ESR STUDIES OF 1,1«DITH'IOL
COMPLEXES WITH NIOBIUMOV)

Dissertation for the Degree of Ph. D.

MICHEGAN STATE UNIVERSITY
KIRBY KIRKSEY
1975

LIBRARY

 

This is to certify that the

thesis entitled

ESR Studies of 1,1-Dithiol Complexes with
Niobium( Iv)

presented by

Kirby Kirksey

has been accepted towards fulfillment
of the requirements for

Ph.D. degepm Chemistry

 

   
 

Major professor

0-7539

 
   
    

‘musmn‘i
V mame«
. ‘ u ' ‘ E_

 

ABSTRACT

ESR STUDIES OF l,l-DITHIOL COMPLEXES WITH NIOBIUM(IV)

by

Kirby Kirksey

Eight coordinate Nb(pipdtc)4 was isolated from the
reaction of ammonium piperdinyldithiocarbamate (NH4pipdtc)
with niobium tetrahalides. The infrared spectrum indicates
that pipdtc- is bidentate. The electronic spectrum exhibits
four d-d transitions and supports a D2d dodecahedral con—
figuration for the complex. Esr spectra confirm this
geometry with the parameters <g> = 1.9677, 9" = 1.9155,
gi = 1.9938, and <a> = 0.0110 cm‘l. By using molecular

orbital theory developed for D complexes, metal-ligand

2d
bonding parameters were obtained which indicate strong mixing
of metal and ligand orbitals.

When two moles of NH4pipdtc were allowed to react with
the tetrahalides, complexes of varying stoichiometries were
obtained. In each, the pipdtc- was bidentate as determined
from infrared spectra. Far infrared spectra showed bands

which could be assigned as 0(Nb-S) and 0(Nb-X) (X = Cl, Br,

I). Two bands in the near infrared-visible region were

Kirby Kirksey

assigned as d-d transitions. Esr spectra of these species
were unresolved single peaks at both ambient and 77°K
temperatures. The chloro compound when diluted into a
solution of the disubstituted zirconium complex, gave a
powder esr spectrum which was indicative of an exchange-
coupled dimer. The esr parameters are 9|] = 1.7873 and

All = 0.00642 cm‘l. The zero field splitting is 0.05778 cm’1

0

which corresponds to a niobium-niobium separation of 3.30 A.

ESR STUDIES OF 1,1-DITHIOL COMPLEXES WITH NIOBIUM(IV)

BY

Kirby Kirksey

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements

for the degree

DOCTOR OF PHILOSOPHY

Department of Chemistry

1975

ACKNOWLEDGMENTS

The author wishes to express his sincere appreciation
to Professor James B. Hamilton for patience, guidance, and
encouragement during the course of this study.

Special thanks go to Dr. R. N. McGinnis and
Mr. B. L. Wilson, whose discussions on various aSpects of
the universe were both invaluable and enjoyable.

Appreciation is also extended to Ms. B. L. Robbins

for considerable help in the preparation of this thesis.

ii

TABLE OF CONTENTS

INTRODUCTION
REVIEW OF PREVIOUS WORK
PURPOSE OF THIS WORK

EXPERIMENTAL

Materials
Analytical
Spectra
Syntheses

RESULTS AND DISCUSSION

Characterization of Eight Coordinate Species
Vibrational Spectra
Electronic Spectra
Electron Spin Resonance Spectra
Characterization of Halogen Complexes
Vibrational Spectra
Electronic Spectra
Nuclear Magnetic Resonance Spectra
Electron Spin Resonance Spectra

SUMMARY AND CONCLUSIONS

BIBLIOGRAPHY

iii

12

14

14
15
15
17

19

19
19
26
28

41
48
53
54
62

64

Table 1.

Table 2.

Table 3.

Table 4.

Table 5.
Table 6.

Table 7.

Table 8.

Table 9.

Table 10 .

Table 11.

Table 12.

LIST OF TABLES
Infrared Spectral Data for pipdtc
(4000- 600 cm“1 )

Infrared Spectral Data for Nb(pipdtc)4
(4000- 600 cm"1 )

Far Infrared Spectral Data for pipdtc
and Nb(pipdtc)4 (600- 100 cm"1 )

Electronic Spectral Data for Nb(pipdtc)4
in Benzene

Esr Parameters for NbB4 Complexes
Esr Spectral Parameters of Nb(pipdtc)4

Esr Parameters for Eight Coordinate
Niobium(IV) Complexes

Calculated “e Values for Eight
Coordinate NbTIV) Complexes

Infrared Spectral Data for Halogen Complexes
(4000— 600 cm-1)

Far Infrared Spectral Data for Halogen
Complexes (600- -100 cm"1 )

Electronic Spectral Data for Halogen
Complexes

Esr Parameters for the Chloride Complex
Diluted into ZrC12(pipdtC)2

iv

21

22

25

26
31
33

35

37

42

44

52

60

Figure

Figure
Figure

Figure

Figure

Figure

Figure
Figure
Figure
Figure
Figure
Figure

Figure

Figure

LIST OF FIGURES
Apparatus for Determination of Electronic
Spectra
Far Infrared Spectrum of NH4pipdtc
Far Infrared Spectrum of Nb(pipdtc)4

Electronic Spectra of Nb(pipdtc)4 in
Benzene

Esr Spectrum of NbB Complexes in Benzene
at Ambient Temperat re

Esr Spectrum of NbB
at 77°K

4 Complexes in Benzene

Far Infrared Spectrum of Chloride Complex
Far Infrared Spectrum of Bromide Complex
Far Infrared Spectrum of Iodide Complex
Electronic Spectra of Chloride Complex
Electronic Spectra of Bromide Complex
Electronic Spectra of Iodide Complex

Esr Spectrum of Halogen Complexes at
Ambient Temperature

Esr Spectrum of the Chloride Complex
Diluted into ZrC12(pipdtc)2 at 139°K

16
23

24

27

29

30
45
46
47
49
50

51

55

56

INTRODUCTION

The niobium tetrahalides, which, except for the

fluoride species, contain metal-metal bonded szx8 pairs,

are diamagnetic chains formed by NbX octahedra sharing two

1-3

6
opposite edges.

