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DESIGN AND SYNTHESIS OF IRON(I|) TERPYRIDYL
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Lindsey Louise Jamula

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DESIGN AND SYNTHESIS OF IRON(II) TERPYRIDYL COMPLEXES FOR
APPLICATION IN DYE-SENSITIZED SOLAR CELLS

By

Lindsey Louise J amula

A THESIS

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

MASTER OF SCIENCE
Chemistry

2010

ABSTRACT

DESIGN AND SYNTHESIS OF IRON(II) TERPYRIDYL COMPLEXES FOR
APPLICATION IN DYE-SENSITIZED SOLAR CELLS

By
Lindsey Louise Jamula

The threat of global climate change is highly concerning as societies continue to
rely heavily on fossil fuels to fulfill the world’s energy needs. It is pertinent to look to
alternative energy sources to meet the increasing global energy requirements. Solar
energy may be a viable option to meet electricity demands and is an area of research
worth pursuing. Titanium dioxide-based dye-sensitized solar cells (DSSCs) provide a
promising alternative to high cost silicon-based solar cells. In order to achieve
cost/efficiency ratios in DSSCs necessary for them to compete with fossil firels we set out
to explore the use of first row transition metal-based chromophores.

A series of iron(II) terpyridyl complexes have been designed to gain a better
understanding of the ultrafast dynamics of potential chromophores in DSSCs. Iron(II)
polypyridyl complexes inherently have short-lived charge transfer excited states. New
terpyridyl ligands have been prepared, in which steric hinderance has been introduced to
impose strain along a torsional coordinate in the iron(II) adducts for the analysis of a
proposed relaxation pathway of the charge-transfer excited state. Details on the
development of the synthesis of these compounds are presented along with full
characterization of the ligands and iron(II) adducts. The fundamental understanding that
we hope to gain from the forthcoming analysis of these complexes is pertinent toward the

development of iron(II) based sensitizers in DSSCs.

Copyfightby
LINDSEY LOUISE JAMULA

2010

To Mom and Dad
for all your love and support

iv

ACKNOWLEDGEMENTS

First of all I would like to thank my advisor Jim McCusker for all his guidance
and support over the past few years. His enthusiasm for science is contagious and it is a
pleasure to be a part of his research group. I would like to thank the McCusker group for
sharing their knowledge and for their support during group meetings and seminar
preparations. They are not only a great group to work with; they have become my good
friends. A few of my group members deserve special thanks. I am grateful to Allison for
collecting photophysical data on all of my molecules; without her expertise my project
would go nowhere. I want to thank Dong and Kate for sharing their synthetic knowledge,
and Rick and Drew for all of their help with calculations.

I want to thank my entire family for their support, especially my parents who have
given me the confidence and drive to achieve anything I set out to do. Mom, you are the
best mother and fiiend a person could hope to have. In all the great qualities you possess,
you have shown by example how to be the person I would like to be. Dad, you are the
best father and hardest working person I have ever known. I get my drive and my
aptitude for science and math from you, and I would not have made it this far without
you. Jessica, my other half, I always looked up to you while we were growing up and
thank you for always being there for me. Your support and friendship means so much to
me; I don’t know who I’d be without you in my life. Rob, you are the most charismatic
person I have ever known. We always have so much fun and I appreciate you, even with
your relentless need to pick on me. Joshua, you have grown into such a gentleman and I

appreciate you in my life. I would also like to thank my grandparents. Granny, you are

an amazing woman and I strive to be just like you. Grandma and Grandpa, your
encouragement and support are greatly appreciated and I hope I make you proud.

I want to acknowledge Brenda, my role model and my fiiend, and Jeanette, both
for showing me that hard work and effort pays off. I want to acknowledge Greg, my best
friend for so many years, thank you for everything. To the rest of the Mason family,
thank you all for being there for me. I am also grateful to my friends: Jenny, Leigha,
Michael, Kristen, Jen, Lisa, Rick, Drew, Alli, Kate, Joel, Scott, and Troy for making all
these years in school bearable. Without all the fun times I would not have made it this
far.

Last, but certainly not least, I want to thank Mike for renewing my confidence and
showing me that I deserve the best in life. I admire your ambition and you have had a
positive influence on mine. Thank you for everything you do for me and for what you

bring to my life. You are a wonderful man and I am so grateful to have met you.

vi

TABLE OF CONTENTS

List of Tables ................................................................................................................... viii
List of Figures .................................................................................................................... ix
List of Schemes ................................................................................................................. xii
Chapter 1. Introduction to the Design of Iron(II) Terpyridyl Complexes .......................... 1
1.1 Introduction ............................................................................................... 1

1.2 Relaxation Dynamics of Iron(II) Complexes ............................................ 3

1.3 Synthesis of Terpyridine ........................................................................... 8

1.4 Contents of Thesis ................................................................................... 12
REFERENCES .................................................................................................................. 13
Chapter 2. Design and Synthesis of Alkyl-Substituted Terpyridine Ligands ................... 15
2.1 Introduction ............................................................................................. 15

2.2 Experimental ........................................................................................... 18

2.2.1 Synthesis ........................................................................................ 18

2.3 Results and Discussion ........................................................................... 23

2.3.1 Synthesis of Terpyridine Ligands .................................................. 23

2.3.2 Synthesis of Bis(Terpyridine)iron(II) Complexes ......................... 30

2.4 Concluding Comments ............................................................................ 34

APPENDIX ........................................................................................................................ 35
REFERENCES .................................................................................................................. 49
Chapter 3. Design and Synthesis of Aryl-Substituted Terpyridine Ligands ..................... 51
3.1 Introduction ............................................................................................. 51

3.2 Experimental ........................................................................................... 53

3.2.1 Synthesis ........................................................................................ 53

3.3 Results and Discussion ........................................................................... 62

3.3.1 Synthesis of Terpyridine Ligands .................................................. 62

3.3.2 Synthesis of Bis(Terpyridine)iron(II) Complexes ......................... 70

3.4 Concluding Comments ............................................................................ 73

APPENDIX ........................................................................................................................ 74
REFERENCES .................................................................................................................. 90
Chapter 4. Concluding Comments and Future Directions ................................................ 92
4.1 Concluding Comments ............................................................................ 92

4.2 Future Directions .................................................................................... 93
REFERENCES .................................................................................................................. 96

vii

Table 2-1.

Table 3-1.

LIST OF TABLES

Oxidation and First Reduction Potentials (mV) of Fe(II) complexes.
Measurements were taken by DPV vs Ag/AgNO3, 0.1 M TBAPF6 in

acetonitrile, then corrected to ferrocene/ferrocenium ................................ 33

Oxidation and First Reduction Potentials (mV) of Fe(II) complexes.
Measurements were taken by DPV vs Ag/AgNO3, 0.1 M TBAPF6 in

acetonitrile, then corrected to ferrocene/ferrocenium ................................ 73

viii

Figure 1-1.

Figure 1-2.

Figure 1-3.
Figure 1-4.

Figure 1-5.

Figure 2-1.

Figure 2-2.

Figure 2-3.

Figure 2-4.

Figure 2-5.
Figure 2-6.
Figure 2-7.
Figure 2-8.

Figure 2-9.

Figure 2-10.

LIST OF FIGURES

Diagram of the Gréitzel cell. Green arrows represent forward processes
that allow the cell to function regeneratively. The red dashed arrows
represent processes that hinder cell function .............................................. 2

Structures of [Ru(dcbpy)2(NCS)2]2+ (N3) and [Ru(tcterpy)2(NCS)3]2+
(black dye) .................................................................................................. 4

Representation of the energetics of sensitizer excited states and TiOz ...... 5

Trigonal faces of an octahedron [adapted from ref. 10] ............................. 7

Structures of 2,2’:6’,2”-Terpyridine and 2,2’:6’,2”-Terpyridine bound to
iron (the second planar terpyridine ligand bound to iron was removed for
clarity) ......................................................................................................... 7

Structure of [Fe(5,5”-R-terpy)2](PF6)2 where R=t-butyl (1), adamantyl
(2) .............................................................................................................. l6

Optimized Structures of [Fe(5,5”-R—terpy)2](PF6)2 where R = H, t-butyl

(l), adamantyl (2) (left to right). (Top) View looking down the C2 axis.
(Bottom) Side view ................................................................................... 17

Electronic absorption spectra of [Fe(terpy)2]2+(—), [Fe(tbuterpy)2]2+(l)

(---), and [Fe(adterpy)2]2+ (2) (- -) in acetonitrile .................................... 32
1H NMR spectrum of tbuterpy ................................................................. 35
13C NMR spectrum of tbuterpy ............................................................... 36
Direct probe mass spectrum of tbuterpy ................................................... 37
IR spectrum of tbuterpy ............................................................................ 38
1H NMR spectrum of [Fe(tbuterpy)2](PF6)2 ........................................... 39
1331 mass Spectrum of [F6(tbuteFPY)2l(PF6)2 ........................................... 40
IR Spectrum of [Fe(tbuterpy)2](PF6)2 ...................................................... 41

ix

Figure 2-11.

Figure 2-12.
Figure 2-13.
Figure 2-14.
Figure 2-15.
Figure 2-16.
Figure 2-17.

Figure 3-1.

Figure 3-2.

Figure 3-3.

Figure 3-4.
Figure 3-5.
Figure 3-6.
Figure 3-7.
Figure 3-8.
Figure 3-9.
Figure 3-10.

Figure 3-1 1.

1H NMR spectrum of adterpy. Water is present to enhance solubility and

enable resolution of the splitting pattern ................................................... 42
13 C NMR spectrum of adterpy ................................................................. 43
Direct probe mass spectrum of adterpy .................................................... 44
IR spectrum of adterpy .............................................................................. 45
1H NMR spectmm of [Fe(adterpy)2](PF6)2 ............................................ 46
E31 mass Spectrum of [Fe(a<1t¢rpy)2l(1’F 6)2 ............................................ 47
IR Spectrum of [Fe(adterpy)2](PF6)2 ....................................................... 48

Structure of [Fe(5,5”-R-terpy)2](PF6)2 where R= manisyl (3), panisyl
(4) .............................................................................................................. 52

Optimized Structures of [F e(5,5”-R-terpy)2](PF6)2 where R = adamantyl

(2), manisyl (3), panisyl (4) (left to right). (Top) View looking down the
C2 axis. (Bottom) Side view ..................................................................... 53

Electronic absorption spectra of [Fe(terpy)2]2+(—), [Fe(maniterpy)2]2+(3)

(- -) ,and [Fe(paniterpy)2]2+ (4) (...) in acetonitrile ............................. 72
1H NMR spectrum of 2-Bromo-5-panisylpyridine ................................... 74
13 C NMR spectrum of 2-Bromo-5-panisylpyridine ................................. 75
Direct Probe mass spectrum of 2-Bromo-5-panisylpyridine .................... 76
IR spectrum of 2-Bromo-5-panisylpyridine .............................................. 77
1H NMR spectrum of maniterpy .............................................................. 78
IR spectrum of maniterpy ......................................................................... 79
1HNMR spectrum oftFe<maniterpy>21<PFs>2 ........................................ 80
1351 maSSS Spectrum of [Fe(maniterp>')2](PF6)2 ...................................... 81

Figure 3-12.

Figure 3-13.

Figure 3—14.
Figure 3-15.
Figure 3-16.
Figure 3-17.
Figure 3-18.

Figure 3-19.

IR Spectrum of [Fe(manitcrpy)2](PF6)2 ................................................... 82

1H NMR spectrum of paniterpy ............................................................... 83
13 C NMR spectrum of paniterpy ........................ I ...................................... 84
Direct probe mass spectrum of paniterpy ................................................. 85
IR spectrum of paniterpy .......................................................................... 86
1H NMR spectrum of [Fedaxfiterpynrrraz ......................................... 87
E81 mass Spectrum of [Fe(paniterpy)2](PF6)2 ......................................... 88
IR Spectrum of [Fe(PaniteIpy)2](PF6)2 .................................................... 89

Images in this thesis are presented in color.

xi

Scheme l-l.

Scheme 1-2.

Scheme 2-1.
Scheme 2-2.
Scheme 2-3.
Scheme 2-4.
Scheme 2-5.
Scheme 2-6.

° Scheme 2-7.

Scheme 3-1.

Scheme 3-2.

Scheme 3-3.
Scheme 3-4.
Scheme 3-5.
Scheme 3-6.
Scheme 3-7.

Scheme 3-8.

Scheme 4-1.

LIST OF SCHEMES

Krtihnke terpyridine synthesis via central ring cyclization reactions from
a) acylmethylpyridinium salts and a,B-unsaturated ketones and b) 1,5-
diketones ................................................................................................... 10

Terpyridine synthesis via central ring cyclization reactions using a) the
Potts methodology and b) the Jameson methodology ............................... 11

Terpyridine synthesis through Stille coupling .......................................... 24
Terpyridine synthesis via the Sasaki method of the Krohnke reaction ..... 25
Synthesis of 2-tert-butylpropenal ............................................................. 26

Alternate synthesis of 2-tert-butylpropenal as described by Breit et al.... 27

Proposed mechanism of organocatalytic a-methylenation reaction ......... 28
Synthesis of 2-(1 -adarnantyl)acrylaldehyde ............................................. 29
Synthesis of [Fe(5,5”-R-terpy)2](PF6)2 where R=t-butyl (1), adamantyl
(2) .............................................................................................................. 31
Potts methodology of Krohnke central ring cyclization for the preparation
of 5,5”-bis(4-methoxyphenyl)-2,2’ :6’,2”-terpyridine ............................... 63
Synthesis of starting materials for the preparation of 5 ,5”-bis(4-
methoxypheny1)-2,2’:6’,2”-terpyridine ..................................................... 63

Synthesis of 5,5”-diaryl-terpyridines via an aza-Diels-Alder reaction ..... 64

Synthesis of 4-bromo-3,5-dimethy1anisole ............................................... 66
Synthesis of maniterpy via Negishi coupling procedures ......................... 67
Synthesis of 3,5-diisopropylphenol .......................................................... 68
Synthesis of 1-bromo-2,6-diisopropyl-4-methoxybenzene ...................... 69
Synthesis of [Fe(5,5”-R-terpy)2](PF6)2 where R=mani (3), pani (4) ...... 70

Preparation of [bpy.bpy.bpy] according to the procedure described by
Lehn and coworkers .................................................................................. 93

xii

Scheme 4-2. Synthetic route for the preparation of a meta-linked bipyridine cage
strirrrttrrr: ..................................................................................................... 5941

xiii

Chapter 1. Introduction to the Design of Iron(II) Terpyridyl Complexes

1.1 Introduction

To date, society has relied heavily on fossil fuels to fulfill the world’s energy
needs. With global energy consumption on the rise along with increasing concerns of
global climate change, it is necessary to turn to renewable, carbon-neutral sources. The

sun is the most scalable among the available sources, providing enough energy in one day

l

to supply the world’s energy needs for one year. Silicon based solar cells are an

efficient device to capture and convert solar energy and have dominated the market.2
The cost of the production of such devices limits the ability for them to compete
effectively with fossil fuels.

A breakthrough was achieved in 1991 with the introduction of the nanocrystalline
Ti02 dye-sensitized solar cell (DSSC) or Griitzel cell.3 The Gratzel cell utilized wide
band semiconductor nanoparticles sensitized a by metal based chromophore adsorbed to
the surface. The Ti02 based DSSC exhibited a 7% light harvesting efficiency. Dye-
sensitization was not a new concept; the significant advance was the use of nanoparticles
of TiOz which greatly increased the surface area of the semiconductor. The efficiency of
DSSCs has been improved to just over 11% since the initial discovery, but does not
compare to the efficiencies of silicon based solar cells. The practicality of these devices
lies in the possibility to utilize inexpensive materials for some components of the cell.

