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This is to certify that the

thesis entitled

PART I: THE SYNTHESIS AND CHARACTERIZATION OF SOME DIHYDRD-
BIS(PYRAZOL—1-YL) BORATE COMPLEXES OF NIOBIUM(IV).

PART II: ESR STUDIES OF SOME SULFUR DONOR COMPLEXES OF

NIOBIUMUV).
presented by

Bobby L. Wilson

has been accepted towards fulfillment
of the requirements for

Ph.D. degree in Chemistry

 

  

Date 5/13/76 i:

0-7639

 

1“.

ABSTRACT

PART I: THE SYNTHESIS AND CHARACTERIZATION OF SOME
DIHYDROBIS(PYRAZOL-1-YL)BORATE COMPLEXES OF
NIOBIUM(IV)

PART II: ESR STUDIES OF SOME SULFUR DONOR COMPLEXES
OF NIOBIUM(IV)

By
Bobby L. Wilson

Eight-coordinate Nb[H2B(Pz)2]4 was isolated from the
reaction of potassium dihydrobis(pyrazol-l-yl)borate
(K[H2B(Pz)2]) with niobium tetrahalides. The infrared
spectrum indicates that H2B(Pz)z' is bidentate. This is
supported by the nmr spectrum of the analogous zirconium
complex. The electronic spectrum exhibits one d-d transi-
tion and one strong band at 340 nm. This intense band in
the ultraviolet region is due to ligand charge transfer.
This assignment was confirmed by observing an identical
band at 340 nm in the analogous zirconium complex. The
esr spectra support a D2d dodecahedral configuration for
the complex with the parameters <g> = 1.955, gll = 1.903,
gi = 1.982 and <a> = 0.0148 cm—l.

Complexes of the type NbX2[H2B(Pz)2]2 (X = Cl, Br and
I) were obtained from the reaction of the niobium tetra-
halides with two moles of K[H2B(Pz)2]. The infrared spectra

indicate that H2B(Pz)2- is bidentate in all three complexes.

Bobby L. Wilson

This is supported by the nmr spectra of isomorphous
ZrClZ[H2B(Pz)2]2. Electronic spectra exhibited one d—d
band in the chloride complex and two d-d bands in the
bromide and iodide complexes. In addition, a band at
340 nm was observed in each case. This band in the ultra-
violet region is due to ligand charge transfer. The assign-
ment of this band was confirmed by observing an identical
band in isomorphous ZrC12[HzB(Pz)2]2. The nmr spectrum of
isomorphous ZrC12[H2B(Pz)z]2 suggests a trans geometry.
This assignment is also supported by the far infrared and
electronic spectra. The esr spectra proved that all the
complexes are monomeric by exhibiting ten lines at ambient
temperature. The esr spectral parameters Show that gi is
greater than gll in all cases. By using the molecular
orbital theory developed for octahedral complexes, metal—
ligand bonding parameters were obtained which indicate
strong mixing of metal and ligand orbitals.

The infrared spectra, obtained by allowing one mole of

K[H2B(Pz)2] to react with one mole of NbX indicate that

4,
H2B(Pz)2- is acting as a bidentate donor. Electronic spectra
exhibited one d-d vibration and one ligand charge transfer
band at 340 nm. The esr solution spectra indicate the
presence of a monomer by exhibiting ten lines at ambient

temperature. The esr Spectra gL values were observed to be

greater than the gll values in all cases.

Bobby L. Wilson

The esr spectra of NbX4(dth)2 (X = Cl, Br and I;
dth = 2,5-dithiahexane) were investigated to determine the
structures of these complexes in toluene or excess dth
solution. This investigation confirmed a trigonal dodeca-
hedral structure, which had been proposed based on the
electronic spectra, with the parameters <g> = 1.954 and
1.973, gll = 1.917 and 1.960, gL = 1.972 and 1.974, and
<a> = 0.0131 and 0.0124 cm’1 for NbCl4(dth)2 and NbBr4(dth)Z
respectively. Metal-ligand bonding parameters, obtained
from the molecular orbital theory developed for D2d com-
plexes, indicate strongly mixed metal and ligand orbitals
in these complexes.

[NbC12(dmtp)2]2 results from the reaction of niobium
tetrachloride with two moles of Nadmtp (dmtp = dimethyl-

dithiophosphate). When [NbCIZCdmtp) is diluted into a

2]2
solution of the analogous zirconium complex, a powder esr
spectrum which is indicative of an exchange coupled dimer
is obtained. The esr parameters are gi‘ = 2.092 and

1

All = 0.0110 cm- 1

The zero field splitting is 0.07325 cm-
which corresponds to a niobium—niobium separation of 3.38 A.
Examination of Nb(dtc)4 diluted into the corresponding
diamagnetic zirconium(IV) matrix by esr methods gives
spectra which are anisotrOpic with two overlapping sets of
ten lines. The esr spectra support the presence of a
trigonal dodecahedral geometry with the parameters <g> =

1.948, gll = 1.902, gi = 1.971 and <a> = 0.0110 cm'l.

PART I

THE SYNTHESIS AND CHARACTERIZATION OF SOME
DIHYDROBIS(PYRAZOL-l-YL)BORATE COMPLEXES OF NIOBIUM(IV)

PART II

ESR STUDIES OF SOME SULFUR DONOR COMPLEXES
OF NIOBIUM(IV)

by

Bobby L. Wilson

A DISSERTATION

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

DOCTOR OF PHILOSOPHY

Department of Chemistry

1976

To The Family

ii

ACKNOWLEDGMENTS

The author wishes to express his sincere appreciation
to Professor James B. Hamilton for being a continual source
of guidance, knowledge, wisdom, and encouragement during
the course of this study.

Special thanks go to Dr. R. N. McGinnis and
Dr. K. Kirksey, whose suggestions, discussions, assistances
and friendships were invaluble and to Professor
Thomas J. Pinnavaia for being my second reader.

It would be impossible to express my gratitude to my
wife, Mary, for her constant love, encouragement and under-
standing because mere words cannot express feelings of such
magnitude. Suffice it to say that I am forever grateful.

I wish to thank my three children, Tony, Fae and Missy for
their love; it has made me a rich man. I wish also to
thank my parents, Mr. and Mrs. Johnny B. Wilson, for their
love, encouragement and prayers.

Sincere appreciation is also extended to Bonnie for
her helpfulness in the preparation of this dissertation,
and to men everywhere whose fundamental purpose is achieve-

ment.

iii

TABLE OF CONTENTS

INTRODUCTION

REVIEW OF PREVIOUS WORK

Complexes of Niobium(IV)
A. Eight-Coordinate Addition Complexes
B. Eight-Coordinate Substitution Complexes

C. Six and Lower Coordinate Complexes of
Niobium(IV)

D. Poly(pyrazol-l-yl)borate Complexes of
Group 2 and d-Transition Elements

PURPOSE OF THIS WORK
EXPERIMENTAL

Materials

Analytical Determinations
Molecular Weight Determinations
Conductance Measurements

Electron Spin Resonance Spectra
Nuclear Magnetic Resonance Spectra
Electronic Spectra

Vibrational Spectra

X-Ray Powder Diffraction Analysis
Syntheses

PART I: THE SYNTHESIS AND CHARACTERIZATION OF
SOME DIHYDROBIS(PYRAZOL—1-YL)BORATE
COMPLEXES OF NIOBIUM(IV)

RESULTS AND DISCUSSION
Preparation and Properties of Dihydro-
bis(pyrazol-l-yl)borate Complexes of Niobium(IV)
Vibrational Spectra
A. Infrared Spectra

B. Far Infrared Spectra (600—100 cm—1

)

iv

15

22
24

24
25
25
26
26
26
27
27
27
29

34
35

35
38
38
43

NMR Spectra of Zr[H2B(Pz)Z]4 and ZrC12[H2B(Pz)2]2 61
X-Ray Powder Diffraction Analyses of

M[H2B(Pz)2]4 and MX2[H2B(P2)2]2 63
Electronic Spectra of Nb[H2B(Pz)2]4 64
Electronic Spectra of NbX2[H2B(Pz)2]2 66
Electronic Spectra of "NbX3[H2B(Pz)2]" 74
Electron Spin Resonance Spectra of Nb[H2B(Pz)2]4 74
Electron Spin Resonance Spectra of NbX2[H2B(Pz)2]2

1n Dichloromethane 84

Electron Spin Resonance Spectra of NbX2[H2B(le2]2
in Ethanol 91

Electron Spin Resonance Spectra of "NbX3[H2B(Pz)2]"

in Dichloromethane 96
PART II: ESR STUDIES OF SOME SULFUR DONOR COMPLEXES

OF NIOBIUM(IV) 102

RESULTS AND DISCUSSION 103

Preparation and Properties of
1,2-bis(methylthio)ethane Complexes of Niobium(IV) 103

Preparation and Properties of Dimethyldithio-

phosphate Complexes with Niobium(IV) 103
Preparation and Properties of Dimethyldithio-
carbamate Complexes with Niobium(IV) 104
Electron Spin Resonance Spectra of NbX4(dth)2 105
Electron Spin Resonance of NbC12(dmtp)2 110
Electron Spin Resonance of Nb(dtc)4 116
SUMMARY AND CONCLUSIONS 119
SUGGESTIONS FOR FURTHER WORK 122

BIBLIOGRAPHY 123

Table

Table

Table

Table

Table

Table

Table

Table

Table

Table

Table

Table

10.

ll.

12.

LIST OF TABLES

Far Infrared Spectral Data for K[H2B(Pz)2]
and Nb[H2B(Pz)z]4 (600-100 cm‘l)

Far Infrared Spectral Data for K[HZB(P2)2]
and NbX2[H2B(Pz)2]2 (600-100 cm'l)

Far Infrared Spectral Data for K[H2B(Pz)2],
ZrC12[H2B(Pz)2]2 and Zr[H2B(Pz)

1 214
(600-100 cm" )

Far Infrared Spectral Data for K[H2B(Pz)2]
and NbX3[H2B(Pz)2] (600-100 cm'l)
Proton Chemical Shifts (T)

Electronic Spectral Data of Nb[HZB(Pz)

1n CHZCI2

214

Electronic Spectral Data of Nsz[H2B(Pz)

1n CHZCI2

212
Esr Spectral Parameters of Nb[H2B(Pz)2]4
Esr Parameters for Eight-Coordinate

Niobium(IV) Species

Calculated “eff Values for Eight-Coordinate
Niobium(IV) Complexes

Solution Esr Spectral Parameters of NszB2
Complexes

Esr Spectral Parameters of NbC12[H2B(Pz)
Dissolved in Ethanol

2]2

vi

45

52

56

60

61

64

72

79

81

83

89

92

Table

Table

Table

Table

Table

13.

14.

15.

16.

17.

Solution Esr Spectral Parameters of
Nb(OCH2CH3)4[HzB(Pz)(Pz-H)]2in Ethanol

Solution (CHZCIZ) Esr Spectral Parameters
of "NbX3[H2B(Pz)2]" Complexes

Esr Spectral Parameters of Niobium(IV)
Complexes

Esr Parameters for NbC12(dmtp)2 Diluted
into ZrC12(dmtp)2

Esr Parameters for Nb(dtc)4 Diluted into
2r(dtc)4

vii

95

96

105

115

117

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

10.

11.

12.

LIST OF FIGURES

Apparatus for Determination of

Electronic Spectra

Infrared Spectrum of K[H2B(Pz)2]

(Nujol)

Infrared Spectrum of Nb[H2B(Pz)

(Nujol)

Infrared Spectrum of NbC12[H2B(Pz)

(Nujol)

214

212

Infrared Spectrum of NbBr3[H2B(Pz)Z]

(Nujol)

Far Infrared Spectrum
(Nujol)

Far Infrared Spectrum
(Nujol)

Far Infrared Spectrum
(Nujol)

Far Infrared Spectrum
(Nujol)

Far Infrared Spectrum
(Nujol)

Far Infrared Spectrum
(Nujol)

Far Infrared Spectrum
(Nujol)

viii

of

of

of

of

of

of

of

K[H2B(Pz)2]

Nb[H2B(Pz)2]4

NbC12[H2B(Pz)2]2

NbBr2[H2B(Pz)2]2

NbI2[H2B(Pz)2]2

Zr[H2B(Pz)2]4

ZrC12[H2B(lez]Z

28

39

40

41

44

46

47

49

50

51

54

55

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

13.

14.

15.

16.

l7.

18.

19.

20.

21.

22.

23.

24.

25.

Far Infrared Spectrum of NbC13[H2B(Pz)2]
(Nujol)

Far Infrared Spectrum of NbBr3[H2B(Pz)2]
(Nujol)

Far Infrared Spectrum of Nb13[H2B(Pz)2]
(Nujol)

0

Electronic Spectra of Nb[H2B(Pz)2]4 1n
CHZCI2

Distortion of a Cube

The Crystal Splitting for D2d and D4d
Symmetries

Electronic Spectra of NbC12[H2B(Pz)2]2
in CHZCIZ

Electronic Spectra of NbBr2[H2B(Pz)2]2

1n CHZCI2

Electronic Spectra of NbIZ[H2B(Pz)

1n CHZCI2

2]2
Esr Spectrum of Nb[H2B(Pz)2]4 in Toluene
at Ambient Temperature

Esr Spectrum of Nb[H2B(Pz)2]4 in Toluene
at 77 K

Esr Spectrum of Solid Nb[H2B(Pz)2]4 at
Ambient Temperature

Esr Spectrum of NbBr2[HzB(Pz)2]2 in

CHZCI2 at Ambient Temperature

ix

57

58

59

65

67

68

69

70

71

75

76

77

85

Figure 26. Esr Spectrum of NbC12[H2B(Pz)2]2 in

CHZCI2 at 77 K 86

Figure 27. Esr Spectrum of NbBr2[H2B(Pz)2]2 in

CHZCI2 at 77 K 87

Figure 28. Esr Spectrum of NbI2[H2B(Pz)2]2 in

CHZCI2 at 77 K 88

Figure 29. Esr Spectrum of Nb(OCH2CH3)4[H2B(Pz)(Pz-H)]2
in CHSCHZOH at Ambient Temperature 93

Figure 30. Esr Spectrum of Nb(OCH2CH3)4[H2B(Pz)(Pz-H)]2

in CH3CH20H at 77 K 94
Figure 31. Esr Spectrum of NbI3[H2B(Pz)2] in CHZCI2

at Ambient Temperature 97
Figure 32. Esr Spectrum of NbC13[HZB(Pz)2] in CHZCI2

at 77 K 98
Figure 33. Esr Spectrum of NbBr3[H2B(Pz)2] in CHZCIZ

at 77 K 99
Figure 34. Esr Spectrum of Nb13[H2B(Pz)2] in CHZCI2

at 77 K 100
Figure 35. Esr Spectrum of NbC14(dth)2 in Excess

2,5-dithiahexane at Ambient Temperature 106
Figure 36. Esr Spectrum of NbC14(dth)2 in Excess

2,5-dithiahexane at 77 K 107
Figure 37. Esr Spectrum of Solid NbC14(dth)2 at

Ambient Temperature 108
Figure 38. Esr Spectrum of [NbC12(dmtp)2]2 diluted

into ZrC12(dmtp)2 at 77 K 111

Figure 39. Proposed Structure for Nb2(dmtp)4X4 112

Figure 40. Esr Spectrum of Nb(dtc)4 Diluted into
Zr(dtc)4 at 77 K 118

xi

INTRODUCTION

The niobium tetrahalides of fluorine, chlorine, bro-
mine, and iodine have each been prepared by several
methods.1-3 The preparation used in our laboratory involves

the thermal gradient method.4’S

The niobium(IV) halides
are diamagnetic polymers whose basic unit can be represented
as Nb2X8. They are dark solids and will thermally dis-
proportionate to form the corresponding trihalides and
pentahalides.4-9
Many niobium(IV) halide adducts and complexes have been
formed from the niobium tetrahalides or as a result of the
reduction of the pentahalides. The magnetic properties of
these complexes have proven most interesting. The expected
spin-only magnetic moment of a d1 transition element is
1.73 B. M. Although this value is found in some of the
complexes, many show lower values. Some are diamagnetic
and this behavior has been attributed to Nb-Nb interactions
in the polymeric compounds.5’10’11
The coordination number of niobium(IV) is usually six
but as expected for relatively larger early second row
transition elements, complexes with higher coordination
numbers exist. There is even evidence12 for nine-coordinate
Nb(acac)4 dioxane. Eight—coordinate complexeslz‘18 are more

common and have been prepared, studied and shown to be para—

magnetic with magnetic moments ranging from 1.6 to 1.9 B. M.