This structural feature has dominated the chemistry
which the tetrahalides undergo. Most of the complexes
formed are either:

a) Simple addition compounds in which the metal-
metal bond is cleaved. These complexes are
paramagnetic.

b) Simple addition compounds in which the metal-
metal interaction is only slightly altered.
These compounds are usually dimeric and may
by either diamagnetic or antiferromagnetic.

c) Substitution products in which the metal-
metal bond is cleaved. These complexes are
usually paramagnetic monomers.

d) Partially substituted products in which the
metal-metal interaction may or may not be
altered. These complexes may be monomers
dimers, etc., and may exhibit either para-
magnetic, antiferromagnetic, or diamagnetic
behavior.

Relatively few of these complexes have been investigated by
esr methods. The limited studies which have been done have
focused upon alkoxo-complexes formed by NbCl4 in alcohol

solutions.4'S

More recently esr methods have proven to be useful in

investigations of complexes of both type (a) and (c) above.

This particularly has been the case for eight coordinate
complexes formed via reactions of type (a) and (c). Esr
has been demonstrated to be a powerful tool for elucidating
the structure of eight-coordinate complexes - both in solu-
tion and in the solid state. Studies of exchange coupled

1 and d9

dimers for a number of d systems have utilized esr
methods.6

With niobium(IV), 1,1-dithiols have proven to be quite
versatile ligands, forming both paramagnetic eight-coordinate
complexes,7 and in at least one instance, exchange-coupled
dimeric species.8 In each instance, esr has proven to be a

valuable tool for investigating such systems.

REVIEW OF PREVIOUS WORK

There have been several reviews on the chemistry of
niobium halides and their complexes.9-ll Reviews of the
coordination chemistry of l,l-dithiols have also been

recorded.12’l3

What is lacking in these reviews are dis—
cussions of complexes formed by niobium(IV) with l,l-dithiols
and related sulfur donor ligands. This review will focus
upon niobium(IV)sulfur complexes, related dithiocarbamate-
metal complexes, and dimeric, niobium(IV) systems.

Hamilton and McCarley in their studies of thioether

14'15 found that the thioethers would form both

complexes
paramagnetic and diamagnetic complexes of the general for-
mulations NbX4L2 and [NbX4L]2. Anomalous behavior was found
with the methyl sulfide complexes. Under a dynamic vacuum,
the diadduct which was recovered from the original reaction
mixture would lose one mole of methyl sulfide over a period
of twelve hours, yielding the monoadduct. The ethyl sulfide
would only form a monoadduct. All of these species were
weakly paramagnetic. Fowles16 also studied these systems

and obtained similar results, the one exception being the
postulation of the monoadducts as antiferromagnetic exchange-
coupled dimers. Further study of this system in solution by

Chen and Hamilton17 by use of esr methods, revealed that in

solution both the ethyl and methyl monoadducts were strongly

4

paramagnetic, but exhibited no resonances at g~4 as expected
for a dimeric exchange-coupled species.18

Fowlesl6 also investigated the reactions of tetrahydro—
thiOphene (tht) with niobium tetrahalides. This ligand
two different forms

formed diadducts with NbCl and NbBr

4 4‘
of the bromide complex were obtained. The far infrared
spectra of the chloride and a-bromide species were similar.

In both NbCl4(tht)2 and a-NbBr4(tht) the expected v(Nb-X)

2:
and 0(Nb-S) were observed in the region 340-240 cm-l, with
the appropriate shift for the change in the halide. The
B-bromide complex gave only one strong band at 227 cm-1.
The latter complex was assigned a trans configuration and
the other complexes cis. Although most bis-adducts of niobium
tetrahalides have been assigned as cis structures, there has
been a trans structure reported for DMFlg complexes.
Bereman,20 on the basis of esr data, preposed a trans
structure for certain pyridine adducts.

Douglas and Green21 studied the product from the
reaction of methyl mercaptan and bis(n5-cyc10pentadieny1)-
niobium dichloride. The resulting complex, (nS-Cp)2Nb(SCH3)4
was examined by esr. The esr spectrum of the complex gave
ten lines with <g> = 1.991 and <a> = 25.1 gauss. The phenyl-
thio-derivative gave similar esr parameters. Of interest
was the small <a> values reported. Except for the hexa-
haloniobium(IV) complexes, the usual <a> values are ~150-200

gauss. The authors made no comment as to what contributed

to these lower values. These complexes could be oxidized

by the addition of iodine.
Bidentate sulfur ligands typically form eight coordinate

15 used l,2-bis(methylthio)-

species. Hamilton and McCarley
ethane (dth) to prepare Nbx4(dth)2. Far infrared data on
metal-halogen and metal-sulfur vibrations show the ligand
is bidentate. Molar magnetic susceptibilities ranged from
1.28-1.60 B.M. and did obey the Curie law. Solid-state esr
data of the chloride complex indicated the structure was a
dodecahedron. Further investigation of the complexes in
frozen solution by esr22 supported this prOposed structure.
From the frozen solution spectra of the chloride and bromide
complexes, the fact that 9'] < gi was taken as evidence of
an idealized triangular dodecahedron as has been found with
similar complexes.23

Tetrasubstituted dialkyldithiOphosphate complexes were
studied by esr..7 Esr spectra of both the solid and solution
gave <g> = 1.955. At 77°K, 9" < gl was what is expected
for a dodecahedron. Far ir studies assigned the strong
bands at 356 and 274 cm.1 as 0(Nb-S) modes. The presence of
two bands so assigned is further support of the structure.
When the acid form of the ligand was used, only viscous oils
resulted.

Machin and Sullivan24 used a variety of ligands to
study the tetrahalides. Three different compounds were
formed by thiourea (TU), NbCl4(TU)2, NbI4(TU)3, and an

unidentifiable NbBr4 polymer. The magnetic moment of the

chloride Species determined by the Faraday method, 1.19 B.M.,

was consistent with those found for other octahedral com-
plexes.14 No esr work was performed. The iodide compound
had a molar conductivity of 220 ohm-lineal x 10-3 molar

solution in acetonitrile, in the range expected for a 1:1

electrolyte. Bands in the infrared at 695, 1510 cm.1 and

700, 1510 cm-1 for the chloride and iodide adducts respec-

tively, as well as the absence of a band at 1080 cm-1,

supported sulfur bonding.
One class of sulfur donor ligands which will be treated
separately is the dithiocarbamates. This ligand is readily

prepared by the reaction:

R NH + cs + on” = R NCS + H

2 2 2 2 0 (l)

2

This ligand has the ability to exist in the following

canonical forms.