The Gratzel cell is depicted in Figure 1-1. The chromophore is chemically bound

to the Ti02 semiconductor which is coated on a conductive glass support. Upon

absorption of a photon the sensitizer is promoted into an excited state that can inject an

 

 

 

 

 

 

 

 

CB
Conductin
lass g ~ (‘— * + \Counter
g —— Tr- S /S electrode
\‘ ‘ \ ‘ I
\ ‘J
. ‘\ I‘ t \
\ I ‘\
\ l S e'
X : A_IL_‘ "— u
\
—- SO/S+
VB
v TiOz
4 LOAD

 

 

 

 

 

 

 

 

 

V

Figure 1-1. Diagram of the Gratzel cell. Green arrows represent forward processes that allow the cell to
function regeneratively. The red dashed arrows represent processes that hinder cell function.

electron into the conduction band of TiOz causing the charge separation, then the

oxidized sensitizer is regenerated by the redox couple. Ideally, the electron will percolate

through the nanocrystalline Ti02 to the back contact and flow through the cell to the

external load. Residual current will flow to the counter electrode where it can reduce the
oxidized sensitizer. Unwanted processes occur in the cell as well. Relaxation of the
sensitizer may occur prior to injecting an electron. Upon injection, charge recombination
is possible reducing either the oxidized sensitizer or the redox couple. There is a
multitude of research on the various components of the cell to improve overall efficiency.

The focus of a significant amount of the research on DSSCs has been on the
sensitizer. The sensitizer must be able to absorb solar radiation strongly in the visible or

near—IR region and must be able to bind strongly to the TiOz. The dye must have charge

separation capability: a high energy excited state above the conduction band of TiOz to

achieve injection and must have a low energy HOMO (highest occupied molecular

orbital) in order to be regenerated by the redox couple. The metal-to-ligand charge

transfer (MLCT) excited states of cl6 coordination compounds have been found to be

efficient for solar harvesting and sensitization of wide-bandgap semiconductors due to
their charge separation ability.4 To date the most efficient Ti02 DSSCs utilize

ruthenium(II) polypyridyl chromophores. In the ruthenium based cells, the dye accounts
for a large portion of the cost of production of the cell. In order to produce low cost

DSSCs, it is of interest to look to the first row transition metals. In 1998, Ferrere and

Gregg reported the observation of a photocurrent from an iron(II) based chromophore.5

This promising result led to the focus of our research to prepare an iron(II) based

chromophore.

1.2 Relaxation Dynamics of Iron(II) Complexes

To design an iron(II) based chromophore for a DSSC we must first look to the
extensively studied ruthenium based chromophores. The “N3” and “black dye”
complexes shown in Figure 1-1 contain polypyridyl ligands; specifically carboxylic acid
functionalized 2,2’-bipyridine and 2,2’:6’2”—terpyridine. These complexes illustrate how
charge separation in the dye plays a role in the kinetics of electron transfer. The
polypyridyl ligands have low lying 1t* molecular orbitals capable of accepting an electron
upon excitation of the complex. The carboxylic acid group can form ester linkages to the

Ti02 resulting in strongly bound dyes with good electronic communication between the

dye and semiconductor.

 

 

N3 Black dye

Figure 1-2. Structures of [Ru(dcbpy)2(NCS)2]2+ (N 3) and [Ru(tcterpy)2(NCS)3]2+ (black dye).

We know from ligand field theory that the d orbitals in a transition metal ion are
degenerate. In the presence of an octahedral field, the d orbitals split into non-bonding

(tZg) and antibonding (eg*) sets of orbitals. In second and third row transition metal

complexes this octahedral splitting is large resulting in high energy antibonding orbitals
regardless of the identity of the ligands. Charge transfer states are lower in energy than
the ligand field states making ruthenimn(II) chromophores an ideal choice for DSSCs. In
first row transition metal complexes the octahedral splitting is not as great resulting in
lower energy ligand field states that are more accessible, therefore the charge separation

dynamics are much different as shown in Figure 1-3.

 

 

 

 

 

 

 

LF CB
CT fir - ->» 4- - - CT
’ LF
—— Ti02 ——
Ru(II)/(III) VB Fe(II)/(III)

Figure 1-3. Representation of the energetics of sensitizer excited states and TiOz.

In order to achieve a photocurrent in a DSSC, an excited state of a chromophore
must lie above the conduction band of TiOz and must be sufficiently long lived for an
electron to be injected into the semiconductor before relaxing to a lower lying state. In
these ruthenium based chromophores, absorption of a photon results in a MLCT excited
state; an electron is promoted from a metal based d orbital into a 1t* orbital of the ligand.
The MLCT states are the lowest lying excited states, they lie above the conduction band
of TiOZ, and are sufficiently long lived (ns) for injection to occur. In iron(II) complexes,
photoexcitation results in promotion of an electron into an MLCT state followed by

ultrafast relaxation into lower lying ligand field states and it is questionable as to whether

efiicient injection can occur.6

The ligand field states in iron(II) complexes have unfavorable electronic overlap

with Ti02 therefore “hot injection” or injection from an upper lying excited state prior to

any relaxation would be required. Lian and coworkers observed evidence for electron

injection from vibrationally hot excited states in ruthenium, iron, and rhenium

7

complexes. The focus of our research is to study the relaxation from the hot excited

state in iron(II) complexes with the overall goal of slowing down relaxation and
promoting injection.

In low spin iron(II) complexes there are six electrons in the t2g nonbonding
orbitals populating the 1A1 ground state. In the 5T2 high spin state, there are only four
electrons in the t2g nonbonding orbitals and two of the electrons are in the eg“ 0-
antibonding orbitals. This results in an increase in metal — ligand bond length when a
complex converts from the 1A1 to the 5T2 state. This observation leads to the possibility

that spin state interconversion is coupled to the breathing mode of the complex. The

hypothesis of our current study originates from the work by Purcell and Vanquickenbome
who established a connection between enantiomerization of d6 complexes and spin state
interconversions’ 9 McCusker et al. proposed that torsional modes play a central role in
the kinetics of spin state interconversion between the 5T2 and 1A1 states and indeed
found evidence that suggests there is a correlation.10 In attempts to determine the
deactivating coordinate of the MLCT excited state, we are investigating the extent to
which this torsional coordinate might modulate MLCT to ligand field kinetics. The

torsional twisting mode is shown in Figure 1-4 and can be described as the motion two

trigonal faces of an octahedron with respect to each other.

 

b-- ----

 

 

 

Figure 1-4. Trigonal faces of an octahedron [adapted from ref. 10].

A series of iron(II) terpyridyl complexes has been prepared in order to study the
relaxation dynamics of potential sensitizers. To investigate the role that torsional modes
play in relaxation dynamics we set out to introduce steric bulk to terpyridine ligands to
explore steric effects on the charge transfer to ligand field state conversion. Iron(II)
terpyridyl complexes are an ideal choice to investigate steric effects on rotational

freedom about the C2 axis. Terpyridine is a tridentate ligand and readily coordinates to

iron(II) in a 2:1 stoichiometry. The terpyridine ligand is planar over the three ring system

and binds meridinally, remaining planar when bound to the iron(II) center (Figure 1-5).

 

 

Figure 1-5. Structures of 2,2’:6’,2”-Terpyridine and 2,2’:6’,2”-Terpyridine bound to iron (the second
planar terpyridine ligand bound to iron was removed for clarity).

To impose steric hindrance on rotational freedom we chose to substitute the

terminal rings of the terpyridine in the meta- or 5,5”-positions. All iron(II) terpyridyl

complexes substituted in the 4,4” or 5,5” positions have a low spin ground state.‘1 We
wanted to avoid substitution at the 6,6”-positions because this may obstruct the ability of
the terpyridine to bind to the iron(II) center or if binding does occur, the complex will
likely exist in the high spin ground state.

Ensuring a low spin ground state is not just relevant to the design for current
study but for future applications of these molecules to be used in DSSCs as well. An
iron(II) sensitizer with a low spin ground state will have better metal-ligand orbital
overlap than a high spin complex. This is beneficial in that it provides good electronic
communication between the metal and ligand: increasing the intensity of the metal to
ligand charge transfer (MLCT) transition upon photoexcitation. Also, iron(II) low spin
complexes are quite stable while high spin complexes are more easily oxidized to

iron(III) in air, which is problematic for long term stability in a DSSC.

1.3 Synthesis of Terpyridine
Polypyridines have been of interest in many areas of research. 2,2’-Bipyridine
has been known since the late 1800s and 2,2’:6’,2”-terpyridine was first isolated by

Morgan and Burstall in the early 19305 as a by-product of a reaction to produce 2,2’-

bipyridine.12 The reaction of iron(III) chloride and pyridine under high pressure (50 atm)
and high temperature (340°C) in a steel autoclave produced more than 20 different
compounds. After multiple distillations, extractions, and recrystallizations pure
2,2’:6’,2”-terpyridine was obtained. As a method of characterization a bis(2,2’:6’,2”-

terpyridyl)iron(II) bromide complex was prepared; showing that from the onset of the

discovery of the molecule, it was already shown to be a strongly coordinating tridentate
ligand with iron(II). The chemistry of terpyridine went undeveloped for many years
following this first discovery and research on the molecule was sparse. It wasn’t until the

development of an efficient synthetic route by Krohnke in the 19705 that the synthesis of

terpyridine attained a great deal of attention. ‘3

Pyridine is a potential building block for substituted terpyridines, but the nature of
pyridine makes it difficult to functionalize. Pyridine is an unreactive aromatic imine due
to the electronegative nitrogen in the ring lowering the energy of all of the orbitals. The
low energy of the orbitals in the 1: system makes electrophilic attack on the ring difficult.
Another problem is that the nitrogen lone pair is basic and is a reasonably good
nucleophile, making it susceptible to protonation in the acidic reaction conditions
commonly used for electrophilic aromatic substitution. On the other hand, the low
energy (lowest unoccupied molecular orbital (LUMO) in the 1t system of pyridine makes
it reactive toward nucleophilic substitution, particularly in the ortho- and para— positions.
We set out to prepare meta-substituted terpyridines so alternative methods were sought.

Among the various procedures, the two most common routes to prepare

substituted terpyridine are central ring cyclization reactions and coupling reactions.14

For decades various ring cyclization reactions have been developed and are prevalent in
the literature. More recently palladium catalyzed cross-coupling procedures have been
employed. 4’-substituted terpyridines are the most prevalent functionalized terpyridines
in the literature, likely due to the ease of substitution at the para position of terpyridine

and the simplicity and applicability of central ring cyclization.

In 1961, Krdhnke and Zeher reported a new and efficient method to prepare
highly substituted pyridines.15’ 16 The method was widely applicable for a range of

fimctionalities and evolved to include a variety of oligopyridines.13 This synthetic
method has become the most popular route to prepare terpyridine. The method involves
the condensation of acylmethylpyridinium salts with a,B-unsaturated ketones and an
ammonia source to give substituted pyridines (Scheme l-la). Similarly, the method may
be applied following initial construction of 1,5-diketones and subsequent ring closure
(Scheme 1-1b). 4,4”- and 6,6”-substituted terpyridines may be prepared via these routes,
but the inaccessibility of meta- substituted pyridine starting materials results in the

difficulty of these routes for the preparation of 5,5”-substituted terpys.

 

 

Scheme l-l. Krohnke terpyridine synthesis via central ring cyclization reactions from a)
acylmethylpyridinium salts and (LB-unsaturated ketones and b) 1,5-diketones.

In 1987, Potts et al. developed an a-oxoketene dithioacetal methodology to

prepare substituted terpyridines.l7 This route introduced a convenient one pot synthesis

of a 4’-methylthio-terpy which can be readily converted to 4’-methyl-terpy which is a

10

good building block to functionalize the 4’-position of terpyridine (Scheme 1-2a).

Jameson and Guise followed up on the Potts methodology utilizing an enaminone instead

of a dithioacetal (Scheme 1-2b).18 This new route resulted in improved scalability of
terpy since it avoids the harsh reductive removal of the thiomethyl group, however, both
of these procedures have the same inadequacy of applicability to 5,5”-substituted-terpys
as the Krohnke central ring cyclization resulting in the need to look at alternative routes.
1. t-BuOK,
mm; %SCH3
2. CH3l

SCH3

SCH3

O

\
1)! /N

2) NH4OAc
HOAc

 

 

1|\

 

2) NH4OAc
HOAc

 

Scheme 1-2. Terpyridine synthesis via central ring cyclization reactions using a) the Potts methodology
and b) the Jameson methodology.

11

Symmetric functionalization of the terminal rings can be obtained through
terminal ring cyclization and/or coupling reactions. The terpyridine ligands presented
herein were prepared via terminal ring cyclization and Negishi coupling. The design and

synthesis of these molecules will be discussed.

1.4 Contents of Thesis

The focus of the research described in this thesis involves the design, synthesis,
and characterization of polypyridyl ligands and the corresponding iron(II) adducts for the
analysis of ultrafast relaxation dynamics. Chapter 2 examines the synthetic possibilities
for the preparation of terpyridines functionalized with bulky aliphatic groups, with the
key focus on a terminal ring cyclization reaction. Two previously unknown terpyridine
ligands have been prepared and the development of the synthetic route is discussed.
Included in the discussion is the preparation and characterization of the corresponding
iron(II) bis-chelate complexes.

Chapter 3 examines the synthetic possibilities for the preparation of bulky aryl-
functionalized terpyridines, with a focus on palladium catalyzed coupling reactions. A
previously known terpyridine fits the criteria of our study has been prepared along side a
new terpyridine. As in chapter 2, the development of the synthetic route and the
preparation and characterization of the corresponding iron(II) bis-chelate complexes are
discussed.

Chapter 4 introduces the current work on the project to develop iron(II)
complexes to study relaxation dynamics. As a follow up to the introduction of steric bulk
to alter relaxation, we set out to design iron(II) polypyridyl complexes with fixed cage

ligand structures. Concluding comments and future directions are also discussed.

12

10.

11.

12.

13.

14.

REFERENCES

Report of the Basic Energy Sciences Workshop on Solar Energy Utilization;
Department of Energy: Washington DC, 2005.

Goncalves, L. M.; Bermudez, V. d. 2.; Ribeiro, H. A.; Mendes, A. M., Energy
Environ. Sci. 2008, 1, 655.

O'Regan, B.; Gratzel, M., Nature 1991, 353, 737.
Ardo, S.; Meyer, G. J., Chem. Soc. Rev. 2009, 38, 115-164.
Ferrere, S.; Gregg, B. A., J. Am. Chem. Soc. 1998, 120, 843.

Juban, E. A.; Smeigh, A. L.; Monat, J. E.; McCusker, J. K., Coora'. Chem. Rev.
2006, 250, 1783.

Asbury, J. B.; Wang, Y.-Q.; Hao, E.; Ghosh, H. N.; Lian, T., Res. Chem.
Intermed. 2001, 27, 393—406.

Purcell, K. R., J. Am. Chem. Soc. 1979, 101, 5150.
Vanquickenbome, L. G.; Pierloot, K., Inorg. Chem. 1981, 20, 3673.

McCusker, J. K.; Rheingold, A. L.; Hendrickson, D. N., Inorg. Chem. 1996, 35,
2100.

Halcrow, M. A., Polyhedron 2007, 26, 3523—3576.
Morgan, G. T.; Burstall, F. H., J. Chem. Soc. 1932, 20-30.
Krohnke, F., Synthesis 1976, l.

Schubert, U. S.; Hofineire, H.; Newkome, G. R., Modern T erpyridine Chemistry.
Wiley-VCH: 2006.

13

15.

16.

17.

18.