The formation of direct Nb-Nb bonds in these complexes is
hindered by the fact that the niobium is coordinatively
saturated.
Lower coordination numbers are also known for niobium—
(IV).19’20

The six-coordinate complexes, NbX4LZ, could have D4h or
C2V symmetry and both exist. Fowles, et al. have assigned
the geometry of a series of niobium(IV) compounds, mainly
with nitrogen donor ligands,21 while Hamilton and McCarley19
have worked with sulfur donor ligands. They have shown that
sulfur donor ligands form complexes in which the coordina-
tion number of niobium(IV) varies from a low of five to at
least eight. More recent studies with 1,1-dithiols, for
example, dialkyldithiophosphates18 and dialkyldithiocarba-

mates13,22,23

reveal for the first time complexes of
niobium(IV) in which a significant metal-metal interaction

has a substantial impact on observable magnetic behavior.

REVIEW OF PREVIOUS WORK

This section shall consist of (l) a discussion of known
complexes of niobium(IV) and (2) a consideration of some
poly(pyrazol-l-yl) borate complexes formed by transition
metals in the +2 and +4 oxidation states. There have been
several reviews on the chemistry of niobium halides and

3’24’25 A recent review26 by Trofimenko

their complexes.
considers the coordination chemistry of the poly(pyrazol-l-yl)
borate ligands. In this review therefore primarily those

aspects are discussed which are relevant to later discussions

of results.

Complexes of Niobium(IV)

 

A. Eight-Coordinate Addition Complexes

Clark and coworkers15 prepared eight—coordinate
complexes with the bidentate ligand o-phenylenebis(dimethyldi-
arsine) (or diarsine). These complexes were prepared by

heating NbX4, NbX5 or NbOX with diarsine in a sealed

3

evacuated tube. The NbX4-2 diarsine complexes are isomorphous
with the dodecahedral titanium(IV), zirconium(IV), and
hafnium(IV) halide diarsine complexes.27 The diffuse

reflectance spectra exhibit four d-d transitions as expected

4

for dodecahedral complexes. The magnetic moment of 1.69
Bohr Magnetons, calculated from the esr spectrum of the

chloride complex, is consistent with a single d electron
in an orbitally non-degenerate ground state.

Deutscher and Kepert16 obtained eight-coordinate com-
plexes using 4-methyldiarsine and 4-ethyldiarsine as ligands.
These compounds were assumed to be dodecahedral based on the
similarity of the visible spectra to those of the diarsine

complexes. Solutions of NbCl4(Etdiars)2 exhibit maxima at

9.75 kK, E

\)

10; v = 12.8 kK, E = 26; and

max max

17.5 kK, 5

v 6. The spectrum of NbBr4(Etdiars)2

max
exhibits maxima at 12.72 and 15.92 kK and the iodide complex
gives maxima at 10.1 and 12.7 kK. The effective magnetic
moments are 1.63, 1.67 and 1.80 B. M. for the chloride,
bromide and iodide complexes, respectively, as expected for
a single d electron in a non—degenerate ground state. The
high moment of the iodide complex was attributed to a large
temperature independent paramagnetic contribution.

Hamilton and McCarley14 obtained eight-coordinate com~
plexes with composition NbX4(dth)2 using the sulfur donor
ligand 1,2—bis(methylthio)ethane (more commonly referred to
as 2,5-dithiahexane or dth). The adducts are all paramagnetic.
The far-infrared spectra indicate that the ligand is bidentate
and metal-halogen and metal-sulfur stretching frequencies

were assigned. The solid state electronic spectra are similar

to those reported by Clark for NbX4(Diars)2.15 The magnetic

S
moments for the chloride, bromide and iodide complexes were
reported to be 1.60, 1.61 and 1.28 B. M. respectively. Esr
spectra were observed only for the solid (powders) complexes.
The spectrum of the solid chloride complex consists of a
broad asymmetric band with <g> = 1.92, gi = 1.98 and gll = 1.80.
The complexes were proposed to be dodecahedral on the basis
of the solid state electronic spectra and the esr spectrum
of the chloride. This was later confirmed by Wilson and
Hamilton28 who obtained solution esr spectra and found
gi > gll in the case of the chloride and bromide. In the
case of the iodide the esr spectra were inconclusive. It
has been demonstrated that gi > gll for a dodecahedral
complex and that gll > gi for a square antiprismatic com-

29-31

plex. All of the complexes were assumed to be dodecahedral

since gL > gll.

B. Eight-Coordinate Substitution Complexes
A number of eight-coordinate substitution complexes
have been produced using beta-diketonates. Deutscher and

Kepertlz’32

prepared niobium(IV) complexes of acetylacetone
(Acac), benzoyltrifluoroacetone (Bta),thenoyltrifluoroace-
tone (Tta) and dibenzoylmethane (Dbm). Complexes with the
ligands 8-hydroxyquinoline (OX) and trOpolone (T) were also
prepared. The complexes were prepared by allowing the appro-
priate niobium tetrahalide to react with the free ligand in

an acetonitrile-triethy1amine solution. The infrared spectra

proved the ligands to be bidentate and equivalent in all

cases. The diffuse reflectance and solution spectra do not
indicate any d-d transitions. The ambient temperature mag—
netic moments range from 1.43 to 1.66 B. M. except for NbT4
which has a magnetic moment of 0.74 B. M. Powder X-ray
patterns indicate Nb(acac)4 is not isomorphous with the

33

square antiprismatic Zr(acac)4; Nb(Tta)4 is not isomorphous

34

with square antiprismatic Zr(Tta)4; and Nb(Dbm)4 is not

isomorphous with the square antiprismatic Th(Dbm)4.35 The
esr spectra of Nb(acac)4 and Nb(Dbm)4 Show gi > gll. All
of the niobium complexes were assumed to be dodecahedral
since gi > gll and they are not isomorphous with known
square antiprismatic complexes.

Podolsky36 prepared the tetrakisdipivaloylmethane (me)
complex of niobium(IV) by reaction of niobium tetrachloride
with the free ligand in acetonitrile. The visible spectrum
exhibits only two d-d transitions at 14.2 kK, gmax = 280
and at 15.3 kK, gmax = 636. Ten lines are observed in the

esr spectrum with <g> = 1.95 and a hyperfine splitting of

110 gauss. The anisotropic constants are found to be:

gL = 1.928, gll = 1.997, A1 = 141 gauss, and All = 53 gauss.
The structure of Nb(me)4 was proposed to be a D4 square
antiprism on the basis of gll > gi and the observation of

only two d-d transitions. This was later confirmed by
X-ray structural analysis37 and represents the first reported

case of a square antiprismatic niobium(IV) complex.

A number of workers have prepared eight-coordinate
niobium(IV) complexes with diethyldithiocarbamates

(Detc).13,22,23

The ir spectrum indicates the ligand is
bidentate. A band observed at 360 cm"1 is assigned as a
metal-sulfur stretching mode. The visible spectrum exhibits
one d-d band at 363 mu with gmax = 48. The magnetic moment
is 1.57 B. M. indicating a single unpaired electron. No

esr was reported.

Kirksey38 isolated from the reaction of ammonium piper-
dinyldithiocarbamate (NH4pipdtc) with niobium tetrahalides
the eight-coordinate Nb(pipdtc)4 complex. The infrared
spectrum indicates that piptc' is bidentate. The electronic
spectrum exhibits four d-d transitions and supports a D2d
dodecahedral configuration for the complex. Esr spectra
confirm this geometry with the parameters <g> = 1.9677,
gll = 1.9155, gi = 1.9938, and <a> = 0.0195 cm 1, where
gl’gH'

McGinnis and Hamilton18 isolated eight-coordinate
Nb(Detp)4 from the reaction of sodium diethyldithiophosphate
(NaDetp) with niobium tetrahalides. The infrared spectrum
indicates that Detp- is bidentate. The electronic spectrum
exhibits three d-d transitions and supports a D2d dodeca-
hedral configuration for the complex. The esr spectra values
of <g> = 1.952, gll = 1.895, gi = 1.982 and <a> = 0.014 cm’1

confirm this geometry with 81 > gll.

17’39 isolated the stable

Griffith and coworkers
K4[Nb(CN)8], ZHZO salt by the reduction of methanolic
solution of niobium pentachloride at a mercury pool cathode
followed by reaction with concentrated aqueous potassium
cyanide. The salt is paramagnetic with a magnetic moment
of 1.69 B. M. at 293K. The esr spectra of the salt indicate
a change in configuration from D2d in the solid state to D4d

in solution. The vibrational and electronic spectra support

this structural change.

C. Six and Lower Coordinate Complexes of Niobium(IV).

In addition to the large number of mononuclear, para-
magnetic, eight-coordinate niobium(IV) complexes, a number
of mononuclear, paramagnetic, six-coordinate niobium(IV) and
dimeric niobium(IV) compounds have been prepared. The mag-
netic behavior of the mononuclear six-coordinate complexes
is very similar to the magnetic behavior of the eight-
coordinate complexes but the dimeric complexes all have
small magnetic susceptibilities indicating retention of the
metal-metal bond.

40-41

Safonov and Khorschunov prepared the binary

systems NbCl -MC1 (M = Na, K, Rb and Cs) by the direct re—

4
action of niobium(IV) chloride with the alkali metal halides
and found evidence for the congruently melting compounds
MszCl6. Morozov and Lipatova42 prepared the ammonium,

rubidium, and cesium hexachloroniobate(IV) by mixing to-

gether concentrated hydrochloric acid solutions containing

niobium tetrachloride and the salt MCI (M = NH4+, Rb+ and
Cs+).

Walton and coworkers21 prepared the hexahalo salts
[(CZH5)4N]2NbX6 by treating the apprOpriate acetonitrile
complex NbX4°2CH3CN with tetraethyl ammonium halides using
chloroform-acetonitrile as solvent. The octahedral species
NbX6-2 is used as a "model" for comparison of related
measurements on coordination complexes of the types NbXZ'ZL
and MX4°B, where M = Nb or Ta; X = C1 or Br; L = acetoni-
trile, tetrahydrofuran, tetrahydropyran, or 1,4-dioxane;
and B = 2,2'-bipyridyl or 1,10-phenanthroline.

Knox and Brown43

obtained the complex anions Nb(NCS)6-,
Nb(NCS)6-Z, and Ta(NCS)6‘, respectively when potassium

thiocyanate was allowed to react with NbCl NbC14, and

5’
TaCl5 in acetonitrile. Infrared data indicate that the
thiocyanate groups are nitrogen bonded in all cases. A

detailed study of the conductance of these compounds in

acetonitrile showed that dissociation takes place at low

concentrations.
Reductions of NbCl5 in concentrated hydrochloric acid
solutions by using mercury electrode were reported by Cozzi

and Vivarelli.44

At 13N HCl, the solutions were red-orange.
As the HCl concentration was lowered, the solutions turned

blue.

10

The niobium(IV) species in the blue solutions gave absorption
maxima at 14.3 kK and the species present was believed to he
NbOCl4-2. The red-orange species exhibited a band at 20.8 kK.
Wentworth and Brubaker45 prepared complexes of the
formula Nb(OR)ClS-2 by the electrolytic reduction of NbCl5
in HCl-saturated alcohols. The compounds exhibited spin-only
paramagnetism for the d1 ion. The molar magnetic suscepti—
bilities exhibited Curie-Weiss dependence upon reciprocal
temperature in all cases. Rasmussen, Kuska, and Brubaker46
obtained esr spectra of the methoxo complex at ambient
temperature and 77 K. From the spectra it was found that
gll = 1.965, gi = 1.809, All = 248 gauss and A1 = 144 gauss.
A molecular orbital treatment was consistent with covalent
chlorine-niobium Sigma bonds and appreciable n-bonding by
the chlorine atoms. The molecule was assigned C4V symmetry.
Wentworth and Brubaker47 found that treatment of the
above reduced alcoholic solutions with pyridine produced a
diamagnetic species NbCl(OCH2CH3)3Py. On the basis of the
nonlability of the chlorine atoms, the molecular weight
data and the low susceptibility, the complex was formulated
to be dimeric with bridging chlorine atoms and a direct
metal-metal bond. When the dimer was treated with sodium
ethoxide, Nb(OCH2CH3)4 was obtained. This compound is also
diamagnetic with a corrected susceptibility of -100 x 10-6

c.g.s. units. Direct metal-metal bonds were preposed to

account for the observed magnetic behavior.

11

Djordjevic and Katovic48 isolated paramagnetic
Nb2C15(OCH2CH3):5 (bipy)2 (bipy = 2,2'-bipyridine) from an
ethanol solution containing NbCl4 and bipyridine. Properties
of the compound suggested its formulation as an ionic deriv-
ative containing the ions (Nb(OCH2CH3)2(bipy)2+2 and
(NbC15(OCH2CH3))2-.

McCarley and coworkers49 prepared NbX4(py)Z (X = Cl and

Br; py = pyridine) by reactions of NbX with excess pyridine.

5
McCarley and Torp5 obtained the same species as well as
NbI4(py)2 from reactions of NbX4 with pyridine at ambient
temperature. Visible spectra of pyridine solution exhibited
bands with maxima at 20.6 kK for the chloride and 20.7 and
13.9 kK for the bromide. With larger extinction coefficients
than expected for ”d-d" transitions, the bands were attributed
to either pyridine-to-metal or metal-to-pyridine charge
transfer.

