [4] A for Fe(II), Co(II), Ni(II) and Mn(II)
respectively. The M-N distances to the pyridyl moiety are 1.952 [4], 2.052 [3], 2.094 [4]
and 2.241 A for Fe(II) Co(II), Ni(II) and Mn(II) respectively. The gradual increase in the
MN distances is due to an increase in the radii of the metal centers.

The ligand unit itself undergoes considerable distortion upon coordination. The
pyrazine ring is not planar in any of the compounds, as determined from X-ray structural
determinations (11)) (Figure 48). The greatest distortion is found in
[Co(tppz)2][PF6]2-CH3CN (17); in this case, there is a torsion angle of 107° (4)). The
second largest distortion is exhibited by [Ni(tppz)2][ClO.1]2 (20), with a value of 98° ((11),
followed by [Mn(tppz)2][ClOa]2, 55° (0), and finally the smallest distortion is found in
[Fe(tppz)2][ClO.1]2 (18) at 48° (0) (Table 18). The planes between the coordinated
pyridyl groups are not coplanar in these compounds with the exception of
[Fe(tppz)2][C104]2 (18). In the Fe2+ compound, the pyridyls rings are nearly coplanar with

a dihedral angle of only 02°. The Mn2+ and Co2+ analogues display the largest deviation

146

 

from coplanarity with 13.5 and 132° angles respectively (K, Table 18). The compound
[Ni(tppz)2][ClO4]2 (20) displays a deviation of 6.120 (K, Table 18). The non-coordinated
pyridyl rings exhibit much larger dihedral angles (B) between the planes (Figure 48)
These values are 137°, 443°, 582° and 728° for Co", Ni", Mn” and Fe" respectively (B.

Table 18) (Figure 48).

 

 

 

 

 

 

 

Table 18. Values of the distortions exhibited by the tppz ligand by coordination to
Co(II) (17), Ni(II) (l6), Mn(II) (19) and Fe(II) (18).

(1’ (°) 9 (°) [3 (°) K (°)
[Co(tppz)2]2+ 10.7 44.9 13.74 13.2
[Ni(tppz)2]2+ 9.8 66.9 44.3 6.1
[Mn(tppz)2]2+ 5.5 63.4 58.2 13.5
[Fe(tppz)2]2+ 4.8 59.6 72.8 0.2

 

 

 

 

 

 

 

147

 

 

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Table 19. Selected bond distances (A) and angles (°) for [Ni(tppz)2][NO3]2 (16).

 

Nil-N(2)#l
Nil-N2
Nil-N(6)#l

N2-Nil-N(2)#1

N(2)#l-Ni l-N(6)#l

N2-Nil-N(6)#l
N(2)#l-Ni1-N6
N2-Nil-N6

N6-Nil-N(6)#l
N(2)#l-Ni1—Nl

2.002(4)
2.002(4)
2.093(4)

177.7(2)
104.0l(l6)
77.63(l6)
77.63(l6)
104.01(l6)
90.4(2)
100.22(l6)

Ni 1-N6 2.093(5)
Nil-N1 2.118(4)
Nil-N(l)#l 2.118(4)
N6—Ni1-Nl 98.37(16)
N(6)#l-Ni l-Nl 155.49(16)
Nl-Ni l-N(l)#1 83.0(2)
N2-Ni 1 -N1 78.05( 16)
N6-Ni1-N(1)#l 155.49(16)
N(6)#l-Ni 1-N(1)#1 98.37( 16)

Symmetry transformations used to generate equivalent atoms: #1 -x+2,y,-z+l/2

 

Table 20. Selected bond distances (A) and angles (°) for [Co(tppz)2][PF6]2-CH3CN (l7).

 

Col-N2
Col-N5
Col-N1

Nl-Col-N(l)#1
NZ-Co l -N(6)#l
N5-Co l -N(6)#1
N 1-Col-N(6)#l
N2-Co 1 -N5
N 2-Co 1 -N1
NS-Co 1 -N1
N2-Co l -N( l )#1

1.859(4)
1.925(4)
1.970(3)

164.49(l7)
100.80(8)
7920(8)
88.57(12)
180.02(5)
8225(8)
97.75(8)
8225(8)

Col-N6 2.134(3)
Col-N( 1) #1 1.970(3)
Col-N(6)#1 2.134(3)
N(1)#l-Col-N (6)#l 94.33( 12)
N2-Col -N(6)#1 100.80(8)
N5-Col-N6 7920(8)
N1-Col-N6 94.33(12)
N l -Col-N(6)#l 88.57(12)
N6-Col-N(6)#l 158.3906)
NS-Col-Nl 97.75(8)

Symmetry transformations used to generate equivalent atoms: #1 -x+l,y,-z+l/2

149

 

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Table 21. Selected bond distances (A) and angles (°) for [Fe(tppz)2][ClO4]2 (18).

 

Fel-N2 1.865(6) Fel-N3 1.952(7)
Fel-N12 1.876(6) Fel-N7 1.954(6)
Fel-Nll 1.945(6) Fel-Nl 1.955(6)
N2-Fel-N12 178.6(3) N1 l-Fel-N7 162.3(3)
N2-Fe1-N1 l 98.3(3) N3-Fel-N7 88.6(3)
N12-Fel-Nll 81.4(3) N2-Fe1 -Nl 81.4(3)
N2-Fe 1 -N3 81 .5(3) N12-Fel-Nl 100.0(3)
N12-Fel-N3 97.2(3) N1 l-Fel-N l 87.0(3)
N11-Fel-N3 942(3) N3-Fel-N1 162.8(3)
N2-Fel—N7 99.4(3) N7-Fe1-N1 95.4(3)
N12—Fe1-N7 80.9(3)

 

Table 22. Selected bond distances (A) and angles (°) for [Mn(tppz)2][ClO4]2 (l9).

 

Mn(l)—N(2)#1 2.227(8) Mn(1)-N(l)#1 2.241(11)
Mn(1)-N(2) 2.227(8) Mn( 1)-N(1)#2 2.241(11)
Mn(1)-N(1) 2.241(11) Mn(l)-N(1)#3 2.241(11)
N(2)#1-Mn1-N2 180.0 N(1)#1-Mnl-N(1)#2 144.7(4)
N(2)#1-Mnl-N(l)#l 72.3(2) N l-Mnl-N(1)#2 9529(12)
N2-Mn1-N(1)#1 107.7(2) N(2)#1-Mn1-N(l)#3 107.7(2)
N(2)#1-Mnl-N(1) 107.7(2) N2-Mn 1 -N(1 )#3 72.3(2)
N2-Mn1-N1 72.3(2) N(1)#l-Mn1-N(l)#3 9529(12)
N(l)#1-Mnl-N1 9529(12) N1-Mnl—N(1)#3 144.7(4)
N (2)#1-Mn1-N(1)#2 72.3(2) N (l)#2—Mn1—N(l)#3 9529(12)
N2-Mn1-N(1)#2 107.7(2)

Symmetry transformations used to generate equivalent atoms: #1 y,-x+l,-z ; #2 -y+1,x,-z
#3 -x+1,-y+l,z; #4 x+0,-y+1/2,-z+1/4

Another aspect that should be considered is that the nitrogen atoms of the non-
coordinated, pyridyls rings are rotated away from each other, and do not form a terpy

coordination environment.

The final angle to note in these compounds is the one between the coordinated

and non-coordinated pyridyl rings. These deviations from coplanarity are quite large (0 in

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Table 18 and Figure 48). All the values are near 60° with the exception of the Coll
analogue. The specific values are 449°, 596°, 634° and 66.90 for Co", Fe", Mn" and
Ni'l respectively. This deviation is in response to the steric interaction between the 2’
hydrogen atoms of the pyridyl rings.

It is obvious from the X-ray structures of compounds (16)-(20) that the tppz
moiety in these complexes experiences a great deal of distortion upon coordination. This
observation helps to explain the difficulties encountered in using these as precursors for
the self-assembly of larger supramolecular entities and the lack of electronic coupling
between the two ligands, as evidenced by spectroscopic methods and cyclic voltammetry

as previously discussed.

 

Table 23. Selected bond distances (A) and angles (°) for [Ni(tppz)2] [C104]; (20).

 

Nil-N2 1.981(4) Ni l-N3A 2.091(5)
Ni l-N2A 1.990(5) Ni l-N 1A 2.094(5)
Ni 1-N3 2.082(5) Ni l-N 1 2.101(5)
N2-Ni l-N 2A 176.46(19) N3-Ni1—N1A 94.990 8)
N2-Ni1-N3 78.70(l8) N3A-Nil-N1A 157.24(19)
N2A-Ni 1-N3 99.59(18) N2-Nil-N1 78.39(18)
N2-Ni1-N3A 97.81(18) N2A-Nil-N1 103.3608)
N2A-Ni1-N3A 78.9908) N3-Nil-N1 157.0508)
N3-Ni1-N3A 88.3008) N3A-Ni l-Nl 96.0909)
N2-Ni1-N1A 104.9208) NlA-Ni l-Nl 89.6308)
N2A-Ni1-N1A 7826(19)

155

 

 

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B. Synthesis of the dinuclear compound [Ni2(tppz)(CH3CN)6][BF4]4.

B.l. Synthesis

In all previous cases, attempts to synthesize a dinuclear compound of the form
[Mz(tppz)(CH3CN)6][X]4 where M = Mn", Fe" and Coll were unsuccessful regardless of
the ratio of reagents, solvent, anion or temperature conditions. The exception to this
situation is Ni". In this particular case, the choice of anion appears to play an important
role in whether the product is mononuclear or dinuclear, as previously discussed. In the
presence of [BF.;]', the dinuclear product is formed rather than the mononuclear form.
The reaction was performed under anaerobic conditions in acetonitrile in a 2:1 ratio of

[Ni(CH3CN)6][BF4]2 and tppz (Eq. 9).

CH3CN _
2[Ni(CH3CN)6] [1312,12 + tppz > [N12(tppz)(CH3CN)s][BF414 (Eq. 9)

yield: 66%

 

The reaction proceeds instantaneously, with the solution changing from a pale blue to a
dark yellow color within minutes. Solvent is another variable that has to be taken into
consideration. Alcohols and water promote the formation of insoluble products. Aprotic
solvents, such as acetone or dichloromethane promote the formation of the mononuclear
form as indicated by IR and UV-Visible spectroscopies. The only conditions that appear

to favor the assembly of the dinuclear complex are the combination of CH3CN and [BF.;]'

Crystals of [Ni2(tppz)(CH3CN)6][BF4]4 were obtained by adding diethyl ether to a

concentrated solution of the compound in acetonitrile and placing it in the freezer at

157

 

 

 

 

-10°C for 10 days. The crystals are highly hygroscopic and readily decompose in the
presence of moisture.
B.2. Spectroscopic and cyclic voltammetric studies.

UV-Visible spectroscopic studies of the dinuclear compound
[Ni2(tppz)(CH3CN)6][BF4]4 in acetonitrile led to the observation of three electronic
transitions. Two of the transitions are nearly identical to the mononuclear species
[Ni(tppz)2][C104]2 (20) and appear at 7km, = 354 and 292 nm with e = 2.5 x 104 and 2.4 x
104 L-(mol°cm)'l respectively. In addition to the previous two transitions, there is a third
feature at 74m. = 241 nm (8 = 4.2 x 104 (L-(mol°cm)"). As in the previous case, these
transitions can be assigned to MLCT transitions between the metal center and low lying
7t* orbitals in tppz.

The cyclic voltammogram exhibits one reversible reduction at Er}, = -0.4 V, a
quasireversible reduction at Etc = -1.10 V and a reduction wave at Ep,c = —1.5 volts
without accompanying wave. These studies indicate that weak electronic coupling
between the two Ni" centers is occurring. As will be pointed out in the next section, the
weak coupling can be attributed to the disruption of planarity of the pyrazine ring upon
coordination of both Ni" centers, with the subsequent disruption of the n-system.

B.3. X-ray crystallographic studies.

The Ni" center resides in a highly distorted octahedral geometry with the angle
between the pyridyl and pyrazine rings and the metal center of N(3)-Ni(l)-N(2) = 78.1
(3)° (Figure 60). The angle between the acetonitrile ligands is close to the ideal value 90°
(N(5)-Ni(l)-N(6) = 88.3 (2)°). The shortest M-N distance is to the pyrazine ligand at

Ni(l)-N(2) = 2.025 (2) A, followed by the M-N(CH3CN) distances at an average value of

158

 

 

 

 

 

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2.083 [3] A. The longest M-N distance is to the pyridyl ring (2.104 [3] A). The two

coordinated pyridyl rings are not coplanar to each other, posessing a dihedral angle of 9.2

°. The central pyrazine ring is twisted at an angle of 129°.

 

Table 24. Selected bonds (A) and angles (°) for [Ni2(tppz)(CH3CN)6][BF4]4-CH3CN (21).

 

Ni 1-N2 2.025(2) Nil-N1 2.100(2)
Ni 1-N5 2.043(2) Ni 1-N3 2.108(2)
Ni 1-N6 2.090(3) Ni l-N4 2.115(3)
N2-Ni1-N5 177.3100) N5-Nil-N1 100.08(9)
N2-N i l-N6 8994(9) N6-Nil-N1 94.64(9)
N5-Ni l-N6 8826(10) N2-Ni1-N3 7806(8)
N 2-Ni l ~N1 7808(8) NS-Ni l-N 3 103.84(9)

C. Reactivity studies of [M(tppz)2]2+ (M = Ni“, Fe", Co", Mn") and

[NizttpszCHaCN).l‘*.

As stated earlier, the goal of synthesizing and characterizing the mononuclear
species [M(tppz)2]2+ and the dinuclear compounds represented by [Ni2(tppz)(CH3CN)6]4+
is to use them in the assembly of extended arrays. Several approaches were undertaken to
connect these precursors through their accessible coordination sites, (see Figure 6,
Chapter 1). One attempt involved the reaction of [Ni2(tppz)(CH3CN)6]4+ with two
equivalents of the mononuclear species [Co(tppz)2]2+, with the aim of synthesizing the
tetranuclear species [(tppz)Co(tppz)Ni(tppz)Ni(tppz)Co(tppz)]8+. Various reaction
conditions were used, including different solvents such as acetonitrile, acetone,
dichloromethane and alcohols, and various temperature conditions. The outcome in every
case was the isolation of mononuclear species such as [Ni(tppz)2]2+, [Co(tppz)2]2+ as

corroborated by X-ray crystallography. Another approach that was taken is to use

160

FeC13(tpy) as the source of the second coordinating metal. In this particular case,
Fe(tpy)Cl3 was reacted with the [M(tppz)2]2+ complexes in a 2:1 ratio. The products that
were isolated are [Fe(tppz)2]2+, Fe(tppz)Cl2 and [Fe(tpy)2]2+, which result from ligand
redistribution and reduction of Fe(III) to Fe(II). These results indicate the that the
[Fe(tppz)2]2+ species is quite stable.