Zecher, W.; Krdhnke, F., Chem. Ber. 1961, 94, 690.

Zecher, W.; Krdhnke, F., Chem. Ber. 1961, 94, 698.

Potts, K. T.; Usifer, D. A.; Guadalupe, A.; Abruna, H. D., J. Am. Chem. Soc.
1987, 109, 3961.

Jameson, D. L.; Guise, L. E., Tetrahedron Lett. 1991, 32, 1999.

14

Chapter 2. Design and Synthesis of Alkyl-Substituted T erpyridine Ligands

2.1 Introduction

In order to produce low cost dye-sensitized solar cells (DSSCs) we are looking to
the first row transition metals for cheaper materials. Since ruthenium(II) polypyridyl
chromophores constitute a large portion of the cost of the current DSSCs, we are
interested in looking at iron(II) polypyridyl chromophores. Many of the first row

transition metals are more abundant and cheaper than ruthenium, but iron shares similar

coordinative ability since they are both d6. Relaxation processes kinetically compete with
injection in iron(II)-based chromophores as opposed to their ruthenium(II) counterparts.
The first step in designing a chromophore is to gain more understanding of the
mechanism for the ultrafast decay of iron(II) MLCT states. We propose that torsional
modes are involved in the relaxation from the initial excited state to the longer-lived
ligand field excited state. In order to study the torsional modes of iron(II) terpyridyl
complexes, we set out to introduce steric bulk to the terminal rings of terpy.

2,2’:6’,2”-Terpyridine (terpy) as well as various substituted terpys are

commercially available and are widely described in the literature.1

Terpyridine can be
functionalized on the central ring, symmetrically or unsymmetrically on the terminal
rings, or on all rings. Substitution at the 4’—position is the most common with a wide
range of substituents. 4,4”- and 6,6”- substituted terpys are quite common as well since
the para and ortho positions are susceptible to functionalization through a variety of
synthetic routes. The research on 5,5”- substituted terpys is much more limited due to

inactivity of the meta position; however, the meta position is an ideal choice for

functionalization of the terminal rings of terpy to introduce steric bulk.

15

Large aliphatic substituents were chosen to impose steric constraints without
introducing any unwanted electronic effects into the system. We wanted to avoid
aromatic groups which may extend the 7: system and introduce conjugation. The 4,4”-
and 6,6”-positions are synthetically accessible but will not give the intended result.
Functionalizing the 4,4”- positions would introduce bulk on the outer edges of the
complex and would likely not hinder the torsional motion. Substituting the 6,6”-
positions would introduce sterics that would hinder binding to the iron(II) center.
Although less synthetically studied, the 5,5”- positions are ideal; they lie away from the
iron(II) center and introduce sterics along the torsional coordinate of interest. tert-Butyl
and adamantyl groups are sterically bulky, aliphatic substituents that will provide us with

the desired result (Figure 2-1).

      
 

 

 

F 1
WM _5
§\ 2

Figure 2-1. Structure of [F e(5,5”-R-terpy)2](PF6)2 where R=t-butyl (l), adamantyl (2).

Though the meta position substituted terpys are the least common, they are
accessible through a variety of routes; ring cyclization reactions and coupling reactions

are both feasible. Due to the reasonable accessibility of starting materials through

16

minimal steps, the Krdhnke terminal ring cyclization reaction is the most practical route.2
The previously unknown compounds 5,5”-di-tert-butyl-2,2’:6’,2”-terpyridine (tbuterpy)
and 5,5”-bis(l-adamantyl)—2,2’:2’,6”-terpyridine (adterpy) have been prepared as well as
the corresponding bis(terpyridyl)iron(II) complexes.

Space filling models of the optimized structures are shown in Figure 2-2.3 The
introduction of steric bulk that would hinder torsion about the C2 axis is clearly shown.
The iron(II) terpyridyl complexes have a low spin ground state and can be readily
prepared by a standard procedure of combining a 2:1 mixture of ligand with iron(II)

chloride by standard Schlenk line techniques.

 

 

fidfird‘l‘

Figure 2-2. Optimized Structures of [Fe(5,5”-R-terpy)2](PF6)2 where R = H, r-butyl (1), adamantyl (2)
(left to right). (Top) View looking down the C2 axis. (Bottom) Side view.

 

2.2 Experimental

2.2.1 Synthesis

General. All reagents were of reagent grade and used as received without further
purification unless otherwise noted. Solvents were purchased from Aldrich, Jade
Scientific, Spectrum, Mallinckrodt, EMD Chemical, or CCI and were distilled by
standard purification techniques. All air sensitive reactions were carried out under inert

atmosphere using standard Schlenk techniques utilizing thoroughly deoxygenated

solvents that were degassed by the freeze—pump—thaw method. 1H and 13 C NMR
were variously recorded with either a Varian Inova-300 or a Varian UnityPlus-300 MHz
spectrometer. Melting points were obtained on a Mel-Temp apparatus. Ground state
absorption spectra were obtained on a Varian Cary 50 spectrophotometer. IR spectra
were obtained on a Mattson Galaxy 5000 FTIR. Elemental analysis and direct probe
mass spectra were obtained through the Analytical Facilities at Michigan State
University. Electrospray mass spectra (ESI-MS) were obtained from the staff of the
MSU Mass Spectrometry Facility. Characterization data for previously unknown
terpyridine ligands and iron(II) complexes can be found in the appendix at the end of this
chapter.

2,6-Bis(pyridinioacetyl)pyridine diiodide. The pyridinium salt was prepared
from 2,6-diacetylpyridine (1.05 g, 6.41 mmol; Aldrich) according to a published

procedure.4 The pyridinium salt was obtained as a beige powder and recrystallized from

95% ethanol. Yield: 3.36 g (92%). Mp 172-174°c. 1H NMR (DMSO-d6, 300MHz): 8

9.12 (d, 2H), 8.78 (t, 2H), 8.44 (s, 3H), 8.33 (t, 4H), 6.65 (s, 4H).

18

3,3-Dimethylbutanal. The aldehyde was prepared by modification of a

previously published procedure.5 3,3-dimethylbutanol (6.0 mL, 50 mmol; Aldrich), dry
benzene (30 mL), dry DMSO (165 mL), dry pyridine (4.0 mL, 50 mmol), trifluoroacetic
acid (2.0 mL, 27 mmol), and freshly distilled dicyclohexylcarbodiimide (DCC; 33 g, 160
mmol; Alfa Aesar) were added in order to a round bottom flask and stirred for 18 hours
under nitrogen. Benzene (30 mL) was added and the solid dicyclohexylurea by-product
was filtered off. The impure aldehyde was obtained as a yellow oil by fractional
distillation from 101-106°C and used without further purification. Yield: 2.39 g (48%).
1H NMR (CDC13, 300MHz): 8 9.85 (t, 1H), 2.28 (d, 2 H), 1.10 (s, 9H).
2-tert-Butylpropenal. The acrolein was prepared from 3,3-dirnethylbutana1 (0.50
mL, 4.0 mmol) according to a literature procedure.6 The product was obtained as yellow
oil and used without purification. Yield: 0.35 g (79%). 1H NMR (CDCl3, 300MHz): 8
9.54 (s, 1H), 6.18 (s, 1H), 5.95 (s, 1H), 1.10 (s, 9H).
5,5”-Di-tert-butyl-2,2’:6’,2”-terpyridine (tbuterpy). The ligand was prepared
by adaptation of previously reported procedures.7’ 8 To a solution 2,6-bis(pyridinio-
acetyl)pyridine diiodide (0.598 g, 1.04 mmol) in formamide (6 mL), ammonitun acetate
(1.24 g, 16.1 mmol) and 2-tert—butylpropenal (0.612 g, 5.46 mmol) were added. The
reaction mixture was heated at 80°C for 1 hour. After cooling, the beige precipitate was
filtered, washed with distilled water, and dried. The product was recrystallized from
ethyl acetate as beige needles. Yield: 0.178 g (50%). Mp 174-175°C. 1H NMR(CDC13,
300MHz): 5 8.72 (dd, 2H, J= 2.4, 1.0 Hz), 8.50 (dd, 2H, J= 8.4, 1.0 Hz), 8.37 (d, 2H, J

= 7.8 Hz), 7.91 (t, 1H, J = 7.8 Hz), 7.84 (dd,'2H, J = 8.4, 2.4 Hz), 1.35 (s, 18H). 13C

19

NMR (CDC13, 300MHz): 6 155.7, 153.9, 147.0, 146.0, 138.0, 134.1, 120.8, 120.6, 33.9,

31.3. Direct probe EI MS m/z 346.3 [MH+]. IR (KBr, cm’l): 2958 s, 2906 m, 2868 m,
1590 m, 1573 m, 1553 s, 1485 m, 1447 s, 1380 s, 1361 m, 1267 m, 1122 m, 1078 w,
1023 m, 870 w, 819 s, 761 s, 711 w, 657 w, 574 w. Elemental Analysis: Calculated:
C23H27N3: C, 79.96; H, 7.88; N, 12.16. Found: C, 77.42; H, 7.15; N, 11.03.
1-Adamantaneacetaldehyde. The aldehyde was prepared from l—adamantane

ethanol (2.50 g, 13.9 mmol; Alfa Aesar) according to the published procedure.9 The
yellow oil was purified by column chromatography on silica gel with 5% ether/pentanes
resulting in clear colorless oil. Yield: 2.00 g (82%). 1H NMR (CDC13, 300MHz): 5 9.85
(t, 1H), 2.11 (d, 2 H), 1.92-2.05 (br m, 3H), 1.62-1.72 (m, 12H).
2-(1-adamantyl)acrylaldehyde. The acrolein was prepared by modification of a
published procedure.6 In a screw cap conical vial l-adamantaneacetaldehyde (0.55 mL,
3.6 mmol) was added to isopropanol (1.0 mL) followed by the addition of 37%
formaldehyde (aq) (270 uL, 3.6 mmol), propionic acid (27 11L, 0.36 mmol), and
pyrrolidine (30 pL, 0.36 mmol). The reaction mixture was loosely capped and heated at
45° C. The reaction mixture completely solidified after 1 hour so 1.0 mL of isopropanol
was added to redissolve the precipitate and heating was continued for 3 more hours.
NaHCO3 (aq) (2.0 mL) was added and the mixture was extracted with CHZCIZ (5 x 5
mL). The extract was then washed with NaCl (aq), dried over Na2804, and evaporated

to yield off-white powder. The acrolein was purified by short pass column

chromatography on silica gel with ether. Yield: 0.62 g (90%). Mp 69.5-72°C (Lit. 69-

20

71 °C)‘°. 1H NMR (CDC13, 300MHz): 8 9.52 (s, 1H), 6.18 (s, 1H), 5.88 (s, 1H), 2.00 (m,
3H), 1.50-1.90 (m, 12H).

5,5”-Bis(l-adamantyl)-2,2’:2’,6”-terpyridine (adterpy). The ligand was
prepared by modification of the tbuterpy preparation. To a solution of 2,6-bis(pyridinio-
acetyl)pyridine diiodide (0.725 g, 1.27 mmol) in formamide (8 mL), ammonium acetate
(1.56 g, 20.2 mmol) and 2-(l-adamantyl)acrylaldehyde (0.486 g, 2.55 mmol) were added.
The reaction mixture was heated at 80°C for 5 hours. After cooling, the beige clay-like
precipitate (0.731 g) was filtered, washed with distilled water, and dried. The crude

product was recrystallized by dissolution in CHC13 and hexane, followed by addition of
MeOH until cloudy. Off-white powder product precipitated upon standing for 24 hours.
Yield: 0.331 g (45%). Mp 340°C (decomposes). 1H NMR (CDC13, 300MHz): 8 8.69
(dd, 2H, J = 2.0, 1.0 Hz), 8.50 (dd, 2H, J = 8.4, 1.0 Hz), 8.36 (d, 2H, J = 7.8 Hz), 7.90 (t,

1H, J= 7.8 Hz), 7.8l(dd, 2H, J= 8.4, 2.0 Hz), 2.10 (br m, 6H), 1.52-1.90 (overlap with

water signal, m, 24H). 13c NMR (CDC13, 300MHz): 8 155.5, 153.7, 146.5, 137.7, 133.5,

120.6, 120.3, 42.8, 36.6, 28.7. Direct probe El MS m/z 502.3 [MH+]. IR (KBr, cm'l):
2908 s, 2855 m, 1585 m, 1573 m, 1554 s, 1484 m, 1448 s, 1379 m, 1344 w, 1317 w,
1245 w, 1103 m, 1015 m, 809 m, 761 m, 709 w, 659 w, 633 w, 572 w. Elemental
Analysis: Calculated: C35H39N3-H20: C, 80.89; H, 7.95; N, 8.09. Found: C, 80.49; H,
7.48; N, 7.94.

[Fe(L)2](PF6)2. The iron(II) complexes were prepared by modification of a

11

previously reported procedure. An air free flask was charged with 2 equivalents of

ligand and deoxygenated 1:1 MeOH/HZO (30 mL). The solution was heated to promote

21

dissolution. A separate air free flask was charged with 1 equivalent of FeClz-ZHZO and
1:1 MeOH/HZO (15 mL). The Fe(II) solution was transferred via cannula to the ligand

mixture. The reaction was allowed to proceed until the solution reached a deep purple
color. If unreacted solid ligand remained, it was filtered from the reaction mixture. A

separate air free flask was charged with 10 equivalents of NH4PF 6 and 1:1 MeOH/HZO

(15 mL). This solution was transferred via cannula to the reaction mixture resulting in
immediate formation of dark purple precipitate. The precipitate was filtered and rinsed

with H20. Purification by ether diffusion into a saturated acetone solution of the

hexafluorophosphate complexes produced crystals.

[Fe(tbuterpy)2](PF6)2 (l). The solution of tbuterpy (0.11 g, 0.28 mmol) and
FeC12-2H20 (0.024 g, 0.14 mmol) was stirred for 1 hour at room temperature resulting in
the deep purple solution. The general procedure was followed producing leaf-like
crystals. Yield: 0.12 g (76%). 1H NMR ((CD3)2CO, 300MHz): 5 9.26 (d, 4H, J = 7.8
Hz), 8.91 (t, 2H, J = 7.8 Hz), 8.74 (dd, 4H, J = 8.4, 1.0 Hz), 8.12 (dd, 4H, J = 8.4, 2.0
Hz), 7.15 (dd, 4H, J = 2.0, 1.0 Hz), 0.94 (s, 36H). MS [ESI, m/z (rel int)]: 373 (100)
[Fe(tbuterpy)2]2+, 891 (15) [Fe(tbuterpy)2(PF6)]+ IR (KBr, cm“): 3095 w, 2967 m, 2872
m, 1608 m, 1555 w,1464 m, 1382 m, 1276 m, 1253 m, 1208 w, 1135 m, 1102 w, 1040
w, 980 w, 838 br, 748 m, 604 w, 558 s. Elemental Analysis: Calculated:
C46H54N6FeP2Flz-HzO-CH3OH: C, 51.94; H, 5.56; N, 7.73. Found: C, 51.66; H, 5.20;
N, 7.80. UV-Vis (CH3CN) Me): 325 (54000), 366 (4500), 506 (5600), 553 (9300), 612

(2200)

22

[Fe(adterpy)2](PF6)2 (2). The mixture of adterpy (0.147 g, 0.294 mmol) and
FeC12-2H20 (0.0239 g, 0.147 mmol) was heated at 70°C for 18 hours resulting in the

deep purple solution. Solid remained so the unreacted ligand was filtered off and the

general procedure was followed. Crystallization by ether diffusion resulted in
microcrystalline product. Yield: 0.0786 g (40%). 1H NMR ((CD3)2CO, 300MHz): 8
9.26 (d, 4H, J = 7.9 Hz), 8.95 (t, 2H, J = 7.9 Hz), 8.74 (d, 4H, J = 8.5 Hz), 8.12 (dd, 4H, J
= 8.5, 2.0 Hz), 7.09 (d, 4H, J = 2.0 Hz), 2.05 (overlap with acetone signal, m, 12H), 1.50-

1.80 (m, 48H). MS [E81, m/z (rel int)]: 529 (100) [Fe(adterpy)2]2+, 1204 (5)

[Fe(adterpy)2(PF6)]+ IR (KBr, cm'l): 2904 s, 2849 m, 1604 m, 1551 w, 1457 m, 1377 m,
1318 m, 1262 w, 1245 w, 1185 w, 1047 w, 1032 m, 977 w, 840 br, 753 m, 590 w, 558 s.
Elemental Analysis: Calculated: C70H78N6FeP2F12-2HZO: C, 60.70; H, 5.97; N, 6.07.
Found: C, 59.91; H, 5.48; N, 6.22. UV-Vis (CH3CN) 2(8): 326 (58000), 366 (5000), 506

(5000), 555 (8200), 612 (2200).
2.3 Results and Discussion

2.3.] Synthesis of Terpyridine Ligands

The most common route to prepare terpyridine is via central ring cyclization. As
described in chapter 1, the method allows for 4,4”- and 6,6”-substituted terpyridines to be
prepared via these routes, but the limited accessibility of meta- substituted pyridine
starting materials results in the difficulty of these routes for the preparation of 5,5”-

substituted terpys.