Brown and Newton50 studied the reactions NbCl4 and
NbX4 (X = Cl, Br and I) with triethylamine and N,N,N',N'-
tetramethylethylenediamine respectively. 1:1 adducts were
isolated which were diamagnetic in the case of the triethylamine.
This indicated that the metal—metal bond of the tetrahalide
had been retained. The diamine gave products of the formula
MX4'B. The visible spectra of the bidentate complexes were

interpreted by using a tetragonally distorted octahedral

model.

12
Bradley and Thomas51 isolated Nb(NR2)4 (R = CH3,

CH CH CH3(CH2)2 and CH3(CHZ)3) from the direct reaction

3 2’

of LiNR and NbCl . The oxidation state of niobium in these

2 5
compounds was determined by treatment of their HZSO4-ethanol

solutions with excess FeCl followed by titration of the

3
FeCl2 formed with standard ceric sulfate solution.

Machin and Sullivan22

reported substitution reactions
of the tetrahalides with potassium cyanate, thiocyanate,
cyanide and borohydride and sodium diethyldithiocarbamate
(Na dtc). Nb(NCS)3C1 was obtained from the reaction of
niobium tetrachloride with potassium thiocyanate. The
magnetic susceptibility is extremely small on the order of

6

40 x 10' c.g.s. units and independent of field strength.

This value is very close to NbCl indicating similar

4
structures. The diffuse reflectance spectrum exhibits an
asymmetric band at about 18,000 cm“1 from the reaction of
niobium tetrachloride with potassium cyanate Nb(CNO)3C1
was obtained. The magnetic susceptibility for this com-
pound is also low, be being of the order of 90 x 10.6 c.g.s.
units. Four bands were observed, at 9,000; 15,000; 22,000;
and 30,000 cm—1 in the diffuse reflectance spectrum. The
origin of the bands is not clear. When potassium cyanide
was allowed to react with niobium tetrachloride in the
acetonitrile solution, a compound which supports the for-
mulation NbC13(CN)(CH3CN)2 was obtained. The magnetic
susceptibility is only 50 x 10_6 c.g.s., which is very low

for a formulated six-coordinate monomer. The diffuse

13

reflectance spectrum Shows only one band at 22,000 cm 1.
The reaction between NbCl4 and a two-fold molar excess of
KBH4 in acetonitrile solution produced (CHSCN)2NbC12(BH4)Z.
This compound is also diamagnetic with a be of only

40 x 10.6 c.g.s. units. When less than four moles of sodium
diethyldithiocarbamate are allowed to react with niobium
tetrabromide, Nb2(Detc)5Br3 is obtained. This complex is

a weak 1:1 electrolyte in nitromethane and [Nb2(Detc)5Br2]Br
is proposed. The magnetic susceptibility is markedly field
dependent. The diffuse reflectance spectrum shows bands at

1 and 19,500 cm.1 with a shoulder at 16,000 cm—l.

52

23,500 cm-

Fowles, Tidmarsh and Walton obtained NbCl4°S(CH

3)2

by the direct reaction of NbCl and dimethylsulfide. The

4
complex was reported to be antiferromagnetic with a magnetic

moment of 0.44 B.M. at ambient temperature. The electronic
spectrum exhibited two bands at 11.2 and 16.0 kK and were
asSigned as d-d transitions. The magnetic moment and far
infrared spectrum of NbC14'S(CH3)2 suggest that this species

is structurally similar to the related dl antiferromagnetic

and TiCl -ZSC H 53

3 3)2 3 4 8'

The reactions of tetrahydrothiOphene (tht) with NbCl4 and

NbBr4 were also investigated. This ligand formed diadducts

titanium(III) derivatives TiCl °ZS(CH

with both NbCl4 and NbBr4. However, two different forms of
the bromide complex were obtained. The far infrared spectra
of the chloride and a~bromide species were similar. The

expected v(Nb—X) and v(Nb-S) were observed in the region

14

340-240 cm.1 with the appropriate shift for the change in
the halide in both NbCl4(tht)2 and a-NbBr4(tht)2. In the
B-bromide complex only one strong band at 227 cm.1 was
observed. The NbCl4(tht)2 and a-NbBr4(tht)2 were assigned
a cis configuration and the B-NbBr4(tht)2 was assigned a
trans configuration. Although most bis-adducts of niobium
tetrahalides have been assigned as cis structures, there
has been a trans structure reported for DMF complexes.54
Bereman55 proposed a trans structure for certain pyridine
adducts on the basis of esr data.

Hamilton and McCarley19 also investigated some mono-
dentate alkyl sulfides including S(CH3)Z. From the reaCtion
of dimethylsulfide with NbCl4 and NbBr4 complexes of two

types NbX4[S(CH3) and NbX4[S(CH3)2]2 were obtained in

2]
benzene. Under a dynamic vacuum the diadduct recovered

from the original reaction mixture loses one mole of dimethyl-
sulfide over a period of twelve hours yielding the monoadduct.
The monoadducts are weakly paramagnetic with room temperature
magnetic moments of 0.36 and 0.50 B.M. for the chloride and
bromide complexes, respectively. While no evidence was

found for an antiferromagnetic interaction as proposed by

52the same molecular structure was

Fowles and coworkers,
proposed.

McGinnis56 prepared complexes of the general formula
Nb2X4(dmtp)4 (X = Cl, Br and I; dmtp = dimethyldithiophosphate)

by allowing stoichiometric amounts of the tetrahalides to

15

react with sodium dimethyldithiophosphate. (The chloride and
bromide complexes were reported to be diamagnetic while the
iodide exhibited antiferromagnetism with a singlet-triplet
separation of -l40 cm.1 and a magnetic moment of 2.32 B.M.
at ambient temperature. The esr spectrum of the iodide
complex showed both the normal AmS = i 1 transition and the
"forbidden” AmS = i 2 transition. This is the first
reported niobium(IV) electron-exchange coupled species.
Kirksey38 prepared complexes of varying stoichiometries
when two moles of ammonium piperdinyldithiocarbamate
(NH4pipdtc) were allowed to react with the tetrahalides.
In each complex the pipdtc_ was bidentate as determined
from infrared spectra. Two bands were assigned as d—d
transition in the visible spectrum. Esr spectra of these
species were unresolved single peaks at both ambient and
77°K temperatures. However, when the chloro compound was
diluted into a solution of the disubstituted zirconium
complex a powder esr spectrum was obtained which was
indicative of an exchange-coupled dimer. The esr parameters
are gll = 1.7873 and All = 0.00642 cm-l. The zero field

splitting is 0.05778 cm.1 which corresponds to a niobium-

niobium separation of 3.30 A.

D. Poly(pyrazol-1-y1)borate Complexes of Group 2 and d-
Transition Elements.

Since TrofimenkoS7 first reported the poly-

(pyrazol—l-yl)borate ligand a wide variety of Group 2 and

l6

d-transition-element complexes have been described.58 The
poly(pyrazol—l-y1)borates, anions of the general structure
[HnB(Pz)4_n], where pz = l—pyrazol—l-yl and n = 0, l and 2,
have established themselves as a remarkably versatile class
of ligands as summarized in a 1971 review.59 By appropriate
control of substituents they can be made to function as
bidentate ligands, analogous to B-diketonates, as tridentate
ligands of C3v symmetry analogous to the cyclopentadienide
anions and even as tetradentate (bis-bidentate) ligands in
binuclear complexes.60

The poly(pyrazol-l-y1) borate ligands are readily
available by the reaction of an alkali metal borohydride
with pyrazole, the extent of substitution depending on the

61

reaction temperature as shown in Scheme 1. These salts

are remarkable in that, on acidification, they yield
isolable and stable free acids (unlike any other BR4-
species) which may be converted via neutralization with
NR4OH to quaternary ammonium salts, unavailable by the
direct route.

Ligands containing C substituents are prepared by using
an appropriately substituted pyrazole in the above scheme,
while B-substituted ligands are obtained by starting with

a [BRnH4-n] spec1es 1nstead of BH4

 

17

Scheme I

— /
3H4. 2(\,,

 

 

 

 

HN—N
_ 12ml _...
H H
\B/
>1\N/ \N»N
<9 Q
180° HPz
7‘ I/A/ ‘—
H N-N
\
B
N~N/ N—-N
/ \
-9 \ 1
220° HPz
(S C»
\N N N /
\B/ N
N-—-N/ \N-—-N

<2 Q

 

18

The reaction of the parent bidentate ligand H2B(Pz)2-

with most first row transition metal ions in the 2+ state gives

59,61

rise to monomeric chelates [H2B(Pz)2]2M. The isomorphous

Ni and Cu chelates are square planar, while chelates of Mn,
Fe, Co, and Zn are tetrahedral.62 An X-ray structure
determination of [H2B(Pz)2]2Co confirmed the above assign-
ment.63 These chelates are precipitated immediately when
aqueous solutions of KH2B(Pz)Z and the appropriate metal ion
are mixed. Similarly, the compounds M[H2B(Pz)2]2 are pre-

cipitated when M is Pb2+ or Cd2+, but not when M is Mg2+,

Ca2+, Sr2+ or Ba2+.61 Ag+, Pd2+

and Hg2+ ions are reduced
to the free metals. Compounds M[H2B(Pz)2]2 are extractable
with organic solvents, particularly well with methylene
chloride. They are stable to air and moisture and can be
stored in the solid state for years without decomposition,
with the exception of the unstable, air-sensitive
Mn[H2B(Pz)2]2 and Fe[H2B(Pz)2]2 derivatives.64
Compounds of the structure M[HB(Pz)3]2 are precipitated
immediately upon mixing solutions of an alkali hydrotris-
(pyrazol-l-yl)borate and of a divalent transition metal ion.

In addition, such compounds are also precipitated with Mg2+,

Pb2+, Cd2+ and Pd2+ ions. The compound AgHB(Pz)3 is obtained
using Ag+ ion. On heating the Ag and Pd compounds decompose
readily with formation of the free metal. From more con-

centrated solutions the Mgz+ and Ca2+ ions are also pre—

cipitated by the HB(Pz)3' ion.

19

The transition metal compounds of M[HB(P2)3]2 are all
solids with high-melting points and are sublimable in vague.
They are sparingly soluble in polar solvents such as alcohols
or acetone but are readily dissolved by halocarbons and
aromatic hydrocarbons and may be conveniently recrystallized
from them. The nmr spectra Show only one kind of pyrazolyl
group. The octahedral nature of these compounds is also
supported by electronic spectra, magnetic data, and nmr

studies of paramagnetic compounds.62 X-ray crystal structure

determination on Co[HB(Pz)3]2 confirmed the above structure.65
The B(Pz)4- ligand can act in a bidentate, tridentate
or tetradentate fashion. As a tridentate ligand compounds
M[B(Pz)4]261 are formed which have the same octahedrally
coordinated structure around the metal ion as M[HB(Pz)3]2
but in many other ways they are different. They are prepared
by substitution and have the same colors as their M[HB(Pz)3]Z
counterparts. They, too, are sublimable but are more
thermally stable, less soluble in organic solvents and have
higher melting points. In addition to divalent first-row

2+ 2+

transition metal ions, Cd Pd , Hg2+ and Ag+ are readily

precipitated from aqueous solutions. In the alkaline earth
group Mg2+ is precipitated readily, Ca2+ less readily, and
Sr2+ and Ba2+ are precipitated from very concentrated
solution. The nmr and infrared spectra have proved that
one of the four pyrazolyl groups attached to boron is

different from the other three.63’66

20

The B(Pz)4- acts in tetradentate fashion when pairs of
N termini are bridged by appropriate four-coordinate species.
Trofimenko60 obtained [LZPd(Pz)2B(Pz)ZPdLZ]+ (L = n—allyl)
when B(Pz)4' was allowed to react with two equivalents of
n—allylpalladium chloride dimer. In this species the ligand
acts in a bis-bidentate (tetradentate) fashion. The nmr
Spectrum of this cation is fluxional, limiting spectra being
observed at 87 and -44°. At 87° all four pyrazolyl groups
are spectroscopically equivalent, while at -44° two different
types of pyrazolyl groups are present.

Cotton and coworkers67 reported the spectrosc0pic studies
on [B(Pz)4](CSH5)(CO)2Mo which, together with preliminary
results from a single crystal X-ray crystallographic study,
show the B(Pz)4- ligand to be bidentate. The six-membered
metallocyclic ring can exist in two conformers in solution,
and these interconvert, with an activation energy of the
order of 10 kcal/mol, thus giving rise to extensive variations
in the proton nmr spectrum as the temperature is varied.

Examples of complexes of these ligands with elements in
oxidation state IV appear to be unknown except for some
recently reported uranium(IV) complexes. Bagnall and
coworkers68 obtained U[HB(Pz)3]4, U[H2B(Pz)2]4 tht, and
[UC12(HB(P2)3)2] when uranium(IV) tetrachloride was allowed
to react with stoichiometric quantities of the potassium
salt of the appropriate anion in tht. These complexes are
soluble in dichloromethane and dimethylsulphoxide (dmso).

The tetrakis complexes are also soluble in acetone, benzene,

dme and thf.

21

Steric effects play an important role in polypyrazolyl-

borates. In bidentate chelates the B(NN)2M ring is puckered

59

in the boat form. This leads, in the case of CSHS-

3, C2F5, CFBCFZCFZ,

69

CoRf(Pz)nBH (R = CF and (CF3)2CF) to

4-n f
isolable geometric isomers. It also brings boron
substituents into interaction distance with the metal,
including an example of an aliphatic three—center two electron
C—H-M bond in EtZB(Pz)2Mo(CO)2-n-CH2C¢CH3 such bonding being
even able to compete effectively with olefinic n—bonding.7O
The tridentate RB(Pz)3— ligands form a host of half-
sandwich complexes resembling those derived from CSH5 but,
generally, much more stable. This characteristic was
exploited in preparing a stable cepper carbonyl, HB(Pz)3CuCO71
and a variety of stable, five—coordinate Pt(II) complexes.
Of interest are also hybrid sandwiches exemplified by

73

[B(Pz)4RuC6H6]PF6. The examples cited above indicate the

RnB(Pz)4_n- ion to be a most versatile ligand.

PURPOSE OF THIS WORK

While a number of eight-coordinate mononuclear, neutral
complexes of niobium(IV) have been prepared by the complete
substitution of the niobium tetrahalides with oxygen and
sulfur donor bidentate ligands no such species has been
reported with a nitrogen donor ligand. In order to prepare
such a complex a uninegative bidentate ligand is needed.
Therefore potassium dihydrobis(pyrazol-l-yl)borate
(K[H2B(Pz)2]) was used. This ligand has been found to form

58,68 It

a wide variety of d-transition-element complexes.
was anticipated that by replacing all of the halogen atoms
in NbX4 an eight-coordinate complex could be prepared.