/,3 s s

+ ¢¢ /,
R N=C (—'——-) R NC Q———') R NC

2 \ (2)

S
Chatt and his colleagues25 have demonstrated the ability of
this type of ligand to form complexes with a large number
of metals in which it bonds bidentately.

Several workers have prepared tetrakisdiethyldithio-

26-28 Bradley26 prepared the complexes

carbamatoniobium(IV).
by allowing the pentakisdimethylamidoniobium(IV) to react
with CS2 while the other authors used the sodium salt of

diethyldithiocarbamate. Characteristic ir bands occurring

 

at ~1500, 1000 and 360 cm'1 were assigned as v(C - - - N),

.7

V(C ——— S) and 0(Nb ———-8) respectively. The magnetic data

26

were in conflict with Bradley and Machin27 finding

28

z 0.5 B.M. while Brown found ”eff = 1.57 B.M. No

"eff
comment was made as to the cause of the low magnetic moments.
Another study of the stereochemical lability of eight-
coordinate complexes29 confirmed the equivalence of the alkyl
groups by use of 1H and 13C nmr, and established a dodeca-
hedral structure.

The lack of work with sulfur-donor ligands is clearly
illustrated by these citations. Except when the
dithiocarbamates were used as precipation reagents,30
a similar situation exists for niobium - dithiocarbamate
complexes. However, with other (11 systems, in particular
V(IV), as well as other metal systems such as Ti(IV), Zr(IV)
and Cu(II), a great deal of work has been reported.

Tetrakis-N,N-dialkyldithiocarbamate complexes of Ti(IV)
and V(IV) have been reported in two papers by Bradley.26'31
These complexes were prepared via an insertion reaction
involving CS2 and the metal dimethylamines. Some con-
tamination of the tetrakisdiethylvanadium(IV) compound by
the presence of the tris species caused ”eff to be 2.24 B.M.,
which is much greater than the expected value 1 1.73 B.M.
for a V(IV) dl species. This contamination was due to the
method of preparation which involved refluxing the reaction
mixture. Esr data, which gave 9" < 91' and All > Ai'

supported a structure based on a dodecahedral model. When

the vanadium complex was dOped into the isomorphous titanium

compound, the esr spectrum showed g-anisotropy corresponding

51

to an axially symmetric species with resolved V nuclear-

hyperfine splitting. Electronic spectra, which showed a

maximum at 13,600 cm-l, assigned as a 2B1 + 2E transition,
were considered consistent with a dx2_y2 ground state. In
the infrared region, three bands of importance were noted
for the complexes. These bands, occurring at ~1500, ~1000

and 850, and ~360 cm.1 were assigned as "thioureide"

C - - - N, C ——— S, and M ——— S respectively. The tris-

 

(N,N-diethyldithiocarbamato)vanadium(III) compound was
assigned a distorted octahedral structure based on three
transitions found in the electronic spectra.

Fay and his co-workers32 prepared a seven-coordinate

titanium(IV) compound, TiX(R2NCS by the reaction of the

2’3'
tetrahalide with a stoichiometric amount of the anhydrous
sodium dialkyldithiocarbamate. The ethyl derivative was
found to be monomeric in benzene. Nmr spectra over a tem-
perature range indicated the ligands were non-rigid on the
nmr time scale. Although symmetry requirements should make
some of the alkyl protons inequivalent, no splitting was
found as low as -80°C. Further evidence as to the coordina-
tion was found from the infrared data. The single bands
found at 1500 and 1000 cm-1 were indicative of bidentate
coordination. The authors did not assign the compound a

structure based on any idealized geometry for seven-

coordination.

Additional work on the titanium(IV) dithiocarbamates
by Fay33 produced compounds of the stoichiometry
Ti(SZCNR2)nC14_n where n = 2,3, or 4. The nature of the
species formed was only dependent on the amount of dithio-
carbamate allowed to react with TiCl4. Conductance measure-
ments showed the complexes were nonelectrolytes and molecular
weight determinations in benzene established that they were
monomers. Infrared data were indicative of bidentate coor-
dination by the ligand. Proton nmr experiments on the iso-
propyl derivatives reveal the presence of two equally
populated i-Pr sites at low temperatures.

Several investigators12

have examined Cu(II) complexes
of dialkyldithiocarbamates. The compounds were reasonably
stable in water, having stability constants of ~23.0. The
diethyl species was monomeric in both benzene and chloro-
form. However, a crystal structure34 showed the copper to
be five coordinate, a distorted tetragonal-pyramid of four
sulfur atoms, at a distance ranging between 2.297 and

2.339 A and a fifth apical sulfur atom at 2.851 A. The
solid state study determined that this species was a dimer.

+

The importance of the canonical form, ---82-C = NR2 as
25

established from infrared study was also supported.
Dimeric niobium(IV) complexes occur quite frequently

due to the structurecfifthe tetrahalides. In some cases,

there is conflict concerning the nature of the compounds.

14,16,17

Different reports on the compound, NbC14°S(CH3)2.

10

found it was diamagnetic14 in the solid, and paramagnetic

in solution,17 while other authors16 showed it was anti-

ferromagnetic.

This type of ambiguity was found when NbBr4 was allowed
to react with less than four moles of sodium diethyldithio-
carbamate (Na dtc).27 A species was prepared which was formu-

lated as NbZBr3(dtc)5. The molecular weight of the compound in
chloroform was found to be 1240, compared to the calculated
value of 1166. The infrared spectrum was similar to that of
Nb(dtc)4, indicating the ligands were bidentate. The molar
conductance in nitromethane lead the authors to propose a
formulation, [Nb2(dtc)SBr2]Br. The magnetic susceptibility
was field dependent.