23

Palladium catalyzed coupling reactions are a reasonable alternative for the
preparation of functionalized terpyridines. For instance, 4,4’,4”-tri-tert-butyl-2,2’:6’,2”-
terpyridine has been prepared by a palladium (10% on charcoal) catalyzed coupling
reaction of 3 equivalents of 4-tert—butylpyridine.12 Negishi, Suzuki, and Stille coupling
are also widely applicable for the preparation of functionalized terpys. Stille cross-
coupling has become a popular route due to its simplicity, efficiency, and substitution
possibilities.l 5,5”-dimethyl-2,2’:6’,2”-terpyridine has been prepared in high yield via

Stille coupling (Scheme 2-1).13

 

\ 1) BuLi, -78°c, THF \
> I
/
N/ Br 2) BU3SnCl, -78°C N SnBU3
| \
/
Br N Br
Pd(Ph3)4
fl,

 

 

toluene, 110°C

Scheme 2-1. Terpyridine synthesis through Stille coupling.

Looking into adapting this procedure for the preparation of tbuterpy and adterpy was
problematic due to the accessibility of starting materials. Appropriate 5-methylpyridyl
reagents are commercially available or synthetically accessible while halogenated S-tert-

butylpyridyl reagents are not known. 3-tert—butylpyridine is known and has been

prepared through a five step sequence from 3,3-dimethylbutanol via ring cyclization.14

Since ring cyclization. leads to the appropriate substituted pyridine it is reasonable to

pursue a terminal ring cyclization route for the terpyridine.

24

In 1998, Adrian et al. developed a four step, high yield preparation of 5,5”-

dimethyl-2,2’:6’,2”-terpyridine from 2,6-diacetylpyridine.15 The commercially available
2,6-diacetylpyridine provides the central ring, allowing for the two terminal rings to be

symmetrically cyclized. In 1999, Sasaki et al. published a simpler and efficient two-step

7 This route seemed

Kriihnke-type terminal ring cyclization from 2,6-diacetylpyridine.
the most promising for the preparation of tbuterpy and adterpy. The synthetic strategy
begins with the preparation of 2,6-bis(pyridinioacetyl)pyridine diiodide; which Krdhnke

designated as the Ortoleva-King reaction.16

The bispyridinium salt is obtained by
precipitation from a reaction of 2,6-diacetylpyridine, iodine, and pyridine in high yield.
The terpyridines may then be formed by reacting the bispyridinium salt with 01,0-

unsaturated aldehydes as shown in Scheme 2-2.

 

 

NH4OAC
formamide

80°C

Scheme 2-2. Terpyridine synthesis via the Sasaki method of the Krbhnke reaction.

5,5”-Di—tert—butyl-2,2’:6’,2”-terpyridine (tbuterpy) was prepared from 2-tert-
butylpropenal in 40% yield via the Sasaki method. Tbuterpy was fully characterized, but
the elemental analysis result was a little concerning and did not fit the empirical formula
of the compound. A few recrystallization techniques were attempted so a variety of

solvents could have been present, including methanol, water, ethyl acetate, and diethyl

25

ether. Another possibility is the sample may have contained trace amounts of the fitted
filter which could have led to the lower values obtained. The rest of the characterization
data was acceptable and since the subsequent iron(II) complex was prepared in pure
form, the elemental analysis was less of a concern.

The 2-tert—butylpropenal starting reagent was obtained from 3,3-dimethylbutanol

via an initial oxidation followed by a-methylenation from known procedures as shown in

Scheme 2-3.5’ 6

 

 

   

O
excess DCC H/U\H O
’2 > ” ’
OH DMSO/Benzene /</Ii\ 10% pyrrolidine! H
reflux 18 h propionic acid
48% i-PFOH; 4 h
79%

Scheme 2-3. Synthesis of 2-tert—butylpropenal.

The dimethyl sulfoxide (DMSO) mediated oxidation is known as the Pfitzner-Moffatt
oxidation. The reaction proceeds through activation of DMSO by dicyclohexylcarbodi-
imide (DCC) in the presence of a pyridinium salt catalyst (py-TFA). 3,3-Dimethyl-
butanol is activated and transformed to an alkylsulfonium ylide, which undergoes
intermolecular decomposition to yield 3,3-dimethylbutanal. Most of the dicyclohexyl
urea by-product may be filtered off, but the liquid 3,3-dimethylbutanal remains in
solution and must be isolated by fractional distillation, which has proven to be quite
difficult. DMSO begins to decompose at high temperatures producing impurities such as
dimethyl sulfide during the isolation process. The decomposition of DMSO was
minimized by reducing the quantity of the lower boiling co-solvent, benzene, in the

reaction, thereby reducing the time of the lengthy distillation. The more pertinent

26

problem during isolation is the presence of pyridine. Pyridine distills at 115°C which is
very close to the product (bp. 101-106°C) resulting in contamination of the aldehyde
product. The impure aldehyde was used without further purification in the subsequent
methylenation reaction.

The first attempt to prepare 2-tert-butylpropenal was by a known Mannich

reaction described by Breit et al.17 Though the authors quoted a 40% yield, this route
proved to be inefficient and isolation difficult since the product is a liquid and required
multiple distillations. The inadequate quantity of 3,3-dimethylbutyraldehyde was a
limiting factor and a small scale reaction was necessary. Consequently, scaling down the
reaction resulted in no isolable product (Scheme 2-4).

0

o /Ik 0

’x

H H a,
’a
O

37% (aq)

’/
I,
I
I
I
I
O
a

        

’

H Me2NHZCI

H20 (pH 7)
(40%)

Scheme 2-4. Alternate synthesis of 2-tert-butylpropenal as described by Breit et al.

An extremely efficient organocatalytic method was published by Erkkiléi and

Pihko in 2006.6 The reaction of formaldehyde with (ll-substituted aldehydes can produce
(ll-substituted acroleins, but can also form the aldol product. The authors screened a
variety of amine/carboxylic acid combinations, as well as amino acids and dipeptides, to
develop a mild catalytic method that would produce the intended acrolein exclusively and
prevent any unwanted aldol or polymerization reactions. The acrolein 2-tert—

butylpropenal was prepared in high yield from 3,3-dimethylbutanal and formaldehyde

27

utilizing a pyrrolidine/propionic acid catalyst. The authors found that under the mild
conditions, the reaction is applicable to a wide variety of substituted acroleins and is
efficient regardless of sterics, functionalities, and protecting groups. I found that the
preparation of 2-tert-butylpropenal proved to be highly efficient, even in the presence of
the pyridine impurity producing 79% yield. In 2007, Erkkila and Pihko proposed a

Knoevenagel-Mannich type mechanism for the (ll-methylenation reaction (Scheme 2-

5).18
V (””3" HEN “A
0 cw
.. . 43>. 54:33

HX

1.2

Scheme 2-5. Proposed mechanism of organocatalytic a-methylenation reaction.

The authors performed a series of kinetic studies and determined that the reaction most
likely proceeds through a mechanism in which amine molecules activate both the
formaldehyde molecule and the aldehyde. The acceptor aldehyde is likely the irninium
species of formaldehyde while the donor is the enamine species of the a—substituted

aldehyde. The exceptional efficiency of this reaction for a wide range of (ll-substituted

28

aldehydes convinced me to pursue the same route for the preparation of 2-(1-

adamantyl)acrylaldehyde (Scheme 2-6).

 

O
TEMPO H/U\H
9'3 0 37% (aq) ‘ 0
OH CH2C|2 H 10% pyrrolidine! H
82% propionic acid
i-PrOH; 4 h
90%

Scheme 2-6. Synthesis of 2-( 1 -adamantyl)acrylaldehyde.

l-Adamantane ethanol is oxidized to l-adamantaneacetaldehyde through a

TEMPO catalyzed reaction as described by Beeson et al.9 TEMPO (2,2,6,6-tetramethyl-
1-piperidinyloxy) is a commercially available nitroxyl radical that catalyzes the oxidation
reaction in the presence of the hypervalent iodine reagent (diacetoxyiodo)benzene (DIB).

The mild conditions of the reaction and ease of workup made this route ideal opposed to

other alternatives such as the well known Swem oxidation.l9 l-Adamantaneacetaldehyde
was then transformed to 2-(1-adamantyl)acrylaldehyde by the organocatalytic 01-
methylenation reaction in 90% yield by a slight modification of the procedure. The solid
product precipitates from the reaction within 1 hour, but I found that a higher yield may
be obtained by adding more isopropanol and allowing the reaction to proceed for 3 more
hours.

A similar method to the route mentioned above by Breit et al. for the preparation

of 2-tert-butylpropenal was a potential alternative for the preparation of 2-(1-

adamantyl)acrylaldehyde.10 The isolation problems that arose during the synthesis of 2-

-tert-butylpropenal can be avoided since the 2-(1-adamantyl)acrylaldehyde is a solid at

29

room temperature. Utilization of this alternative method was not necessary since the
organocatalytic a-methylenation reaction produced such high yield.
5,5”-Bis(1-adamantyl) -2,2’:6’,2”-terpyridine (adterpy) was prepared analogously
to tbuterpy by the Sasaki method of the Krdhnke reaction in 45% yield. A longer
reaction time was necessary due to the poor solubility of the 2-(1-
adamantyl)acrylaldehyde. Furthermore, the purification of adterpy was complicated due
to solubility issues. After various failed recrystallization attempts to isolate the product,
the crude clay-like product was ultimately recrystallized by dissolving in chloroform and
hexane and precipitated with methanol. Terminal ring cyclization proved to be a

convenient route for the preparation of 5,5”-aliphatic-substituted terpyridines.

2.3.2 Synthesis of Bis(Terpyridine)iron(II) Complexes

It is well know that tridentate terpyridine will readily form a coordination
compound in the presence of to iron(II). Due to the lability of iron(II), the preparation of
heteroleptic complexes may be problematic, but homoleptic complexes are readily
produced. Iron(II)chloride is a highly sensitive to oxidation in the presence of air so it is
pertinent to keep the reaction oxygen free until the metathesis is complete. A solution of
iron(II)chloride is added to a solution of terpyridine and the solution immediately turns
purple upon complexation to the iron(II). Excess ammonium hexafluorophosphate is

then added to metathesize the pure complex, which can then be filtered (Scheme 2-7).

30

      
 
 

1) FeCI2'2H20
1:1 MeOH/HZO

 

 

2) NH4PF5

\z

 

Scheme 2-7. Synthesis of [Fe(5,5”-R-terpy)2](PF6)2 where R=t-butyl (l), adamantyl (2).

Complex 1 was prepared in 76% within an hour while complex 2 proved to be
more difficult. Again, the solubility of adterpy was problematic and upon heating the
reaction for 18 hours, yielded only 40%. Since complex 2 was obtained in its pure form,
the reaction conditions were not further optimized. A solution of methanol and water is
chosen since the iron(II) chloride and ammonium hexafluorophosphate reagents are
soluble while the product complex is insoluble and precipitates from the reaction. A
different solvent choice may have improved the reaction yield, but may have complicated
the isolation. The complexes were determined to be stable and may be filtered from the
reaction mixtures in the presence of air. Microcrystalline products are obtained through
recrystallization by ether diffusion into saturated acetone solutions. All attempts to grow
X-ray quality single crystals have been unsuccessful since the complexes crystallize in

leaf-like patterns; therefore, X-ray crystal structures have not been determined.

31

The complexes were characterized by 1H NMR, ESI-MS, IR, elemental analysis,
and UV-Vis spectroscopy. Iron(II)chloride readily oxidizes to iron(III) in the presence of
air so it is pertinent to ensure that there are no iron(III) impurities mixed in with the

product complexes. The ESI/MS data show no evidence of any iron(III) impurities. The

diarnagnetic (16 metal complexes are characterized by 1H NMR and the spectra show no
evidence of any paramagnetic impurities. Functionalizing the terpyridine ligands in the
5,5”-positions resulted in the desired low spin iron(II) coordination compounds upon
complexation.

Aliphatic substituents were chosen to introduce steric effects, without introducing
any electronic effects into the system. Ground state absorption measurements were made

to ensure that there are no significant electronic effects. Figure 2-3 shows the absorption

spectra of [Fe(terpy)2]2+, [Fe(tbuterpy)2]2+, and [Fe(adterpy)2]2+ in acetonitrile.20

 

Ground State Absorption

 

 

 
  

 

 

 

 

7" 12000 ~ e -
s 1 .—evv l
‘72 10000 —, ‘ ------- tbuterpy I
1 .' -. :
S 8000 4.: ,5 '. ”‘5‘: - adterpy
E El "
e 6000 43‘
o .
8 '~
S 4000 ‘1 .
CD
3 2000 4
2
0 = l I ' l
350 400 450 500 550 600 650 700 750 800 ‘

 

Wavelength (nm)

Figure 2-3. Electronic absorption spectra of [F e(terpy)2]2+(—), [Fe(tbuterpy)2]2 +(1) (- . -), and
[Fe(adterpy)2]2+ (2) (- -) in acetonitrile.

32

All three complexes possess the lowest energy broad feature assigned to lMLCT

transitions. The sharp feature overlaid with the broad feature at around 550 nm is typical

of [M(terpy)2]n+ transitions and is a result of the ability of terpyridine to delocalize the
electron over the three rings.11 The complexes have maxima at 550 nm, 553 nm, and 555

nm for [Fe(terpy)2]2+, [Fe(tbuterpy)2]2+, and [F e(adterpy)2]2+ respectively allowing us to

conclude that the differences arise from geometry changes and it is unlikely that there are

unwanted electronic effects. Electrochemical data reinforces this conclusion (Table 2-1).

Table 2-1. Oxidation and First Reduction Potentials (mV) of Fe(II) complexes. Measurements were taken

 

 

by DPV vs Ag/AgNO3, 0.1 M TBAPF 6 in acetonitrile, then corrected to ferrocene/ferrocenium.20
Complexes Oxidation E 1/2 (mV) First Reduction E 1/2 (mV)
[Fe(terpy)2](PF6)2 +704 -1641
[Fe(tbuterpy)2](PF6)2 (1) +692 -l684
[Fe(adterpy)2](PF6)2 (2) +682 -1722

 

 

 

 

 

A single substituent introduced to a terpyridine ligand can modulate the redox

properties of a corresponding iron(II) bis-chelate adduct to a large extent.” The

reduction potentials involve reduction of a terpy ligand, while the oxidation is metal-

based but is strongly dependant on the nature of the ligand.22 As shown in Table 2-1, the
difference between the oxidation and first reduction potentials of complexes 1, 2, and

[Fe(terpy)2](PF6)2 vary minimally indicating the absence of significant electronic

effects.