It also seemed of interest to investigate the replace-
ment of only one or two of the halogen atoms in NbX4. By
replacing two halides either a six-coordinate monomer or a
seven-coordinate dimer with bridging halogen atoms could be
obtained. By replacing only one of the halogen atoms either
a monomeric five-coordinate complex or a six-coordinate
dimer would be expected.

The primary goal of this work then was to gain insight
into the structure and magnetic behavior of disubstituted
and fully substituted complexes formed by reaction of

K[H2B(Pz)2] with NbX (x = Cl, Br, I).

4
22

23

In order to attempt to determine the structure and mode
of bonding in the eight—coordinate adducts NbX4(dth)Zl4’28
(X = Cl, Br and I; dth = l,2-bis(methylthio)ethane, more
commonly referred to as 2,5—dithiahexane), the esr spectra
of the solids dissolved in toluene or excess ligand were
investigated and -- in conjunction with the electronic
spectral data -- the esr data are used to determine the
applicability of an ionic model to the systems.

In an attempt to investigate further exchange coupling
in niobium(IV) complexes, the dimethyldithiophosphate
system56 was reinvestigated. It was hoped that by diluting
NbC12(dmtp)2 into ZrC12(dmtp)2 powder (dmtp = dimethyl-
dithiophosphate) an exchange-coupled complex could be pre-
pared from which more insight into the metal-metal bonding
which characterizes much of the chemistry of niobium(IV)
would be obtained. The dimethyldithiocarbamate analogue

was also investigated in the corresponding zirconium(IV)

powder.

EXPERIMENTAL

All the compounds synthesized during this study were
extremely sensitive to oxygen and moisture. It was,
therefore, essential that all manipulations of these com-
pounds be effected under a high vacuum or in a Vacuum
Atmospheres Corporation nitrogen filled drybox containing

less than 1 ppm water and oxygen.

Materials. Niobium pentachloride, zirconium tetrachloride

 

and high purity (99.9%) niobium metal were purchased from

Alfa Inorganics. Niobium pentabromide, niobium pentaiodide
and the three niobium tetrahalides (NbCl4, NbBr4
were prepared by using procedures previously described.

and NbI4)
5,6

Practical grade pyrazole (98%) obtained from Aldrich
Chemical Company and potassium borohydride obtained from
Pfaltz and Bauer, Inc. were used as received. Analytical
grade methylene chloride was purchased from J. T. Baker
Chemical Company and was dried by refluxing over calcium
hydride. It was distilled under nitrogen atmosphere and
stored over molecular sieves. l,2-bis(methy1thio)ethane
purchased from Columbia Chemicals, was dried and deoxygenated.
Toluene and hexane were standard reagent grade chemicals.

Potassium dihydrobis(pyrazol—l-yl)borate (K[H2B(Pz)2])
was prepared by previously described methodsé1 via the

reaction:

24

25

KBH4 + Z HN-*N ———————9 N-—-N

/ ..
m z120°C m +
HZB 2 K

(1)

The salt was dried, analyzed by use of nmr and infrared

spectra and stored in the drybox.

Analytical Determinations. Preliminary Analyses were per-

 

formed to determine halogen. Samples of the complexes were
added to aqueous ammonia and heated until solution was com—
plete. The samples were cooled and acidified with dilute
nitric acid. The solutions were filtered and the filtrates
analyzed for halogen ion by potentiometric titration with a
standard silver nitrate solution. A Beckman expanded scale
pH meter was used in conjunction with a silver indicator
and a saturated calomel reference electrode. Niobium was
not determined due to the presence of boron in the dihydro-
bis(pyrazol-l—yl)borate complexes. Final microanalyses were
performed by Galbraith Laboratories, InC., Knoxville,

Tennessee.

Molecular Weight Determinations. The molecular weight of

 

the complexes was determined cryoscopically in dry benzene.
Recrystallized benzil was used as the calibrating solute in
the determination of the molal freezing point depression

constant of benzene (5.38°C m‘l). Freezing point depressions

26

were measured in the concentration range 0.01 to 0.1 m with
a Beckman differential thermometer graduated at 0.01°
intervals. Temperature readings were estimated to i 0.001°

with the aid of a magnifying thermometer reader.

Conductance Measurements. Molar conductivities were measured

 

with a Beckman Model RC-16B2 bridge. A Freas type conduc-
tivity cell with bright platinum electroles was used. The
cell constant was determined to be 0.2275 cm.1 at 25°C by

using a standard KCl solution.

Electron Spin Resonance Spectra. All esr spectra were
obtained on solutions and powders by use of a Varian Model
E-4 spectrometer with an operating frequency range of 8.8

to 9.6 GHz and equipped with a field dial-regulated magnet.
Low temperature spectra were obtained by use of a liquid
nitrogen insert dewar or a Varian Model V 4540 variable
temperature controller. Samples were sealed in pyrex or
quartz tubes under a nitrogen atmosphere. The magnetic field

was calibrated by using strong pitch (g = 2.0028).

Nuclear Magnetic Resonance Spectra. Proton nmr spectra
were obtained by use of a Varian Model A56/60D spectrometer

operated at 60 MHz.

27

Electronic Spectra. Solution spectra were recorded by using

 

a Cary Model 17 spectrophotometer. Cylindrical fused silica
cells, 1.0 cm long and adapted for use at low pressure
(Figure l), were used. Saturated solutions were loaded in
the drybox. The cell assembly was then evacuated to SE 10-5
torr. After sealing off the cell assembly, solvent and/or

solutions of various concentrations could be distilled

through a medium porosity frit into the cell.

Vibrational Spectra. Solid state infrared spectra were
1
)

spectrophotometer. Samples were prepared in the drybox and

 

obtained by use of a Perkin-Elmer 457 (4000-250 cm-

were mounted on Nujol mulls between cesium iodide plates.
Mulls were prepared immediately before recording the spectra.
Far infrared spectra were obtaind by using a Block
Engineering Company Model-FTS 16 (3900-20 cm-l) far infrared
Fourier-transform spectrophotometer. High density poly—

ethylene was used for windows.

X-Ray Powder Diffraction Analysis. Powder diffraction

 

patterns of the sample were routinely obtained with a Haegg
Type Guinier forward-focussing camera (radius 80 mm) and Cu

K81 radiation, 161 = 1.54051 A, t = 24 : 1°C. The X-radia—

tion source was a fine focus X-ray tube powered by a Picker
809 B generator. Sample preparation and Guinier techniques

have been reported e1sewhere.74’75

The oxygen and moisture-
sensitive materials were protected by a thin film of sodium—

dried Nujol.

28

Figure 1

   
  
  
 

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CHAMBER. QUARTZ

SPECTROSCOPY
CELL

(Taken from Reference 5) Apparatus
for Determination of Electronic Spectra

29

Syntheses. Tetrakis[dihydrobis(pyrazol-l—yl)borate]-
niobium(IV): A 3.72 g sample of potassium dihydrobis—
(pyrazol-l-yl)borate [K(H2B(Pz)2] was introduced into a
round bottom flask containing a magnetic stirring bar and

1.2 g of NbC14, 3.0 g in the case of NbI and 2.06 g in the

4
case of NbBr4. The flask was evacuated to ca. 10'5 torr

and 50-80 ml of methylene chloride was vacuum distilled into
the flask. The flask was isOlated from the vacuum system
and mixture was stirred for 4-5 days at ambient temperature.
A greenish-brown solution and a dark green precipitate were
Obtained. The precipitate was removed by filtration and the
filtrate was evaporated to dryness in vacuo. The filtrate
was extracted with methylene chloride, and a greenish-brown

solid was recovered by removal of methylene chloride in

vacuo .

Anal. Calculated for Nb[H2B(Pz)2]4: Nb, 13.65;
C, 42.33; H, 4.75; N, 32.92; B, 6.35. Found:
Nb, 14.24; C, 39.30; H, 4.54; N, 29.79; B, 5.36.
Molecular weight: Calculated, 680.87. Found,
626 in benzene. M.P., 70°-75°C. Molar conduc-
tance in CHZClZ: 0.48 Ohm"1 moles”1 cmz.

Dichlorobis[dihydrobis(pyrazol-l—yl)borate]niobium(IV):

A mixture of 1.20 g of NbCl and 1.86 g of K[H2B(Pz)2] were

4
placed in a round bottom flask in the same manner as above
and stirred for 7-8 days. The solution and solid Obtained

were both brown. The complex was recovered as described

above.

30

Anal. Calculated for NbCl [H2B(Pz) ] : Nb, 20.30;
C, 31.47; H, 3.53; N, 24. 7; Cl, 15.48; B, 4.72.
Found: Nb, 21.58; C, 30.82; H, 3.79; N, 23.72;
C1, 15.60; B, 4.51. M.P., 130-137°C. Molgr con—
ductance in CHZCIZ: 1.38 Ohm'1 moles‘1 cm

Dibromobis[dihydrobis(pyrazol-l—yl)borate]niobium(IV):
The bromide complex was prepared in exactly the same manner
as the chloride complex. A mixture of 2.06 g Of NbBr4 and
1.86 g of K[H2B(Pz)2] was allowed to react. The resulting

brown solution contained a greenish-brown solid.

Anal. Calculated for NbBrZ[H2B(Pz)2] : Nb, 16.99;
C, 26.36; H, 2.96; N, 20.50; Br, 29. 3; B, 3.95.
Found: Nb, 17.62; C, 25.02; H, 3.33; N, 18.97;
Br, 27.18; B, 3.30. M.P., 155—160°C. Molgr con-
ductance in CHZCIZ: 1.17 Ohm’1 moles'1 cm

Diiodobis[dihydrobis(pyrazol-l-yl)borate]niobium(IV):
The iodide complex was also prepared in the same manner as

the chloride complex. Mixture of 3.0 g of NbI and 1.86 g

4
of K[H2B(Pz)2] was allowed to react. The resulting solution
and solid were a dark brown.

Anal. Calculated for Nb12[H2B(Pz)2]2: Nb, 14.50;

C, 22.49; H, 2.52; N, 17.49; I, 39.61; B, 3.37.

Found: Nb, 19.49; C, 26.76; H, 3.43; N, 19.22;

I, 18.90; B, 3.80. M.P., 14l-150°C. Molar con—
ductance in CHZClZ: 2.36 Ohm‘1 moles‘1 cmz.

Trichlorodihydrobis(pyrazol-l-y1)borate niobium(IV):
The trichloro complex was prepared in exactly the same
manner as the dichloro complex. A mixture of 1.2 g of NbCl4
and 0.93 g of K[H2B(Pz)2] was allowed to react. The re-

sulting solution was a yellowish—green containing a pink

solid.

31

Molar conductance in CHZCIZ: 0.56 ohm'1 moles'1 cmz.

M.P., ZED-255°C.

Tribromodihydrobis(pyrazol-l-yl)borate niobium(IV):
This compound was prepared in a manner (2.06 g of NbBr4 and
0.93 g of K[H2B(Pz)2] analogous to the previous description.
It appeared greenish in solution and was isolated as a dark

green-brown solid.

Molar conductance in CHZCIZ: 2.4 ohm‘1 moles'1 cmz.

M.P., 230-235°C.

Triiodo[dihydrobis(pyrazol 1-yl)borate niobium(IV):
A dark brown solid was Obtained by using the procedures

(3.0 g of NbI and 0.93 g of K[H2B(Pz)2]) applied for

4
synthesis of the analogous chloride and bromide compounds.

1

Molar conductance in CHZCIZ: 4.2 ohm’ moles.1 cmz.

M.P., 140-145°C.

Tetrakis[dihydrobis(pyrazol-l-yl)borate]zirconium(IV):
This compound was prepared in a manner analogous to the
previous description of the niobium(IV) compound. It was

isolated as a cream-white solid.

Molar conductance in CHZCIZ: 3.3 Ohm.1 moles-1 cmz.

M.P., 55-60°C.

Dichlorobis[dihydrobis(pyrazol-l—yl)borate]zirconium(IV):
A pinkish-white solid was Obtained by using the procedure
applied for the synthesis of the analogous niobium(IV)
chloride complex.

Molar conductance in CHZCIZ: 6.2 Ohm_1 moles.1 cmz.
M.P., 210-216°C.

32

Tetrahalobis[1,2-bis(methylthio)ethane]niobium(IV):

Compounds14

of composition NbX4(dth)2 [x = Cl, Br, and I;
dth = 1,2-bis(methylthio)ethane (more commonly referred to
as 2,5-dithiahexane)] were obtained by direct reaction of
NbX4 with a solution of excess dithiahexane in SE: 50 m1 of

dry toluene. A 2-3 g quantity of NbX was introduced into

4
a 100 m1 round bottom flask to which an excess (10 ml) of
dithiahexane had been added. A magnetic stirring bar was
introduced and the flask was evacuated to ga. 10‘5 torr,
toluene was distilled in and the flask was isolated from

the vacuum system. This mixture was stirred continuously

at ambient temperature for 4-5 days. During this period

all the tetrahalide reacted.

With dark brown NbCl4 a tan precipitate began forming
after one hour. After about one day no unreacted tetra-
chloride could be observed. The reaction was allowed to
proceed for three more days. Excess ligand and toluene were
removed into cold traps and the residual tan solid was dried.

From NbBr4 a green precipitate with composition
NbBr4(dth)2 was isolated.

A brown precipitate approaching the composition
Nbl4(dth)2 was Obtained after a total of ten days of reaction
of NbI4 and the toluene solution Of dithiahexane.

Dichloro—u-dichlorotetrakis(dimethyldithiophosphato)-
diniobium(IV): A two to one molar ratio of sodium dimethyl-

dithiophosphate: MCI M = Nb, Zr) was placed in a round

4 (
bottom flask containing a magnetic stirring bar and 75 ml of

33

dry toluene. The mixture was stirred for 7—8 days in the
drybox at ambient temperature. A clear colorless solution
and violet precipitate were Obtained. The precipitate was
removed by filtration. The original mixture was prepared so
that the ZrCl4 to NbCl4 ratio was ten to one.

Tetrakis(dimethyldithiophosphato)niobium(IV): The
four to one molar ratio complexes were prepared in the same
manner as above. A yellow solution and a violet precipitate
were obtained. The precipitate was recovered by filtration.
A pinkish-yellow solid was obtained from the filtrate by
removal of toluene in vague.

Tetrakis(dimethyldithiocarbamato)niobium(IV):
Essentially the same procedure was used here as has been
described for the analogous dimethyldithiophosphate complex.
The final product was a colorless solution containing a
violet precipitate. The mixture was treated as described
above.

Dichloro-u-dichlorotetrakis(dimethyldithiocarbamato)-
diniobium(IV): By using the same technique involved in the
preparation of two to one (molar ratio) dimethyldithio-
phosphate complex the dimethyldithiocarbamate complex was

isolated as a violet powder.