Additional attempts to prepare oligomers were performed in which the included
reactions of [M(tppz)2]2+ with two equivalents of [M(CH3CN)6][X]2 or [M(H20)6][X]2
(M 2 Ni”, Co", Fe"; X = [BF4]’, [004]"). This reaction is based on the fact that two the
dangling coordination sites on [M(tppz)2]2+ cations could allow for a trinuclear product to
be assembled. In most cases, an insoluble product was obtained, suggesting that a
polymer or higher order oligomer is being formed. On several occasions, crystals were

isolated, but these were inevitably the mononuclear species [M(tppz)2][X]2.

D. Reactions of C00; with tppz and tpy ligands.

D.1. Synthesis.

After noting that [Co(tppz)2][PF6]2 (17) exhibits spin-crossover behavior we
became interested in adding two additional Co(II) centers at the open coordination sites
to produce a trinuclear complex (Figure 55). The presence of three Co" ions instead of
one could lead to cooperative spin crossover or even multiple step spin crossover. In
pursuit of the proposed trinuclear compound, a Coll starting material that possesses three
open coordinating sites, namely [mer-Co(tpy)(CH3CN)3][BF4]2 was used. In an attempt to
prepare the unknown compound [Co(tpy)(CH3CN)3][BF4]2, CoClz was treated in a 1:1

ratio with tpy in order to first prepare [Co(tpy)Clz(CH3CN)]. The next step was to

161

 

 

 

 

abstract the Cl' ions from [Co(tpy)C12(CH3CN)] with AgBFa in acetonitrile so that the

compound [Co(tpy)(CH3CN)3][BF4]2 could be obtained.

 

 

 

 

 

_ _ _2+
1 / \ 2* / 4 / 4 Q Q
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09‘ 4 ,0.

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Figure 55. Schematic drawing of the proposed synthesis of [C03(tpy)2(tppz)2]6+ from the
reaction of [Co(tppz)2][PF6]2 with two equivalents of
[Co(tpy)(CH3CN)3][BF4]2.

 

 

We reasoned that this mononuclear partially solvated species, would be an ideal
building block because of the presence of the tpy capping ligand. Instead of the

anticipated [Co(tpy)(CH3CN)3][BF4]2 compound however, the dinuclear compound

 

[Coz(tpy)2(CH3CN)2Cl2][BE]; (26) with two bridging Cl' ions was obtained instead. The

162

 

 

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163

reaction of [Coz(tpy)2(CH3CN)2C12][BF4]2 with [Co(tppz)2][BF4]2 in the presence of an
excess of [n-BuaN][PF6]afforded the polymer {[C02(tppz)2(CH3CN)2C12][BFa][PF6]}..
(22). The product is a l-D chain with alternating bridging Co(u-Cl)2Co and Co(tppz)Co
units.

Once the existence of {[Coz(tppz)2(CH3CN)2Cl2][BF4][PF6] }. (22) had been
confirmed by X-ray crystallography, a more direct route to its synthesis was attempted.
The polymeric material CoClz was used as the source of Co" and reacted in 1:1 ratio with
tppz. The outcome of this reaction is an unexpected rearrangement of the tppz moiety, as
shown in Figure 63. This ligand rearrangement allows for the coordination of two Co(II)
centers in a molecular square (Figure 66).

D2. Spectroscopic and cyclic voltammetric studies.

The cyclic compound [Coz(tppz’)2][PF6][C02Cl7] (25) displays two electronic
transitions in the UV-visible spectrum, at 70m = 680 nm (e = 3.7 x 102 L°(mol-cm)") and
Am. = 588 nm (e = 3.7 x 102 L°(mol-cm)"). Both transitions can be assigned to MLCT
transitions in which electrons from the d orbitals of the metal are promoted to the low
lying 7t* LUMO of the tppz ligand.

The cyclic voltammogram displays an irreversible reduction wave at Ep,c = —0.75
V; there is no evidence of an oxidation of the metal up to 2.0 V.

D.3. X-ray crystallographic studies.

In the tpy derivative [C02Clz(tpy)2(CH3CN)2][BF4]2 (26), the C001) center resides
in a highly distorted octahedral environment as illustrated in Figure 57. The pyridyl and
the pyrazine interaction with the metal is N(l)—Co(1)-N(2) = 76.15 (3)°, which is a 138°

deviation from an ideal 90° angle. The acetonitrile and the pyrazine moieties are

164

 

 

 

separated by N(4)-Co(l)-N(2) = 99.92 (2)°, which is a 99° deviation from ideal
octahedral coordination. Finally, the two bridging chlorine angle Cl(1)-Co(l)-Cl(1)* =
86.24 (3)°. The Co—N distances (in increasing order) are N(2)—Co(1) = 2.078 (3) A, N(4)-
Co(l) = 2.127 (3)/3., and N(1)-Co(l) = 2.146 (3) A for pyrazine, acetonitrile and pyridyl
interactions, respectively. The metal centers are 3.562 A apart. It is worth noting that the
midpoint of the dinuclear unit resides on an inversion center; thus, only one-half of the

molecule is unique.

 

Table 25. Selected bond distances (A) and angles (°) for
[C02C12(IP)')2(CH3CN)2] [BF412- (26)-

 

Col-N2 2.078(3) Col-N1 2.150(3)
Col-N4 2.127(3) Col-C11 2.3679(9)
Col-N3 2.141(3) Col-C12 2.510(1)
N 2-C01-N4 99.9200) N3-C01-Nl 151.3600)
N2-Col-N3 75.6400) N2-Col—Cll 172.58(7)
N4—Col-N 3 90.6700) N4-Col-Cll 8729(8)
N 2-Col-N 1 76.1500) N3-Col-Cll 102.68(7)
N 1 -Co 1 -C11 105.93(8) N2-Co 1 -C11 8649(7)
N4-Col-N1 89.3800) N4-Col-Cll 173.17(7)

The compound {[CozClz(tppz)(CH3CN)2][BF4][PF6] - CHZClz}... (22) was
obtained from the reaction of [C02Cl2(tpy)(CH3CN)2][BF4]2 (26) in a 2:1 ratio with
[Co(tppz)2][PF6]2 in acetonitrile. This compound is similar to
[CozCl2(tpy)(CH3CN)2][BF4]2 (26), but, in this case, the tpy ligands have been
substituted by tppz units, which allows for the growth of a l-D polymer (Figure 58). The
Co(H) centers are linked on one side by bridging Cl‘ ions and on the other side by a tppz
unit. The two Co(II) centers linked by the Cl- ions are 3.509 A apart from each other,

which is 0.053 A shorter than in the dinuclear compound [Coz(tpy)Clz(CH3CN)2][BF4]2

165

 

 

 

 

 

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166

(26) (Figure 57). The distance between Co" centers linked by the tppz unit is much larger,

namely 6.792 A, which is nearly twice the distance between Coll centers bridged by two

Cl' ions. Each metal center is in a very distorted octahedral coordination environment

that consists of one tppz, one CH3CN and two Cl' ions. Three of the coordination sites are

occupied by the tppz ligand, one by an acetonitrile molecule, and finally two bridging

chlorine atoms (Figure 64).

 

 

Table 26. bond distances (A) (°) for
{1C02(IPPZ)(CH3CN)2C12] [BF411PF6] ° CH2C12 }. (22)-

Co 1 —N 5 2.085(6) C02-N2 2.081(6)
Co 1 -N6 2.102(6) CoZ-Nl 2.1 14(6)
Col -N8 2.1 12(7) C02-N7 2.122(7)
Co 1 -N4 2.1 13(6) C02-N 3 2.126(6)
Co 1 -Cll 2.326(2) Co2-C12 2.319(2)
Co 1 -C12 2.563(2) Co2-Cll 2.561(2)
NS-Co 1 -N6 75.4(2) N2-C02-Nl 76.7(2)
N 5-Co 1 -N8 95.8(2) N2-C02-N7 94.9(2)
N6—Co 1 -N8 88.5(2) N l -C02-N 7 91 .9(2)
NS-Co 1 -N4 76.0(2) N2-Co2-N3 75.2(2)
N 6-Co 1 -N4 151.3(2) N l -C02-N 3 151.7(2)
N8-Co 1-N4 922(2) N7 -Co2-N3 87.3(2)
N 5-Col-Cl 1 171.7206) N 2-Co2—Cl2 171.6707)
N6-Co 1 -Cll 103.63( 16) N1-Co2-Cl2 102.7006)
N 8-Co 1 -Cl 1 92.34(19) N7-C02-C12 93 .4609)
N4-Col-Cll 105.0106) N3-C02-C12 105.6006)
N5-Col-Cl2 83.65(16) N2-Co2-Cll 83.3406)
N 6-Col-Cl2 89.66(17) N l -Co2-Cll 90.1006)
N8-Col-C12 178.1209) N7-C02-Cl1 176.9109)
N4-Co l -Cl2 89.47(16) N 3-Co2-Cl 1 89.8007)
Cll-Col-CIZ 88.14(7)

The angle between the pyridyl and pyrazine ring in this polymer is 75.7 [3]°.

which represents a 143° difference from the ideal value of 90°. The two chlorine atoms

are asymmetrically bound, as evidenced by the two distances Co(1)-Cl( l) = 2.326 (2)A

167

 

 

 

 

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168

and Co(1)-Cl(2) = 2.563 (2)A. In the case of the Co(2)-Cl angles, the inverse situation is
encountered (Figure 58). The Cl(2)-Co(l)-N(4) angle is 89.5 (2)°, which is close to the
ideal value of 90°. On the other hand, the Cl(1)-Co(1)—N(6) angle is 103.6 (2)°, which is a
large deviation from 90°. The M-N distance trend was the same as that observed in
[Coz(tpy)C12(CH3CN)2][B13412 (26). The angles are Co(1)-N(5) = 2.086 (6) A, Co-
N(6)/(4) = 2.108 [3] A, and Co(l)-N(8) = 2.113 (6)A for pyrazine, pyridyl and

acetonitrile interactions, respectively.

 

Table 27. Selected bond distances (A) and angles (°) for {[Coz(tppz’)2][PF6][C02CI7]

 

(25).

Col-N1 2.112(3) Col-N2A 2.149(3)
Col-NlA 2.112(3) Col-N2B 2.149(3)
Col-NIB 2.112(3) Col-NZC 2.149(3)
Nl-Col-NIA 98.86(11) Nl—Col-NZB 88.6402)
Nl-Col-NIB 98.86(11) N2A-Col-N2 95.84(12)
NlA-Col-NlB 98.86(11) NlA-Col-N2C l71.99(12)
Nl-Col-N2 88.6402) NlB-Col-N2C 8864(12)
NlA-Col-N2 7705(12) NIB-Col-NZC 7705(12)
NlB-Col-N2 l71.99(12) N2B-Col-N2C 95.84(12)
NlA-Col-NZB 7705(12) N2-Col-N2C 95.84(12)
NIB-Col-NZB l71.99(12)

In [Coz(tppz’)2][PF6][C02Cl7] (25), the tppz’ ligand is the result of a
rearrangement of tppz in the presence of C00; (Figure 59). The rearranged tppz ligand is
composed of two equivalent units linked by a C-C tether (Figure 59a). Each one of these
units is composed of a pyridyl ring joined to a six-membered ring which is fused to a
five-membered ring (Figure 59). These two equivalent units can rotate around the C-C
bond that links them together, which allows them to coordinate to two Co(II) metal

centers 90° from each other. Two more units can be added to complete the cyclic unit

169

 

 

 

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170

viewed in Figure 62a. The distance Co(1)-N(2) = 2.149 A is longer than the metal
interaction to the fused rings at Co(1)-N(l) = 2.112 A. The metal center resides in a
highly distorted octahedral geometry composed of three equivalent chelating interactions
from a pyridyl and a five-membered ring. The angle between the pyridyl and the five
membered ring N (1*)-Co(1)-N(2) = 77.1 (2)0 (Figure 59).

E. Reactions of cis-[Rh2(02CCH3)2(CH3CN)6][BF4]2 and

[Rh2(CH3CN)101[BF 414 With tppz.

E.l. Synthesis

A careful search of the literature in the last 40 years reveals that tppz has been
used exclusively in mononuclear metal complexes'é“ Reports by Pruchnnik and
coworkers17 and observations in our own laboratories with bipyridine and terpyridine
ligand on [Rh2]4+ prompted us to react tppz with metal-metal bonded compounds. In one
such reaction, [Rh2(OzCCH3)2(CH3CN)6][BF4]2 was selected as a source of RM” because
of its availability and stability. The reaction with tppz was performed in a 1:1 ratio in
H20. At room temperature no reaction was observed, but, with heating, the purple
solution of the starting material gradually changes to a pale brownish red color and
finally to green. At this point, the reaction must be ceased, otherwise cleavage of the
metal-metal bond with concomitant oxidation to Rh(III) bond takes place, as evidenced
by isolation of [Rh(tppz)2]3+. The product is recovered by evaporation of the H20. A
methanol solution of the product was layered with toluene and green platelet crystals
were observed to grow over a period of two months (yield 15%). X-ray studies revealed
that these crystals are the “dimer of dimers”

[Rh4(02CCH3)2(IPPZ)2(CH30H)4][PF616'CH30H (24)-

171

 

 

 

With the successful isolation of the [Rha]4+ compound from
[Rh2(OzCCH3)2(CH3CN)6][B134]; we turned to another source for the [Rha]4+ core,
namely [Rh2(CH3CN)to][BF4]4. The 1:1 reaction between [Rh2(CH3CN)m][BF4]4 and
tppz was performed in acetonitrile, and, upon mixing, the orange solution changed
immediately to a brown color solution from which the mononuclear Rh'" species, was
isolated [Rh(tppz)2][BF4]3. In an attempt to avoid the presumed disproportionation
reaction, the reaction was performed at low temperature and in the absence of light, but
we were not able to avoid the formation of [Rh"'(tppz)2]3+. The fate of the other half of
the reaction, namely the Rhl species is not known.

E2. Spectroscopic studies.

The mononuclear species [Rh(tppz)2][BF4]3 (23) exhibits interesting electronic
spectral properties. Four electronic transitions were located in the range of 200-800 nm.
The lowest energy transition occurs at Am. = 749 nm (e = 2.9 x 103 L°(mol'cm)"). The
others appear at Km“ = 517 nm (e = 1.6 x 104 L-(mol-cm)"), at Am = 437 nm (e = 1.4 x
104 L-(mol-cm)"), and at Am = 405 nm (e = 1.45 x 10‘ L-(mol-cm)°'). All of these
transitions are presumed to be MLCT transitions based on their high molar absorptivity
coefficients.