33

2.4 Concluding Comments

The Sasaki adaptation of the Krtihnke terminal ring cyclization was suitable for
the preparation of 5,5”-alkyl-substituted terpyridines. The recent developments in
organocatalytic chemistry have been demonstrated to be very useful toward the synthesis
of these ligands. The previously unknown tbuterpy and adterpy ligands were prepared
and complexation of the ligands to iron(II) resulted in the intended low spin complexes
appropriate for the analysis of ultrafast relaxation dynamics. The large aliphatic
substituents were shown to introduce steric bulk without introducing unwanted electronic
effects. The complexes are suitable for the study of the correlation between relaxation of

the initial charge transfer excited state and torsion about the C2 axis.

34

APPENDIX

 

 

 

 

 

 

 

'''''''''''''''''''''''''

 

 

 

 

 

 

Figure 2-4. 1H NMR spectrum of tbuterpy.

35

 

 

 

 

 

 

 

 

 

- jTY w ‘ 'TT I v 1 v I v v T v I r171 T v rfi 1 v T‘V’ I

200 180T 160 140 120 Taggfso 60 40 20 0

 

 

13

Figure 2-5. C NMR spectrum of tbuterpy.

36

 

 

100"

96FS 0

 

50

 

129A4

14334

157£1

 

 

01W--3,----,-

33013

 

 

 

150

34533

 

 

250 350
n02

 

Figure 2-6. Direct probe mass spectrum of tbuterpy.

37

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 2-7. IR spectrum of tbuterpy.

38

1001 H W
% {T
T
r -
a 656.4
n
s -
m
i _ 1121.4
t
1 1552.8
3 - ..
n 2957.3
C
e ~ 1446.6 816.5
65 -
3900 3400 2900 2400 1900 1400 900 400
_,___Wavenumbers_, '

 

 

 

 

 

 

 

 

 

 

 

9.5" V8.5Y'V7I5W'615
ppm
'11 J I l A n l
V100" '90 I 80"70'1 6.0f'510I'4T01730‘V20 '10 ' 0.01

 

Figure 2-8. 1H NMR spectrum of [Fe(tbuterpy)2](PF6)2.

39

 

 

 

 

373.2

 

 

AJAA‘A
erY vvvv

400 m,z 500

Figure 2-9. ESl mass spectrum of [Fe(tbuterpy)2](PF6)2.

4O

.1 1 my I I l 1 L. x

TOF MS ES+

 

 

 

 

 

 

 

 

100‘ A-
r “VT“ IRWIN/1 “WV ‘ M «I
WM” ’1’”): llm’l Id Wt“ If W .4
o d 11’ 1608.0 [1 ~ II I I" l
/o l 1" 1275.5 I
T 2966.9 1. 1
r -
3 1453.1 I j
n .
S s \ ’1 557.6
m ' .
i I,
t I i
t l
a - p ‘
n I
° 1
e -
l
I
837.7
30 1
3900 3400 2900 2400 1900 1400 900 400
Wavenumbers

 

Figure 2-10. IR spectrum of [F e(tbuterpy)2](PF6)2.

41

 

 

 

 

#fi—rTj'v'v'v'vl-fir ''''''''

8.8 8.6 8.4 8.2 8.0 7.8
ppm

 

 

 

 

 

 

 

 

 

oppm.

 

Figure 2-11. 1H NMR spectrum of adterpy. Water is present to enhance solubility and enable resolution of
the splitting pattern.

42

 

 

 

 

 

 

 

 

 

 

vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv

 

 

 

13

Figure 2-12. C NMR spectrum of adterpy.

43

 

 

 

 

 

 

 

 

100 a 501.3
193.6
1 444.2
%FS 3 79.0
I 502.3
, 93.0
222.7
350.2
I
o * ' ...........
1 00 200 300 400 500 600
m/z

 

 

 

Figure 2-13. Direct probe mass spectrum of adterpy.

44

700“

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 2-14. IR spectrum of adterpy.

45

7 '\
Wfl‘ f/h/fl‘w'w/ “MM/‘1‘ pl {1 fl” flNI‘ fix”
% 7’ 1.1111100 , : l
T — I II “1111., 1’ IN! III ' ;
'I l l‘ ' ' ‘
r ‘l l 'l I l a
. ll ll
n I I | . b .
f}, I I 1378.9
i "1
t E
t 1 1553-2 1014.6
a 1' 808.4
11
c s
e
1448.0
201
2901.2
3900 3400 2900 $100 1900 1400 900 400
L_ A? _ a _Wavenumb_e_rs*

 

 

 

 

9.5 ' 9.0 ' 8:5 ' 80 7.5 7.0
ppm

 

 

 

 

 

 

 

''''''''''''''''''''''''''''''''''''''''''''

 

 

 

Figure 2-15. 1H NMR spectrum of [Fe(adterpy)2](PF6)2.

46

Flgl

 

 

100 ‘

96.

 

 

 

 

529.3 TOF MS ES+
197.0
156.0
dd 1203.6
1111., _. 4.1.9.3. $1,111, -1 .................................. - ......... .+.
200 400 600 800 1000 1200
n02

 

Figure 2-16. ESI mass spectrum of [Fe(adterpy)2](PF6)2.

47

 

 

 

 

100 7

W“ H m

1 WIN \ {Av/15,1.i 'Ww/IMJTV V \ A n
‘ .1

 

 

 

 

I l
l .
~ I. 1603.7Il 10323 1 ;
% il l l
T l l ' ‘
' l l
2 “ I ll
s 1 ' '
m — l
i l
t 2903.8 1
t 3
a l
" l
c .
e 4|
l
40 840.2
3900 3400 2900 2400 1900 1400 900 400

Wavenumbers

 

 

 

Figure 2-17. IR spectrum of [F e(adterpy)2](PF6)2.

48

LA)

.J.‘

REFERENCES

Schubert, U. S.; Hofrneire, H.; Newkome, G. R., Modern Terpyridine Chemistry.
Wiley-VCH: 2006.

Krdhnke, F ., Synthesis 1976, l.

Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.;
Cheeseman, J. R.; J. A. Montgomery, J .; Vreven, T.; Kudin, K. N.; Burant, J. C.;
Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.;
Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.;
Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajirna, T.; Honda, Y.;
Kitao, 0.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.;
Bakken, V.; Adamo, C.; Jaramillo, J .; Gomperts, R.; Stratrnann, R. E.; Yazyev,
0.; Austin, A. J.; Carnmi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;
Morokurna, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J .; Zakrzewski, V. G.;
Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A.
D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J .; Stefanov, B. 8.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, 1.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Chen, B. J. W.; Wong, M.
W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision D. 01, Gaussian, Inc.:
Wallingford, CT, 2004.

Ziegler, M.; Monney, V.; Stoeckli-Evans, H.; Von Zelewsky, A.; Sasaki, I.;
Dupic, G.; Daran, J .-C.; Balavoine, G. G. A., J. Chem. Soc., Dalton Trans. 1999,
667.

Cheung, C. K.; Wedinger, R. S.; 16 Noble, W. J. J. Org. Chem. 1989, 54, 570.

Erkkila, A.; Pihko, P. M., J. Org. Chem. 2006, 71, 2538.

Sasaki, 1.; Daran, J. C.; Balavoine, G. G. A., Synthesis 1999, (5), 815.

Fallahpour, R.-A.; Neuburger, M.; Zehnder, M., Polyhedron 1999, 18, 2445.

Beeson, T. D.; MacMillan, D. W. C., J. Am. Chem. Soc. 2005, 127, 8827.

49

10.

11.

12.

l3.

14.

15.

l6.

l7.

l8.

19.

20.

21.

22.

Avery, T. D.; Macreadie, P. I.; Greatrex, B. W.; Robinson, T. V.; Taylora, D. K.;
Macreadieb, I. G., Bioorganic & Medicinal Chemistry 2007, 15, 36—42.

Smeigh, A. L. PhD Dissertation, Michigan State University, 2007.

Hadda, T. B.; Bozec, H. L., Inorganica Chimica Acta 1993, 204, 103.

Schubert, U. S.; Eschbaumer, C.; Hochwimmer, G., Synthesis 1999, (5), 779.

Fujii, T.; Hiraga, T.; Yoshifuji, S.; Ohba, M.; Yoshida, K., Chem. Pharm. Bull.
1978, 26, 3233.

Adrian Jr., J. C.; Hassib, L.; Kimpe, N. D.; Keppens, M., Tetrahedron 1998, 54,
2365.

Krtihnke, F., Angew. Chem. Int. Ed. 1963, 2, 225.

Breit, B.; Heckmann, G.; Zahn, S. K., Chem. Eur. J. 2003, 9, 425.

Erkkiléi, A.; Pihko, P. M., Eur. J. Org. Chem. 2007, 4205.

Luly, J. R.; Dellaria, J. F .; Plattner, J. J.; Soderquist, J. L.; Yi, N., J. Org. Chem.
1987, 52, 1487.

Data collected by Allison M. Brown.

Chambers, J.; Eaves, B.; Parker, D.; Claxton, R.; Ray, P. S.; Slattery, S. J.,
Inorganica Chimica Acta 2006, (359), 2400—2406.

Braterman, P. S.; Song, J .-I.; Peacock, R. D., Inorg. Chem. 1992, 31, 555.

50

Chapter 3. Design and Synthesis of Aryl-Substituted Terpyridine Ligands

3.1 Introduction

To obtain a larger series of molecules to study the correlation of torsional modes
with relaxation dynamics two additional sterically hindered iron(II) terpyridyl complexes
have been prepared. As with tbuterpy and adterpy, it is pertinent to design ligands
substituted at the 5,5”—positions to study the effects of torsion about the intended
coordinate on relaxation dynamics. Aryl-functionalized terpyridines are well known
throughout the literature. Aryl substituted heterocyclic ligands in metal complexes make

attractive chromophores since they are known to increase the extinction coefficients of
the MLCTs of complexes.1 Amongst many other applications they are very appealing as

building blocks in metallo-supramolecular chemistry.2

Initially, we chose to exclude aromatic substituents in order to focus exclusively
on steric effects and avoid introducing any unwanted conjugation into the ligands, which
could potentially introduce competing relaxation pathways through intraligand
delocalization. McCusker and coworkers have shown that the extent of electron

delocalization in a conjugated system can be modulated by imposing geometric

constraints.3 By introducing steric constraints to an aromatic ring, the ability to achieve
planarity with the rest of the conjugated system will be hindered, thereby reducing the
intraligand delocalization.

A disubstituted phenyl group with bulky substituents in the ortho positions would
be an ideal functional group to impose the intended the steric constraints. The initial
design involves a terpyridine functionalized with a 2,6—diisopropylphenyl group.

Adapting the Krtihnke terminal ring cyclization to aromatic substituents may be quite

51

 

difficult due to inaccessibility of starting materials. The synthesis of such starting
reagents would be complicated by the necessity to prepare multifunctional phenyl groups
as opposed to the more straightforward preparation of the t-butyl- and adamantyl-
analogs. Palladium catalyzed cross-coupling reactions are a reasonable alternative to
pursue as there are many will know methods for coupling aryl groups. In 2005, a new

route was developed for the preparation of 5,5”-aryl-terpys based on the conversion of

1,2,4-triazines through an aza-Diels-Alder reaction.4

Of all the available routes, I came across an ideal system with a wide range of

synthetic adaptability in the work by Loren and Siegel.5 By a convenient Negishi
coupling methodology, they prepared a variety of polypyridines utilizing a 4-methoxy-
2,6-dimethylphenyl or “manisyl” firnctional group. I prepared the 5,5”-dimanisyl-
2,2’:6’,2”—terpyridine (maniterpy) as described in the literature and set out to adapt the
procedure to obtain an unknown 2,6-disopropyl-4-methoxyphenyl or “panisyl” analog:
5,5”-dipanisyl—2,2’:6’,2”-terpyridine (paniterpy). The corresponding bis(terpyridyl)-

iron(II) complexes were also prepared (Figure 3-1).

 

 

Figure 3-1. Structure of [F e(5,5”-R—terpy)2](PiF6)2 where R= manisyl (3), panisyl (4).

52

Space filling models of the optimized structures are shown in Figure 3-2.6 The

introduction of steric bulk that would hinder torsion about the C2 axis is clearly shown as
compared to [Fe(adterpy)2]2+ (2). The bis(terpyridyl)iron(II) complexes have a low spin

ground state and can be readily prepared by a standard procedure of combining a 2:1
mixture of ligand with iron(II) chloride by standard Schlenk line techniques. Complex 3

is known and the crystal structure has been published while complex 4 is previously
I
1 C2 !

Figure 3-2. Optimized Structures of [Fe(5,5”-R-terpy)2](PF6)2 where R = adamantyl (2), manisyl (3),
panisyl (4) (left to right). (Top) View looking down the C2 axis. (Bottom) Side view.

7

unknown.

 
 

 

3.2 Experimental

3.2.1 Synthesis
General. All reagents were of reagent grade and used as received without further

purification unless otherwise noted. Solvents were purchased from Aldrich, Jade

53

Scientific, Spectrum, Mallinckrodt, EMD Chemical, or CCI and were distilled by
standard purification techniques. All air sensitive reactions were carried out under inert

atmosphere using standard Schlenk techniques utilizing thoroughly deoxygenated

solvents that were degassed by the freeze—pump—thaw method. IH and 13 C NMR
were variously recorded with either a Varian Inova-300 or a Varian UnityPlus-300 MHz
spectrometer. Melting points were obtained on a Mel-Temp apparatus. UV-vis spectra
were obtained on a Varian Cary 50 spectrophotometer. IR spectra were obtained on a
Mattson Galaxy 5000 FTIR. Elemental analysis and direct probe mass spectra were
obtained through the Analytical Facilities at Michigan State University. Electrospray
mass spectra (ESI-MS) were obtained from the staff of the MSU Mass Spectrometry
Facility. Characterization data for the previously unknown compounds, terpyridine
ligands, and the iron(II) complexes can be found in the appendix at the end of this
chapter.

3,5-Dimethylanisole. The compound was prepared from 3,5-dimethylphenol (50

g, 0.40 mol; Aldrich) according to the literature procedure.8 Caution! Highly toxic and

reactive dimethyl sulfate is used. The anisole was prepared then purified by vacuum
distillation at 30°C yielding colorless oil (43.06 g, 77%). 1H NMR (CDCl3, 300MHz): 5

6.65 (s, 1H), 6.59 (s, 2H), 3.81 (s, 3H), 2.34 (s, 6H).
4-Bromo-3,5-dimethylanisole. 3,5-Dimethylanisole (22.50g, 0.1652 mol) was

8

brominated according to the literature procedure. The product was isolated from the

dibromo by-product by vacuum distillation at 82-85°C yielding a colorless oil. Yield:

20.12 g (57%). 1H NMR (CDC13, 300MHz): 5 6.66 (s, 2H), 3.78 (s, 3H), 2.40 (s, 6H).

54

 

2-bromo-5-iodopyridine. The compound was prepared from 2,5-

dibromopyridine (5.02 g, 21.1 mmol; Aldrich) according to the literature procedure.5

The brown solid was recrystallized from ethanol to pure light brown plate crystals.
Yield: 3.93 g (66%). 1H NMR (CDCI3, 300MHz): 8 8.57 (d, 1H), 7.78 (dd, 1H), 2.25 (d,
1H).