PART I

THE SYNTHESIS AND CHARACTERIZATION OF SOME
DIHYDROBIS(PYRAZOL-l-YL)BORATE COMPLEXES OF NIOBIUM(IV)

34

RESULTS AND DISCUSSION

Preparation and Properties of Dihydrobis(pyrazol-l-yl)borate
Complexes of NiobTUm(IV)

 

 

The reaction of the niobium(IV) halides with stoichio-
metric amounts of potassium dihydrobis(pyrazol-l-yl)borate
in dichloromethane or toluene proceeds according to

equation 2.

NbX + 4K[HZB(P2)2] + Nb[H2B(P2) + 4KX (2)

4 2]4

The complex was isolated as a dark green powder which is
quite soluble in ethanol dichloromethane and toluene. The
complex is air and water sensitive as indicated by a color
change from green to white. The complex melts over the
range of 70-75°C without decomposition.

When two moles of potassium dihydrobis(pyrazol-l-yl)-
borate are allowed to react with one mole of NbX4 (X = Cl,

Br, 1) reaction occurs according to equation 3.

NbX + 2K[H2B(Pz)2] + NbX2[H2B(Pz)

4 + 2KX (3)

2]2

Solid species were isolated as brown, greenish—brown and
dark brown powders for the chloride, bromide, and iodide
respectively. The complexes are air and/or water sensitive
as indicated by color changes when exposed to the atmosphere
or placed in water. The melting point ranges of 130—137°C,

155-160°C and 141-150°C are Observed respectively for the

35

36

chloride, bromide and iodide. The complexes are only
slightly soluble in toluene and dichloromethane. The
NbX2[H2B(Pz)2]2 complexes interact with ethanol to produce
a deep blue solution. The following equation is proposed
based on spectral data to be subsequently discussed.

CHSCHZOH
NbX2[H2B(Pz)2]2

 

> Nb(CH2CHZO)2[H2B(Pz)2]2 + 2HX (4)

Spectral data also indicate that when either two or
four moles of potassium dihydrobis(pyrazol-l-yl)borate are

allowed to react with one mole of NbX X = Cl, Br, and I)

4 (
in ethanol the same product is obtained in solution. In
each case the solution was tan in color and all efforts

to recover a solid product from the solution resulted in a
red-brown thick Oil or tar-like substance. The following
equation is proposed for the production Of the paramagnetic
species.

NbX + 2K[H2B(Pz)2]-+4CH CH OH +

4 3 2
Nb(OCH2CH3)4[H2B(Pz)(Pz-H)]2 + 2HX + 2KX (5a)

Upon standing or when solvent is removed a diamagnetic
species results, for which the following equation is pro-

posed.

Nb(OC2H5)4[H2B(Pz)(Pz-H]2 + [Nb(OC2HS)4]n + 2 H B H

37

Brubaker47 reported the formation of similar complexes by
electrolytically reducing NbX5 in ethanol saturated HCl to
which pyridine was later added. Nb(OCH2CH3)4 was Obtained
by adding NaOCHZCH3 dissolved in ethanol to the reduction
product [NbCl(OCH2CH3)3(C5H5N)]2 Obtained from the above
solution.
If only one mole of potassium dihydrobis(pyrazol-l-yl)-

borate is present reaction occurs according to equation 6

in dichloromethane solutions.

NbX4 + K[H2B(Pz)2] + NbX3[H2B(Pz)Z] + KX (6)

The complexes are obtained as pinkish-brown, dark greenish-
brown, dark brown powders for the chloride, bromide, and
iodide respectively. These complexes are also air and
moisture sensitive. The melting point ranges of 250-255°C,
230-235°C and 140-145°C are observed for the Chloride,
bromide, and iodide respectively. Like the NbX2[H2B(Pz)Z]2
complexes, these complexes are only slightly soluble in
toluene and dichloromethane. For all practical purposes,
the chloride complex appears to be insoluble in ethanol
but the bromide and iodide turn a dark greenish-brown and are
quite soluble in ethanol.

In a manner analogous to the reaction Of the niobium(IV)
halides with stoichiometric amounts of potassium dihydro-
bis(pyrazol—l-yl)borate in dichloromethane the zirconium(IV)

complex is Obtained according to equation 7.

38

ZrCI4 + 4K[H2B(Pz)2] + Zr[HZB(Pz) + 4KC1 (7)

2]4

The species was isolated as a cream-white powder with a high
solubility in dichloromethane. The complex is air and water
sensitive as indicated by a color change. The complex melts
over the range of 55-60°C.

When only two moles of potassium dihydrobis(pyrazol-l-
yl)borate are allowed to react with one mole of ZrCl

4
reaction occurs according to equation 8.

ZrCl + 2K[H2B(Pz)2] + ZrClZ[H2B(Pz) + 2KC1 (8)

4 2]2

A pinkish-white solid was obtained, which is air and mois-
ture sensitive as indicated by a color change when exposed
to the atmosphere. This complex melts over the range 210-

216°C and is only slightly soluble in dichloromethane.

Vibrational Spectra

 

A. Infrared Spectra (4000-600 cm‘1

)
There has not been a detailed study Of potassium
dihydrobis(pyrazol-l-yl)borate in the infrared region. The
spectra of the ligand, the completely substituted complex,
and disubstituted chloride complex are presented in Figures
2, 3, and 4 respectively. The infrared spectra of the di-

substituted chloride may serve as representative Of the

bromide and iodide spectra.

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42

In complexes containing the dihydrobis(pyrazol-l-yl)-
borate anion several regions are of interest in the infrared.
A strong BH2 stretching multiplet is Observed at 2230-

2460 cm_1 and resembles that of pyrazabole. A strong band
at 2900 cm.1 is due to 6(C-H) of Nujol and the multiplet at
3100 cm.1 is a combination of vibrations due to the 6(C-H)
Of the aromatic rings of the dihydrobis(pyrazol-l-y1)borate.

The infrared spectra of the analogous zirconium com-
plexes were recorded. The infrared spectra Of all the
niobium and zirconium complexes are very similar. However,
closer examination reveals that these compounds can be
divided into two groups, the spectra within each group being
virtually identical. To the first group belong the
M[H2B(Pz)2]4 complexes and to the second group complexes of
the general formula MX2[HZB(P2)2]2 (M = Nb; X = Cl, Br, and
I: For M = Zr; X = Cl only). These are three regions Of
the Spectrum where differences are apparent: (1) the band
around 1300 is a doublet in M[H2B(Pz)2]4 but a singlet in

MXZ[H2B(Pz)2]2, (2) the 1100-1250 cm-1 region is distinctly

different in each group; (3) the band at 950 cm.1 in
M[H2B(Pz)2]4 1s absent 1n MXZ[H2B(Pz)2]2.

As the same ligand is involved in all these compounds,
and the ionic radii of the metal ions are comparable, the
Observed Spectral differences are ascribed to differences

in molecular geometry, and the presence of halogen atoms in

MX2[H2B(P2)2]2.

43

The infrared spectra of the niobium and zirconium com-
plexes are also very similar to those Observed for other
metal dihydrobis(pyrazol-l-yl)borates.61 Comparison of the

1 with other known chelated

multiplet at 2230-2460 cm-
structures61 indicate that the ligand is acting as a
bidentate donor.

Infrared spectra were also recorded on the products
obtained by the interaction of one mole of K[H2B(Pz)z] with
NbX4 (X = Cl, Br, and I) in dichloromethane. The representa-
tive spectrum is presented in Figure 5. This spectrum is
virtually identical to spectra obtained from the NbXZ-
[HZB(Pz)2]2 complexes and identical for the chloride, bro-
mide and iodide complexes. The region at 1100—1200 cm—1 is
the only region which shows any difference in NbX3[H2B(Pz)2]
and NbX2[H2B(Pz)2]2 complexes. Therefore, one can predict

similar behavior of the ligand in these complexes.

B. Far Infrared Spectra (600-100 cm-l)

The far infrared spectra for the potassium salt and
the complexes were recorded. The data are presented in
Table 1 and the spectra are shown in Figures 6 and 7 for

the ligand and M[H2B(Pz) complex respectively. Two bands

2]4
at 331 and 270 cm.1 were present in the spectrum of the
complex and are not present in the Spectrum Of the ligand.

These bands have been assigned as v(Nb-N) vibrations.

44

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45

Table 1

Far Infrared Spectral Data for K[HZB(Pz)2] and

Nb[H2B(Pz)2]4 (600-100 em'l)
K[H2B(Pz)2] Nb[H2B(Pz)Z]4
540 b 540 b
505 sh 505 sh
440 s 438 S
400 m 384 S
385 s 346 sh
348 m v(Nb-N)331 m
335 sh 316 sh
324 s 305 s
304 m 294 m
292 w 283 sh
282 m v(Nb-N)270 s
268 w 254 m
257 S 230 m
227 sh 217 sh
204 sh 206 m
143 b 195 w
125 sh 180 w
175 w
154 s
141 m
120 sh
b = bread, 5 = strong, m = medium, w = weak and
sh = shoulder.

46

 

ABSORBANCE (arbitrary units)

 

 

 

 

 

 

Figure 6
1 1 1 l
600 500 400 300 200 I00
u(cm")
Far Infrared Spectrum of K[H2B(Pz)2] (Nujol)

47

 

ABSORBANCE (arbitrary units)

 

 

 

 

Figure 7
i
1 l J l
600 500 400 300 200 100
v (cm’U
Far Infrared Spectrum of Nb[H2B(Pz)2]4 (Nujol)

48

The far infrared spectra of NbX2[H2B(Pz) are shown

212
in Figures 8, 9, and 10, and the data are presented in

Table 2. The band at 263 is assigned as a v(Nb-N) vibration
in the disubstituted chloride. In the case of the disubsti-
tuted bromide complex, it was not possible to distinguish
clearly a band which could be assigned as a v(Nb-N) vibra-

tion. The band at 333 cm-1

is assigned as a 0(Nb-N) vibra-
tion in the iodide complex. The presence of only one
v(Nb-N) band suggests a trans stereochemistry. While this
suggestion is only tentative, it is pertinent to point out
that for a cis complex (approximating to C2V symmetry) four

0(Nb-N) bands are allowed in the infrared (2A1 + B + B2)

1
whereas for a trans (D4h) complex, only one v(Nb—N) vibra—
tion is allowed (Eu).

Halogen sensitive bands are also found when the spectra
of the complexes are compared. In NbC12[H2B(Pz)Z]2 the band
at 328 cm.1 has been assigned as the 0(Nb-Cl) vibration

because of the intensity, general shape and absence from

the spectra of the tetrakis compound and ligand.

By using the ratio v(Nb~Br)/v(Nb~Cl) 0.76, that was

found for the monodentate thioether adducts Of NbX4,19 one

expects to find a v(Nb-Br) mode at 22- 249 cm-1. A broad

intense band found at 253 cm"1 is assigned as the v(Nb—Br)
mode because of its absence in the chloride and iodide

complexes. The ratio v(Nb-I)/v(Nb-Cl), calculated as 0.56
1

predicts a 0(Nb-I) mode pg. 184 cm- A strong band is

observed at 192 cm-1 and is assigned as 0(Nb-I).

49

 

 

 

 

 

Figure 8
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0
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<1
(D
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L l J 1
600 500 400 300" 200 IOO
V (cm")
Far Infrared Spectrum of NbC12[H2B(Pz)2]2 (Nujol)

SO

 

 

 

 

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600 500 5 400 300 200 IOO
v (cm")
Far Infrared Spectra of NbBr2[H2B(Pz)2]2 (Nujol)

51

Figure 10

 

 

 

ABSORBANCE (arbitrary units)

 

 

L l l L
600 500 400 300 200 (00

 

v (cm")

Far Infrared Spectrum of NbIZ[H2B(Pz)2]2 (Nujol)

52

Table 2

Far Infrared Spectral Data for K[H2B(Pz)2] and
l

NbX2[H2B(Pz)2] (600-100 cm‘ )
K[H2B(Pz)2] X = C1 X.= Br X =
540 b 540 b 540 b 540
505 sh 505 sh 505 sh 505
440 s 436 s 439 s 438
480 m 388 sh 384 s 395
385 s 375 sh 335 m 383
348 m 358 sh 303 s 367
335 sh v(Nb-Cl)328 s,b 293 w v(Nb-N)333
324 s 315 sh 277 Sh 303
304 m 288 w 265 sh 293
292 w v(Nb-N)263 s v(Nb-Br)253 s,b 268
282 m 227 m 230 m 222
268 w 200 m 205 w v(Nb-I)192
257 s 150 m 194 w 155
227 sh 140 m 154 m 142
204 sh 140 w
143 b
125 sh

b = broad, s = strong, m = medium, w = weak and
sh = shoulder.

S

SEC/35w

53

Data for Zr[H2B(Pz) and ZrC12[H2B(Pz)2]2 are given

214
in Table 3 and the spectra are shown in Figures 11 and 12.
The metal-nitrogen vibrations are found at 252 and 362 cm-1
in Zr[HzB(Pz)2]4. These bands were assigned as 0(Zr-N)
bands because they are not present in K[H2B(Pz)2]. Bands
at 255 and 325 cm.1 are assigned as v(Zr-N) and v(Zr-Cl)

vibrations respectively for the ZrC12[HZB(Pz) The far

212'
infrared indicates that bonding in niobium and zirconium
complexes is very similar and possibly identical structures
are present in each group of compounds. Actually the in-
tensity and general shape of these spectra are virtually
identical to those of the corresponding niobium complex

with very little shift in some of the peaks.

The products Obtained by allowing one mole of potassium
dihydrobis(pyrazol-l—yl)borate to react with one mole of
NbX4 (X = Cl, Br, and I) produced niobium-halogen stretch-
ing bands identical to those produced by NbX2[H2B(Pz)2]2.
However, due to other changes in the spectra of the
NbX3[H2B(Pz)2] complexes, it was not possible to assign
the v(Nb-N) vibration in the chloride complex, the bro-
mide complex v(Nb-N) band was clearly assigned at 362 cm"1
and the iodide complex v(Nb—N) band was tentatively
assigned at 330. Note in the NbX2[H2B(Pz)2]2 complexes
the v(Nb-N) assignments were made clearly for the chloride
and iodide, and the bromide complex was unassigned. The

spectra are presented in Figures 13, 14, and 15 and the

parameters are given in Table 4.

54

 

 

 

 

 

 

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3
U.)
0
Z
.4
m
a:
C>
(.0
CD
‘1
J_ 1 1
600 500 400 300 200
v(cm"')
Far Infrared Spectrum of Zr[H2B(Pz)Z]4 (Nujol)

55

Figure 12

 

ABSORBANCE (arbitrary units)

 

 

 

L 1. 1 1
600 500 400 300 200 I00

v(cm")

Far Infrared Spectrum of ZrC12[H2B(Pz)2]2 (Nujol)

Far Infrared Spectral Data for K[H2B(Pz)2], ZrC12[H2B(Pz)

K[H2B(Pz)2]

b
s

h

540
505
440
400
385
348
335
324
304
292
282
268
257
227
204
143
125

b
sh
s
m

(I)

and Zr[HZB(Pz)

ZrClZ[H2

535

506

438

395

383

365
v(Zr-C1)325
305

293

282
v(Zr-N)255
228

203

174

153

56

Table 3

214 (600-100 cm'l)

B(Pz)2]2 Zr[H2B(Pz)
b 530 b
sh 505 w
s 440 5
sh 383 s
m v(Zr-N)362 m
sh 345 m
s,b 323 w
m 307 s
w 280 w
w 268 sh
s,b v(Zr-N)252 s,b
m 228 w
m 205 m
w 192 w
m 152 s
b 140 m

126

= bread, 5 = strong, m = medium, w = weak and

= shoulder.