Wentworth and Brubaker35

prepared two compounds,
[NbCl(OC2H5)3(CSH5N)]2 and Nb(OC2H5)4, which were dimers.
The former species had five possible isomers, two octahedra
with a shared edge, bridged by two chlorine atoms. When it
was allowed to react with sodium ethoxide, Nb(OC2H5)4 was
formed. The oxidation state of (IV) was demonstrated by the
lability of the compound to chloride ion to form NbC15(OC2H5)2-
which is known to contain Nb(IV). Esr studies of the same
system by Gunthard5 confirmed the existance of the latter
compound, although these authors did not isolate the species.
McGinnis' study8 of the dimethyldithiophosphates (dmtp)
found that dimeric niobium(IV) species could be formed with

the composition Nb2X4(dmtp)4. The compounds were prepared

by allowing stoichiometric amounts of the tetrahalides to

11

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 room temperature magnetic
moment of 2.32 B.M. The esr spectrum of the iodide complex
showed both the normal AmS = i 1 transition and the "for-
bidden" Ams = i 2 transition. This is the only reported
niobium(IV) electron-exchange coupled species.

The use of piperidyldithiocarbamate as a ligand has

37 This ligand, as well as

been reported in only one paper.
the 2-, 3-, and 4- methyl derivatives reacted

with alcoholic solutions of Cr(III). Tris-substituted
compounds were prepared and showed a great deal of metal-

1igand covalency. Each of the compounds exhibited bands

indicative of bidentate character for the ligand.

PURPOSE OF THIS WORK

Thus far in this brief review, several facts are

apparent:

a)

b)

C)

d)

There has been relatively little study of
niobium(IV) complexes with sulfur-donor
ligands.

Of the work which has been reported, a
significant trend is the formation of
paramagnetic six-coordinate or para-
magnetic eight-coordinate complexes.

Esr characterization of the paramagnetic
species has been limited to a small number
of systems.

A significant anomaly is the relative
frequency of occurrence of either dia-
magnetic or weakly paramagnetic dimeric
species or antiferromagnetic exchange-
coupled species (especially when the halo-
gens are partially substituted by 1,1-
dithiols).

In view of the ability of sulfur donor ligands to stabilize

low oxidation states while exhibiting a high degree of

covalency with transition metals, it is surprising that the

reactions of l,l-dithiols with niobium(IV) halides have not

been studied in more detail.

The

preparation of dialkyldithiophosphate complexes

of niobium(IV),8 and the series of compounds,

T1(SZCNR2

)nCln_4 (where n = 2,3,4)32 suggests that the

1,1-dithiols may be ideal ligands for investigation. In an

12

13

attempt to study compounds of a similar nature, the
uninegative, bidentate salt, ammonium piperdinyldithio-

26-28 which

carbamate (NH4pipdtc) was used. Previous studies
used straight-chain dialkyldithiocarbamates did not include
examination of the complexes by esr.

This ligand when allowed to react with the tetrahalides
could form species in which

a) all of the halogens would be substituted.

b) disubstituted dimeric Species with only
the non-bridging halogen being substituted are formed.

c) disubstituted, monomeric Species are formed.

All of these cases could, by electron spin resonance
spectroscopy, provide further insight into the nature of

the chemistry which characterizes niobium(IV).

 

EXPERIMENTAL
Due to the sensitivity of the compounds to oxygen and
water, all the compounds were handled under high vacuum or
in a Vacuum AtmOSpheres Corporation nitrogen filled drybox
containing less than 1 ppm water and oxygen.

Materials. Niobium pentachloride, ZrCl and high purity

4’
niobium metal were purchased from Alfa Inorganics. NbBr

 

5!
NbIS, and Nbx4 (X = Cl, Br, and I) were prepared by using

procedures previously described.38'39

Practical grade piperidine (98%), 2-, 3-, and 4-
methylpiperidine were obtained from Aldrich Chemical Company.
Benzene, aqueous ammonia, carbon disulfide, anhydrous
ethanol, and methylene chloride were standard reagent grade
chemicals.

Ammonium piperdinyldithiocarbamate (NH4pipdtc) and the
methyl-substituted species were prepared by previously

40

described methods via the reaction:

RNH + cs + NH3(aq) + NH4(52CNR) (1)

2
Potassium ethyl xanthate was prepared by the reaction:

H CCH OH + cs + KOH + H o + K(H3CCH

3 2 2 2 ocsz) (2)

2

14

15

Analytical. Microanalyses and molecular weight determina-

 

tions were performed by Galbraith Laboratories, Inc.,
Knoxville, Tennessee.

Preliminary analyses were performed to determine niobium.
The niobium was determined gravimetrically as niobium(V)
oxide. Samples of the complexes were added to aqueous ammonia
and heated for two hours. After the samples cooled to ambient
temperature, they were acidified with dilute nitric acid.
The white anhydrous niobium oxide was filtered on ashless
filter paper, washed three times with dilute nitric acid,

ignited at 900°C for two hours and weighed.

Spectra. Esr spectra were obtained with samples in benzene
solutions at ambient temperature and 77°K 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 mag-
net. The magnetic field was calibrated by using strong
pitch (9 = 2.0028).

Electronic spectra were recorded by using a Cary Model
17 spectrophotometer. Cylindrical fused silica cells, 1.0 cm
pathlength and adapted for use at low pressure (Figure l),
were used.38 Saturated solutions were loaded in the drybox.
The cell assembly was then evacuated to ca. 10"5 torr. After
sealing off the cell assembly, solvent and/or solutions of

various concentrations could be drawn through a medium

porosity frit into the cell.

16

TO HIGH VACUUM MANIFOLD

—_._._.—

STOPCOCK

T BALL-SOCKET JOINT
SEAL OFF HERE AFT
LOADNG ER

 

QUARTZ - PYRE X
GRADEO SEAL

  
 
 
 

MIXING
CHAMBER

Figure 1. (Taken from Reference 38) Apparatus for
Determination of Electronic Spectra

 

17

Solid state infrared spectra were obtained by use of a
Perkin-Elmer 457 (4000-250 cm-l) spectrophotometer. Samples
were prepared in the drybox and were mounted as Nujol mulls
between cesium iodide plates. Mulls were prepared immediately
before measuring the Spectra.

Far infrared spectra were obtained by using a Block
Engineering Company Model-FTSl6 (600-20 cm-l) far infrared
spectrOphotometer. High density polyethylene was used for
windows.

Proton nmr spectra were obtained by use of a Varian

Model A56/60D spectrometer operated at 60 MHz.