2-bromo-5-manisylpyridine (manisyl = 4-methoxy-2,6-dimethylphenyl). The
compound was prepared from 4-bromo-3,S-dimethylanisole (1.51 g, 7.02 mmol) and 2-

bromo-S-iodopyridine (1.93 g, 6.81 mmol) according to a literature procedure.5 The

yellow crystalline product was isolated by column chromatography on silica gel with 3:2
CHzClz/hexanes. Yield: 1.46 g (74%). Mp 79-80°C. 1H NMR (CDC13, 300MHz): 8
8.16 (d, 1H), 7.53 (d, 1H), 7.34 (dd, 1H), 6.66 (s, 2H), 3.80 (s, 3H), 2.00 (s, 6H).
5,5”-dimanisyl-2,2’:6’,2”-terpyridine (maniterpy). The ligand was prepared
from 2-bromo-5-manisylpyridine (0.64 g, 2.2 mmol) and 2,6-dibromopyridine (0.25 g,
1.0 mmol; Aldrich) according to the published procedure.5 The white powder precipitate
was collected by filtration and rinsed with hexane to obtain the pure ligand. Yield: 0.17
g (32%). Mp 212-214°C. 1H NMR (CDC13, 300MHz): 8 8.70 (dd, 2H, J = 8.0, 1.0 Hz),
8.50 (dd, 2H, J = 2.0, 1.0 Hz), 8.49 (d, 2H, J = 8.0 Hz), 8.02 (t, 1H, J = 8.0 Hz), 7.67 (dd,
2H, J = 8.0, 2.0 Hz), 3.83 (s, 6H), 2.07 (s, 12H). IR (KBr, cm'l): 2995 w, 2940 m, 2836
w,1604 s, 1588 m, 1546 m, 1474 m, 1446 s, 1366 w,1318 s, 1273 m, 1192 m, 1154 s,
1073 m, 1034 w, 999 m, 934 w, 857 w, 823 m, 765 m, 710 w, 657 w, 585 w.
4-Bromo-2,6-diisopropylaniline. The aniline was prepared from 2,6-diiso-

propylaniline (5.0 mL, 26.5 mmol; Aldrich) according to the published procedure.9 The

55

081

mi

611

C0

_\\’

compound was obtained as pale yellow oil and used without purification. Yield: 6.1 g
(92%). 1H NMR (CDC13, 300MHz) 8= 7.09 (s, 2H), 3.69 (br s, 2H), 2.86 (sept, 2H),
1.23 (d, 12H).

1-Bromo-3,5-diisopropylbenzene. The compound was prepared by modification

of the literature procedure.9 4-bromo-2,6-diisopropylaniline (6.60 g, 25.8 mmol) was

added to 2 M HCl (70 mL) and cooled to -5°C. NaN02 (4.48 g, 64.9 mmol) was added
portion wise over 2 hours with stirring followed by the addition of 50% H3P02 (aq) (30

mL). The reaction was left at 0°C for 24 hours and then stirred at room temperature for
24 hours. The two layer mixture was separated and the aqueous layer extracted with

ether (3 x 100 mL). The combined organic layers were dried with MgSO4 and
concentrated in vacuo. The viscous brown oil obtained was purified by two sequential

vacuum distillations at 95-98°C yielding a yellow oil. Yield: 5.12 g (82%). ‘H NMR

(CDC13, 300MHz): 8 7.16 (d, 2H), 6.97 (t, 1H), 2.83 (sept, 2H), 1.21 (d, 12H).
2,4,6-Tris(3,5-diisopropylphenyl)cyclotriboroxane. The boronic acid was

prepared from 1-bromo-3,5-diisopropylbenzene (3.85 g, 16.0 mmol) according to the

literature procedure.9 The published reaction produced the off white solid boronic acid
which was mostly converted to the cyclotriboroxane by heating under vacuum at 100°C

for 11 hours. The white powder product mixture was used without firrther purification.

Yield: 2.42 g (81%). Cyclotriboroxane: 1H NMR (CDCl3, 300MHz): 8 7.39 (d, 6H),

7.21 (t, 3H), 2.90 (sept, 6H), 1.25 (d, 36H). Boronic acid: 1H NMR (CDC13, 300MHz): 8

7.90 (d, 2H), 7.30 (t, 1H), 3.03 (sept, 2H), 1.33 (d, 12H).

56

 

3,5-Diisopropylphenol. The phenol was prepared by modification of the

9

published procedure. The cyclotriboroxane (3.39 g, 6.00 mmol) was slowly added

portion wise with stirring to 30% H202 (10 mL) over 5 hours. The reaction mixture was
heated at 70°C until light brown oil was present (~45 minutes). After cooling to room
temperature the oil solidifies and 0.5 M HCl (100 mL) was added and the mixture was
extracted with ether (3 x 30 mL). The combined extracts were dried with MgSO4 and
concentrated in vacuo. The waxy beige residue may be used without firrther purification
(Yield: 2.76 g, 86%). Mp 46-49°C. The residue may be purified by column

chromatography on neutral alumina with 9:1 cyclohexane/ethyl acetate to give colorless
waxy solid. Yield: 1.61 g (50%). Mp 50-51°c.IO 1H NMR (CDC13, 300MHz): 8 6.65 (t,

1H), 6.50 (d, 2H), 4.53 (br. s, 1H), 2.81 (sept, 2H), 1.20 (d, 12H).
3,5-Diisopropylanisole. The anisole was prepared from 3,5-diisopropylphenol

8

(0.77 g, 4.3 mmol) by adaptation of a known procedure. Caution! Highly toxic and

reactive dimethyl sulfate is used. The brown oil was purified by vacuum distillation at

48-50°C to yield the yellow oil anisole. Yield: 0.57 g (75%). 1H NMR (CDC13,

300MHz): 8 6.71 (t, 2H), 6,62 (d, 1H), 3.83 (s, 3H), 2.85 (sept, 2H), 1.24 (d, 12H).
1-bromo-2,6-diisopropyl-4-methoxybenzene. The compound was brominated

according to the general procedure described in the literature.9 3,5-diisopropylanisole

(2.61 g, 13.6 mmol) was dissolved in 3:2 CHzClz/MeOH (85 mL). An addition funnel
was charged with a solution of TBABr3 (6.55 g, 13.6 mmol) in 3:2 CH2C12/MeOH (70

mL). The orange bromine solution was added dropwise with stirring over 4.5 hours and

allowed to stir until colorless (approx. 30 minutes). The solvent was evaporated in vacuo

57

and the remaining oil was dissolved in ether (100 mL), washed with water (3 x 50 mL),
dried with MgSO4, and concentrated in vacuo. The crude yellow oil (3.30 g, 90%)

contained trace evidence of a dibromo by-product. The product was isolated by column

chromatography on neutral alumina with 9:1 cyclohexane/ethyl acetate yielding colorless
oil. Yield: 3.06 g (83%). 1H NMR (CDC13, 300MHz): 8 6.68 (s, 2H), 3.80 (s, 3H), 3.47
(sept, 2H), 1.41 (d, 12H). UV (CH3CN). Elemental Analysis: Calculated: C13H19BrO:
C, 57.6; H, 7.06; N, 0.00. Found: C, 59.16; H, 7.41; N, 0.00.
2-Bromo-5-panisylpyridine (panisyl = 2,6-diisopropyl-4-methoxyphenyl). The
compound was prepared by a modified procedure adapted from the literature.5 An air
free flask was charged with 4-bromo-3,5-diisopropyl-1-methoxybenzene (1.29 g, 4.77
mmol) and dry, deoxygenated THF (25 mL) and the solution was cooled to -78°C. 1.6

M n-BuLi in hexanes (3.00 mL, 4.80 mmol; Fluka) was added dropwise via syringe over

20 minutes with stirring. A separate air free flask was charged with anhydrous ZnC12

(0.655 g, 4.80 mmol; Aldrich) and dry THF (25 mL) and cooled to 0° C. The Zn solution
was transferred to the reaction flask via cannula and the reaction mixture was allowed to

warm to room temperature (approx. 1.5 hours). A separate air free flask was protected

from light then charged with 2-bromo-5-iodopyridine (1.30 g, 4.54 mmol), Pd(PPh3)4

(0.300 g, 0.260 mmol; Strem), and dry THF (15 mL). Upon dissolution the contents
were transferred to the reaction flask via cannula. The reaction mixture was heated at
reflux for 18 hours under nitrogen in the dark. The solution was allowed to cool to room

temperature and then added to CH2C12 (50 mL) in a separatory funnel. The solution was
washed vigorously with EDTA (aq) and basified with NaHCO3 (aq). The layers were

separated and the organic layer washed with H20, dried with MgSO4, and concentrated

58

C111

“'3

in vacuo. The product was isolated from the crude orange residue by column

chromatography on silica gel with 3:1 followed by 3:2 hexanes/CH2C12. The compound

was isolated as pure colorless cube crystals. Yield: 1.60 g (70%). Mp 128-130°C. 1H

NMR (CDC13, 300MHz): 8 8.18 (dd, 1H, J= 2.4, 1.0 Hz), 7.54 (dd, 1H, J= 8.1, 1.0 Hz),
7.51 (dd, 1H, J= 8.1, 2.4 Hz), 6.74 (s, 2H), 3.84 (s, 3H), 2.48 (sept, 2H, J= 6.8 Hz), 1.06

(dd, 12H, J= 6.8, 1.5 Hz). 13C NMR (CDC13, 300MHz): 8 159.6, 150.6, 148.3, 140.0,

139.8, 135.1, 127.1, 108.0, 54.7, 30.1, 23.5. Direct probe EI MS m/z 349.1 [MH+]. IR

(KBr, cm'l): 3036 m, 2961 s, 29228 m, 2868 m, 2836 w, 1604 s, 1574 s, 1546 m, 1447 s,
1362 m, 1339 m, 1303 3,1248 w, 1197 m, 1172 m, 1130 m, 1081 s, 1043 s, 996 m, 880

w, 850 m, 757 m, 579 w, 495 w. Elemental Analysis: Calculated: C13H22NBrO: C,

62.1; H, 6.37; N, 4.02. Found: C, 62.2; H, 6.53; N, 3.99.
5,5”-dipanisyl-2,2’:6’,2”-terpyridine (paniterpy). The ligand was prepared by
modification of the maniterpy preparation. An air free flask was charged with 2-bromo-
5-panisylpyridine (0.504 g, 1.44 mmol) and dry, deoxygenated THF (15 mL) and the
solution was cooled to -78°C. 1.6 M n-BuLi in hexanes (Fluka, 0.900 mL, 1.44 mmol)
was added dropwise via syringe over 10 minutes with stirring. A separate air fiee flask

was charged with anhydrous ZnC12 (Aldrich, 0.196 g, 1.44 mmol) and dry THF (15 mL)

and cooled to 0° C. The Zn solution was transferred via cannula to the reaction flask and
the reaction mixture was allowed to warm to room temperature (approx. 1.5 hours). A
separate air free flask was protected from light then charged with 2,6-dibromopyridine

(0.166 g, 0.699 mmol; Aldrich), Pd(PPh3)4 (0.0900 g, 0.0779 mmol; Strem), and dry

THF (15 mL). Upon dissolution the contents were transferred via cannula to the reaction

59

flask. The reaction mixture was heated at reflux for 20 hours under nitrogen in the dark.

The solution was allowed to cool to room temperature and then added to CH2C12(50 mL)

in a separatory funnel. The solution was washed vigorously with EDTA (aq) and

basified with NaHCO3 (aq). The layers were separated and the organic layer washed
with H20, dried with MgSO4, and concentrated in vacuo. The product was isolated from

the crude orange residue by column chromatography on silica gel with 1:1

hexanes/CH2C12 followed by 100% CHZCIZ, then 3:2 CH2C12/MeOH. Upon evaporation
cream colored solid precipitates from the brown CHzClz/MeOH fraction. The precipitate
was purified by dissolving in CHZCIZ followed by the addition of MeOH until cloudy.
The white crystalline product precipitated upon standing for 24 hours. Yield: 0.0613 g
(14%). Mp 225-228°C. 1H NMR (CDC13, 300MHz): 8 8.71 (dd, 2H, J= 8.1, 1.0 Hz),
8.52 (dd, 2H, J= 2.0, 1.0 Hz), 8.51 (d, 2H, J= 8.1 Hz), 8.00 (t, 1H, J= 8.1 Hz), 7.68 (dd,
2H, J = 8.1, 2.0 Hz), 6.78 (s, 4H), 3.87 (s, 6H), 2.64 (sept, 4H, J = 6.8 Hz), 1.10 (dd,
24H,J= 6.8, 1.5 Hz). 13C NMR (CDCl3, 300MHz): 8 159.9, 155.3, 154.6, 150.1, 148.9,
138.6, 136.5, 128.1, 120.8, 120.5, 108.4, 55.1, 30.6, 24.0. Direct probe EI MS m/z 614.6

[MH+]. IR (KBr, cm'l): 2959 s, 2867 m, 1603 s, 1576 m, 1547 m, 1463 s, 1446 s, 1383
w,1337 m, 1304 m, 1249 m, 1196 m, 1173 m, 1129 m, 1046 m, 1000 w, 934 w, 866 m,

818 m, 764 m, 718 w, 661 w, 579 w. Elemental Analysis: Calculated: C41H47N3OZ: C,

80.22; H, 7.72; N, 6.85. Found: C, 79.57; H, 7.76; N, 8.19.

[Fe(L)2](PF6)2. The iron(II) complexes were prepared according to the general

procedure described in chapter 2.

60

[Fe(maniterpy)2](PF6)2 (3). The mixture of maniterpy (0.031 g, 0.061 mmol)

and FeClz-ZHZO (0.0049 g, 0.030 mmol) was heated at 50°C for 24 hours resulting in the

deep purple solution. Solid remained so the unreacted ligand was filtered off and the

general proCedure was followed. Crystallization by ether diffusion resulted in needle like
crystals. Yield: 0.026 g (64%). 1H NMR ((CD3)2CO, 300MHz): 8 9.18 (d, 4H, J = 8.1
Hz), 8.91 (d, 4H, J = 8.2 Hz), 8.74 (t, 2H, J = 8.1 Hz), 8.01 (dd, 4H, J = 8.2, 2.0 Hz), 7.37
(d, 4H, J = 2.0 Hz), 6.63 (s, 8H), 3.78 (s, 12H), 1.58 (s, 24H). MS [ESL m/z (rel int)]:
529 (100) [Fe(maniterpy)2]2+, 1203 (35) [Fe(maniterpy)2(PF6)]+ IR (KBr, cm'l): 2952
w, 2987 w, 2844 w, 1601 s, 1453 s, 1383 w, 1315 s, 1280 m, 1243 w, 1192 w, 1156 s,
1073 m, 1032 m, 843 br, 758 m, 558 m. Elemental Analysis: Calculated:
C66H62N6O4F6P2F12'2H20: C, 57.23; H, 4.80; N, 6.07. Found: C, 56.88; H, 4.82; N,
5.85. UV-Vis (CH3CN) M6): 326 (38000), 500 (4500), 534 (5500), 555 (7000), 612
(2000).