212

214

57

 

 

 

 

 

 

 

 

Figure 13

’2

'E

a

. i

E

:22:

3 u

LU

0

:Z

'4

CD

(I

<3

(I)

in

.q

. l 1 i 1
600 500 400 300 200 iOO
v (cm")

Far Infrared Spectrum of NbCl3[H2B(Pz)2] (Nujol)

$8

 

 

 

 

 

Figure 14
E
'E
a
2‘
E
E
32
(1.1
U
Z
id
a:
a:
(D
(D
an
id
_ 1 1 1 1
600 500 400 300 200 IOO
11km-”
Far Infrared Spectrum of NbBr3[H2B(Pz)2] (Nujol)

59

Figure 15

 

 

ABSORBANCE (arbitrary units)

 

 

1 ii i_ L
600 500 .400 300 200 IOO

”(cm-'7

 

Far Infrared Spectrum of Nb13[H2B(Pz)2] (Nujol)

60

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61

NMR Spectra of Zr[HzB(Pz)2]4 and ZrC12[H2B(Pz)2]2

 

The 1H nmr spectra of the tetrakis and his complexes
were recorded using DCCl3 as solvent. The spectra of the
diamagnetic chelates each consisted of two doublets and a
triplet (made up of two overlapping doublets) in a 1:1:1

ratio, and an upfield singlet. The chemical shifts are

shown in Table 5 relative to TMS.

Table 5

Proton Chemical Shifts (T)

Assignments: Zr[HzB(Pz)2]4 ZrClZ[H2B(Pz)2]2
B-Ha 9.90 4.90
B-Hb 9.90 4.90
C-H3 2.92* 4.00*
C-H4 4.22 2.58
C-H5 2.60* 3.92*

Measurements in DCC13 solution, relative to internal tetra-
methylsilane. *They could not be assigned unequivocally.

62

There are two possible conformations for the B(N-N)ZZr
six-membered rings, one is the boat and the other is the
chair. The boat76 conformation allows, on the one hand,
planarity of the nitrogen atoms and planarity for the rings.
This results consequently in stabilization due to the de—
localization of 6-r-electrons in each of the five-membered
organic heterocyclic rings. In addition, a less angularly
strained environment is obtained for the four-coordinate
boron atoms. Such a structure places the hydrogen on boron
in two unique environments. Such differently located
hydrogen atoms should be clearly distinguishable by 1H nmr.
The fact that only one singlet is observed in these com-
plexes implies that the two hydrogens on boron are equiv-
alent. The equivalence of the two hydrogen atoms suggests
that these complexes are stereochemically non-rigid in
solution. Stereochemical nonrigidity has been proposed to
account for similar behavior observed in many other transi-

tion metal complexes.61’62’67

The fact that only one singlet
is observed for the protons attached to the boron atom in

the ZrC12[H2B(Pz)2]2, is also direct evidence that the trans
isomer is present in solution. A cis isomer would give a
doublet (two singlets) for the two boron protons since

these protons would be in two unique environments under all
conditions of temperature. Solid state far infrared in-
dicated previously that a trans isomer is present. Such

an arrangement may also be used to explain the downfield

shift of the protons attached to boron in the bis complex

63

as arising from protons interacting with the chloride.
Trofimenko77 has noted previously a downfield shift of half
the methylene protons in the 1H nmr spectra of the boron
derivatives, Ni(R2B(Pz)2]z (R = Et, Bu), and postulated an
interaction of these protons with the nickel atom to account
for it.

The absence of a second set of aromatic protons in the
1H nmr spectra indicates that the ligand is acting as a
bidentate donor in both complexes. This is in agreement

with the far infrared spectra.

X-Ray Powder Diffraction Analyses Of M[H2B(Pz)2]4 and

 

MX2[H2B(P2)2]2

 

Guinier powder patterns taken Of the MX2[H2B(Pz)z]2

(M = Nb; X = Cl, Br, I) and (M = Zr; X = Cl) samples dis-
played considerable similarity for the zirconium(IV) chloride
and the niobium(IV) chloride and bromide complexes but the
pattern for the niobium(IV) iodide complex did not show any
evidence of structure.

The Zr[HzB(Pz)2]4 samples exhibited a powder pattern
quite different from the above patterns and the Nb[H2B(Pz)Z]4,
like the bis iodide complex, was devoid of structure. The
fact that NbX2[HZB(Pz)2]2 (X = Cl, Br) proved tO be isomor-
phous with ZrC12[H2B(Pz)2]2 and that the ligand is acting
bidentately in ZrC12[H2B(Pz)2]2 Supports the earlier con-

clusion, that H2B(Pz)- is acting as a bidentate ligand in the

niobium complexes. It also supports a proposed trans

64

configuration for the NbX2[H2B(Pz)2]2complexes.

Electronic Spectra of Nb[H2B(Pz)2]4

 

Visible and near infrared spectra Of Nb[H2B(Pz)2]4 were
studied by using the technique described in the experimental
section. These studies were carried out with dichloromethane
solutions of the complexes. The spectra and wave number
maxima of the Nb[H2B(Pz)2]4 complex are given in Figure 16

and Table 6 respectively.

Table 6
Electronic Spectral Data of Nb[H2B(Pz)2]4 in CHZCl2
1(nm) 5(cm-1) x 103 5(ch1 M—l)
540 18.52 126.00
340 29.41 538.67

The qualitative features of the spectra include one
weak broad band at 540 nm. In addition a strong band is
Observed at 340 nm. There are three possibilities for
assigning these bands. The bands may be due to (l) d-d
transitions, (2) metal to ligand charge transfer, and (3)
ligand to metal charge transfer. In view of the low in-
tensities of the band at 540 nm (e = 126.00) it can be
assigned confidently as arising from a d—d transition. The
extinction coefficient is in the range that other authors

24

have assigned as d-d transitions. The intense band in the

65

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nxym

N

:0 ea e.N
“ES 4

000 CON. 000 000

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ea eesmaa

66

ultraviolet region is due to ligand charge transfer. This
assignment was confirmed by observing an identical band at
=340 nm for a dichloromethane solution Of the analogous
zirconium complex.

Two common symmetries are Observed for eight-coordinate
transition metal complexes: the D2d triangular dodecahedron
and the D4d square antiprism. These structures are formed
by distorting the cube. Figures 17 and 18 show the most
common structures and the crystal field splitting diagrams
for a D2d dodecahedron and a D4d square antiprisng’78
respectively. In the case of D4d symmetry, two d-d transi-
tions are predicted while for D2d symmetry three transitions
are expected. Since only one band was observed which is

assigned as a d—d transition no structure assignment can be

made on the basis of the spectrum.

Electronic Spectra of NbX2[H2B(Pz)2]2

 

The solution spectra and wave number maxima Of

NbX2[H2B(Pz) (X = Cl, Br, and I) complexes are given in

2]2
Figures 19, 20 and 21 and Table 7 respectively. The dichloro-
methane solution spectra consist Of two bands for the chloride
species, and three bands each for the bromide and iodide
complexes. The ligand charge transfer band is observed in

all three complexes $3., 340 nm. This assignment was again

confirmed by observing an identical band at about 340 nm in

the dichloromethane solution spectra of the isomorphous

0930 w MO GOHHHOHmHQ

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67

 

   

  

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68

Figure 18
D2d 94d
—- dxy —dxz — dyz
dxz dyz
.......... 922
-—-d 2
"—dx2-y2 z

The crystal field splitting for D2d and D4d symmetries

69

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Electronic Spectral Data of NbX2[H2B(Pz)

Complex
X = Cl

X = Br

X = I

X(nm)

480
340
800
500
340
700
560
360

72

Table 7

0(cm’1) x 103
20.
28.
12.

20.

28

14.
17.
27.

83
41
50
00

.41

29
86
78

212

e:(Cm-1

180

380.
17.
75.

399.
23.
61.

163.

in CH Cl

M
.93
61
54
00
56
21
81
92

-1

2

)

2

73

ZrC12[H2B(Pz)2]2. This band also corresponds to the charge
transfer band found in the spectrum of Nb[H2B(Pz)Z]4. In
addition, a band at 480 nm is present in the chloride com-
plex. Bands are located at 800 nm and 500 nm, and 700 nm
and 500 nm in the bromide and iodide respectively. On the
basis of the observed extinction coefficients Of these bands
they are assigned as due to d-d transitions.

The spectra of the trans-NbX2[H2B(Pz)2]2 complexes can

19

be discussed relative to crystal field theory. It has

been shown, at least qualitatively, that the spectra of trans-

MXZB2 molecular complexes can be discussed as tetragonally

79

(D4h) perturbed complexes. The effect of a tetragonal

component in the ligand field upon terms arising in Oh cause

the ZTZg level to resolve into 2B and 2Eg levels and the

28
excited 2Eg level to 2B1g and zAlg' Hence one would expect

three transitions from the splitting of the d—manifold.

Treating these trans complexes as D4h symmetry the expected

transitions will be from the ground 2B2g level to the excited

2 2 2
Eg’ A1g and B1g levels. In no case are three bands

observed. The absence of some of the d-d transitions are

80’81 in related systems. Fowles80 et al. have

well-known
reported that the acetonitrile adducts of the tetrahalides
have halogen(n) - niobium(d) transitions throughout the

visible region and as a result the d-d transitions may be

masked. Hence it is conceivable that a similar situation

exists in these systems.

74

Electronic Spectra Of "NbX3[H2B(Pz)2]"

 

The visible and near infrared spectra of the species,
Obtained by allowing a one to one molar ratio of
K[H2B(Pz)2]:NbX4 (X = Cl, Br, and I) to react in dichloro-
methane, consisted of two bands in each case. A band observed
at 340 nm was assigned as a ligand to metal charge transfer
vibration as in the bis and tetrakis niobium complexes. The
band ranging from 470 nm for the chloride complex to 570 nm
for the iodide complex was assigned as a d-d transition.

The extinction coefficient Of each complex was in the range
which other authors have assigned as d-d transitions.24
Neither trigonal bipyramidal nor square pyramidal geometry
can account for the spectra. At least two transitions are
expected for both trigonal bipyramidal and square pyramidal
complexes. As proposed in the NbX2[H2B(Pz)2]2 systems the
d—d transitions could be masked by halogen(n)—niobium(d)
transitions in these species.

These results are by no means conclusive, but they are
indicative of the fact that a species is present which is

different from both Nb[H2B(Pz) and NbX2[H2B(Pz)

2]4 212'

Electron Spin Resonance Spectra of Nb[HzB(Pz)2]4

 

Esr studies were performed on Nb[H2B(Pz)2]4 as described
in the Experimental section. The spectra are presented in
Figures 22, 23 and 24. Since the hyperfine splittings are

on the order Of 160 gauss the high field approximation cannot

75

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76

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78

be rigorously applied and second order corrections should be
employed. The perturbation of the Zeeman transition resulting

from the hyperfine interactions was corrected by means of the

 

following equations:82
hv = gBHO (9)
for isotrOpic g' H ' = H + <a>m + :33— [I(I+l) — m 2] (10)
' m m I ZHm I
Alz
_ 2
o '= _— "
for gll. Hm Hm + AllmI + ZHm [I(I+l) mI ] (ll)
A||2+Alz 2
. t: _
for gi. Hm Hm + AimI + ( 4Hm ) [I(I+1) mI ] (12)

where Hm is the corrected magnetic field position, Hm is

the experimental position of the esr line due to the compo-
nent mI of the nuclear spin I, v is the klystron frequency
and <a>, All’ and A1 are the hyperfine splitting constants.
The calculations are necessarily reiterative and were carried
out by use of a desk calculator. Normally three iterations
were sufficient. The hyperfine splitting constants were
determined from the positions of the fifth and sixth, fourth
and seventh, third and eighth, second and ninth, and first
and tenth lines where resolution permitted. The separation
of the hyperfine components in gauss is related to the energy
splitting in cm“1 between adjacent hyperfine levels as

follows:

A (cm-1) = g x 4.6686 x 10‘5 A (gauss) (13)

79

The experimental esr parameters are listed in Table 8 with

the corrections due to second order effects.

Table 8
Esr Spectral Parameters of Nb[H2B(Pz)2]4
<g> g: l 81 *Au “‘1
Experimental Solid 12.012 1.892 2.072 227 113
Corrected +1.955 1.903 1.982 254 93
Experimental Soln. 2.012 1.892 2.072 153 227 113
Corrected 1.955 1.903 1.982 148 254 93

*Hyperfine splittings are given in units Of 10'4 cm 1.

The esr spectral parameters were identical for the solid
at both 298 and 77 K.

<> values were obtained from spectra recorded at 298 K and
the II and i values were obtained from spectra recorded at
77 K.

+<g> is assumed to be the same in the solid as in the
solution.

In dichloromethane or toluene glass at 77 K the esr

spectrum may be described by the spin Hamiltonian with

axial symmetry:83

H = gIIBHZSz + gi(HXSX + HySy) + AIISZIZ

+ AL(SXIX + Sny) (14)

80

where S = 1/2, 1(93Nb; 100%) = 9/2. At room temperature in
liquid solution, the anisotrOpies are averaged tO zero, and

the Hamiltonian becomes:

H = <g>BH°S + <a>I-S (15)

Theoretical calculations of esr parameters for a D2d
dodecahedron and a D4d square antiprism show that gL > gll

for a dodecahedronzg’30

and gll > gi for a square anti-
29,31

prism. For Nb[H2B(Pz)2]4, gl = 1.982 and gll = 1.903
indicates dodecahedral symmetry. The gll and gL values for
the solid powder esr spectra are the same as the above values
indicating the stereochemistry remains the same in the solid
and in solution. In comparison to other known eight co-
ordinate complexes of Nb(IV) listed in Table 9, the data
observed here fit well. Two complexes where gll > gL,

36,37 17’39 have been reported.

Nb(dpm)4 and Nb(CN)84‘ (soln)
Single-crystal x-ray studies of these compounds have
determined the structure of the latter species as dodeca-
hedral while the former is antiprismatic. However, recent
work39 confirmed the belief that the Nb(CN)84- species
actually change symmetry in solution. The esr spectrum of
a magnetically dilute solid solution of K4[Nb(CN)8]°2H20 in
isomorphous K4[MO(CN)8]°2H2
with theoretical predictions.