 

Synthesis. A four to one molar ratio of NH4pipdtc:

Nbx4(x = Cl, Br, I), and a magnetic bar were introduced

into a round bottom flask. The flask was evacuated to ca.
10.5 torr and 50—80 m1 of benzene were vacuum distilled into
the flask. The flask was isolated from the vacuum system
and the mixture was stirred for 4-5 days at ambient tempera-
ture. A deep maroon solution and orangish precipitate were
obtained. The precipitate was removed by filtration and

the filtrate was evaporated to dryness in vacuo. The
filtrate was extracted with benzene, and a reddish-purple
solid was recovered by removal of benzene in vacuo. The
product was then washed with dry pentane and dried in vacuo.
Similar results were found when the acid form of the ligand

was prepared in situ in a slurry of Nbx4 in benzene.

18

The methyl-substituted derivatives and ethyl xanthate
species were prepared in a similar manner.

A two to one molar ratio of NH4pipdtc: NbX4 (X = Cl,
Br, I) was placed in a round bottom flask in the same manner
as above and stirred for 7-8 days. The solution, which was
purple after two hours of stirring, became yellowish-brown
after two days with an orangish precipitate being visible
in the mixture. The species were recovered as above.

Two different two to one molar ratios of NH4pipdtc:
MCl4 (M = Nb, Zr) were prepared. Each mixture was allowed
to stir for 7-8 days. They were filtered to remove the
precipitates and then mixed. The original mixtures were

prepared so that the ZrCl4 to NbCl4 ratio was ten to one.

Anal. Calculated for NbC H40N4S : Nb, 12.66;

C, 39.25; H, 5.51; N, 7.64; S, 3 .95. Found:

Nb, 13.09; C, 39.95; H, 5.65; N, 7.61; S, 28.07.
Molecular weight: Calculated, 734. Found, 698 in
HCC13. MP, 128°C with decomposition.

RESULTS AND DISCUSSION
Characterization of Eight Coordinate Species

The reaction of the niobium(IV) halides with stoichio-
metric amounts of ammonium piperdinyldithiocarbamate in

benzene proceeds according to the equation:

4NH4pipdtc + Nbx + Nb(pipdtc)4 + 4NH X (3)

4 4

The complex was isolated as a reddish-purple powder which

is soluble in benzene, toluene, methylene chloride, and

acetonitrile. The species is either air and/or water sensi-

tive as indicated by a color change and the distinctive

odor of carbon disulfide on exposure to the atmosphere. The

melting point is over the range 112-128°C with decomposition.
When the methyl-substituted species are employed as

well as the ethyl xanthate salt, the reaction appears to go

as stated above.

Vibrational Spectra

1)

Ammonium piperdinyldithiocarbamate has not been studied

Infrared Spectra (4000-600 cm-

in the infrared region. The Spectrum of the sodium salt
dihydrate is published in the Sadtler Standard Spectra.41
Assignments were made by comparing the spectrum of the

sodium salt with published correlation charts of infrared

19

20

spectra.42 The assignments for the ammonium salt were based
on this information. Infrared data of the ligand and complex
are presented in Tables 1 and 2 respectively.

In complexes containing pipdtc and other dithiocarbamates,
two regions are of interest in the infrared, the "thiouride"

1 and the v(C=S) band at 1000 cm-1. The com-

plex showed vibrations at 1490 cm.1 and at 997 cm‘l. The

band at 1490 cm-

1ower energy vibration has been used to determine whether

the ligand is acting as a bidentate. Bonati and Ugo43 in a

study of organotin(IV) dithiocarbamates, compared the ir
2)25n and EtzNCSZEt. The chelated compound
showed only one strong band at 995 cm.1 while the ester shows

Spectra of (EtZNCS

a doublet (1005 and 983 cm-l). An absence of a doublet in
this region has been confirmed for other known chelated

1 O I I
1s a comb1natlon

structures.33 The strong band at 730 cm-
of vibrations due to the 6(C-H) of both Nujol and the piper-

idine ring.

Far Infrared Spectra (600-100 cm-l)

 

The far infrared Spectra for the ammonium salt and the
complex were recorded. The data are presented in Table 3
and the spectra are shown in Figures 2 and 3 for the ligand

and complex respectively. Two bands at 385 and 360 cm-1

were present in the spectrum of the complex and are not
present in the ligand spectrum. These bands have been

assigned as 0(Nb-S) vibrations. Combination bands involving

ligand vibrations and v(Nb-S) overtones were present at

Infrared Spectral Data for pipdtc- (4000-600 cm- )

Assignment

v(OH)
v(C-H)
v(N-C)
5 (HOH)
+
v(C=N)
V(C-N)
5 (CH2)
v(C=S)

v(NCSz)

0(NC32)
V(NCSZ)

5 (CH2)

*Interferring vibrations from NH4

21

Table 1

Na pipdtc-2H

3333
2924
2096
1626
1468
1420
1361
1274
1263
1227
1130
1109
1070
1024
1008

966

949

919

884

855

s = strong

b = broad,
m = moderate, w = weak
sh = shoulder

20

<2
m

0"

ring
deformation

m a: m i: a i: m a) a E: a

s
I
a

5 (0 B 23 m 53 8

+

NH

*

*

*

*

*

4

2025
1590
1430
1390
1305
1275
1235
1207
1155
1115
1065
1025
995
967
947
937
883
865
848
800
730

OS
pa
U1

1

pipdtc

S E! a 23 B 53 B (n

U‘

2

22

Table 2

Infrared Spectral Data for Nb(pipdtc)4 (4000—600 cm' )

Assignments

v(N-C)
ring deformation
v(C=N)
v(C-N)
6(CH2)
V(C=S)
V(NCSZ)
v(NCSz)

v(NCSZ)

0(Ncsz)

v(C=S)

6(CH2)

b = broad,
sh =

s = strong,
shoulder

2022
1585
1490
1390
1390
1275
1253
1232
1160
1125
1103
1030
1015
997
947
887
850
800
725

m = moderate, w =

1

U)

sh
m,sh

m,sh

m 53 a E! S 0)

weak

23

 

 

 

 

 

ABSORBANCE (arbitrary units)

 

 

1

l l 1
I00 200 300 400 500 600
1! (cm")

 

Figure 2. Far Infrared Spectrum of NH4pipdtc

24

 

 

 

 

7..