B.3. X-ray crystallographic studies.

Compound (24), [Rh4(OzCCH3)2(tppz)2(CH3OH)4][PF6]6-CH30H, consists of a
rectangular unit based on two Rh-Rh bonds and two M-tppz edges. The coordination
sphere of the Rh" ions consists of three nitrogen atoms from the tppz ligand, two oxygen
atoms, (one from the bridging acetate and the other from 21 methanol molecule), and

finally the Rh-Rh bond (Figure 61). The angle between the pyridyl and pyrazine

172

 

 

nitrogens, Rh(2)-N(2)-N(3) is 81.2 (3)°. The N(l)-Rh(2)-N(3) = 160.3 (3)° angle, which
involves the pyridyl rings, is far from the ideal 180°. The shortest of the three Rh-N
distances is the Rh-pyrazine distance of Rh(2)-N(2) = 1.934 (7) A, followed by the Rh-

pyridyl distance Rh(2)-N = 2.030 [8] A (Figure 60).

 

Table 28. Selected bonds distances (A) and angles (°) for
[Rh4(02CCH3)2(tppz)2(CH30H)4][PF6]6-CH3OH (24)-

 

 

Rhl-N2A 1.929(7) Rh2-N2 1.934(7)
Rh l-N l A 2.041(7) Rh2-N1 2.010(8)
Rh 1-02 2.054(7) Rh2-N3 2.047(8)
Rh 1—03 2.245(7) Rh2-Ol 2.053(7)
Rh 1-Rh2 2.6058(11) Rh2-O4 2.216(8)
N2A-Rh l -N3A 81.7(3) N 2-Rh2-N1 80.7(3)
N2A-Rh1-N1 A 81.0(3) N2-Rh2-N3 81.2(3)
N 3A-Rh l -N l A 160.6(3) N1-Rh2-N3 160.3(3)
N2A-Rh1-02 179.0(3) N2-Rh2-Ol 178.5(3)
N3A-Rh1-02 98.8(3) N l -Rh2-Ol 1002(3)
N 1 A-Rhl-02 98.4(3) N3-Rh2-Ol 97.8(3)
N2A-Rh1-O3 93.0(3) N 2-Rh2—O4 95.3(3)
N3A-Rh1-O3 88.4(3) N1-Rh2-O4 89.6(3)
N1A-Rhl-O3 83.8(3) N3-Rh2-O4 84.4(3)
OZ-Rhl-OB 862(3) Ol-Rh2-O4 83.5(3)
N2A-Rh1-Rh2 95.9(2) N 2—Rh2-Rh1 962(2)
N3A-Rh1-Rh2 91.2(2) N 1—Rh2-Rhl 90.6(2)
N1A-Rh1-Rh2 99.3(2) N 3-Rh2-Rh1 99.0(2)
02-Rh 1 -Rh2 85.0(2) Ol-Rh2-Rh1 85.0(2)
O3-Rh 1-Rh2 171.0(2) O4-Rh2-Rhl 168.4(2)

 

The molecular cation is a rectangle in which each Rh-Rh unit is a short edge of
the rectangle and tppz units are the long edges. The Rh-Rh bond distance is 2.606 (1)A,
which is within the expected range for a Rh-Rh single bond.‘8 The distance between tppz
bridged Rh" atoms along is 6.749 A and the diagonal of the rectangle is 6.977 A. The
tppz units are highly distorted, with the planes of the two coordinating pyridyl units on

the same Rh atom forming a dihedral angle of 27.4°. The pyrazine ring exhibits a dihedral

173

 

 

 

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angle of 19.46 °. A close look at the space filling diagram in Figure 68 shows the

remarkably good fit of the two twisted tppz units in this molecule.

 

 

Table 29. bond distances (A) and angles (°) for
[Rh(tPPZh] [BF413'2CH3CN'C4H80 (23)-
Rh 1 -N2A 1.962(6) N2-C19 1.345(10)
Rh 1-N2 1.975(6) N4-C7A 1.347(10)
Rhl-N3 2.035(7) N4A-C13A 1.348(9)
Rhl-Nl 2.032(7) N4-C13 1.335(10)
Rh l-NlOA 2.054(6) N4-C7 1.355(10)
Rh 1-N3A 2.066(7) N2A—C19A 1.334(10)
N 2-N 6 1.334(9)
N2A-Rh 1 -N2 178.0(3) N3-Rh l -N10A 91 .3(3)
N2A-Rhl- 100.7(3) Nl-Rhl-NIOA 91.8(3)
N2-Rh1-N3 80.3(3) N2A-Rhl-N3A 80.7(2)
N 2A-Rh 1 -N1 98.9(3) N2-Rh l-N3A 97.5(3)
N2-Rh1-Nl 80.2(3) N3-Rh l -N3A 91 .9(3)
N3-Rh 1 -N1 160.4(2) N 1 -Rh 1 -N 3A 91 .3(3)
N2A-Rh1-N10A 80.5(3) N lOA-Rh 1 -N3A 161.2(2)
N2-Rh1-N10A 101.3(3)

Compound (23) contains a mononuclear Rh(HI) cation in a distorted octahedral

geometry (Figure 69). The angle between the pyrazine and pyridyl rings and the Rh atom

is N(l)-Rh(l)-N(2) = 80.2 °, which is much less than the ideal 90°. The angle N(l)-Rh(1)-

N(3) between two pyridyl rings is 160.4°, also far from the ideal. The tppz experiences

severe distortions, with two of the non-coordinating pyridyl rings forming an angle of

37.11° between them. The pyrazine ring is twisted by 9.68 °. In the case of coordinated

and non-coordinated pyridyl rings, they are twisted as far as possible at 48.31 °. The

driving force for this is most likely the steric interaction of the 4,4’ hydrogen atoms.

176

 

 

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F. A hybrid ligand of 2,2’ azobypyridine and tetrapyridyl pyrazine:
2,4,6-Tris(2-pyridyl)-l,3,5-triazine (tptaz).

F.1. Synthesis

The ligand 2,4,6-tris(2-pyridyl)-l,3,5-triazine (tptaz) contains three possible
coordination sites as depicted in Figure 46. The most likely candidate for the first
coordination is the tridentate site. The three compounds isolated in this work are
mononuclear species with tptaz using only the tridentate coordination site. In all cases,
formation of the product is the same, regardless of the ratio of reagents or reaction

conditions used.

 

 

 

. CH CN . .

[Ni(CH3CN)6][BF4]2 + tptaz 3 = [N1(tptaz)(CH3CN)2(H20)][BF4]2 (Eq. 10)
yield: 62%

[Fe(H20)6][ClO4]2 + 2 tptaz CH3CN : [Fe(tptaz)2][ClO4]2 (EQ- 11)
yield: 82%

CH3CN

[Mn(phen)2][CF3SO3]2 + tptaz > [Mn(tptaz)(H20)][CF3SO3]2 (Eq. 12)

yield: 91%

In a typical reaction, [Ni(CH3CN)6][BF4]2 was dissolved in acetonitrile to form a
bright blue solution, and one equivalent of tptaz was added. The color of the reaction
changed to pale yellow and finally to pale yellow green within 8 hours (Eq. 10). Layering
of the solution with toluene, benzene or diethyl ether affords crystals of
[Ni(tptaz)(CH3CN)2(H20)2][BF4]2 within days. It must be pointed out that the acetonitrile
cannot be completely dry, otherwise, the compound will not crystallize. Several attempts

to coordinate a second Ni" center to the already formed [Ni(tptaz)(CH3CN)2(H20)]2+ (27)

178

 

 

molecule were unsuccessful. This may be due to the inductive effect of the first metal on
the triazine ring, which renders the triazine ring much less basic.

The compound [Fe(tptaz)2][ClO4]2, (28), results from the reaction of
[Fe(H20)6][ClO4]2 with tptaz in a 1:2 ratio (Eq. 11). The reaction takes place
instantaneously, with the color changing from a colorless Fe(II) solution to a deep blue
solution within minutes. All attempts to prepare polynuclear compounds by varying the
reaction conditions were unsuccessful. Crystals of the mononuclear compound were
obtained by layering with diethyl ether or THF or by slow evaporation of the reaction
mixture.

For the synthesis of [Mn(tptaz)(phen)(H20)][O3SCF3]2 (29), a slightly different
approach was taken. Instead of using a “naked” cation (in which all six coordination sites
are occupied by labile water or acetonitrile molecules), [Mn(phen)2](O3SCF3]2 was
employed. This precursor contains four coordination sites occupied by phenanthroline
molecules (capping ligands), and two sites occupied by weakly coordinating triflate
([CF3SO3']) anions. By using this precursor, it was intended that the tptaz would
coordinate in its middle bidentate coordination site (Figure 46), allowing for the
coordination of a second metal center in a subsequent step in the tridentate site. The
reaction was performed anaerobically, with a 1:1 ratio of [Mn(phen)2(O3SCF3]2] and
tptaz in acetonitrile (Eq. 12). The reaction was instantaneous, as judged by an immediate
color change to light yellow. The outcome of the reaction is the mononuclear species
[Mn(tptaz)(phen)(H20)][O3SCF3]2 (29) in which the tptaz ligand is coordinated through
its major coordination site (tpy mode), which requires the displacement of one

phenanthroline molecule. Further attempts to coordinate a second Mnll metal center on

179

 

 

 

the available bpy coordination site (middle coordination site, Figure 46) by adding an
excess of either [Mn(CH3CN)4][BF4]2 or [Mn(phen)2(03SCF3]2] (with and without
refluxing) were unsuccessful. The mononuclear product was always obtained as
evidenced by X-ray crystallographic studies.
F.2. Spectroscopic and cyclic voltammetric studies

UV-Visible spectroscopy of [Ni(tptaz)(CH3CN)2(H20)2][BF4]2
[Fe(tptaz)2][ClO4]2, [Mn(tptaz)(phen)(H20)][O3SCF3]2 revealed numerous electronic
transitions. The tptaz ligand exhibits two electronic transitions in the UV region, the
lowest energy one being at Amax = 279 nm (e = 3.0 x 104 L-(mol'cmf') and the higher
energy one at Am“ = 244 nm (e = 3.0 x 104 L-(mol-cm)"). Both transitions are assigned
to 1t—> 1t* electronic transitions. [Ni(tptaz)(CH3CN)2(H20)][BF4]2 (27) displays two
electronic transitions at km, = 293 nm (e = 5.9 x 104 L'(mol-cm)") and Am” = 253 nm (e
= 2.5 x 102 L-(mol-cm)"). The transition at A = 293 nm corresponds to a MLCT
transition, based on its location and its absorption coefficient. The higher energy
transition at Am“ = 253 nm is assigned to a rt—> 1t* transition of the ligand.
[Fe(tptaz)2][ClO4]2 (28) exhibits three electronic transitions in the UV-Visible spectrum
at Am” values of 568 nm, 341 nm and 243 nm with 8 values of 6.1 x 102, 6.5 x 105 and
4.3 x 103 L-(mol-cm)", respectively. The two lower energy transitions are assigned to
MLCT transition and the higher energy one at km“ = 243 nm is a 1t—-> 1t* transition, as
inferred from a comparison to the free ligand spectra. Finally,
[Mn(tptaz)(phen)(I-120)][O3SCF3]2 (29) displays three electronic transitions at Am = 227

and 272 nm (assigned to 1t—> 1t* electronic transitions) and at Am“ = 292 nm, attributed to

180

4“

 

 

a MLCT transition. The e values are 5.0 x 10“, 7.0 x 10“, and 6.7x 104 L-(mol-cm)" for

Am, = 292, 272 and 227 nm, respectively.

 

Table 30. UV-Visible spectrosc0pic data for compounds (27)-(29).

 

 

 

 

 

 

 

 

 

 

Mnm) 8 L-(mol-cm)’l
279 3.0 x 104
Tptaz 244 3.0 x 104
293 5.9 x 104
[Ni(tptaZXCHsCNXHzOH[BF412 253 2.5 x 102
568 6.1 x 102
[Fe(tptazhHClOdz 341 6.5 x 105
243 4.3 x 103
227 6.7 x 104
[Mn(tptaZXPhenXHzOH[038CF312 272 7.0 x 10‘
292 5.0 x 10‘

 

The presence of energetically accessible 1t* orbitals leads to a rich
electrochemistry for these molecules. Cyclic voltammetric studies show that
[Fe(tptaz)2][ClO4]2 (28) exhibits the richest electrochemistry. The free ligand exhibits
only a reversible reduction (Er, = -l.2 V), but upon coordination to Fe(II), four one-
electron reductions became accessible as depicted in Figure 63. The four one-electron
reductions of [Fe(tptaz)2][ClO4]2 (28), are found within the range —O.4 to -l.85 V. These
reduction couples occur at El)” = -0.62 , Ewes = -0.75, E(3)% = -l.47 and EN)” = -l.7lV
(Figure 63). In addition, there is an irreversible oxidation at E = +1.54 V that corresponds
to the oxidation of Fe(II) to Fe(III) . An examination of the separation of the four couples

allows for the calculation of the comproportionation constants for the two intermediates

181

 

 

 

as shown in scheme 4, based on the Robin and Day approach.19 The first two couples lead
to a comproportionation constant of 1.57 x 102, which corresponds to a value on the
borderline of class I and H. For the next two reversible couples, the Kc is 1.14 x 104,

which falls in the category of class 11 (Scheme 4).

 

8x10.6 1 I 1
6x10'6
4x10"’

2x10'6

 

 

 

 

4 x 10 '6 1 . 1 J
-0.4 -0.8 -1.2 -l.6 -2

Volts

Figure 63. Cyclic voltammogram of [Fe(tptaz)2][ClO4]2 in 0.1 M TBAPF6 acetonitrile at
3 Pt disk electrode versus Ag/AgCl.
In marked contrast to the Fe(II) derivatives, [Mn(tptaz)(phen)(HzO)][OgSCF3];
(29) exhibits only one reversible reduction at Ev, = -1.47 V. There are three irreversible
waves at Ep,c = —0.55, -O.72 and -0.80 V without corresponding anodic return waves.
Finally, [Ni(tptaz)(CH3CN)2(H20)][BF4]2 exhibits a reversible reduction at EV2 =

-O.5 V and a cathodic wave at EN = -l.03V with a return wave at E = -0.85V.