0 shows gL > gIl in agreement

81

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82

In most systems data from the esr studies can be used
in conjunction with the electronic spectral assignments to
determine the applicability of an ionic model to the system.
It has been demonstrated that while the dodecahedral model
has D2d symmetry, it can be considered as arising from the
distortion of a cube (see Figure 15). If a metal atom is
at the center, the net effect is a tetragonal distortion.

For a dx2_y2 ground state, the gyromagnetic ratios are given

by the equations:30
8A 2A
g = 2.0023 - ——— g = 2.0023 - ——— (16)
|| AE3 L 4E2
where A is the free ion coupling constant, 0E3 = (2B2 - 2B1),

and AEZ = (2E3 - 2Bl). It was not possible to qualitatively
assess the applicability of an ionic model in the case of
Nb[HZB(Pz)2]4 because only one d-d transition (AE) was ob-
served in the electronic spectrum. The above equations will
be used to assess a different system later in this study.
Another guide to the delocalization Of the electron
from the metal to the ligand is the amount of deviation of
”eff from the spin-only value Of 1.73 B.M. by use of the

equation:

ueff(B.M.) = g/S(S+l) (17)

in which 8 is the absolute value of the spin quantum number
and g is the experimental gyromagnetic ratio, a value for
“eff can be Obtained. Table 10 lists calculated ”eff values

for the compounds listed in Table 9. From the table, the

83

Table 10

Calculated “eff Values for
Eight-Coordinate Niobium(IV) Complexes

Compounds ueff(B.M.)
NbCl4(dth)228 1.73
Nb(dmtp)456 1.71
Nb(dpm)436’37 1.69
Nb(CN)84‘ (soIid)17’39 1.72
Nb(CN)84- (soln)17’39 1.71
Nb(pipdtc)438 1.70

1.69

Nb[H2B(Pz)2]4
ability of nitrogen-donor ligands to form fairly covalent
species with Nb(IV) is illustrated.

Further confirmation of the bonding is Obtained by

using Equation 18 developed by McGarvey.29

All = P[-K- 4/7 + (gll—2.0023) + 3/7(gL-2.0023)]

A = P[-K-+2/7 + ll/l4(g -2.0023)] (18)
l l

where P = gegNBeB <r—3>ave and is defined as positive for
93Nb which has a positive nuclear moment, <r—3>ave is the

reciprocal cube of the average radical distance of the outer
electrons from the nucleus, and K is the isotrOpic contri-
bution to the hyperfine constant due to polarization of the

inner electron spin density by the unpaired d electron.

84

Agreement with experimental data is found for values of

K = 0.818 and P = 169.6 x 10-4 cm—l. Comparing these values
with those for a Nb4+ free ion, K = 1.0 (pure d orbital) and
P = 192.0 x 10.4 cm-l,84 the smaller experimental value for

P indicates that the unpaired electron is more delocalized,

hence more covalent in bonding orbitals.

Electron Spin Resonance Spectra of NbX2[H2B(Pz)2]2 in

 

Dichloromethene

 

Representative esr spectra of NbX2[H2B(Pz) X = Cl,

212 (
Br, and I) are presented in Figures 25, 26, 27 and 28. Since
the hyperfine splittings are large, second order corrections
were employed according to equations 9-12 to correct for

the perturbation of the Zeeman transition resulting from

the hyperfine interaction. The hyperfine components in

gauss were converted to cm-1

by use of equation 13. The
experimental esr parameters are listed in Table 11 with the
corrections due to second order effects.

Examination of these data revealsthe fact that in all
casesgll is less than gL. It is of interest to note that of
those esr studies of octrahedral niobium(IV) complexes which
have been reported most have gll greater than gL. Exceptions
to this appear to occur with transition metal complexes
where the ligands bonded in both axial and equatorial sites

have similar electron donating prOperties.SS’85

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. Eu v-0H mo mafia: :H co>Hw CHE mwcfluuwfimm mafimHomxmr

HI
OO NON eNH HNO.H HOO.H eeO.H NONHOZH
OO eON NNH OHO.N HHO.H NOO.H NmNHez
ON NHN mNH OOO.H OOO.H ONO.H NmNemezt
HO OON ONH OOO.H ONO.H ONO.H NmNemez
OO OmN eeH OOO.N ONN.H HHO.H NmNHue2+
OOH eNN HmH OOO.N NON.H OOO.H NmNHuez
A<r __<« AmvR Hm __m AmV mwcsomEou

moonmEou Nmmxoz mo mHOHoEmHmm Hmhuoemm Hmm :oHusHom

HH OHQHH

90

Data from the esr Studies can now be used in conjunction
with the electronic spectral assignments for NbX2[H2B(Pz)2]2
to determine the applicability of an ionic model to these
systems. For a de-yz ground state, the gyromagnetic ratios

are given by equation 16. Taking the spin—orbital coupling

4+ 1 3

constant for Nb as 748 cm' and AE and AE as 20.00 x 10

2 3
and 12.50 x 103 cm.1 respectively for NbBr2[H2B(Pz)2]2 and

AE and AE as 14.29 x 103 and 17.86 x 103 respectively for

2 3
NbIZ[H2B(Pz)2]2, the calculated values of gll and gi are

1.52 and 1.93 respectively for NbBr2[H2B(Pz) and 1.58 and

2]2

1.92 respectively for NbIZ[H2B(Pz) It was not possible

2]2°
to assess the chloride complex because only one d-d transi-
tion (AE) was observed in the electronic spectrum. These
values are much lower than the experimental values and

indicates the inadequacy of the ionic model.

It is possible to qualitatively assess the amount of

covalent bonding via equation 19.30
2 2 2 2
_ 8A0! B __ 2A0. y
g - 2.0023 - -—————— g — 2.0023 - ——————- (19)
|| AE3 L AEZ
The parameters a2, 82 and y2 are associated with B E and

1’

B2 molecular orbitals formed by linear combinations of metal

and ligand orbitals of appropriate symmetry. The range of
possible values for each of the parameters is 1.0 (ionic

bond) to 0.50 (covalent bond). Agreement with the experi-

mental g values is found for Ozyz = .699 and 0282 = .195

for NbBr2[H2B(Pz)2]2, and azyz = .374 and 0282 = .266 for

91

Nb12[H2B(Pz) For pure covalent bonding in the ground

212°
. 2 2 _ 2 2 _ .
and exc1ted states a y — a B - 0.0625. Thus, It appears
that the niobium d-Orbitals are strongly mixed with ligand

orbitals in the formation Of NbBr2[H2B(Pz) and

212

NbIZ[H2B(Pz)2]2.

By using equation 1829 agreement with experimental data

is found for values of K and P at 0.870 and 146.6 x 10'4 cm—1

0.767 and 149.2 x 10‘4 cm”1

4

respectively for NbC12[HzB(Pz)2]2,

respectively for NbBr2[H2B(Pz) and 0.864 and 132.7 x 10’

212
cm respectively for NbIZ[H2B(Pz)

In each case the P
4+ 84

212‘
value is smaller than the P value for the Nb free ion.
This indicates that the unpaired electron is more delocalized,
hence more covalent in nature. The results are in good
agreement with the conclusion made using the electronic

spectral data in conjunction with esr spectral data.

Electron Spin Resonance Spectra of NbX2[H2B(Pz)2]2 in Ethanol

 

Analyses of the esr spectra obtained from the deep blue
solution produced by dissolving NbX2[H2B(Pz)2]2 (X = Cl, Br
and I) in ethanol, give esr parameters quite different from
the parameters obtained in dichloromethane. Identical
spectra are obtained in all three cases. The esr spectra

parameters are presented in Table 12.

92

Table 12
Esr Spectral Parameters Of NbClZ[HZB(Pz)2]2
Dissolved in Ethanol
<g> g: l g1 ”I I ”I
1.907 1.859 1.931 156 262 93

*Hyperfine splitting are given in units of 10.4 cm-l.

<> values were obtained from spectra recorded at 298°K and

the II and 1 values were obtained from spectra obtained at
77°K.

It is apparent from the table that <g> and <a> are larger
than the <g> and <a> for the corresponding complexes dis—
solved in dichloromethane. In contrast, when the tetrakis
complex was dissolved in ethanol no change (relative to the
esr results in CHZCIZ) was observed in <g> and <a>. It is
then reasonable to suggest that some interaction between the

ethanol and halogen atoms in NbX2[H2B(Pz) occurred. The

212
fact that the same spectra is obtained from each of the three

solutions, is evidence of a new common species as proposed in

equation 4.

Efforts to prepare Nb[H2B(Pz)Z]4 and NbXZ[H2B(Pz)2]2
complexes in ethanol result in the formation of the same
species in solution. This Species produces extremely clear
esr solution spectra. The spectra are shown in Figures 29

and 30 and the esr parameters are listed in Table 13.

93

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94

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om musmflm

95

Table 13
Solution Esr Spectral Parameterscxf
Nb(OCHZCH3)4[HzB(Pz)(Pz-H)]2 in Ethanol
<g> gll gi *<a> *All *Ai
1.938 1.870 1.972 133 218 88

*Hyperfine splittings are given in units of 10'4 cm-l.

<> values were obtained from spectra recorded at 298 K and

the II and L values were obtained from spectra recorded at
77 K.

It is clear from the table that these parameters are dif-
ferent from the parameters obtained by similar procedures
carried out in toluene or dichloromethane using a two or

four to one molar ratio of ligand to niobium tetrahalide.

The fact that the solutions become diamagnetic upon standing,
implies that the paramagnetic species is only an intermediate
or is very unstable under these reaction conditions.

47 has previously reported similar results with

Brubaker
niobium(IV) chloride solutions in ethanol. Two diamagnetic

compounds, [NbC1(OCH2CH3)3(C5H5N)]2 and Nb(OCHZCH were

3)4,
obtained from the niobium(IV) chloride solutions. The
former was prepared by the addition of pyridine to niobium(IV)
chloride solutions in ethanol. The structure of the dimer

is thought to involve chloride bridging on the basis of its
chemical properties. Nb(OCHZCH3)4 was prepared by the
reaction of NaOCHZCH3
compound is thought to be polymeric in nature. In view of

with [NbC1(OCH2CH3)3(C5H5N)]2. This

96

the above results, it is reasonable to propose the formation
of diamagnetic [Nb(OCHZCH3)4]n when attempting to prepare
NbX2[H2B(Pz)z]2 and Nb[HZB(Pz)z]4 1n ethanol solutions. The
paramagnetic intermediate is thought to be six-coordinate
monomeric Nb[OCH2CH3)4[H2B(Pz)(Pz-H)]2 which decompose to

produce [Nb(OCHZCH3)4]n as shown in equation 5b.

Electron Spin Resonance Spectra of "NbX3[HZB(Pz)2]” in

 

Dichloromethane

 

The esr spectra of the species, obtained by allowing a

one to one molar ratio of K[H2B(Pz)]:NbX X = Cl, Br and I)

4 (
to react in CH2C1Z’ were recorded as described earlier.
Representative spectra are presented in Figures 31—34. Since
the hyperfine splittings were large, the high field approxi—
mation could not be applied and second order corrections

were employed by means of equations 9—12. The corrected

esr parameters for the complexes are listed in Table 14.

Table 14
Solution (CHZClZ) Esr Spectral Parameters of
"NbX3[H2B(Pz)2]" Complexes

Com ounds < > *<a> *A *A
p g g|| g1 ll 1

NbC13[H2B(Pz)2] 1.940 1.700 2.060 122 234 52
NbBr3[H2B(Pz)2] 1.943 1.860 1.984 121 220 68
NbI3[H2B(Pz)Z] 1.940 1.876 1.972 123 204 80

*Hyperfine splittings are given in units of 10'4 cm'l.

<> values were obtained from spectra recorded at 298 K and
the II and L values were obtained from spectra recorded at
77 K.

97

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98

4 J o
m an on ouunzu cu Hmfiuavmmzwrauoz co escouoam emu

 

 

 

 

 

 

 

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1.

Nm 3&5

99

8
«more :2 _NflNacmN:_mtmnz do escuumgm 2mm

 

 

 

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100

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000w

A.II

 

 

 

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101

For the series chloride, bromide and iodide, the <g> values
are the same within experimental error. The significance of
this observation is not clear.

The esr spectra are consistent with five-coordinate
monomers, but in the absence of their molecular weights and
analytical data one cannot preclude the presence of dimers
or some other species. Efforts to prepare a pure species
always resulted in the formation of NbX2[H2B(Pz)2] as a
contaminant. This was apparent from the observation of a

second set of lines cn' shoulders in the esr spectra.

PART II

ESR STUDIES OF SOME SULFUR DONOR COMPLEXES WITH NIOBIUM(IV)

102

RESULTS AND DISCUSSION

Preparation and Properties of 1,2-bis(methy1thio)ethane
Complexes of Niobium(IV)

 

The reaction of the niobium(IV) halides with excess
l,Z-bis(methy1thio)ethane (commonly referred to as 2,5-dithia-

hexane (dth)) proceeds according to equation 20.

NbX4 + Zdth + NbX4(dth)2 (20)

The complexes14 were isolated as tan, green and brown solids
for the chloride, bromide and iodide complex respectively.
They are all air and water sensitive as indicated by a color
change and the distinctive odor of carbon disulfide on

exposure to the atmosphere.

Preparation and Properties of Dimethyldithiophosphate
Complexes with Niobium(IV)

 

 

The reaction56 of niobium(IV) chloride and zirconium(IV)
chloride with a four to one molar ratio of NadmtpzMX4 (M =
Nb, Zr; Nadmtp = sodium dimethyldithiophosphate) in toluene

proceeds according to equation 21.

NbCl4 + lOZrCl4 + 44Nadmtp + Nb(dmtp)4

+ lOZr(dmtp)4 + 44NaCl (21)

103

104

The mixture was prepared so that the ZrCl4 to NbCl4 ratio
was ten to one. A yellow solution containing a violet
precipitate was obtained. The precipitate was recovered by
filtration. A pinkish-yellow solid was obtained from the
filtrate upon removal of the toluene.

The two to one molar of NadmtpzMX4 in toluene proceeds

according to equation 22.

NbCl + lOZrCl + 22Nadmtp + NbC12(dmtp)2

4
+ lOZrC12(dmtp)2 + 22NaCl (22)

A violet precipitate was obtained by filtration leaving a
colorless liquid. Removal of toluene in vagug produced no
recoverable solid. These complexes are air and water sensi-
tive as indicated by a color change and the distinctive odor

of dithiophOSphoric acid.

Preparation and PrOperties of Dimethyldithiocarbamate
Complexes with Niobium(IV)

 

 

Spectral data indicate,that when either a four to one

or a two to one molar ratio of NadtczMX4 (Nadtc = sodium

dimethyldithiocarbamate) is used, the reaction proceeds

according to equation 23 in toluene solutions.