[Fe(paniterpy)2](PF6)2 (4). The mixture of paniterpy (0.049 g, 0.080 mmol) and
FeClz-ZHZO (0.0070 g, 0.043 mmol) was heated at 50°C for 18 hours resulting in the

deep purple solution. Solid remained so the unreacted ligand was filtered off and the

general procedure was followed. Crystallization by ether diffusion resulted in needle like
crystals. Yield: 0.021 g (31%). 1H NMR ((CD3CN), 300MHz): 8 9.18 (d, 4H, J = 8.1
Hz), 8.95 (d, 4H, J= 8.1 Hz), 8.67 (t, 2H, J= 8.1 Hz), 8.10 (dd, 4H, J= 8.1, 1.5 Hz), 7.29
(d, 4H, J = 1.5 Hz), 6.69 (s, 8H), 3.79 (s, 12H), 1.87 (sept, 8H, J = 6.8 Hz), 0.94 (d, 24H,

J = 6.8 Hz), 0.61 (d, 24H, J = 6.8 Hz). MS [ESL m/z (rel int)]: 641 (100)

[Fe(painterpy)2]2+, 1428 (65) [Fe(paniterpy)2(PF6)]+ IR (KBr, cm"): 2961 m, 2929 m,

61

2872 w, 1602 s, 1461 m, 1451 m, 1386 w, 1367 w, 1337 m, 1307 m, 1254 m, 1196 m,
1174 m, 1130 w, 1039 w, 1012 w, 843 br, 558 s. Elemental Analysis: Calculated:
C32H94N6O4FeP2F12-H20: C, 61.88; H, 5.28; N, 6.08. Found: C, .60.23; H, 5.20; N,
5.10. UV-Vis (CH3CN) 2(6): 276 (54000), 329 (50000), 500 (5300), 535 (6500), 555

(8800), 610 (2600).
3.3 Results and Discussion

3.3.1 Synthesis of Terpyridine Ligands

Aryl substituted terpys appeared in the initial work by Kr8hnke though the

applicability of the method to functionalize the 5,5”-position had not yet been realized.11
In 1991, Sauvage and Ward utilized the Potts methodology of the Kr8hnke central ring
cyclization to prepare 5,5”-bis(4-methoxyphenyl)-2,2’:6’,2”-terpyridine.12’ 13 The terpy
was prepared in modest yield from 2-acetyl-5-(4-methoxyphenyl)pyridine through the
sequence of reactions in Scheme 3-1. The first step of the Potts methodology was
plagued by side product formation. Instead of the intended a-oxoketene dithioacetal (B),
significant quantities of ethyl ketone, isopropyl ketone, and t-butyl ketone were isolated.
Sauvage and Ward improved on the method with the addition of dibenzo-l 8-crown-6
(DB-18C6) to prevent the repeated attack of the enolate by the methyl iodide. A Michael
reaction then yields the dicarbonyl intermediate which undergoes ring cyclization. As
mentioned in chapter 1, harsh reductive removal of the thiomethyl group is then required

leading to low yield in the final step.

62

— DB-18C6,THF _ _
H co—.——< >——( s: CH
\ N o 2) 682 3 \ N/ o 3

A 3) Mel
B
(57%) SCH3

 

       

 

1) t-BuOK
DB-18C6, THF
A :

 

2) B
3) NH4OAc, HOAc

NiClz, NaBH4
(47%) H3CO

EtOH
(31%)

 

H3CO OCH3

Scheme 3-1. Potts methodology of Krtlhnke central ring cyclization for the preparation of 5,5”-bis(4-
methoxyphenyl)-2,2’:6’,2”-terpyridine.

The low yield of the Potts method is not the only deterrent to utilizing this
procedure; another is the variety of problems that arise during the preparation of the

starting materials. The sequence of reactions is shown in scheme 3-2.

OCH3B
Ni(PPh3)2CI2 H O
_ 2 2
?©_> THF —’H3CO \ N/ a”
c
(75%) (74%)
MgBr
__ Me3Si-CN CH3Mg|
< > < >——> H CO—< >—<\— N/CN>-—
H CO 3
3 \ N/ CH3CN, Et3N Benzene
N\ (62%) (80%)

0

Scheme 3-2. Synthesis of starting materials for the preparation of 5,5”-bis(4-methoxyphenyl)-2,2’:6’,2”-
terpyridine.

63

The first step requires careful control of the highly exothermic reaction to obtain a decent
yield. The third step required a highly toxic cyanide reagent and is also plagued by the
formation of unwanted isomers, and though other solvents were screened, the final step
works best in benzene. Overall, this route leaves much to be desired and other routes
were sought.

The highly efficient route developed by Kozhevnikov et al. for the preparation of

5,5”-diaryl-terpys based on the conversion of 1,2,4-triazines through an aza-Diels-Alder

reaction was promising.4 The authors prepared a varity of terpyridines in good yields
through a relatively simple series of reactions as shown in Scheme 3-3. A benefit of this
route is the structural diversity that may be gained through the ability to functionalize the
various components in the reaction. The route may be applicable to the preparation of a
bis(2,6-diisopropylphenyl)-terpy since the required 2,6-diisopropylacetophenone starting

material is available through synthetic means, albeit in very low yield. ‘4

1) i-PrONO EtONaA \NHZH
Ar 0 o
EtOH (10 e) N2H4-H20

EtOH rt TH AcOH, reflux, 1 min
0

 

\N
2) AcOH |
0

you

Scheme 3—3. Synthesis of 5,5”-diaryl-terpyridines via an aza-Diels-Alder reaction.

I

H

xylene 48h!“r

 

Palladium catalyzed coupling reactions are the most promising for the preparation
of aromatic substituted terpys since there are a wide variety of options to choose from. In

1999, Lehmann et al. described the synthesis of 5,5”-substituted-terpys through and for

64

utilization as building blocks for Stille and Suzuki type coupling reactions.15 Though the
reactions may be hindered by homo-coupling side reaction and problems arise from

difficulty of purification in some cases, the methods are viable. One of these routes

would have been chosen had I not come across the work by Loren and Seigel.5 They
prepared maniterpy, amongst a variety of “manisyl” substituted polypyridines by a
convienient and widely applicable route. I chose to reproduce the preparation of the
maniterpy for our study and apply the route to the preparation of paniterpy as well.

The authors utilized the “manisyl” group to impart two desired features to the
polypyridines. The methoxy group may be conveniently deprotected and realkylated
without incident, allowing for the utilization of these molecules in the synthesis larger
supramolecular structures. The methyl substituents were chosen to enhance solubility of
manisyl polypyridines in organic solvents. The superior solubility is attributed to the
orthogonal conformation about the pyridyl — aryl bond. The steric constraints imposed
by the methyl groups results in the inability to achieve planarity as is seen with mesityl
groups. For our purposes, the hindrance of conjugation is ideal to introduce sterics
without extending delocalization through aromatic substituents; solubility is an added
benefit.

To ensure that the procedure for maniterpy is adaptable to the preparation of

paniterpy, I chose to begin the synthesis from 3,5-dimethylphenol by known methods.

The phenol is initially methylated followed by bromination (Scheme 3-4).8

65

Br

 

1) NaOH/H20 372
> —->
2) (CH3)2$O4 ACOH
77% 57%
OH (92%) OCH3 (69%) . OCH3

Scheme 3-4. Synthesis of 4-bromo-3,5-dimethylanisole.

The procedure is not recommended since the dimethylsulfate reagent is highly toxic, as is
bromine. My yields suffered as compared to the literature since multiple vacuum
distillations were required after each step to obtain pure product. The sequence of the
reaction is important and provides higher yields than initial bromination of the phenol.
With 4-bromo-3,5-dimethylanisole in hand, the maniterpy was prepared through
two sequential Negishi coupling procedures (Scheme 3-5). The first step involves
lithiation and transmetallation to obtain the organozinc reagent which is not isolated. The
palladium catalyzed reaction proceeds via oxidative addition of the halopyridine, the
organozinc undergoes a transmetallation and the product is obtained by reductive
elimination. The 2-bromo-5-manisylpyridine is isolated and purified by column
chromatography. Maniterpy is prepared by the same sequence, but with a different
halopyridine and longer reaction time. The zinc(Il) in solution complexes with the
forming terpy and protects the catalyst from poisoning; ultimately the complexed zinc
must be removed by vigorous extraction with EDTA. The maniterpy is obtained in pure

form, requiring only washing with hexane to remove trace amounts of grease.

66

 

  
 
 
 
 

 

Br ZnCl \N
*1) n-BuLi, -78°C, THF q—QT’W I /
7 5% [Pd(PPh3)4]
OCH3 2) ZnC'2. THF Reflux 15 h THF
-78°C to rt 75% (35%)
f1
1) n-BuLi, -78°C THF B N/ Br _ OCH3
5% [Pd(PPh3)4]
2)_:;°C;2£OT:F Reflux 20 h THF
32% (62%)
H3CO OCH3

Scheme 3-5. Synthesis of maniterpy via Negishi coupling procedures.

The route did not go a smoothly as described, initially the 2-bromo-5-
manisylpyridine was obtained in extremely low yield and maniterpy was not produced at
all. It is extremely important that the reaction be kept dry and the yields greatly
improved after taking the following precautions. THF must be freshly distilled from

sodium benzophenone ketyl pot. The ZnClz was baked for days at high temperature to

remove water and was still not acceptable for this procedure; therefore, 99.99%
anhydrous zinc was purchased and stored in the dry box. The palladium catalyst is light,
heat, and air sensitive and must be freshly opened. 1.6 M n-BuLi in hexanes has a
limited shelf-life and high purity is critical to the success of the reaction. The 2-bromo-5-
ioodopyridine that was freshly prepared resulted in better yield than the commercially
available reagent. The air-free flask containing the solution of halopyridine and catalyst

should be protected from light prior to transfer to the organozinc reagent, and the

67

resulting reaction mixture refluxed in the dark. After optimizing the reaction conditions
the procedure was adapted to the preparation of paniterpy.
The first sequence that was undertaken for the synthesis of paniterpy was the

preparation of the 3,5-diisopropylphenol that was duplicated from a highly efficient

procedure by Diemer et 81 (Scheme 3-6).9 2,6,-diisopropylaniline was brominated in
very high yield followed by a diazotization reaction. The Grignard reagent of l-bromo-
3,5-diisopropyl benzene may be directly oxidized to the phenol in low yield, but a better
method was to prepare the cyclotriboroxane followed by oxidation with hydrogen
peroxide. The cyclotriboroxane product contained some of the boronic acid but can be
used without purification and is converted to the phenol in high yield. The 3,5-
diisopropylphenol may then be brominated, though selectivity at the para position is

complicated by the steric hinderance of the isopropyl groups and a mixture of isomers are

 

 

 

 

obtained.
Br Br
(C4H9l4NBr3 1) HCI, NaN02
CH20'2 2) H3P02 \
92% 82%
NH2 (97%) NH2 (82%)
1) TMEDA, BuLi
-78°C, THF
2) B(OBu)3
OH B—O— ,, 3) HC' BOH2
H202 100°C] 1 Torr
‘—— z
86%
(87%) 81%

3 (67%)

Scheme 3-6. Synthesis of 3,5-diisopropylphenol.

68

While the authors obtained a 3:1 ratio of para and ortho isomers, the selectivity proved to
be worse during my attempt and yielded a 3:2 ratio; only 53% yield of the intended
product. I decided to methylate prior to bromination and greatly improved the selectivity
and obtained 83% yield of the intended product with only a trace amount of the ortho

isomer (Scheme 3-7).

 

OH OCH3 OCH3
1) NaOH/HzO (C4H9)4NBr3>
2) M82304 CH2CI2. MeOH
75% 83%

Scheme 3-7. Synthesis of 1-bromo-2,6-diisopropyl-4-methoxybenzene.

The sequence of Negishi coupling reactions was adapted to the preparation of
paniterpy with a few silght changes. The reaction time was varied to optimize yield
during the preparation of 2-bromo-5-panisylpyridine and the highest yield of 70% was
obtained at 18 h. The orange product mixture was purified by column chromatography
just as the manisyl analog without incident.

Upon completion of the reaction to prepare the paniterpy, the isolation proved to
be quite problematic compared to the maniterpy analog. A white precipitate was present
which was vigorously extracted with EDTA just as before, but the precipitate was
insoluble in both layers and was not the desired product. There was evidence of the
product in the crude reaction mixture, so the extraction was performed and the solvent
removed. The mixture was separated by silica gel column chromatography with 1:1

hexanes/CH2C12 followed by 100% CHZCIZ providing no evidence of product. The left

over brown residue was rinsed from the column with methanol and left overnight, and

69

paniterpy crystallized from the brown residue providing 14% yield. I attribute the low
yield to the Aldrich n-BuLi reagent and coupling by-products. Following the initial
failed attempts, two successful reactions produced enough paniterpy for full
characterization and preparation of the iron(II) complex so the reaction was not further
optimized. Despite the problems, the Negishi reaction conditions proved useful, though

another palladilun catalyzed reaction may be more optimal.

3.3.2 Synthesis of Bis(Terpyridine)iron(II) Complexes

The bis chelate iron(II) complexes are readily prepared by the standard technique.
A solution of iron(II)chloride is added to a suspension of terpyridine and the solution
turns purple upon complexation to the iron(II). Excess ammonium hexafluorophosphate

is then added to metathesize the pure complex, which can then be filtered (Scheme 3-8).

1) FeClz'ZHzO
1:1 MeOH/HzO

L
7

2) NH4PF5

 

 

Scheme 3-8. Synthesis of [Fe(5,5”-R-terpy)2](PF6)2 where R=mani (3), pani (4).

70

Complex 3 was prepared in 64% after heating at 50°C for 24 hours. Complex 4 formed
in 31% yield after heating at 50°C for 18 hours. Since the complexes were obtained in

pure form, the reaction conditions were not firrther optimized. The crystal structure of 3

is published in the literature.7 To date, all attempts to grow X-ray quality single crystals
of 4 have been unsuccessful. The complex crystallized in hair-like structures, therefore,

an X—ray crystal structure of 4 has not been determined.

The complexes were characterized by 1H NMR, ESI-MS, IR, elemental analysis,
and UV-Vis spectroscopy. Since the oxidation of iron(II) is a concern it is pertinent to

ensure the products do not contain any iron(III) impurities. The ESI/MS data show no
evidence of any iron(III) contaminants. The diamagnetic d6 metal complexes are

characterized by IH NMR and the spectra show no evidence of any paramagnetic
impurities. Functionalizing the terpyridine ligands with aryl groups in the 5,5”-positions
resulted in the desired low spin iron(II) coordination compounds upon complexation.

The methyl and isopropyl substituents hinder the ability of the aryl group to
achieve planarity with the rest of the conjugated system, thus reducing the intraligand
delocalization. Ground state absorption measurements were made to ensure that there are

no significant alterations to the electronic structure. Figure 3-3 shows the absorption

spectra of [Fe(terpy)2]2+, [Fe(maniterpy)2]2+, and [Fe(paniterpy)2]2+ in acetonitrile.16

71

 

Ground State Absorption

 

 

 

  

 

12000 - ‘,
‘ L, ,, _
- te
10000 . 1, 1 my, .
j I — — - - manlterpyl
8000 ~ ‘, a ------ paniterpy I
1

Molar Absorptivity (M'10m'1)
O)

 

 

 

4000 ~
2000 ~
0 r . .
350 450 550 650 750
Wavelength (nm)

 

 

 

Figure 38. Electronic absorption spectra of [Fe(terpy)2]2+(—), [Fe(maniterpy)2]2+(3) ( - - ) , and
[1=e(paniterpy)2]2+ (4) (. . .) in acetonitrile.

As with the first two complexes described in chapter 2, these complexes possess
the lowest energy broad feature assigned to 1MLCT transitions. The sharp feature
overlaid with the broad feature at around 550 nm is typical of [M(terpy)2]n+ transitions
and is a result of the ability of terpyridine to delocalize the electron over the three rings. 17
If the aryl groups had introduced further delocalization, the spectra would indicate this.
Complexes 3 and 4 have maxima at 555 nm consistent with [F e(terpy)2]2+ allowing us to

conclude that the differences arise from geometry changes and it is unlikely that there are
unwanted electronic effects. Electrochemical data reinforces this conclusion (Table 3-1).
The difference between the oxidation and first reduction potentials of complexes 3 and 4

are consistent with [Fe(terpy)2](PF6)2, l, and 2; varying minimally across the series

indicating the absence of significant electronic effects.