182

 

 

 

 

 

 

 

 

 

 

[1=e(tptaz)2]2+ ‘ _ L [Fe(tr>tetz)2]+
-e
KC =1.57 x 102
+e'
[Fe(tptaz)2]+ ‘ _ ‘ [Fe(tptaz)2]
-e
[Fe(tptaz)2] ‘ +8. ¥ [Fe(tptaz)2]'
-e
Kc=1.14x104
[Fe(tpta2)2]' ‘ +e ‘ [Fe(tptaz)2]2“
-e
Scheme 4.

E3. X-ray crystallographic studies.

The compound [Ni(tptaz)(CH3CN)2(H20)][BF4]2 (27) consists of 21 Ni" center in
distorted octahedral geometry (Figure 71). The coordination sphere is composed of two
acetonitrile molecules, one water molecule and tptaz ligand coordinated at the tpy site
(Figure 70). The shortest Ni-N bond is to the nitrogen of the triazine ring, (Ni(1)-N(2) =
1.982 (3)/31), followed by the acetonitrile interaction (2.037 [3]A). The longest distance is
to the pyridyl rings at an average Ni-N distance of 2.134 [3]}t. The angle between the
pyridyl and pyrazine rings and the nickel center is N2-Nil-Nl = 77.05 (11)°, which is far
from the ideal 90°. The angle between the pyridyl and the acetonitrile ligands is N(l)-

Ni(1)-N(8) = 103.17 (11)°. The two trans pyridyls are 154.1° apart from each other,

183

 

 

which is 259° from the expected 180°. With respect to the open middle coordination site,

the dihedral angle between the triazine and pyridyl rings is only 9.4°.

 

Table 31. Selected bond distances (A) and angles (°) for [Ni(tptaz)(CH3CN)(HzO)][BF4]2

 

(27).

Nil-N2 1.982(3) Nil-OI 2.063(2)
Ni l-N8 2.027(3) Nil-N3 2.134(3)
Ni l-N7 2.047(3) Nil-N1 2.135(3)
N 2-Ni 1-N8 179.70(12) N7-Ni1-N3 89.0801)
N2-Nil-N7 91.51(12) Ol-NiI-N3 91.6600)
N8-Nil-N7 88.29(12) N2-Nil-N1 77.05(11)
N2-Ni l-Ol 88.84(l l) N8-Ni1-N1 103.l7(l 1)
N8-Nil-Ol 91.37(11) N7-Ni l-Nl 90.68(11)
N7-Ni l-Ol 179.24(11) Ol-Nil-Nl 88.7400)
N2-Nil-N3 77.05(11) N3-Nil-N1 154.0901)
N8-Nil-N3 102.72(11)

In compound [Fe(tptaz)2][ClO4]2 (28), two tptaz units are present in the
coordination sphere. The Fe" center resides in a distorted octahedral environment (Figure
72). The longest Fe(1)-N interaction is to the pyridyl ring, with an average value of 1.984
[3]/K, followed by the triazine with an average value of 1.862 [3]A. The angles
throughout the coordination sphere reflect the distortion from an ideal octahedron. The
angle between the pyridyl and triazine units and the Fe" center is 80.10 [12]° (almost a
10° difference from the ideal 90°). The coordinated pyridyl rings of the same tptaz
molecule are 160.2° apart from each other, which is a 198° deviation from the expected
180°.

The compound
{ [Mn(tptaz)(phen)(H20)] [CF330312 } { [Mn(tptaz)(phen)(H20)] [CF350312 } “H20 (29)

crystallizes in the P-l space group with two independent co-crystallized molecules. Both

184

 

 

 

 

 

 

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molecules contains a Mn(II) ion, but they exhibit different geometries. The first molecule
consists of a Mnll ion in a distorted octahedral environment (Figure 74) whereas the

second molecule contains a heptacoordinate Mn(II) ion.

 

Table 32. Selected bond distances (A) and angles (°) for [Fe(tptaz)2][C104]2-2CH3CN

 

(28).

Fel-N2 1.859(3) Fel-N3A 1.983(3)
Fel-N2A 1.865(3) Fel-Nl 1.988(3)
Fel-N3 1.981(3) Fel-NlA 1.989(3)
N2-Fe1-N2A 178.2303) N3-Fel-Nl 160.3802)
N2-Fel-N3 80.3002) N3A-Fe1-N1 93.2802)
N2A-Fe1-N3 101.0802) N2-Fel-N1A 98.8702)
N2-Fel-N3A 101.1202) N2A-Fe1-N1A 79.9902)
N2A-Fe1-N3A 80.0502) N3-Fe1-N1A 93.0302)
N3-Fel-N3A 89.4202) N3A-Fe1-N1A 159.9902)
N2-Fel-Nl 80.1102) Nl-Fel-NlA 91.0402)
N2A-Fel-N1 98.5302)

A close look at the molecule with seven ligands shows that it has pentagonal bipyramidal
geometry (Figure 73). The equatorial plane is occupied by the tptaz nitrogens atoms
(N(l), N(2), N(3)), a nitrogen from the phenanthroline N(7), an oxygen from a triflate
0(4), and finally the water molecule 0(7). In an ideal case, each atom in the pentagon
should be 72° apart, which is close to what is observed in the structure. Some
characteristic angles found in this plane are N (l)-Mn(2)-N(7) = 76.61 (11)°, N(1)-Mn(2)-
N(2) = 67.00 (10)°, N(3)-Mn(2)-N(2) = 69.24 (11)° and O(3)-Mn(2)-N(7) = 75.11 (11)°.
The two axial sites are occupied by a triflate molecule weakly coordinated through an
oxygen atom, Mn(2)-O(4) = 2.164 (3)/X, and a nitrogen from the phenanthroline ring
Mn(2)-N(8) = 2.277 (3)A. The molecule with the octahedral geometry (Figure 74) does

not contain a coordinated triflate molecule; its coordination sphere is composed of a

186

 

 

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187

tptaz, a phen, and a water molecule. The tptaz ligand and one of the phenanthroline
nitrogen atoms occupy the equatorial plane. The axial sites are filled by a water molecule
and the remaining nitrogen from the phenanthroline. The tptaz and phenanthroline
ligands impose rigid geometric requirements as judged by the angles involving tptaz
(N(2A)-Mn(l)-N(1A) = 70.5 (1)°) and phenanthroline (N(8A)—Mn(l)-N(7A) = 75.7 (1)° )
are far from the ideal 90° in an octahedral environment. Consequently, these coordination
requirements affect the rest of the bonding, e.g., O(l4)-Mn(l)-N(8A) = 162.3 (1)°. The
shortest Mn(1)-N distance is to the triazine ring of the tptaz, Mn(l)—N(2A) = 2.224 (3) A,
followed by the phenanthroline ring with an average M-N distance of 2.255 [3] A.
Finally, the longest interaction is to the pyridyl rings at an average M-N distance of 2.360

[31.31.

 

Table 33. Selected bond distances (A) and angles (°) for
{[Mn(tptaz)(phen)(H20)][CFgSO3]2 }{[Mn(tptaz)(phen)(H20)][CF3SO3]2 ”120(29)-

 

Mnl-Ol4 2.170(3) Mn2-O4 2.164(3)
Mnl-N2A 2.224(3) Mn2-N7 2.276(3)
Mnl-N7A 2.241(3) Mn2—N8 2.277(3)
Mnl-N8A 2.269(3) Mn2-N2 2.339(3)
Mnl-N3A 2.345(3) Mn2-N3 2.357(3)
Mnl-NlA 2.375(4) Mn2-O3 2.511(3)
MnZ-Nl 2.569(3)

014-Mn1-N2A 97.4701) N7A-Mnl-N1A 124.5802)
014-Mnl-N7A 89.7601) N8A-Mnl-N1A 90.7401)
N2A-Mn1-N7A 163.6001) N3A-Mnl-N1A 142.2501)
Ol4-Mnl-N8A 162.3000) O4-Mn2-N7 102.2101)
N2A-Mnl-N8A 99.2001) O4-Mn2-N8 169.2201)
N7A-Mnl-N8A 75.7302) N7-Mn2-N8 74.6902)
Ol4-Mnl-N3A 91.7700) O4-Mn2-N2 89.0000)
N2A-Mnl-N3A 72.0002) N7-Mn2-N2 140.0901)
N7A-Mn1-N3A 93.1602) N8-Mn2-N2 87.2001)
N8A-Mnl-N3A 99.0001) O4-Mn2-N3 100.1500)
Ol4-Mn1-N1A 89.2401) N7-Mn2-N3 142.7001)
N2A-Mnl-N1A 70.4602) N8-Mn2—N3 87.9201)

188

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IV. Conclusions

The ligand 2,3,5,6-tetrapyridylpyrazine (tppz) possesses two tridentate
coordination sites which in principle can allow for the assembly of metal arrays. In our
attempts to prepare such metal arrays, the mononuclear series [M(tppz)2][X]2 (M = Niz+,
Co“, Fe“, Mn“; x = 004’, PFG‘, N03) was obtained by the reactions of [M(S),]2+ (s =
CH3CN or H20) cations with two tppz ligands. The products exhibit interesting
properties in their own right, and could give rise to interesting properties by associative
effects if extended metal arrays can be obtained.

Magnetic studies of [Co(tppz)][PF6]2 (17) revealed that this compound exhibits
spin-crossover behavior. Previous studies with Coll and tpy derivatives have indicated
similar behavior.20 The low spin configuration is obtained at T = 55K with S = '/2 and the
high spin configuration is reached at T = 260 K with S = 3/2. From a structural point of
view, [Co(tppz)2][PF6]2 presents a major difference from other spin-crossover Co(II)
complexes, namely the availability of coordination sites for further chemistry. This
compound is ideal as a potential building block for the construction of higher nuclearity
species that incorporate a building block with spin-crossover behavior.

A cyclic voltammogram of [Fe(tppz)2][C104]2 shows five fully reversible redox
couples, which is quite remarkable. The four reductions indicate electronic
communication between the two ligands. Calculation of the comproportionation constants
show class I/II behavior for the first two reduction couples with Kc = 7.5 x 102. The next
two reduction couples exhibit class II behavior with Kc = 3.7 x 104 (Scheme 3).

Attempts to synthesize the dinuclear products [M2(tppz)2(CH3CN)(,]+4 (M = Ni",

Co", Fe", Mn") were unsuccessful except in the case of [Ni2(tppz)(CH3CN)6][BF4]4. The

191

reaction was performed with a 1:2 ratio of tppz and [Ni(CH3CN)6][BF4]2. The identical
reactions performed with any other anions e.g ([ClOa]', [PF6]', [NO3]') did not afford this
product, but produced only [M(tppz)2]2+ instead. The choice of anion evidently plays a
major role in the self-assembly process.

Reactions of tppz with [Rh2(OAc)2(CH3CN)6][BF4]2 led to the isolation of a novel
rectangle in which the longest sides are occupied by the tppz ligand units and the shorter
sides are defined by the two Rh-Rh bonds. In the case of [Rh2(CH3CN).o][BF4]4
reactions, one observes only metal-metal cleavage and disproportionation with isolation
of [Rh(t992)3l[BF413.

In the tptaz chemistry, all of the metal ions coordinate only to the major tridentate
site. In principle, it should still be possible to use these building blocks in further

chemistry based on metal coordination to the bpy site.

192

 

 

V. References

1. (a) Goodwin, H. A.; Lions, F.; J. Am. Chem. Soc., 1959, 81, 6415.

2. Thummel, R. P.; Chirayil, S. Inorg. Chim. Acta, 1988, 154, 77.

3. (a) Constable, E. C.; Ward, M. D. J. Chem. Soc. Dalton Trans., 1990, 1405. (b) Collin,
J-P.; Lainé, P.; Launay, J-P.; Sauvage, J-P.; Sour, A. J. Chem. Soc. Chem. Commun.,
1993, 434. (c) Ruminski, R. R.; Letner, C.; Inorg. Chim. Acta, 1989, 162, 175. (d)
Ruminski, R.; Kiplinger, J.; Cockroft, T.; Chase, C. Inorg. Chem, 1989, 28, 370. (e)
Tondreau, V.; Leiva, A. M.; Loeb, B.; Boys, D.; Stultz, L. K.; Meyer, T. J. Polyhedron,
1996, 15, 2035.

4. Robin, M. B.; Day, P.; Adv. Inorg. Radiochem, 1967, 10, 247.

5. (a) Graf, M.; Stoeckli-Evans, H.; Escuer, A.; Vicente, R. Inorg. Chim. Acta, 1997,
25 7, 89. (b) Graf, M.; Greaves, B.; Stoeckli—Evans, H. Inorg. Chim. Acta, 1993, 204, 239.

6. Vagg, R. S.; Warremer, R. N.; Watton, E. C.; Aust. J. Chem, 1969, 22,141.
7. Half-Penny, J .; Small, R. W. H. Acta Crystallogr. B, 1982, 38, 939

8. (a) Thomas, N.C.; Foley, B. L.; Rheingold, A. L. Inorg. Chem, 1988, 27, 3426. (b)
Chirayil, S.; Hegde, V.; Jahng, Y.; Thummel, R. P. Inorg. Chem, 1991, 30, 2821. (c)
Berger, R. M.; Ellis, D. D., II. Inorg. Chim. Acta, 1996, 241, 1.

9. (a) Durharrn, D. A.; Frost, G. H.; Hart, F. A. J. Inorg. Nucl. Chem, 1969, 31, 571. (b)
Lerner, E. 1.; Lippard, S. J. J. Am. Chem. Soc., 1976, 98, 5397. (b) Lerner, E. 1.; Lippard,
S. J. Inorg. Chem, 1977, 16, 1546. (c) Embry, W. A.; Ayres, G. H. Anal. Chem. 1968,
40, 1499. (d) Janmohamed, M. J .; Ayres, G. H. Anal. Chem, 1972, 44, 2263. (e) Paul, P.;
Tyagi, B.; Bilakhiya, A. K.; Bhadbhade, M, M.; Suresh, B.; Ramachandraiah, G. Inorg.
Chem, 1998, 37, 5733. (f) Paul, P.; Tyagi, B.; Bilakhiya, A, K.; Dastidar, P.; Suresh, E.
Inorg. Chem, 2000, 39, 14. (g) Paul, P.; Tyagi, B.; Bhadbhade, M. M.; Suresh, E. J.
Chem. Soc., Dalton Trans., 1997 , 2273.

10. Hathaway et al. J. Am. Chem. Soc., 1962, 2444.

11. Case, F. H.; Koft, E. J. Chem. Soc., 1973, 905.

12. Theory and Applications of Molecular Paramagnetism, Boudreaux, E.A.; Mulay,
L.N., Eds; John Wiley & Sons: New-York, 1976.

13. SAINT, Program for area detector absorption correction, Siemens Analytical X-Ray
Instruments Inc., Madison WI 53719, USA 1994-1996.