NbCl + 102rC1 + 44Nadtc + Nb(dtc)4

4 4

+ lOZr(dtc) + 44NaCl (23)

The product was obtained as a violet powder from a colorless

105

liquid. These niobium(IV) complexes formed in the presence
of the zirconium(IV) analog are also air and water sensitive
as indicated by a color change and the distinctive odor of

carbon disulfide.

Electron Spin Resonance Spectra of NbX4(dth)2

 

Esr studies were performed on NbX4(dth)2 (X = Cl, Br
and I; dth = 2,5-dithiahexane) complexes as described in the
Experimental section.

Representative spectra (X = C1 and Br) are presented
in Figures 35, 36 and 37. In the case of the NbI4 the esr
spectra were inconclusive. After corrections for second
order effects by means of equations 9-12, and conversions
of hyperfine splitting from gauss to cm'1 by using equation

13. The corrected esr parameters are listed in Table 15.

Table 15

Esr Spectral Parameters of Niobium(IV) Complexes

Compounds <g> gll gl- *<a> *All *Ai
**NbCl4(dth)2 1.9947
INbC14(dth)2 1.954 1.917 1.972 131 204 102
**NbBr4(dth)2 1.9970
+NbBr4cdth)2 1.973 1.960 1.974 124 189 92

*hyperfine splittings are given in units of 10'4 cm—l.
<> values were obtained from spectra recorded at 298 K and
the II and i values were obtained from spectra recorded at

+spectra data obtained from excess 2,5-dithiahexane solutions.
**spectra data obtained from solids.

106

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3.1

mm ousmfla

107

m an on mcmxmcmneuaeam.m mmmoxm Ca misupvvaonz mo csuuowmm own

 

 

 

 

 

6m mysmfia

108

ououmuomEOB ucmfinad um

 

Nflaoeuaaonz endow mo asuuowmm “mm

 

am enamHm

109

~

It is apparent from the table that <g> soln ~ <g> solid
as expected if the same species is present in each phase.
It is also apparent that gL > gll which is as expected if
the eight-coordinate complexes have the idealized triangular
dodecahedral structure as has been prOposed for other com-
plexes.29’30
In the absence of electronic spectral data in solution,
data from the esr studies were used in conjunction with the
electronic spectral assignments14 for the solid in order to
determine the applicability of an ionic model to the present
system.
By using equation 16 and taking the spin-orbit coupling

constant for Nb4+ as 748 cm.1 and AE and AE as 17.3 x 103

2 3
and 13.8 x 103 cm.1 respectively for NbCl(dth)2 and AE and

2
AE3 as 16.9 x 103 and 13.7 x 103 cm-1 respectively for
NbBr4(dth)2, the calculated values of gll and gi are 1.57
and 1.92 respectively for NbC14(dth)2, and 1.57 and 1.91
respectively for NbBr4(dth)2. These values are much lower
than the experimental quantities thus indicating the in-
adequacy of the ionic model. It is possible to qualitatively
assess the amount of covalent bonding by use of equation 19.30

Agreement with the experimental g values is found for

azyz = 0.368 and 8232 = 0.197 for NbC14(dth)2, and 8272 =
0.307 and aZBZ = 0.098 for NbBr4(dth)2. For pure covalent
bonding in the ground and excited states azYZ = 0282 =

0.0625. Thus, it appears that the niobium d-orbitals are

110

strongly mixed with ligand orbitals in the formation of
NbCl4(dth)2 and NbBr4(dth)2.

Further confirmation of the bonding is obtained by

29 Agreement with experimental data is

found for values of K and P at 1.194 and 109.5 x 10'4 cm—1

respectively for NbC14(dth)2 and 1.10 and 109.1 x 10'4 cm—1

using equation 18.

respectively for NbBr4(dth)z. Comparing these values with

those for a Nb4+ free ion, K = 1.0 (pure d-orbital) and

P = 192.0 x 10.4 cm-l,84 the smaller experimental value for
P indicates that the unpaired electron is more delocalized,
hence appreciable covalent character. The results support

the conclusion made using the electronic spectral data in

conjunction with esr spectral data.

Electron Spin Resonance of NbC12(dmtp)2

 

The powder esr spectrum of NbC12(dmtp)2 diluted into
an isomorphous ZrClz(dmtp)2 is presented in Figure 38. The
19 line esr spectrum is very similar to the spectra obtained

by McGinnisS6

for the exchange-coupled dimer, [Nb12(dmtp)2]2.
The proposed structure is shown in Figure 39.

A necessary condition for obtaining esr spectra due to
dimers is that the paramagnetic ions are magnetically
isolated from each other so as to keep experimental linewidth
small.86 The Spectra of these systems consist of 2nI+1 lines

with a hyperfine splitting of A/2 where n is the number of

atoms present with nuclear spin, 1, and A is the normal

111

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CON t

 

 

 

mm enema;

112

mom monogomom EOH% :omew

¢ ¢
x ASEENEZ 13 6.3036 nmmoqocd

 

 

 

 

m m m
o o
/a\ \o o m
\/ x \6\
m m m _
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m X \ ~ m
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.x. .71
o ” om
m 0
x

mm magmad

 

113

hyperfine splitting for a single metal ion. In addition to
the normal AMS = i 1 transitions, "forbidden” AMS = i 2
transitions arise when the magnetic field is off the symmetry
axis of the molecule by the angle, 0. The intensity and
resolution of these transitions are normally much less than
for the AMS = i 1 transitions. This half field transition

is normally only observed in the frozen solution spectrum
and its presence is considered definitive evidence for an
exchange-coupled system.87 If dimer formation is incomplete,
as is frequently the case, dimer AMs = i 1 spectra are often
considerably obscured by residual monomeric spectra in the

g 2 2 region of the spectrum. This situation is often the
case in powders containing small concentrations of the para-

88

magnetic metal ions. The intensity of the AMS = i 2

6 are also much less than the

86,89,90

transitions of niobium(IV)5

AMS = i 2 for known cases of c0pper(II) complexes.
When two neighboring niobium(IV) ions interact, as

occurs when dimeric complexes are formed, the spin

Hamiltonian for the pair may be written:

H = H1 + H2 + Hint (24)

where H1 and H2 are each of the form:

“1,2 = gl IHZSZ + gL(HXSX + HySy) + AIISZIZ

+ AL(SxIx+ Sny) (25)

114

H representing the interaction energy between two ions,

int’
has the form:

= 2_ 2_2_.
Hint D[sZ 1/38(S+1)] + 13(3x sy) .181 32 (26)

where S = S1 + S2 if the ions are zero field splitting para-
meters. In a complex which contains an even number of elec—
trons the degeneracy of the ground state may be removed in
accordance with Jahn-Teller effect. For a system which
exhibits axial symmetry, it has been found that D >> E and,
in fact, if x and y symmetry axes are equivalent E = 0.
Assuming that the complex has axial magnetic symmetry, an
approximate value for D can be obtained from the separation
of the outer-most pair of lines in the low temperature powder

spectrum by:

H - H 3 2D (27)

Since the D value provides a reasonable measurement of
the intermetal distances which may give structural informa-
tion for the paramagnetic species, the assumption of axial
symmetry is made for this system so that D and the metal-
metal distance can be obtained. Accurate hyperfine and
zero—field splitting parameters cannot safely be extracted
from the spectrum in the absence of single crystal esr
analysis for all of the magnetic parameters. Esr studies

91

of the copper and vanadyl92 tartrates suggest that the

error in the derived esr parameters under the axial symmetry

115

approximation is not very large. If D is attributed to the

magnetic dipolar interaction between two electron spins, it
is expressed as93

2 l-3COS 0
<————————>

D = 3/4gzs (28)

where r12 is the interelectronic distance and e is the angle

between the r12 vector and the magnetic field direction.

Assuming that 6 equals the angle between the niobium-niobium

3
12

niobium—niobium distance, one obtains:

axis and the magnetic field and l/<r > = l/RS, R being the

 

0.325g2l1—coszel]l/3

Rcalc(A) = [ D(cm'1)

(29)

Taking Hll to be along the Nb-Nb axis, and e = 0°, one

obtains the esr parameters listed in Table 16.

Table 16

Esr Parameters for NbC12(dmtp)2 Diluted into ZrClZ(dmtp)2

gll = 2.092
*All = 109.9

*D = 732.5

R = 3.3871

*In units of 10-4 cm-1

116

[NbC12(dmtp)2]2 was reported to be diamagnetic based
on magnetic susceptibility measurements by the Faraday
method, as well as esr measurements in toluene or dichloro-
methane solution.56 However, this data suggest that the
species formed by the solid solution is an exchange-coupled
dimer. The gll value for the compound is similar to that
reported by McGinnis for [NbIz(dmtp)2]2,56 while All is high
compared to All for [NbIz(dmtp)2]2, it is in agreement with
most niobium(IV) compounds. Hence, the effect of diluting
a dimeric compound into a diamagnetic allows for packing
conditions to influence the nature of the complex in the

solid as opposed to the solution. The approximate Nb-Nb

8
4

(3.31 A) and [Nb12(dmtp)2] (3.53 A) but lengthened from the

distance which was comparable to that found for a - NbI
0

value of 3.06 A for NbCl49 illustrates the closeness of the

metal atoms, which under dilute conditions can show electron

exchange.

Electron Spin Resonance of Nb(dtc)4

 

Examination of Nb(dtc)4 diluted into the corresponding
diamagnetic zirconium(IV) matrix by esr methods gives a
spectrum which is anisotropic with some overlapping of two
sets of ten lines at both ambient and 77 K temperatures.
Values for gll, gi, Ail and A1 were obtained directly from
the low temperature spectrum. The values for <g> and <a>

were obtained by using the relationships <g> = 1/3(g|| + ZgL)

117

and <a> = l/3(Al| + ZAL). The esr spectrum is shown in

Figure 40 and the esr parameters are summarized in Table 17.

Table 17

Esr Parameters for Nb(dtcLIDiluted into Zr(dtc)4

<g> g: l 81 *All “‘1
1.948 1.902 1.971 110.0 170.8 78.6

*Hyperfine splittings are given in units of 10.4 cm-l.

It is apparent from the table that gi > gll indicating
a triangular dodecahedral structure as were the cases with

Nb[H2B(Pz) and NbX4[dth)z both included in this study.

214
Esr studies38 of piperdinyldithiocarbamate and a series of
methyl-substituted piperdinyldithiocarbamates reveal that

in each case gll is less than gi.

 

 

 

118

 

 

 

 

 

Doom

 

 

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ea ouswfla

SUMMARY AND CONCLUSIONS

Eight-coordinate Nb[HZB(Pz) was isolated from the

214
reaction of potassium dihydrobis(pyrazol—l-y1)borate
(K[H2B(Pz)2]) with niobium tetrahalides. The infrared
spectrum indicates that H2B(Pz)2- is bidentate. This is
supported by the nmr spectrum of the analogous zirconium
complex. The electronic spectrum exhibits one d-d transi-
tion and one strong band at 340 nm. This intense band in
the ultraviolet region is due to ligand charge transfer.
This assignment was confirmed by observing an identical
band at 340 nm in the corresponding zirconium complex.

The esr spectra support a D2d trigonal dodecahedral con—
figuration for the complex showing gi greater than g|1°

Complexes of the type NbX2[H2B(Pz) X = Cl, Br and

212(
I) were obtained from the reaction of the niobium tetra—
halides with two moles of K[HZB(PZ)2]' The infrared spectra
indicate that H2B(Pz)2_ is bidentate in all three complexes.
This is supported by the nmr spectrum of isomorphous
ZrC12[H2B(Pz)2]2. The electronic spectra exhibited

one d—d band in the chloride complex and two d-d bands in
the bromide and iodide complexes. An intense band in the
ultraviolet region due to ligand charge transfer was
observed in each case. The esr spectra proved that all the

complexes are monomeric by exhibiting ten lines at ambient

temperature. By using the molecular orbital theory

119

 

120

developed for octahedral complexes, metal-ligand bonding
parameters were obtained which indicate strong mixing of
metal and ligand orbitals.

Complexes, obtained by allowing one mole of K[H2B(Pz)2]
to react with one mole of NbX4, were proposed to be
NbX3[H2B(Pz)Z]. In these complexes the infrared spectra
indicate that H2B(Pz)2- is acting as a bidentate donor.
Flectronic spectra exhibited one d-d vibration and an
1ntense band in the ultraviolet region due to ligand charge
transfer. The esr solution spectra indicate the presence
of a monomer by exhibiting ten lines at ambient temperature.

The adducts NbX4(dth)2 were prepared by using pro-
cedures previously described by Hamilton and McCarley14
(X = Cl, Br and I; dth = 2,5-dithiohexane. The esr spectra
of the solid dissolved in toluene or excess ligand were
investigated to determine the structures of these complexes
in solution. This investigation confirmed a trigonal
dodecahedral structure, which had been proposed from
observation of the electronic spectra, with the esr spectra
g1 values observed to be greater than gll values as
expected for D2d symmetry. By using the molecular orbital
theory develOped for D2d complexes, metal—ligand bonding

parameters were obtained which indicate strong mixing of

metal and ligand orbitals.

121

[NbC12(dmtp)2]2 results from the reaction of niobium
tetrachloride with two moles of Nadmtp (dmtp = dimethyl-

dithiophosphate). When [NbC12(dmtp) is diluted into a

212
solution of the analogous zirconium complex, a powder esr
spectrum which is indicative of an exchange coupled dimer
is obtained. The esr spectra were very similar to spectra
obtained by McGinnis56 for [Nblz(dmtp)2]2.

Examination of Nb(dtc)4 diluted into the corresponding
diamagnetic zirconium(IV) matrix by esr methods gives spectra
which are anisotropic with two overlapping sets of ten lines.

The gi value was observed to be greater than the gll value,

which is indicative of D2d dodecahedral configuration.

SUGGESTIONS FOR FURTHER WORK

While there is clearly evidence for the formation of
an eight-coordinate complex in dichloromethane or toluene
X-ray structure data will ultimately be required to confirm
eight-coordination for niobium(IV) in Nb[H2B(Pz)z]4 and
would also provide a basic for esr study of single crystals.

The attempt to prepare Nb[H2B(Pz)2]4 or NbX2[H2B(Pz)2]2
(X = Cl, Br and I) in ethanol resulted in the formation of
a new common paramagnetic species which upon standing turned
diamagnetic and polymeric in nature. This polymeric species
should be investigated further. The nature of the deep blue
paramagnetic complex produced by dissolving NbX2[H2B(Pz)2]2
into ethanol should be investigated by methods other than
esr.

There is evidence for monomeric paramagnetic five-
coordinate complexes of niobium(IV) in dichloromethane or

toluene with NbX2[H2B(Pz) as a contaminant. These

2]2
studies should be extended to other solvents where pure
NbX3[HZB(Pz)2] may have an enhanced stability.

Extension of these studies to other metals should also
be fruitful. Extension to third row transition elements
((11 or d2) will be fruitful where starting materials are

available which are more reactive than the anhydrous halides

themselves.

122

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