E
E.
c
:3
Z‘
8
.o-
‘B
3
5
3:;
m
4

l i L I
'00 200 300 400 500 600
v(cm")

Figure 3. Far Infrared Spectrum of Nb(pipdtc)4

25

Table 3

Far Infrared Spectral Data for pipdtc-

and Nb(pipdtc)4 (600-HM)cm-

1)

Assignments NH4pipdtc Nb(pipdtc)4

Combination: v(C-S) 580
6(C’-N-C’). 6(C-N).
ring deformation

Combination: v(M-S), 545

ring deformation, 510

5(C’-N-C’)

6(C’-N-C’) 445
427

6(C’-N-C’) 415
380

V(M-S)

v (M-S)
322
307
297
285
285
260

b broad, 5 strong, m =

2‘.
II II

s,b 550 s,b
sh 510 w
s 443 m-s
w
m
sh
385 m-s
360 m-s
s
w 307 w
sh
sh
s
w 260 w

moderate,

weak, sh = shoulder, v = very

26

550 and 510 cm-1 as were found in the Cr(III) complexes of

this ligand.37

Electronic Spectra

Visible and near infrared spectra were recorded by using
benzene solutions of the complex. The spectra and wave num-
ber maxima are given in Figure 4 and Table 4 respectively.
The gross features of the spectra include four bands at

11.9, 13.7, 18.8 and 21.7 (x 10*3cm'1).

Table 4

Electronic Spectra Data for Nb(pipdtc)4 in benzene

v(x 10"3 cm-l) s
11.9 52.3+
13.7 75.2+
18.8 378*
21.7 340*
+c = 7.98 x 10'3 M
*c = 2.50 x 10’3 M

Based on the infrared data which have been discussed
earlier, this complex should have either a dodecahedral or
square antiprismatic structure. For a D4d square antiprism,
only two d-d transitions should be observed, while the D2d
triangular dodecahedron should exhibit three d-d transitions.
The fact that this compound has four bands in the near ir-

visible region is not a unique situation for niobium complexes.

27

ocmucom CA vfioupmflmvnz mo muuommm owconuomam .e ousmflm

 

“gnaw/A
000. com 000 00b 000 com 00¢
fl _ u d

 

 

2 Po. x 3

Sub. x m8.

(shun 5401:1010) souvaaosev

 

 

 

28

Other eight coordinate Species, namely NbX4(dth)215 and

NbCl4(diarsine)2,44 have four bands in this region. The
extinction coefficients are in the range that other authors

have assigned as d-d transitions.9

Electron Spin Resonance Spectra

Esr studies were performed as described in the experi-
mental section on the series of compounds NbB4, where B =
pipdtc, 2-Mepipdtc, 3-Mepipdtc, 4-Mepipdtc, and Etxan.
Representative spectra are presented in Figures 5 and 6,
and the esr parameters are presented in Table 5.

Examination of these data points out several features
about the nature of the species. In all cases, 9" is
less than gi. The significance of this observation will be
discussed. For the series, pipdtc, 2-Mepipdtc, 3-Mepipdtc,
and 4—Mepipdtc, the pipdtc compound had the lowest <g> value
as well as the highest All value. This is expected because
the influence of the methyl group on the ring will cause
some steric problems that will affect the amount of de-
localization of the electron from the metal onto the ligand.

Similar results were found in an esr study of NbCl adducts

4
with various substituted pyridines.20 The spectra recorded
for the 2-methyl derivative were not as well resolved as
found for the other members of this series. As a result,

it was not possible to extract the esr parameters at 77°K.
Wasson's study37 of this series of ligands with Cr(III) also

reported anomalous behavior with 2-Mepipdtc. Clearly there

29

ouaumummfime ucownfid um mcmucmm a“
moonmEoo moz mo Eduuowmm Hum .m wusmem

 

 

 

 

 

 

 

 

 

 

 

 

myoc—

 

 

 

 

30

Mosh um ocmwcwm Ce moonmEOU

Ooon

v

moz mo Eduuoomm Hmm

 

 

 

 

 

.w musmflm

31

v.an
m.HhH
N.mOH
mmao.m
mwmm.a

humm.a

cmxpm

m.vn m.mn IIIIII

m.vna m.hnH IIIIII
m.moa N.HHH H.0HH
mmoo.m Hmao.m llllll
mvmm.a mvmm.a IIIIII
mmnm.a mvmm.a mmmm.H
oopmfldozlv Oppmflmmzlm ouUQHmmzlm
mmxmameou gmoz MOM mumuoEmumm umm

m OHQMB

EU

0.00
N.mmH
0.0HH

mmmm.a
mmam.a

nmmm.a

boomed

IOH x a

k.
«__¢

{Amv

:a

Amv

32

are little or no steric effects observed in this system by
the use of alkyl substituted Species or bulky ligands like
piperidine.

Since the hyperfine splittings are on the order of 120
gauss, the high field approximation cannot be applied and
second order corrections should be employed. The pertuba-
tion of the Zeeman transition resulting from the hyperfine
interactions was corrected by means of the following

. 45
equations:

hv = gBHo (4)
f ' t ' H' - H + < > + <a>2 I(I+l)- 2 (5)
or ISO roplc g m — m a mI 2H;_[ mI ]
A 2
2
T = —_ —
for 9" Hm Hm + AIImI + 2Hm[I(I+l) mI ] (6)
2 2
All +A
. = _________ _ 2
for gi Hm Hm + AimI + ( 4Hm )[I(I+l) ml] (7)

where Hm' is the corrected magnetic field position, Hm is
the experimental position of the esr line due to the com-

ponent m of the nuclear spin I, v is the kylstron frequency,

I
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

33

of the hyperfine components in gauss is related to the energy
splitting in cm.1 between adjacent hyperfine levels as

follows:

A(cm-l) = g x 4.6686 x 10’5A(gauss) (8)

The experimental esr parameters for Nb(pipdtc)4 are listed

in Table 6 with the corrections due to second order effects.