193

 

 

14. SHELXL-97 — g.m. Sheldrick, SHELXL — 97 , Program for refining crystal structures,
University of Gottingen, 1997.

15. Sawyer, D. T.; Sobkowiak, A.; Roberts, J. L. Electrochemistry for Chemists. 2nd
edition. John Wiley & Sons, 1995.

16. (a) Tondreau, V.; Leiva, A. M.; Loeb, B. Polyhedron, 1996, I5, 2035. (b) Gourdon,
P. L. A.; Launay, Inorg. Chem, 1995, 34, 5156. (c) Brewer, R. G.; Jensen, G. E.; Brewer,
K. J. Inorg. Chem; 1994, 33, 124. (d) Ruminski, R. R.; Letner, C.; Inorg. Chim. Acta,
1989, 162, 175. (e) Ruminski, R. R.; Kiplinger, J .; Cockroft, T.; Chase, C.; Inorg. Chem,
1989, 28, 370.

17. Pruchnik, F. P.; Robert, F.; Jeannin, Y.; Jeannin, S. Inorg. Chem, 1996, 35, 4261.

18. Cotton, F. A.; Walton, A. R. Multiple Bonds Between Metal Atoms. Second ed., 1993
and references therein.

19. Robin, M. B.; Day, P.; Adv. Inorg. Chem. Radiochem., 1967, 10, 247.

20. (a) Zarembiwithch, J .; Kahn, 0.; Inorg. Chem, 1984, 23, 589. (b) Judge, J. S.; Baker,
W. A.; Inorg. Chim. Acta, 1967, I, 68. (c) Kremer, S.; Henke, W.; Reinen, D.; Inorg.
Chem, 1982, 21, 3013. (d) Heinze, K.; Huttner, G.; Zsolnai, L.; Schober, P. Inorg.
Chem, 1997, 36, 5457.

194

Chapter IV

A Homologous Series of Redox-Active, Dinuclear Cations
[M2(ozccn3)z(pynp)]2+ (M = Mo, Ru, Rh) with the Bridging
Ligand 2-(2-Pyridyl)-1,8-naphthyridine (pynp).

195

 

 

 

I. Introduction

In our quest for nitrogen heterocyclic ligands that are capable of spanning a
dinuclear unit, we found several references on the use of cavity-shaped ligands such as 2-
(2-pyridyl)—1,8—naphthyridine (pynp)l and 2,7-bis(2-pyridyl)-1,8-naphthyridine
(bpnp)6(°)‘(d)'(g) (Figure 68). The pynp and bpnp molecules are relatively rigid tridentate
and tetradentate ligands, that possess, in addition to the naphthrydine bridging unit, one
or two pyridyl binding sites that can be used to bind to the axial positions of a dimetal
unit. The pynp ligand (Figure 68) can act as a tridentate combination bridging/chelating
ligand as in the case of [Rh2(OzCCH3)2(pynp)2]2+ or a chelating, bpy mode in the case of
[Rh2(pynp)3C12][PF45]-CH3CN.lb These compounds were found to exhibit interesting
electronic properties, but, in the intervening years since these examples were published,
no additional reports of M-M bonded pynp compounds have appeared in the literature,
and only one X-ray structure has been reported.lc This chapter is devoted to the study of
the coordination chemistry of 2-(2-pyridyl)—l,8—naphthyridine (pynp) with [M02]4+,
[Rufl‘w5+ and [ha]4+ dimetal complexes and a comparison of the structures and redox

properties of the homologous series [M2(02CCH3)2(pynp)2]2+ (M = M0, Ru, and Rh).

 

/|\
\ /
N |\
N/
bpnp PynP

Figure 68. Schematic drawings of the bpnp and pynp ligands.

196

 

 

11. Experimental Section

All manipulations were performed under an inert atmosphere with the use of
standard Schlenk-line techniques. Acetonitrile was freshly distilled over 3A molecular
sieves and methanol was pre-dried over 4A molecular sieves and then distilled over
NaOMe. The solvents diethyl ether and toluene were freshly distilled over Na/K
amalgam. The ligand 2-(2-pyridyl)-l,8-naphthyridine (pynp) was prepared by a
modified literature procedure.2 The starting materials [Rh2(02CCH3)2(CH3CN)6][BF4]2,3
[Moz(02CCH3)2(CH3CN)6][BF.4]23'4 and Ru2(OzCCH3)4C15 were prepared by literature
methods.

Physical Measurements.

'H NMR spectra were obtained on a Varian VXR BOO-MHz spectrometer Gemini-
200. Electrochemical measurements were carried out by using an H CH Electrochemical
Analyser model 620A. Cyclic voltammetry experiments were performed in CH3CN
containing 0.1M tetra-n-butylammonium hexafluorophosphate (TBAPFs) as the
supporting electrolyte. The working electrode was a BAS Pt disk electrode, the reference
electrode was Ag/AgCl and the auxiliary electrode was a Pt wire. The ferrocene couple
occurs at E”; = +0.52V vs Ag/AgCl under the same experimental conditions. Magnetic
susceptibility measurements were obtained with the use of a Quantum Design SQUID
magnetometer MPMS-XL (housed in the Department of Chemistry at Texas A&M
University). Data was collected in the temperature range 1.8-350 K at 1000 G on finely
divided polycrystalline samples. The raw data was corrected for the contribution of the

sample holder and the diamagnetism of the constituent atoms by the use of Pascal

constants.°

197

 

 

Theoretical Details.

A companion theoretical study for the experimental work in this chapter was
performed by Dr. Lisa Thompson at Texas A&M University. The compounds,
[M(OZCCH3)2(pynp)2]2+ (where M = Moz“, Ruz", Rh”) (30-32), without the solvent
molecules, were subjected to density functional theory (DPT) calculations7 with the
Becke3 hybrid exchange functional and the Lee-Yang-Parr correlation functional
(B3LYP).8 Details of the calculation are contained in Appendix I, which is provided as
supporting information.

Syntheses
Preparation of 2-(2-pyridyl)-1,8-naphthyridine (pynp).

The preparation of pynp requires the precursor 2-aminonicotinaldehyde, which
must be freshly prepared and used immediately after isolation to avoid self—condensation
side reactions.
2-aminonicotinaldehyde.

The method of Caluwe and co-workers2 was followed with some modifications.
Nicotinamide (9.1 g, 0.07 mmol) was mixed with 13.0 g (0.11 mmol) of ammonium
sulfamate in a round-bottomed flask equipped with a condenser. The temperature was
slowly increased to 150 °C until the entire solid had melted. At this point the temperature
was continuously increased until it reached 200 °C, after which time the reaction was
allowed to reflux for 8.5 h. The grayish-white solid obtained from this procedure was
washed with copious amounts of water and diethyl ether to remove unreacted
nicotinamide and sulfamate. The remaining solid was refluxed in 75 mL of 4N HCl for

6.5 hours and the solution was made basic (pH ~ 9.0) by adding portions of a saturated

198

 

 

NaOH solution. The resulting solution was subjected to four extractions with diethyl
ether, and the combined extracts were dried over anhydrous MgSOa for one hour. The
diethyl ether was evaporated and the remaining light yellow solid was sublimed at 60 °C
under vacuum to obtain pure 2-aminonicotinaldehyde; yield, 3.1g (34%). This product
must be stored in an inert atmosphere at 0 °C to avoid self-condensation reactions. IR
(KBr mull) cm": 3412 (br,w), 2924 (br,w), 2750 (w), 1669 (w), 1624 (w), 1568 (w),
1458 (w), 1402 (w), 1377 (w), 1296 (w), 1273 (w), 1194 (w), 1136 (w), 912 (w), 775 (w),
675 (w), 623 (w). 1H NMR spectrum in CDCl3 at 25 °C: 5 9.82 (s, OH), 8.22 (dd), 7.78
(dd), 6.95 (b), 6.72 (dd) ppm.

Preparation of 2-(2-pyridyl)—1,8-naphthyridine (pynp).

An amount of freshly prepared 2-aminonicotinaldehyde (1.6 g, 14.8 mmol) was
placed in a Schlenk flask under nitrogen and dissolved in 25 mL of freshly distilled
ethanol. To this solution was added 1.0 mL of 2-acetyl-pyridine. The solution was
refluxed under nitrogen and treated with one drop of a freshly prepared methanolic
solution saturated with NaOH. A color change from yellow to a pale brown-yellow
immediately ensues. The solution was refluxed overnight under nitrogen and the solution
was concentrated to one half of its original volume. White crystals were observed to form
after a few hours; yield, 1.1 g (43 %). IR (KBr mull) cm": 1550 (m), 1520 (w), 1470 (s),
1350 (m), 1050 (w), 850 (w), 800 (m), 750 (w). lH NMR spectrum in CD3CN at 25 °C, 5
9.1 (q), 8.75 (m), 8.68 (d,d), 8.45 (d), 8.36 (d,d), 7.95 (t,d), 7.56 (q), 7.48 (m) ppm.
[M02(02CCH3)2(PYDP)2][BF412 (30).

A sample of [M02(02CCH3)2(CH3CN)6][BF4]2 (0.2 g, 0.27 mmol) was dissolved

in 20 mL of CH3CN and treated with 114 mg (55 mmol) of pynp, which leads to a color

199

change from a light pink to a dark green. After 12 h, the solution was concentrated and
diethyl ether was added to the point of saturation. The solution was then placed in a
refrigerator to yield green platelets; yield, 0.145 g (75 %). IR (KBr mull) cm": 2924 (br,
s), 2725(w), 1606(w), 1557(w), 1518(w), 1458(8), 1377(s), 1316(w), 1263(w), 1217(w),
1146(w), 1057(br, s), 854(w), 816(w), 781(w), 721(w), 679(w), 521(w) cm". 1H NMR
spectrum in CD3CN at 25 °C: 8 9.05 (d, pynp), 8.85 (dd, pynp), 8.68 (d), 8.2 (td, pynp),
7.72 (m, pynp), 7.66 (m, pynp), 7.52(m, pynp), 2.65(s, CH3-acetate) ppm.
Ruz(02CCH3)2(PynP)2][PFslz (31)-

The starting material [Ruz(02CCH3)4}Cl (0.150 g, 0.32 mmol) was dissolved in
20 mL of MeOH and treated with 0.131 g (0.64 mmol) of pynp. The initial dark brown
solution instantaneously turned to a dark blue color. This solution was stirred overnight
and the volume was decreased to ~ 10 mL, after which time diethyl ether was added to
produce a dark blue crystalline powder. Yield 0.103 g (60%). IR (Nujol, KBr): 2700
(br,s), 2350 (w), 1450 (s), 1380 (s), 1300 (br,w), 845 (br,s), 780 (br,s), 710 (w), 580 (w).
The 1H NMR signals of the product were broad and featureless.
[ha(02CCHa)z(pynp)2l[BF412 (32)-

A sample of [Rh2(02CCH3)2(CH3CN)6][BE]; (0.1 g, 0.13 mmol) was treated with
0.056 g (0.27 mmol) of pynp in 20 mL of CH3CN which led to an instantaneous color
change from pale red to intense red. After stirring overnight, the volume was reduced to
5 mL and the solution was layered with toluene. A crop of red crystals was harvested
after three days. Yield 0.077 g (70%). IR (N ujol, KBr): 2926 (br, s), 2725 (w), 1604 (w),
1650 (w), 1523 (w), 1462 (s), 1377 (w), 1315 (w), 1265 (w), 1147 (w), 1059 (w), 1012

(w), 841 (w), 779 (w), 736(w), 715 (w), 559 (w). lH NMR spectrum in CD3CN at 25 °C:

200

 

 

8 9.70 (d, pynp). 8.87 (dd, pynp), 8.70 (d, Pynp). 8.60 (m, pynp). 8.45 (dd, pynp), 8.35
(td, pynp), 7.48(q, pynp), 2.25 (s,CH3-acetate) ppm.
X-ray data collection and Refinement.

Geometric and intensity data for compound (30) were collected on a Rigaku
AFC6S diffractometer equipped with a graphite-monochromated Mo K0t (ha = 0.71069
A) radiation source. All calculations were performed with VAX computers on a cluster
network using Texsan software package of the Molecular Structure Corporation.9 X-ray
structural studies for compounds (31)-(33) were performed on a SMART 1K area
detector diffractometer equipped with graphite monochromated Mo K01 radiation (3.01 =
0.71073 A). The frames were integrated in the Siemens SAINT software package,'0 and
the data was solved using the direct-methods program S1-IELXS-97.ll Crystal data are
listed for compounds (30)-(33) in Table 34.

M02(02CCH3)2(PYHP)2][BF412'3CH3CN (30).

X-ray quality crystals were obtained within three days from a saturated solution of
the title compound in acetonitrile with diethyl ether at -5 °C. A green rectangular crystal
of dimensions 0.80 x 0.4 x 0.25 mm3 was covered with silicone grease and mounted on
the tip a glass fiber. Cell constants were obtained from a least squares refinement using
25 carefully centered reflections in the range 29<20<37°. Data were collected at —100 i 1
°C, by using the (1) scan method, in the range 45203479 A total of 7493 reflections was
collected of which 7200 were unique. The final full-matrix refinement was based on 4162
observed reflections with Fo>40(Fo) that were used to fit 550 parameters to give R1 =
0.0598 and wR2 = 0.1690. The goodness-of-fit index was 0.999 and the highest peak in

the final difference map was 1.017 e'lA'3.

201

 

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202

Ru2(02CCH3)2(PynP)2][PFslz‘Cnson (31)

X-ray quality crystals were grown by layering a methanol solution of the title
compound with toluene. A blue platelet of dimensions 0.11 x 0.07 x 0.05 mm3 was
secured on the tip of a glass fiber with silicone grease and placed in a N 2(g) stream at 173
i l K. A total of 9867 reflections was collected of which 3429 were unique. A disordered
[PF6]’ anion required modeling in three different orientations. Final least-square
refinement of 242 parameters and 3203 data resulted in residuals of le = 0.0812 and
wR2 = 0.1798. The goodness-of-fit index was 1.078 and the highest peak in the final
difference map was 0.952 e'/A3.

Rh2(OzCCH3)2(pynp)2][BF 412'C7Hs (32)

X-ray quality crystals were obtained by slow diffusion of an acetonitrile solution
of the title compound into toluene. A red, rectangular crystal of dimensions 0.05 x 0.06 x
0.15 mm3 was covered with silicone grease, secured on the tip a glass fiber, and cooled to
173 i 2 K. A total of 7670 reflections was collected of which 3203 were unique. Final
least-squares refinement of 294 parameters and 1840 data resulted in residuals of le =
0.1062and wR2 = 0.2365 and a goodness-of—fit of 1.117. A final difference Fourier map
revealed the highest peak to be 0.952 e'lA3.
[Rh2(OzCCH3)2(PynP)2(CH3CN)2][BF4][PFal'ZCHsCN (33).