72

Table 3-1. Oxidation and First Reduction Potentials (mV) of Fe(Il) complexes. Measurements were taken

 

 

by DPV vs Ag/AgNO3, 0.1 M TBAPF 6 in acetonitrile, then corrected to ferrocene/ferrocenium. l6
Complexes Oxidation E “2 (mV) First Reduction E 1/2 (mV)
[Fe(maniterpy)2](PF6)2 (3) +750 -1580
[Fe(paniterpy)2](PF6)2 (4) +890 -1531

 

 

 

 

 

3.4 Concluding Comments

Negishi cross-coupling reactions were suitable for the preparation of 5,5”-aryl-
substituted terpyridines. Though not pursued, other palladium catalyzed coupling
reactions were viable options as well. The new paniterpy ligand was prepared, as well as
the known maniterpy, and complexation of the ligands to iron(II) resulted in the intended
low spin complexes appropriate for the analysis of ultrafast relaxation dynamics. The
large functionalized aromatic substituents were shown to introduce steric bulk without
introducing unwanted electronic effects. The complexes are suitable for the study of the
correlation between relaxation of the initial charge transfer excited state and torsion about

the C2 axis.

73

APPENDIX

 

IIIIIIIIIIIIIIIIIIII

 

 

 

 

 

 

 

I V T Y r ' T r T r I ' I ' I ' I ' I ' 1 ' r ' I ' T ' T ' I ' I ' 1 ' I ' T ''''''

 

 

Figure 3-4. 1H NMR spectrum of 2-Bromo-5-panisylpyridine.

74

 

 

 

 

 

 

 

 

 

 

 

 

 

200 1 80 160 140 120 1 00 80 60 4O 20
ppm

 

r'TfiT'Tfiv'1'I'I'

 

 

13

Figure 3-5. C NMR spectrum of 2-Bromo-5-panisylpyridine.

75

 

 

 

 

 

 

 

100 - 2531 249.1

‘ 43.1

. 211.1
%Fsd 290.0

167.0
77.0 115.0
1 , «I J
0 {1 1 ’11} i1 1]} i '1 ! ii
50 100 150 200 250 300 350
m/z

 

 

Figure 3—6. Direct Probe mass spectrum of 2-Bromo-5-panisylpyridine.

76

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

100.
“Mi/Ml {WWW‘fl/‘\ N p N J‘) . fl) Wm flfl {\wa
1” [WWW ..‘
:0 ~ ‘ ‘1 "; ‘ . E ‘
r ' i ‘ ‘ ‘ I
a 2 ’ ‘1 ‘ "
2 J ‘ ‘ " 756.5
.m _ 1 .
: “ 1302.5
0 1602.3 10813
e ‘ 2961.2 1446.8 '
301
3900 3400 2900 2400 1910QIVavenumb ‘8149100 900 400

 

 

 

Figure 3-7. IR spectrum of 2-Bromo-5-panisylpyridine.

77

 

 

 

 

 

 

. 8:6 ' 6.2 ' 718
ppm
[Lil 1 JP LJ r

 

 

10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0

 

 

 

Figure 3-8. 1H NMR spectrum of maniterpy.

78

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1100—: I me I h I"
Wt.‘ [WW '1‘ ' 11 J11! NV] {‘1er
o s M ‘ 1W WW“ 1 656.5
T/0 11 ‘1 ‘1 11.1 H 1U} 1‘41!
2 2940.2 ’1.- 111'] ' 1 5
.. I} l 1
:1 If ’1 I 823.6
1 1 W H
t 1 1 1072.9
: 1 1603.8 h
8 1445.5
1154.6
40-
3900 3400 2900 2400 1900 1400 900 400
Wavenumbers

 

 

Figure 3-9. IR spectrum of maniterpy.

79

 

 

 

 

 

vvvvvvvvvvvvvvvvvv

 

 

 

 

 

 

 

 

10.0 9.0 8.0 7.0 6.0 5.0 4.0

 

Figure 3-10. 1H NMR spectrum of [Fe(maniterpy)2](PF6)2.

8O

 

 

 

 

 

 

 

 

 

Figure 3-11. ESI masss spectrum of [F e(maniterpy)2](PF6)2.

81

 

TOF MS ES+

1001 529.1

% -
1203.3
338.3 393.3
0 A A. A, .l.¢..l.,.r.-.l.z. 154.1“ . ,... . .,.. _,.. , ,.. 1" . .-fi

200 400 600 800 1000 1200
a ._ ___m1z__.__ W- - _

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

100 ”fl ..
111 WWW V! N WW“ W

- 1 ’ .
95 2918.6 1 f 1 '
% I
1 - 1 1
r 1 J ‘ 557.6
a 1
n 1453.1 1
S . 1156.0
m 1601.0 1314.7 1
- I
I
t -
t
a
n _
C
e
70. 842.5
65 I I I I I T I

3900 3400 2900 2400 1900 1400 900 - 400

Wavenumbers

 

 

 

Figure 3-12. IR spectrum of [F e(maniterpy)2](PF6)2.

82

 

 

 

 

 

 

 

 

 

 

 

 

 

 

10.0 I 0.07 8.0 V 171.0 6.0 ' .

1.0

0.0 I

 

Figure 3-13. 1H NMR spectrum of paniterpy.

83

 

 

 

 

 

 

 

 

 

 

 

 

 

 

v T v I v r v I fl 7

200' ' ’18'0' ' '160 140 120 1&0“ '8'0' '60' ' '40' '2‘0' ' '0'
m

 

 

 

Figure 3-14. 13C NMR spectrum of paniterpy.

84

 

1001 613.6

%FS .
, 614.6

598.6

 

 

  

 

T

vv~v'—v—vvv'vv

520 540 560 580 600 620" 7340"" 660

mlz

 

 

 

Figure 3-15. Direct probe mass spectrum of paniterpy.

85

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

100W m WW
v 1 1 '1 1 11 111111
1 1 1 ' 11 ‘1
1 11
1
T ‘1 1 1 1 11 1
' 1 1 * 11-
a 1 111
n —.
s 1 . 818.4
1m 1045.4
t 1 . 1173.0
t I
a 1 1303.9
n
C
e
1602.5 1445.6
' 2959.2
501
3900 3400 2900 24700 1900 14100 900 400
Wavenumbers

 

 

 

Figure 3-16. IR spectrum of paniterpy.

86

 

 

 

81111.

9.5 9.0 8.5 8.0 7.5 7.0
ppm

 

 

 

 

 

 

 

 

 

 

vvvvvvvvvvvvvvvvvvvvvvvvvvv fir v 1 v 1 v T r I v I v I T I v I

 

 

 

Figure 3-17. 1H NMR spectrum of [Fe(paniterpy)2](PF6)2.

87

 

100 1 641.3 TOF MS ES+

 

 

 

 

1427.6
%
546.3
0 ...- ...3 3. ...-3-.. .33... . - .. ....... - .... .- . 3...-
200 400 600 800 1 000 1 200 1400

m/z

 

 

 

Figure 3-18. ESI mass spectrum of [F e(paniterpy)2](PF6)2.

88

 

 

 

 

 

 

 

 

 

100 _3333
W 731 1"” 3...... 71.1 11" 1111”“1 W1 1111

1 1 11 1 11111 111 1 1

0 V 1 11 ' '

T" 1 1 1174.0 1

r . 1 1306.9 . 1 1

a 1 1 11 558.1

n 2961.2 1 1450.9 1

s 1

m 1601.8 .

i

t

t

a u.

n

C 1

e '1

85. 842.5

3900 3400 2900 2400 1900 1400 900 400
Wavenumbers

 

Figure 3-19. IR spectrum of [F e(paniterpy)2](PF 6)2-

89

 

REFERENCES

Thompson, A. M. W. C., Coord. Chem. Rev. 1997, 160, 1-52.

Schubert, U. S.; Hofmeire, H.; Newkome, G. R., Modern T erpyridine Chemistry.
Wiley-VCH: 2006.

Damrauer, N. H.; Boussie, T. R.; Devenney, M.; McCusker, J. K., J. Am. Chem.
Soc. 1997, 119, 8253-8268.

Kozhevnikov, V. N.; Kozhevnikov, D. N.; Shabunina, O. V.; Rusinov, V. L.;
Chupakhin, O. N., Tetrahedron Letters 2005, (46), 1521-1523.

Loren, J. C.; Siegel, J. S., Angew. Chem. Int. Ed. 2001, 40, 754.

Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.;
Cheeseman, J. R.; J. A. Montgomery, J.; Vreven, T.; Kudin, K. N.; Burant, J. C.;
Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.;
Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.;
Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.;
Kitao, 0.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.;
Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. B.; Yazyev,
0.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;
Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.;
Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, 0.; Malick, D. K.; Rabuck, A.
D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J .; Stefanov, B. B.; Lin, G.; Liashenko, A.; Piskorz, P.;
Komaromi, 1.; Martin, R. L.; Fox, D. J .; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Chen, B. J. W.; Wong, M.
W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision D.01, Gaussian, Inc.:
Wallingford, CT, 2004.

Loren, J. C.; Gantzel, P.; Linden, A.; Siegel, J. S., Org. Biomol. Chem. 2005, 3,
3105.

Pelter, A.; Drake, R., Tetrahedron 1994, 50, 13775.

Diemer, V.; Chaumeil, C.; Defoin, A.; Fort, A.; Boeglin, A.; Carré, C., Eur. J.
Org. Chem. 2006, 2727.

90

10.

ll.

12.

13.

14.

15.

16.

17.

1e Noble, W. J.; Hayakawa, T.; Sen, A. K.; Tatsukami, Y., J. Org. Chem. 1971,
36, 193.

Kréihnke, F., Synthesis 1976, 1.

Sauvage, J .-P.; Ward, M., Inorg. Chem. 1991, 30, 3869.

Potts, K. T.; Usifer, D. A.; Guadalupe, A.; Abruna, H. D., J. Am. Chem. Soc.
1987, 109, 3961.

Dell'Erba, C.; Gruttadauria, M.; Mugnoli, A.; Noto, R.; Novi, M.; Occhiucci, G.;
Petrilloa, G.; Spinelli, D., Tetrahedron 2000, (56), 4565-4573.

Lehmann, U.; Henze, 0.; Schlfiter, A. D., Chem. Eur. J. 1999, 5, (3), 854.

Data collected by Allison M. Brown.

Smeigh, A. L. PhD Dissertation, Michigan State University, 2007.

91

Chapter 4. Concluding Comments and Future Directions

4.1 Concluding Comments

The overall goal of the research project is to develop low cost sensitizers for
DSSCs in the hopes of attaining cost/efficiency ratios necessary to compete with fossil
fuel-based technology. We set out to ascertain whether iron(II) polypyridyl
chromophores are a feasible option to replace the currently utilized ruthenium(II)
polypyridyl chromophores. The research presented herein has laid the groundwork for a
preliminary study to gain information on the relaxation dynamics of iron(II) polypyridyl
complexes.

Iron(II) polypyridyl complexes inherently have a short lived charge transfer
excited state. In attempts to determine the deactivating coordinate of the MLCT excited

state, we are investigating the extent to which torsional about the C2 axis might modulate

MLCT to ligand field kinetics. A series of iron(II) terpyridyl complexes were designed
impose strain to the coordinate of interest.

Though fimctionalized terpyridines are widespread throughout the literature, three
of the four ligands, and the corresponding iron(II) adducts, described in this thesis were
previously unknown. Various synthetic routes were screened to determine applicability
to the preparation of my target molecules. Ultimately, the alkyl-substituted terpys were
prepared via terminal ring cyclization and the aryl-substituted terpys were prepared
through Negishi coupling procedures. Details of the synthesis and characterization of the
compunds were discussed in detail. The series of four iron(II) complexes provide the
foundation of the forthcoming study to analyze the relaxation dynamics of iron(II)

polypyridyl complexes.

92

4.2 Future Directions

This thesis has examined the synthetic means by which a proposed MLCT
relaxation pathway may be analyzed. The steric bulk on the terminal rings of terpyridine
may not be sufficient to make a significant impact on excited state charge transfer
lifetimes. The next approach that we are pursuing involves fixed ligand cage structures; a
rigid environment would restrict motion along any torsional coordinate.

Lehn and coworkers have developed cryptate—type complexes from bipyridine

1 The preparation of the NaBr complex of the macrobicyclic

and phenanthroline groups.
bipyridine cryptand [bpy.bpy.bpy] is currently being pursued as shown in Scheme 4-1.

The starting 6,6’-dimethyl-2,2’-bipyridine may be obtained through a known procedure

 

 

 

from the literature.2
1 \ 2) NaNOZv "'2O 1 \ 10% Pd/C / \ \ /
y ————>

/ v / BTEACI —N N

N NH2 3) NaOH N 3' NaOH, H20 NBS

Reflux 9d 1 MEN
0 O Benzene

O \S/ (9

— 1 a Na —

/ \ \ / 4 ) \NH / \ \ /
—N N ‘ 2) Reflux EtOH —N N
H2804,120°C.

HN NH 2 h Br Br
/ \ _—
N N— / \ _ \
\
/ \ -N N / —N N /
\ / Br Br N (9 N
Reflux MeCN ' ; Na ;
Na2CO3

 

Scheme 4-1. Preparation of [bpy.bpy.bpy] according to the procedure described by Lehn and coworkers.

The structure of [bpy.bpy.bpy] may make an ideal ligand but does raise a point of

concern. It is known that substitution at the ortho position of polypyridines leads to the

stabilization of the high spin form when bound to iron(II).3 The iron-nitrogen bond
lengths in high spin iron(II) complexes are longer than in low spin complexes due to the
population of the antibonding orbitals. If the cage structure is able to bind to iron(II), it is
our hope that the compound is rigid enough to force the low spin ground state upon
binding. If this is the case, then the rigidity of the molecule may destabilize the high spin
excited state, thereby interfering with the rate of relaxation from the charge transfer to the
ligand field states.

A variation of the Lehn cage structure linked through the meta positions of the

bipyridines also appears in the literature (Scheme 4-2).4 This structure may be better able
to bind to iron(II) with a low spin ground state, than the para substituted cage. The imine
and aliphatic capping groups may provide means of further modulating the flexibility of

the structure.

 

Scheme 4-2. Synthetic route for the preparation of a meta-linked bipyridine cage structure.

94

Other future directions of the project are wide open at this point. The
forthcoming study of the deactivation of the MLCT excited states of the iron(II)
complexes described in this thesis may provide us with results that help guide us in the
right direction. Any insight gained on the relaxation dynamics of iron(II) polypyridyls

will bring us one step closer toward the development of a viable sensitizer for DSSCs.

95

REFERENCES

Rodriguez-Ubis, J .-C.; Alpha, B.; Plancherel, D.; Lehn, J .-M., Helv. Chim. Acta
1984, 67, 2264.

Maheswari, P. U.; Lappalainen, K.; Sfregola, M.; Barends, S.; Gamez, P.;
Turpeinen, U.; Mutikainen, 1.; Wezela, G. P. V.; Reedijk, J ., Dalton Trans. 2007,
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Halcrow, M. A., Polyhedron 2007, 26, 3523—3576.

de Mendoza, J.; Mesa, B.; Rodriguez-Ubis, J.-C.; Vazquez, P.; Vdgtle, F.;
Windscheif, P.-M.; Rissanen, K.; Lehn, J.-M.; Lilienbaum, D.; Ziessel, R.,
Angew. Chem. Int. Ed. Engl. 1991, 30, 1331.

96