Table 6

Esr Spectral Parameters of Nb(pipdtc)4

Experimental Corrected
<g> 1.9677 1.9471
<a>* 110 108
9)) 1.9155 1.8969
gl 1.9938 1.9722
AT|* 195 190
Al* 66 65
4 -l

*Hyperfine splittings in units of 10- cm
In benzene at 77°K, the esr spectrum may be described
by the Spin Hamiltonian with axial symmetry:46
H = gl IBHZSZ + 91(HXSX + Hysy) '1' Al ISZIZ
+ 9
+ Ai(SXIX Sny) ( )

where S =l/2,I(93Nb; 100%) = 9/2. At ambient temperature

in liquid solution, the anisotropies are averaged to zero,

34
and the Hamilton becomes:
H = <g>BH-S + <a>I°S (10)

Theoretical calculations of esr parameters for a D2d

dodecahedron and a D4d square antiprism show that 91,) 9'1

for a dodecahedron23’47

and 9|] > 9i for a square anti-
23,48

prism. For the series of compounds studied in this

work, gi > 91' indicating dodecahedral symmetry in support

of the electronic spectra. In comparison to other known

eight coordinate complexes of Nb(IV) listed in Table 7, the
data observed here fit well. Two complexes where

91" gll' Nb(dpm)449 and Nb(CN)84-(soln)50 have been reported.
Single-crystal x-ray studies of these compounds have determined
the structure of the latter species as dodecahedral while

the former is antiprismatic.

Data from the esr studies can now be used in conjunction
with the electronic spectral assignments for Nb(pipdtc)4 to
determine the applicability of an ionic model to the present
system. It can be demonstrated that while the dodecahedral
model has D2d symmetry, it can be considered as arising
from the distortion of a cube. 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:47

81 21
= 2.0023 - ——— = 2.0023 - ——— (11)
9| | 133 9i AE2

35

HNH
av
hm

onm.a
ooo.N
omm.a

«usmxzovnz
om

mu
vma
Hoa
Nmm.H
ohm.a
5mm.a

+nsmxzuvnz
om

mofloomm A>HVESHQOHZ

H.ooa
mhmm.a
homm.H
homm.H

f

Em
as evnz

o.~a
a.ea~
a.mma
mome.~
emsm.a
Hana.a
ardeeevnz

coflusaom *

UHHOm +

EU OH HO mHHCD CH ¥

at v

m.moa
m.mmH
m.vma
mmmo.m
vamm.a
momm.a

m 4
mm Agape Honz

oumcflpuooutunmflm mom muwuoEmumm Hmm

e means

as

..:e

¥

*AMV
_s
__m

Amv

36

where A is the free ion coupling constant, AB = (232-281),
_ 2 _2
4+

for Nb as 748 cm-1, the calculated values of gi and 9"

3

and AE Taking the spin-orbit coupling constant

are 1.9227 and 1.7265 respectively. These values are much
lower than the experimental values and this indicates the
inadequacy of the ionic model. It is possible to assess

qualitatively the amount of covalent bonding via the

equations:47

2 2 2 2
Bid 8 216 y
9ll 91 422

AE3 (12)

The parameters a2, 82, and Y2 are associated with B1, E, and

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 azyz = 0.069 and a282 = 0.32.

For pure covalent bonding in the ground and excited states,

a2y2 = a282 = 0.0625. It therefore appears that the niobium

d orbitals are strongly mixed with ligand orbitals in the
formation of Nb(pipdtc)4.

Although the structures of the tetrakis(dialky1amido)-

1

niobium(IV) compounds, Nb(NR2)4,5 are tetrahedrons, these

compounds have a similar ground state of dx2_y2. The esr

data for Nb(NCSH <g> = 1.9540 and <a> = 104 x 10.4 cm-l,

10’4'
are similar to the data found in this work for NbB4.

37

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

neff(B.M.) = g /S(S+l) (13)

in which S is the absolute value of the spin quantum number
and g is the experimental gyromagnetic ratio, a value for ”eff

can be obtained. Table 8 lists calculated “eff values for the

Table 8

Calculated ueff Values for Eight Coordinate
Nb(IV) Complexes

Ligand “eff (B.M.)
pipdtc 1.70
2-Mepipdtc 1.72
3-Mepipdtc 1.72
4-Mepipdtc 1.71
Etxan 1.72
dth 1.73
‘dmtp 1.71
dpm 1.69
CN(soln) 1.72
CN(soln) 1.71

compounds in this study as well as other Nb(IV) compounds.
From the table, the ability of sulfur-donor ligands to form
fairly covalent species with Nb(IV) is illustrated. These

values which are nearly equal to the spin-only value of

38

1.73 B.M. is a further indication that the electron is
primarily delocalized onto the ligand.

26’52 stated that

Reports by two different authors
Nb(V) could be reduced to Nb(IV) by dithiocarbamates in
contrast to the experiments by Pantaleo and Johnson in which
Nb(V) compounds were isolated. This prompted an investiga-
tion to determine if pipdtc would also reduce Nb(V). Four—

to-one molar ratios of NH4pipdtc: NbX (X = Cl, Br, I) were

5
prepared under conditions similar to those discussed in the
experimental section for the tetrahalides. The resulting
solutions were examined by esr. In each case, a spectrum
which was identical to the spectrum of the four to one molar

ratios of NH4pipdtc: NbX (X = Cl, Br, I) was obtained.

4

Characterization of Halogen Complexes

When two moles of NH4pipdtc were allowed to react with

one mole of Nbx (X = Cl, Br, I), several observations were

4
made. Initially, the reaction mixture would turn purple as
found in the four-to-one reaction mixtures. After two days,
an orangish precipitate and yellowish-brown solution were
present. In order to establish the pathway through which
the reaction was proceeding, a series of experiments was

performed involving two-to-one molar ratios of NH pipdtc:

4
NbCl4. An original mixture of a four-to-one, ligand to
NbCl4 molar ratio was prepared. After two days of stirring,

this mixture was examined by use of esr. The resulting

39

spectrum was a ten line niobium spectrum with esr parameters
as had been found earlier. The molar ratio was then altered

to two—to-one by the addition of NbCl and the esr spectrum

4
of this mixture was recorded. The resulting esr spectrum of
this solution was the same as had been found when two-to-one
molar ratios were prepared. Further discussion on this
observation will be included in the esr section.