Suitable crystals for X-ray crystallography were obtained by slow diffusion of a
solution of [Rh2(02CCH3)2(CH3CN)6][BF4]2 in acetonitrile layered over a solution of
pynp in dichloromethane. A pale red prism of dimensions 0.15 x 0.05 x 0.1 mm3 was

covered with silicone grease, mounted on the tip of a glass fiber and placed in a N2(g)

cold stream at 173 i 2 K. A total of 23918 reflections was collected, of which 9970 were

203

 

unique. Final least-squares refinement of 598 parameters and 5987 reflections resulted in
residuals of le = 0.0678 and wR2 = 0.1642 and a goodness-of-fit of 0.980. A final
difference Fourier map revealed the highest peak to be 2.209 e'lA3 which is associated

with a disordered [PF6]'.

III. Results and Discussion.

A. A homologous series of redox-active dinuclear compounds.

A.1. Synthesis of compounds [M2(02CCH3)2(pynp)]2+, M = [Mo]2+, [Ru]2+, [Rh]2*.
Synthesis of compounds [M2(02CCH3)2(pynp)]2*, M = M0“, Ru“, Rh“.

Reactions of pynp with [M02(OZCCH3)(CH3CN)6][BF412'°,
[Rh2(02CCH3)2(CH3CN)6][BF412", and Ru2(02CCH3)4Cl5 carried out in a 1:2
metal/ligand ratio (Eqs. 13, 14) reactions proceed instantaneously as judged by the color

changes that occur within minutes of mixing the reagents.

 

CH CN
[M2(0Ac)2(CH3CN)6][BF4]2+2pynp _.3._» [M2(0Ac)2(pynp)2][BF4]2 039-13)
M = (a) Moz+, (b) Rh2+ yield: a = 75%
b = 70%
CH3OH
[Ru2(OAc)4]Cl + 2pynp = [R02(OAC)2(PYDP)2][PF6]2 (Bil-14)

[n-Bu N][PF]
4 6 yield: 60%

The pale violet solution of [Moz(02CCH3)2(CH3CN)6][BF4]2 solution changes to a bright
green color, the purple solution of [Rh2(02CCH3)2(CH3CN)6][BF4]2 changes to a pale red
color, and finally the brown solution of Ruz((02CCH3)4Cl first changes to a dark purple
color and later reverts to dark blue. X-ray and NMR studies (vide infra) revealed that the

reactions proceed by displacement of the equatorial acetonitrile ligands in the former two

204

 

cases and, in the case of the diruthenium chemistry, a loss of two acetate ligands
accompanied by a reduction from [Ruz]5+ to [Ru2]‘”.

The replacement of the six acetonitrile molecules on
[M2(02CCH3)2(CH3CN)6][BF412 10 yield [M2(02CCH3)2(PY"P)2][BF412 (M = Rh, RU)
occurs in methanol, ethanol and acetone regardless of the use of different ligand/metal
ratios or reaction temperature. In the case of the [M02]4+ complex, however,
decomposition occurs at higher temperatures. The synthesis of the dinuclear [Ru2]4+
complex is best performed in alcohols, as reactions preformed in acetonitrile and acetone

lead to much lower yields. In the case of the reaction of [Rh2(02CCH3)2(CH3CN)6][BF4]2
with pynp, the side-product [Rh2(02CCH3)2(pynp)2(CH3CN)2][BF4][PF6]°2CH3CN (33)

was isolated and characterized by single crystal X-ray methods.
A.2. X-ray crystallographic studies.

The molecular structures of the dinuclear cations for compounds (30)-(32) are
essentially identical, as indicated by X-ray crystallography (Figures 69-72). The
compounds consist of a dimetal unit spanned by two cis tridentate pynp ligands, in
addition to two bridging acetate ligands. Such a series of compounds is convenient for
studying the effect on structure and properties of a non-innocent ligand such pynp in a
series of related dinuclear compounds. One interesting aspect to consider is the axial
interaction between the metal and the nitrogen donor of the pyridyl unit. The pseudo-
axial pyridine interaction is relatively long in all three cases, with M-N distances ranging
from 2.204 (4) to 2.439 (8) A and M-M-Nax angles in the range 159.01[2] to l69.6(3)°.
Compound [Moz(02CCI-13)z(pynp)2][BF4]2-3CH3CN (30) exhibits the longest axial M-N

distance of 2.439 [8]A as expected for a [M02]4+ quadruply-bonded complex.12 In the

205

 

case of [Ru2(02CCH3)2(pynp)2][PF6]2.2CH3OH (31) complex, the axial interaction is of
an intermediate value, Ru(l)-N(3)A 2.237(7)A, which is consistent with the fact that this
species is a doubly-bonded compound. Finally, [Rh2(02CCH3)2(pynp)2][BF4]2.C7H3 (32)
exhibits the shortest axial interaction, 2.204 (4) A (Rh(l)-N(3), as expected for the
compound with the weakest M-M bond trans-influence.l2

The strength of the axial interation is closely tied to the M-M distance, as well
illustrated by this series of compounds. The quadruply-bonded compound
[M02(OZCCH3)2(pynp)2][BF4]2-3CH3CN (30) exhibits a M-M bond distance of 2.124(1)
A, whereas the reported M-M distance for the parent molecule Moz(OzCCH3)4 is
2.0934(8) A.” The lengthening of the metal-metal bond by 0.03 A is attributed primarily
to the axial interaction. The [Ruzl‘H derivative in this series
[Ru2(02CCH3)2(pynp)2][PF6]2.2CH3OH (31) exhibits a Ru-Ru distance of 2.298 (1)A,
which is slightly longer than the corresponding distances in [Ruz(02CCH3)4(THF)2] and
[Ru2(02CCI-I3)4(H20)2] which are 2.261(3) and 2.265(3)A, respectively." This
lengthening can be attributed to the fact that the pyridyl ring is a better axial donor than
THF or H20. Finally, in [Rh2(02CCH3)2(pynp)2][BF4]2.C7I-Ig (32) the Rh-Rh distance is
2.408 (2)A which is well within the expected values for a Rh-Rh single bond.'7 This
distance is 0.04 A longer than the corresponding distance in Rh2(02CC3H7)4,'5 which is
the only dirhodium tetra-carboxylate compound without axial coordination to be
structurally characterized. The lengthening of the M-M bond is due to the donation of the
pyridyl moiety into the 0* antibonding orbital of the [Rh2]4+ unit.

The M—Neq interactions with the naphthyridine bridges are considerably shorter

than the M-Nax interactions by comparison, and they decrease as the radius of the metal

206

 

ion decreases. Since Moll possesses the largest radius, the M-N distances are the longest

with the averages being 2.211[8] A followed by Ru" with 20725] A and finally Rh"

with 2.018 [41A

 

 

Table 35. bond distances (A) (°) for
[M02(02CCH3)2(P)’“P)2][BF4]2°3CH3CN (30)-

MOI-03 2.088(6) M02-02 2.092(6)
M01 -01 2.1 14(6) M02-04 2.150(6)
Mol-M02 2.124(1) M02-N2 2.165(8)
Mol-N6 2.184(8) M02-N5 2.245(7)
Mol-Nl 2.251(7) M02-N4 2.450(8)
Mol-N3 2.429(8)

O3-Mol-Ol 88.4(2) 02-M02-N2 88.5(3)
O3-Mol-Mo2 93.5(2) Mol-Mo2-N2 94.7(2)
Ol-Mol-M02 89.66(17) 04-M02-N2 174.9(3)
O3-Mol-N6 88.1(3) 02-M02-N5 175.9(3)
Ol-Mol-N6 174.3(3) Mol-M02-N5 90.7(2)
MoZ-Mol-N6 95.1(2) 04-M02-N5 91.2(3)
O3-Mol-Nl 174.7(3) N2-M02-N5 92.0(3)
Ol-Mol-Nl 87.6(3) 02-M62-N4 106.6(3)
MoZ-Mol-Nl 90.1(2) Mo-MoZ-N4 158.7(2)
N6-Mol-Nl 95.5(3) O4-M02-N4 84.1(3)
O3-Mol-N3 106.1(3) N2-M02-N4 93.4(3)
Ol-Mol-N3 84.3(3) N5-Mo2-N4 69.3(3)
MoZ-Mol-N3 159.3309) Cl-Ol-Mol 119.8(6)
N6-Mol-N3 92.2(3) C1-02-M02 116.8(5)
N1-Mol-N3 69.9(3) C2-03-Mol 116.5(6)
02-M02-Mol 93.33(18) C2-O4-M02 118.7(6)
02-M02-04 88.0(2) C4-N1-Mol 121.6(6)
Mol-M02-O4 89.20(l9) C8-Nl-Mol 119.9(6)

207

 

/\—\
C) C17

 

Figure 69. Thermal ellipsoid plot at the 50% level of the cation in
[M02(02CCH3)2(pynp)2][BF4]2-CH3CN (30). Hydrogens have been
omitted for the sake of clarity.

208

 

 

Table 36. Selected bond distances (A) and angles (°) for
[Ruz(02CCH3)2(PYnP)2][PF612-2CH3OH (31)-

 

Rul-Rul 2.298(1) Rul-Nl 2.071(5)
Ru1-02 2.054(5) Rul-Ol 2.089(5)
Rul—N2 2.072(5) Rul-N3 2.237(7)
OZ-Rul-NZ 90.2(2) Ol-Rul-N3 87.6(2)
OZ-Rul-N 1 176.7(2) OZ-Rul-Rul 91.22(l4)
N2-Rul-Nl 93.0(2) N 2-Ru1-Rul 91. 16(15)
N2-Rul-Ol 175.6(2) Nl-Rul-Rul 89.4405)
Nl-Rul-Ol 90.9(2) Ol-Rul-Rul 86.91(l4)
N2-Rul-N3 95.3(2) N3-Rul-Rul 163.7(2)

As noted earlier, in addition to the [ha]4+ dinuclear complex with two tridentate
pynp units, a by-product was isolated, namely
[Rh2(02CCH3)2(pynp)2(CH3CN)2][BFa][PF6]-2CH3CN (33). This compound co-
crystallizes as a side product from the reaction that produces
[Rh2(02CCH3)2(pynp)2][BF4]2. In this compound, the dirhodium unit possess one pynp
ligand coordinated in the tridentate fashion and a second pynp ligand that acts as a
monodentate ligand to an axial position through one of the N atoms of the naphthyridine
unit (Figure 72). The pseudo-axial distance (Rh(l)-N(4)) between the nl-pynp ligand and
the metal center is 2.158 (5) A, which is 0.16 A larger than the Rh-pyridyl interaction
Rh(2)-N(3) 1.997 (5) A. The weaker RheN interaction is presumably due to the weaker
donor character of the naphthyridine nitrogen atom compared to the pyridine nitrogen

unit.

209

 

 

 

 

 

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Figure 70. Thermal ellipsoid plot of the cation in [Ruz(OzCCH3)2(pynp)2][PF6]2-CH30H
(31) at the 50% probability level. Hydrogen atoms have been omitted for the
sake clarity.

 

210

 

 

 

 

 

Table 37. Selected bond distances (A) angles (°) for
[Rh2(02CCH3)2(PY"P)2] [BF412-C7H8 (32)-

Rh l-Rhl 2.4075( 18) Rh 1 -N1 2.045(12)

Rhl—N2 1.985(12) Rhl-Ol 2.051(11)

Rh 1-02 2.036(11) Rh 1 -N3 2.204(11)

N2-Rh1-02 90.2(5) OZ-Rh 1 -N 3 90.4(4)

N2-Rh1-N 1 92.0(5) N 1 -Rh1-N3 95.6(4)

OZ-Rh 1-N1 173.8(4) Ol-Rhl—N3 101.7(4)

N2-Rh1-Ol 178.7(4) N2-Rh1-Rhl 91 . 1(3)

OZ-Rh l -01 88.6(4) OZ-Rh l-Rhl 86.9(2)

N 1-Rh- Ol 89.2(4) N 1 -Rh1-Rh1 87.3(3)

N2-Rh1-N3 78.9(4) O 1—Rh1-Rhl 88.3(2)

Table 38. Selected bond distances (A) angles (°) for
[Rh2(02CCH3)2(P)’“P)2(CH3CN )2HBF4IP F61'2CH3CN (33)-

Rh 1 -N 8 2.034(6) Rh 1 -N4 2.158(5)

Rh 1 -01 2.084(5) Rh 1-03 2.176(5)

Rh 1 -N1 2.134(6) Rh2-N3 1.997(5)

N8-Rh1-Ol l78.02(17) N1-Rh1-O3 174.15(17)

N8-Rh1-N1 95.6(2) N4-Rh 1-03 94.77(l9)

Ol-Rh 1 -N1 85.1(2) N8-Rh1-Rh2 93.27(14)

N8-Rh 1 -N4 95.5(2) Ol-Rh 1-Rh2 88.46(12)

Ol-Rh 1 -N4 82.67(19) ‘ N1-Rh1-Rh2 97.86(l4)

N1-Rh1-N4 90.9(2) N4-Rh l -Rh2 l66.96(15)

N8-Rh1-O3 85.3(2) O3-Rh1-Rh2 76.30(1 l)

Ol-Rhl-O3 94.23(18)

211

 

 

 

     

”\

C9A .~ \I} "A
“$1., C6A\‘ C5A
\ \\\\1
V

Figure 71. Thermal ellipsoid plot of the cation in [Rh2(02CCH3)2(pynp)2][BF4]2-C7Hg
(32) at the 50% probability level. Hydrogen atoms have been omitted for the
sake of clarity.

212

 

 

Figure

Thermal ellipsoid plot of the cation in
[Rh2(02CCH3)2(PY“P)2(CH3CN)2][BF4HPF612'2CH3CN (32) at the
50% probability level. Hydrogen atoms have been omitted for the
sake of clarity.

213

 

A.3. Theoretical calculations.

To gain insight into the electronic structure of the series [M2(02CCH3)2(pynp)2]2+
(compounds (30), (31), and (32)), single point energy calculations were performed by Dr.
Lisa Thompson at Texas A&M University for the dication, neutral and dianion of these
complexes at the B3LYP level of theory. An orbital analysis of the calculated orbital
occupancy for the dication of [Moz(02CCH3)2(pynp)2]2+, [Ruz(02CCH3)2(pynp)2]2+, and
[Rh2(OzCCH3)2(pynp)2]2+, showed the expected metal orbital occupancy of 021:452,
ozn4825*21t*'n*', and 027t4525*2n*4 respectively. Details of the findings as they pertain to
the experimental findings in this chapter are found in Appendix I. Only a summary of
how the results of the calculations support the spectroscopic and electrochemical results
is included in the relevant sections.

A.4. UV-Visible and N MR spectroscopic studies.

 

Table 39. UV-Visible data for compounds (30)-(32)

 

 

 

 

 

Am 6
nm L-(mol-cm)’l
[M02(02CCH3)2(P)’DP)2[BF4]2 (30) 432 1.8 x 103
1 857 4.8 x 102
[RU2(02CCH3)2(P)’"P)2[PF6]2 (31) 327 3.4 x 104
671 3.6 x 10‘
278 3.1 x 10‘
[Rh2(02CCH3)2(P)’"P)2[BF4]2 (32) 355 8.7 X 103
451 2.4 x 103

 

 

 

 

 

214

The electronic spectra for the three compounds in the series

[M2(02CCH3)2(pynp)2][BF4]2 were recorded in acetonitrile in the range 800-200 nm
(Table 6). The spectrum for [M02(02CCH3)2(pynp)2][BF4]2 (30) exhibits an electronic

transition in the visible range at 432 nm which is a characteristic energy for a 8—55“

'6'” but the evalue is

transition for a quadruply-bonded Moz'"ll complex
uncharacteristically high at 1.8 x 103 L-M'Icm'l an order of magnitude greater than the
typical values, suggesting the involvement of ligand character. The dinuclear [Ruz]“
complex (31) exhibits two electronic transitions located at 327 nm (e = 3.4 x 104 L-M'

lcm") and 671 nm (8: 3.6 x 104 L'M'lcm"). The dirhodium compound

[Rh2(OZCCH3)2(pynp)2]2+ (32) displays three transitions located at 278, 355 and 451 nm

with e values of 3.1 x 10“, 8.7 x 103 and 2.4 x 103 L-M"cm" respectively.

The aforementioned electronic spectral data indicate that the new compounds
exhibit very different electronic properties than the parent tetracarboxylate species.
Electronic transitions for [Moz(02CCH3)4 are located at 435 nm (8 == 102 L-M"cm"), '7
for Rh2(02CCH3)4]L2 at A = 552 nm (e z 2 x 102 L-M"cm") and it = 437 nm (e z 1 x 102
L-rvr"cm"),'8 and finally for Ru2""'(ozccri3). at 2...... = 448 nm (e = 6 x 102 L-M"crn'
l).l2 Clearly the presence of the two pynp ligands has perturbed the electronic structure
of the HOMOand/or LUMO levels, a possibility that was probed by theoretical
calculations (Appendix I).

1H NMR Spectroscopy.

1H NMR spectroscopic studies of the two diamagnetic compounds
[M02(02CCH3)2(PYHP)2][BF412 (30) and [Rh2(02CCH3)2(pYnP)2][BF412 (32) in CD3CN

support the existence of the intact cations [Moz(02CCH3)2(pynp)2]2+ and

215

 

 

[Rh2(02CCH3)2(pynp)2]2+ in solution. The IH NMR spectrum for
[Moz(02CCH3)2(pynp)2][BF4]2 displays an upfield singlet at 5 = 2.65 ppm for the acetate
CH3 groups and seven resonances between 5 = 7.5-9.0 ppm for the aromatic pynp
protons. The [ha]4+ complex (32) exhibits a similar pattern, with an upfield singlet
located at 5 = 2.25 ppm and aromatic resonances in the range 7.5-9.7 ppm. The presence
of only one acetate environment in each case and the 1:1 integration of acetatezpynp
ligands is in accord with the solid-state structure.

A.5. Cyclic voltammetric studies.

Electrochemical studies of Compounds (30)-(32) were performed by the cyclic
voltammetry technique in acetonitrile solutions (Figure 73). The cyclic voltammogram of
[Moz(02CCH3)2(CH3CN)6][BF4]2-3CH3CN (30) exhibits four reversible reduction
couples located at Em“) = -0.43v, Em”) = -O.67 v, Em”) = -1.34v and Bug“) = -1.66V
(Figure 6). In addition, an irreversible oxidation occurs at 13,,a = 0.86V. The most salient
point about these data is that the second reduction occurs at a potential that is 240 mV

more negative than the first couple. This corresponds to a comproportionation constant,

4
Kc, for this reaction of 1.2 x10 (scheme 5), which was class H behavior according to the
Robin-Day classification.l9 Likewise the second set of one—electron reductions are

coupled as judged by the fact that the fourth one-electron reduction occurs at a potential

that is 320 mV higher than the third reduction process (Kc value of 2.70 x 105) (Scheme
5). These comproportionation constants are in the range of what has been observed for
weakly coupled systems.

The compound [Ruz(02CCH3)2(pynp)2][PF6]2 (31) also exhibits four reversible

one-electron ligand based reductions; these are located at Em“) = -0.43 V, Buzz) = -0.82

216

 

V, 131/2‘3) = -1.40 V and E172“) = -1.84 V (Figure 73). The second reduction is shifted to a
more negative potential by 390 mV relative to the first one. This separation corresponds
to a comproportionation constant of Kc = 3.9 x 106, an indication that the unpaired
electron is delocalized over the two pynp units (Class III, Robin-Day behavior).'9
Analogous] y the third and fourth reductions occur at a separation of 420 mV, which leads
to a calculated Kc value of 2.7 x 107. This also corresponds to class III behavior (Scheme
5). In addition, the compound displays a reversible couple at Em = 0.85V which
corresponds to the oxidation from [Ruzm to [Ruz]5+.

+e-

 

 

 

 

 

 

[M2(02CCH3)2(pynp)2]2+ _ _ [M2(02CCH3)2(PY“P)2]+ Emy,
M = a) Mo, Kc(a) = 1.2 x 10‘
b) Ru Kc(b) = 3.9 x 10"
[M2(02CCH3)2(P)’"P)2]+ 4: : [M2(02CCH3)2(PYHP)2] Emr/2
+ - -
[M2(02CCH3)2(pynp)zl _ : [M2(02CCH3)2(pynp)2] Early:
Kc(a) = 2.7 x 105
Kc(b) = 2.7 x 107
[M2(02CCH3)2(PY“P)2]' + :- [M2(02CCH3)2(PY“P)2]2° E(4)y2
Scheme 5

In contrast to the previous two cases, [Rh2(02CCH3)2(pynp)2]2+(3l) exhibits only
two reduction features in the cyclic voltammogram; these are located at Em“) = -0.78 V

and Ema) = -1.42 V (Figure 6c). The first reduction process is associated with a two-

217

 

—.

 

En) | [Rh2(0AC)2(pynp)2][BF4]2 I

E(4)

  
 
  
 

 

I [M02(0Ac)2(pynp)2][BF4]2 I

 

 

 

I [R92(0AC)2(PYDP)2][PF6]2 I

 

 

 

 

 

 

 

 

 

 

AEy2=420 mV
Kc: 2.7 x 107
AEy2= 390 mV
Kc =39 x 106
I l 7 1 l I
t l l I I’
o 0.5 -1 -1 5 -2 -2.5 E (V)

Figure 73. Cyclic voltammograms for compounds (30)-(32) in acetonitrile with 0.1 M
[n-BuaN][PF6] at a Pt disk electrode versus Ag/AgCl.

218

 

electron process whereas the latter one is a one-electron process as determined by
coulometry and differential pulse voltammetry (Figure 74).

A homologous series such as the present one is an ideal situation for making
comparisons based on the differences in metal frontier orbitals. The cyclic voltammetry
results for the M02(II,II) and Ruz(II,II) complexes indicate stable mixed-valence
intermediates of the type [M2(02CCH3)2(pynp)2]+ and [M2(02CCH3)2(pynP)2]'. whereas
for the Rh2(II,II) complex, the mixed valence states are not stable. In order to understand
this situation, theoretical calculations were performed (Appendix I), and the results
indicated that the dirhodium compound is undergoing decomposition as the LUMO is
becoming populated during the reduction processes. In the case of the stable reduced
forms of Moz(II,II) and Ru2(H,II), however, the calculations reveal that the ratios of the
changes in energy between orbitals before and after reduction are in excellent agreement
with the observed differences in potential between the two sets of reductions (neutral/2e

reduction and 2e' reduction/4e reduction).

 

   

 

 

 

 

2.5 x 10 -6 T I I I I j I I I I I fi I I I I I T I q
l- -1
2.0x 10-6 i A r:

O O

t o g :
-6 L g E _‘
1.5 X 10 1' g o 4
C o ‘8 Z
r- O -
1.0 x 10 '6 :- § ‘1
. .
5.0x 10 '7 " 1
0.“) 1 l 1 1 1 J 1 4 1 l 1 1 1 J :

-0.4 -0.6 -O.8 -1 -1.2

Figure 74. Diferential pulse voltammogram of [Rh2(02CCH3)2(pynp)2][BF4]2 in 0.1 M
TBAPF5 acetonitrile at a Pt disk electrode versus Ag/AgCl.

219

 

A 6. Magnetic studies.

In this series, only [Ruz(02CCH3)2(pynp)2][PF6]2 (31) is paramagnetic. According
to previous studies on various [Ruz]4+ compounds, the ground state spin value is S = 1.20
In order to understand the magnetic behavior of these compounds, it is necessary to
understand that a large zero-field splitting is expected due to spin-orbit coupling.24b The

actual data for [Ruz(02CCH3)2(pynp)2][PF6]2 confirms this prediction. Figure 75 presents

 

0.007

IIIIITIIIIIIIIIIIII‘IIITIIIIII1.2

   
     

0.0065

 

0.006 . N
9 f . u-a
O _ .. ’8
E ' {3’ 0.8 a
(2 0.0055 _ .1 . 2
0 I D = 296 cm : O
2 0.005 _- TIP = 350 x 10‘6 emu: 0-6 a
: _3 E
Q _ =
‘22“: 0.0045 _- P 3 "10 5
' " 0.4 2.
0.004 :
" 0.2
0.0035
0.003 1 1 l 1 1 1 1 l 1 1 1 1 I 1 1 1 1 l 1 1_1 1 l O

 

 

100 150 200 250 300
T (K)

Figure 75. Plot of xT versus T (red) and x versus T (blue) for
[Ruz(02CCH3)2(pynp)2][PF6]2 at an applied field of 1000 G.

the data in two different ways, namely as x vs T and xT vs T plots. The plot of xT versus
T shows a constant decrease as the temperature decrease due to the zero field splitting

present in this compound. The plot of x versus T displays an increase in susceptibility

220

 

 

below 30 K that can be attributed to a small amount of paramagnetic impurity (probably

the oxidized Ru(H)-Ru(III) species) which must be taken into account for modeling the

magnetic behavior of compound [RU2(02CCH3)2(P)’nP)2][PF6]2. In order to fit this
experimental data to a theoretical expression of the magnetic susceptibility for an
anisotropic S = 1 system, which accounts for the small paramagnetic impurity and the Tip
observed, the equation 14 was used.

it = (ZNgM21.1132/3161‘){[e’x + (2/x)(1 — e"‘)]/( 1 + 2e"‘)} + Tip + P/T (Eq. 14)

In equation 14 x = D/KT accounts for the zero field splitting and gM was assigned a value

of 2, these assignments were based on previous studies done in [Ruz]+4 systems.24

111. Conclusions

Extensive research over the past forty years has been carried out on the
tetracarboxylate family of compounds of general formula M2(02CCH3)4 (M: Mo“, Ru“,
Rh“).l7 Although these compounds are useful starting materials for a variety of M-M
bonded derivatives, they do not exhibit extensive redox chemistry or unusual electronic
properties. In sharp contrast, the new compounds in this study that contain two pyridyl
naphthyridine ligands pynp in addition to two acetate ligands display vastly different
behavior. In particular, [Moz(OzCCH3)2(pynp)2][BF4]2'3CH3CN (30) and
[Ruz(02CCH3)2(pynp)2][PF6]2-CH30H (31) exhibit four, reversible one-electron
reduction processes. In the case of the [M02]4+ compound (30) the four reductions
indicate Class II behavior based on the Robin and Day scale.'9 For the [Rug]4+ compound,
the potentials of the four, one-electron reduction couples signify Class III behavior,

namely a fully delocalized system.

221

 

 

Density functional theoretical calculations (Appendix I) lend supporting evidence
for the correlation between the degree of mixing of the metal-based orbitals with the
ligand orbtials and the degree of delocalization of the electrons upon reduction. The
Ruz(II,II) complex exhibits the highest degree of mixing, thus the larger
comproportionation constants are observed.

The capability of pynp to coordinate as either a tridentate or bidentate ligand had
earlier been suggested, but not substantiated with X-ray evidence. This work presents the
first well—characterized example of the monodentate coordination mode for the pynp

ligand, which is a likely intermediate in the eventual stabilization of the tridentate form.

222

 

 

IV. References

I. (a) Caluwe, P.; Evens, G. Macromolecules 1979, 12, 803. (b) Tikkanen, W.;
Binamira-Soriaga, B.; Kaska, W.; Ford, P. Inorg. Chem, 1984, 23, 141. (c) Tikkanen,
W.; Binamira-Soriaga, E.; Kaska, W.; Ford, P. Inorg. Chem, 1983, 22, 1147. (d)
Thummel, R. P.; Lefoulon, F.; Williamson, D.; Chavan, M. Inorg. Chem, 1986, 25,
1675. (e) Binamira-Soriaga, E.; Keder, N. L.; Kaska, W. C.; McLoughlin, M. A.; Keder,
N. L; Harrison, W. T. A.; Stucky, G. D. Inorg. Chem, 1990, 29, 2238.

2. (a) Majewicz, T. C; Caluwe, P. J. Org. Chem, 1974, 39, 720. (b) Caluwe, P.; Evens,
G. Macromolecules, 1979, 12, 803.

3. Pimblett, G.; Garner, C. D.; Clegg, W. J. Chem. Soc., Dalton Trans, 1986, 1257.

4. Cotton, F. A.; Reid, A. H.; Schwotzer, W. Inorg. Chem, 1985, 24, 3965.

5. Martin, D. S.; Newman, R. A.; Vlasnik, I. M. Inorg. Chem, 1980, 19, 3404.

6. Boudreaux, E. A.; Mulay, L. N., Eds Theory and Applications of Molecular
Paramagnetism. John Wiley & Sons: New-York, 1976.

7. Parr, R. G.; Yang, W. Density-functional theory of atoms and molecules Oxford
University Press, Oxford, 1989.

8. (a) Becke, A. D J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, W.; Parr, R. G.
Phys. Rev. 3., 1998, 37, 785.

9. TEXSAN-TEXRAY Structure Analysis Package; Molecular Structure Corporation:
The Woodlands, TX 1985.

10. SAINT, Program for area detector absorption correction, Siemens Analytical X-Ray
Instruments Inc., Madison WI 53719, USA 1994-1996.

11. SHELXL-97 - g.m. Sheldrick, SHELXL — 97, Program for refining crystal structures,
University of Gottingen, 1997.

223

 

 

IA AAAAA’AIA’J" 'll
I'll! 1‘

 

 

12. Cotton, F. A.; Walton, A. R. Multiple Bonds Between Metal Atoms. Second ed., 1993
and references therein.

13. Cotton, F. A; Mester, Z. C.; Webb, T. R.; Acta Crystallogr., 1974, B30, 2768.

14. Lindsay, A. J.; Tooze, R. P.; Motevall, M.; Hursthousean, M. B.; Wilkinson, G. J.
Chem. Soc. Commun., 1984, 1383.

15. Cotton, F. A.; Shiu, K. B.; Rev. Chim. Mine’rale, 1986, 23, I4.
16. Trogler, W. C.; Gray, H. B.; Acc. Chem. Res., 1978, II, 232.

17. Trogler, W. C.; Solomon E. 1.; Trajberg, I. B.; Ballhausen, C. J.; Gray, H. B. Inorg.
Chem, 1977, 16, 828.

18. Johnson, 8. A.; Hunt, H. R.; Neumann, H. M.; Inorg. Chem, 1963, 2, 960.
19. Robin, M. B.; Day, P. Adv. Inorg. Chem. Radiochem., 1967, 10, 247.

20. (a) Cotton, F. A.; Miskowski, V. M.; Zhong, B.; J. Am. Chem. Soc., 1989, III, 6177.
(b) Cogne, A.; Belorizky, B.; Laugier, J.; Rey, P. Inorg. Chem, 1994, 33, 3364. (c)
Lindsay, A. J.; Wilkinson, G.; Motevalli, M.; Hursthouse, M. B. J. Chem. Soc. Dalton
Trans., 1987, 2723.

224

 

 

APPENDIX I

Density Functional Calculations on LIST COMPOUNDS.

In order to gain insight into the electronic structure of the compounds
[M2(OZCCH3)2(pynp)2]2+, single point energy calculations were performed at the B3LYP
level of theory for the dication, neutral and dianion versions of all three complexes. In
the case of the Ru(II) analog, calculations were also performed on the monocation and
monoanion species. Analyses of the calculated orbital occupancies for the dications
[M02(02CCH3)2(PYNP)2]2+. [Rh2(02CCH3)2(pynp)2]2+. and [RU2(02CCH3)2(Pynp)2]2+
supports the electronic configurations of 021952, ozn4526*21t*2, and 021:4525*27t*4
respectively.

Correlation with electronic transitions

The calculated energy differences between the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) levels for each of the three
new compounds from DFT (vide infra) calculations are in good to reasonable agreement
with the aforementioned experimental data. In all cases, the lowest energy transition is
one that involves little to significant metal-to-ligand charge transfer (MLCT), which is in
accord with the higher 8 values than those reported for similar metal-metal bonded
compounds. The calculated energy difference of the LUMO and the HOMO of the
dication for complexes (30), (31), and (32), corresponds to the lowest energy transitions
found in the UV-visible spectra for these species (Figure 76). In the case of
[M02(OzCCH3)2(pynp)2]2+ (30), the lowest energy transition is a 8 —> 6* transition, with a
calculated (ELUMO'EHOMO) energy gap of 33.6 kcal/mol as compared to the experimental

value of 33.3 kcal/mol. The extinction coefficient for this transition (e = 4.75 x 102 LM'

225

 

 

Icm") is an order of magnitude greater than those typically found for a pure 5 —-> 8*
transition. The calculation indicates that the ligand character of the LUMO involved in
the first electronic transition of [M02(OZCCH3)2(pynp)2]2+ is much greater than the
ligand character of the HOMO (Table 40). From these results, it is possible to conclude
that the lowest energy transition involves a metal-to-ligand charge transfer (MLCT),
which are much more intense than pure d-d transitions.

The lowest energy transition for [Rh2(OzCCH3)2(pynp)2]2+ was calculated to be a
transition from a metal-based 7t* orbital to a ligand-based 11* orbital (Mn. -> Ln.) with a
value of 63.9 kcal/mol. This is in good agreement with the experimental value of 63.4
kcal/mol (Figure 76). The e value is 2.4 X 103 LM'l cm‘1 which is an order of magnitude
larger than the e value for the first transition of [M02(02CCH3)2(pynp)2]2+. This finding
is consistent with an increase in MLCT character for [Rh2(OzCCH3)2(pynp)2]2+ as
compared to [Mo;g(02CCH3)2(pynp)2]2+ (Table 40). Finally, the lowest energy transition
for [Ruz(OzCCH3)2(pynp)2]2+ was also calculated to be a M“. —) Ln. transition (MLCT)
with an energy of 59.3 kcal/mol. The experimental value is 42.6 kcal/mol, which is in
reasonable agreement with the calculations (Figure 76). The MLCT character for Ruzz“ is
calculated to be slightly smaller than for Rh22+ which is consistent with the slightly
smaller extinction coefficient for Ru;2+ (e = 3.1 X 104 LM" cm") compared to Rh;2+ (e =
3.1 X 104 LM'l cm'l). An overestimation of the excitation energy is not entirely
unexpected for this type of estimation, as the correlation energy at the excited electron is
not used, but it is somewhat surprising given the close agreement of the calculated and
experimental results for the other two compounds. Obviously, solvation of

[Ruz(OZCCH3)2(pynp)2]2+ could lead to subtle geometry changes, which affects the

226

 

 

validity of comparing calculations performed on parameters taken from the solid-state
structure with electronic spectra] measurements measured in solution. One other point
that should be emphasized is that the dication of [Ruz(OZCCH3)2(pynp)2]2+ is an
unrestricted open-shell calculation with two unpaired electrons (S = l with the alpha and
beta orbitals optimized independently), whereas [Moz(OzCCH3)2(pynp)2]2+ and
[Rh2(OzCCH3)2(pynp)2]2+ were both closed shell calculations (3 = 0).

Correlation with Electrochemistry.

DFT single point energy calculations on the X-ray crystal structure of the
dications were performed for three different oxidation states of the Mo and Rh
compounds. namely [MostozccmlztpynphlW“. [RhstosCCHslztpynplzl"*m°’ and
five different oxidation states of the Ru(H) compound [Ru2(02CCH3)2(pynp)2](2m”0"'0‘).
These calculations were analyzed and compared to the electrochemical potentials for the
reduction of the three parent compounds [M2(OZCCH3)2(pynp)2]2+. An open-shell
calculation was unavoidable for [Ruz(OzCCH3)2(pynp)2]2+ (31), because the ground state
was determined experimentally to be a triplet (S = 1), therefore, 1e' and 2e' reductions
were calculated. A single point energy calculation was performed on both the singlet (S
= 0) and triplet (S = l) for (32), and the triplet was found to be 35.4 kcal/mol lower in
energy, in agreement with the magnetic measurements. For compounds
lM02(02CCH3)2(P>'H1))2]2+ (30) and [Rh2(02CCH3)2(Pynphl2+ (32). 29' reductions were
simulated instead of le' reductions to decrease the difficulty of the computations. The 2e"
reductions produce compounds with all electrons spin-paired (S = 0), while le’ reductions
would produce open-shell (S = 1/2) species for (30) and (32), which are harder to

compute than the closed-shell species (8:0).

227

To correlate the calculated energetics with the observed electrochemistry the
change in the AE(LUMO — HOMO) (energy difference between the HOMO and LUMO)
of the unreduced species and the AE(HOMO — HOMO-1) of the reduced species was
originally calculated with the expectation that the larger the calculated change the larger
the splitting in the le' reduction potentials. It was found that the absolute change in
orbital energy differences did not correlate due to the lack of solvation of the charge in
the gas phase calculations. In an attempt to compensate for the effect that the change in
charge has on the orbital energetics, the difference in the change in the frontier orbital
energies was normalized to the average value of the change in the respective orbital
energies. For example, the normalized change in orbital energy (NCOE) for the 2e"
reduction of [Moz(OzCCH3)2(pynp)2]2+ is shown in equation 15.

(E hbmo - EIZIJMO )" (EPOMO-l - E00140)
l

2((E1210M0 _ E80140 )+ (Echomo-l - E(PIOMO »

(Eq. 15)

According to this approach, the calculated NCOE values should be proportional to
the splitting of the le' reduction potentials in the cyclic voltammetry for this series of
compounds. The relevant orbital energies, differences and NCOE values for compound
(31) are listed in Table 41. As mentioned previously, [Ru2(OZCCH3)2(pynp)2]2+ (31) is
paramagnetic with two unpaired electrons (8:1), unlike the [M044] (30) and [Rh‘“] (32)
derivatives. Since the ground state of the [Ru“] complex is an open-shell calculation, the
four 1e' reductions in this series were calculated, as apposed to only calculating two 2e'
reductions. The NCOE for the le' reduction of the dication of (31) was calculated to be
0.45, which indicates a fairly large splitting of the 1e“ process (0e'/ 1e" to 1e'/2e' reduction

potentials). The NCOE for the le' reduction of the neutral species was calculated to be

228

 

 

0.66, indicating a larger splitting of the second le' process (2e'/3e' to 3e'/4e' reduction
potentials). The cyclic voltammogram for (31) exhibits four 1e' reductions with
separations of AB“; = 390mV (le'l2e') and AB“; = 420mV (3e'/4e'), which is consistent
with the calculated larger splitting of the second Ie' process.

To determine if the modeling of 2e' reductions would produce the same trend found
when modeling the le' reductions, the NCOE values for the 2e' reduction of the dication
and neutral species of (31) were calculated. The NCOE value for the 2e’ reduction of the
dication was calculated to be 0.70, and the NCOE value for the 2e' reduction of the
neutral species was calculated to be 0.88, which also indicates that the splitting of the
second 1e' process should be larger than the first. Therefore, modeling the le’ process
with a 2e' reduction should also give a qualitative description of the splitting of the two
1e' reduction potentials.

The orbital energies, differences and NCOE for compounds (30) and (32) are
listed in Table 42. For the dinuclear complex (30), the NCOE for the first 2e' reduction is
0.59, and for the second 2e' reduction, the NCOE is 0.73. The cyclic voltammogram for
[M02(OZCCH3)2(pynp)2][BF4]2 also exhibits four le' reductions with separations of AEm
= 240mV (1e'/2e') and ARI/2 = 320mV (3e'/4e'). The NCOE for the second 2e' reduction
is larger than for the first 2e' reduction, in agreement with the experimental results, which
has a larger splitting of the second le' process. The calculated NCOE values for both of
the le' processes of (30) are smaller than the corresponding NCOE calculated using the
2e' reduction method for (31), a result that is also in agreement with the smaller splitting

found in the cyclic voltammetry of (30).

229

 

 

For the complex [Rh2(OzCCH3)2(pynp)2]2+ (32), the NCOE for the first 2e'
reduction is 0.16, and for the second 2e' reduction is 1.77. As previously mentioned, the
cyclic voltammogram for the [Rh2(OzCCH3)2(pynp)2]2+ complex (32) is very distinct
from its [M02]4+ (30) and [Ruz]4+ (31) counterparts. The calculated NCOE value for the
2e' reduction of [Rh2(OzCCH3)2(pynp)2][BF4]2 (32) is much smaller than the NCOE
values calculated for (30) or (32); which is consistent with the fact that the first two le’
reductions occur at nearly the same potential; this is entirely consistent with the
electrochemical data. The NCOE value of 1.77 calculated for the second le' process (3e'
/4e') for (32) indicates that the splitting of the le' reduction potentials should be
extremely large (Table 42). A differential pulse voltammetry (DPV) experiment
performed on [Rh2(OzCCH3)2(pynp)2][BF4]2 revealed that the first reversible process is a
2e' reduction, whereas the second quasi-reversible couple is only a 1e' reduction (Figure
73) in agreement with the calculated NCOE value. The orbital occupation for the neutral
species of the [Rh2]4+ dinuclear complex (32) is ozn4526*21t*4L(n'2) and the LUMO is an
anti-bonding interaction of the metal-5* with a ligand-11* orbital. Based on the fact that
the electrochemical processes for this complex is quasi-reversible and that the second
reduction is only a le' reduction, it appears that the neutral species Rh2(OzCCH3)2(pynp)2
begins to decompose upon occupation of the anti-bonding LUMO. Overall, the calculated
NCOE values, as calculated by the 2e’ reduction method, for complexes, (30), (31), and
(32), are in qualitative agreement with the experimentally determined splitting of the le’

reduction potentials, shown in the cyclic voltmmograms.

230

Table 40. Ratio of Metal to Ligand (MIL) and Ligand to Metal (UM) character of the
highest occupied molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) as calculated at the B3LYP level of theory for the
dication [M2(02CCH3)2(pynp)2]2+ (M = Mo“, Ru2+ and ha“) of (30), (31),

and (32).
HOMO LUMO HOMO LUMO
MIL M/L IJM L/M
Mo(+2) 1.63 0.36 0.61 2.74
Ru(+2)al 1.53 0.01 0.65 9.87
Rh(+2) 1.33 0.09 0.75 11.5

a HOMO and LUMO for the alpha molecular orbitals.

231

Table 41. Orbital Energies (hartrees) and Ratios for Important Molecular Orbitals in
singlet (S = 0) dinuclear complex Mo(II) (30) and dinuclear complex Rh(II) (32),
dication, neutral and dianion species.

 

 

c1 c2 c3 Alaa AB” NCOE NCOE
M2(+2) HOMO LUMO LUMO+1 AE. AE3
M2(O) HOMO-1 HOMO LUMO AE3 AB, A.;
M2(-2) HOMO-2 HOMO-1 HOMO A85 AE6 A4,,
M02(+2) 0.3745 0.3193 0.3042 0.055 0.015
M02(O) 0.1447 0.1 148 -0.0807 0.030 0.034 0.59
Moz(-2) 0.0713 0.1030 0.1189 0.032 0.016 0.73
Rh2(+2) 0.4109 0.3090 0.3062 0.103 0.003
Rh2(O) 0.1870 0.1003 0.0791 0.087 0.021 0.16
Rh2(-2) 0.0435 0.1210 0.1223 0.078 0.001 1.77

a ABl1 = (orbital energy in column C2 - orbital energy in column C1)
b AB", = (orbital energy in column C3 — orbital energy in column C2)

AB, —AEy

 

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EAEx+AEy

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