The solid species were recovered from the filtrate
by removing the solvent inznumo. In the case of NbI4 +
2NH4pipdtc the solvent, which was condensed into a liquid
nitrogen trap, showed traces of iodine as well as the
presence of carbon disulfide when allowed to warm to
ambient temperature. It is believed that similar results
are possible for the chloride and bromide when treated
in this manner. Apparently, there is an oxidation-reduction
occurring in the reaction mixture when less than four moles

52,54

of the ligand are present. Holah and co-workers studied

the reaction of NbX5 (X = Cl, Br, I) with diethyldithio-

carbamate salts, with particular focus upon the organic

products formed. These authors found that when the ligand-
to-metal ratio was less than four-to-one, complexes of the
type Nb(SZCNEt

and Nb(82CNEt X3'nC H (n = 0.85-0.95)

2’2x3 2’2 6 6

were formed. Further studies confirmed that the ligand
reacts with solvents such as methylene chloride, chloroform,
and acetonitrile. Similar results could possibly occur
even for reactions of NbX . A prOposed mechanism involves

4
the abstraction of a proton from the solvent. Although the

 

‘lllll ‘l. (I I! {1.1

40

solvent used in this study would not undergo this type of
nucleophilic attack, an excellent source (n5 protons is NH4 .
In the two-to—one mixtures, the halides that would
remain bonded to the metal could react with the ligand to
form various organic and halo-organic species. Examples of
the types of final products that could be formed are the thiuram
disulfide (CSHlONCS4CNC5HlO) and monosulfide (CSHIONCS3CN-
C5H10)' Both of these compounds would have infrared spectra
similar to pipdtc. The differences between these species
and the ligand could be observed in the ultraviolet region,
but the metal ion in this system has charge-transfer bands
that would mask this transition. Organo-halogen species such as
CSHloNHCSC1, a thiocarbamyl, can be prepared. The precipi-
tation of all of the halide as NH4+X_ in the four—to-one
reaction mixtures prevents the probable occurrence of these
reactions. This oxidation-reduction process must only
involve the ligand and not the metal, because information that
will be discussed later confirms the oxidation state of
niobium remains +4 in this system.
Solid species were isolated as greyish-brown, dark-
brown, and light-brown powders for the chloride, bromide,
and iodide respectively. The species are air and/or water
sensitive as indicated by color changes when exposed to the

atmosphere. The melting points are 130, 135, and 113°C

respectively for the chloride, bromide and iodide.

41

Analyses of these species point toward a disubstituted
product for the chloride for which %Nb found was 19.95 as
Opposed to a calculated %Nb of 19.21. The bromide and iodide
analyses gave empirical formulas which can be formulated as

NbBr4[C H NCS CNC H

5 lo 3 5 101°2C5H10NH and C

5H10NHZ[NbI(CsHloNC52)2S]'CsHloNH°

Vibrational Spectra

Infrared Spectra (4000-600 cm-l)

 

Infrared data of these complexes are listed in Table 9.
Each of these complexes had the significant bands that are

1 for dithiocarbamates. Con-

expected at 1500 and 1000 cm-
siderable broadening of several peaks occurred in these
species that was not evident in the tetra-substituted
species. The unusual stoichiometries found by analyses
would indicate there should be some change in the spectra.

Machin and Sullivan27

in their report of the compound
szBr5(dtc)3 noted the ir spectrum of this compound did not
differ from the spectrum of Nb(dtc)4.

If the ligand is undergoing reduction to the thiuram
monosulfide, the ir spectra would not differ very much due
to the complexity of the system. This could account for
the broadening of the spectra.

A band not present in the ligand nor in the tetra-

substituted compound appears at 670 cm-1. This band has

been assigned as being due to the rocking of the CH2 units

42

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m mmva

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m oom
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HO

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u m .omouo u o

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43

55

of the ring. Taimsala and Wood found in their study of

alkyl-substituted tin(IV) halides this band was much stronger
for EtZSnCl2 than Et4Sn. Its appearance in the spectra of

these compounds is similar to that found by those authors.

Far Infrared Spectra (600-100 cm-l)

 

Far infrared data for these complexes are presented in
Table 10 and the spectra are presented in Figures 7, 8, and
9 for the chloride, bromine, and iodide complexes respectively.
Metal-sulfur vibrations are found in these complexes at the
same positions as found in the tetrakis compound. This
vibration is shifted to slightly higher frequency for the
chloro complex. No significance can be attributed to this
since the resolution of these bands is not very good.

A new band was present in the chloride compound at
347 cm-1. Because of the intensity, general shape, and its
absence from the spectra of the tetrakis compound and the
ligand, this band has been assigned as v(Nb-Cl). The triplet
formed by the peaks at 303, 290, and 280 cm“1 may also be
indicative of niobium-chloride stretching, but the appearance
of this triplet in the iodide complex at a lesser intensity
prevents the unambiguous assignment.

By using the ratio v(Nb-Br)/v(Nb-C1) = 0.76 that was

found for monodentate thioether adducts of NbX4,14 one ex-

pects to find a 0(Nb-Br) mode ca. 260 cm-1. A band at
260 cm-l,vdfirfllis weak in the ligand, tetrakis species, and

the chloride compound, and of moderate intensity in this

44

Hoodsonm u :m .xmo3 n 3 .oumumooE u E .mcouum u m .omoun u o

n.3 mma n.3 mNH
3 mma 3 bma
E mmH 3 mma E oma
Q.Et3 med E13 mud.oma
3 oma AHIQZV> 9.3 omH 3 omH
E13 mom 3 CAN E how
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3 5mm 3 mom
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OH OHQMB

45

 

 

 

 

ABSORBANCE (arbitrary units)

 

l l l

 

 

l
500 400 300 200
1! (cm")

8
0

Figure 7. Far Infrared Spectrum
of Chloride Complex

I00

46

 

 

ABSORBANCE (arbitrary units)

 

l

I L l
[00 200 300 400 500 600
V(Cl'Tf')

 

 

Figure 8. Far Infrared Spectrum of
Bromide Complex

47

 

ABSORBANCE (arbitrary units)

 

l

l

 

 

8
o

L l
500 400 300
v(cm")

Figure 9. Far Infrared Spectrum of

Iodide Complex

200

IOO

48

complex is assigned as v(Nb-Br).

Broadening of the peaks for these compounds is most
evident for the iodide complex. The ratio of v(Nb-I)/
0(Nb-Cl), calculated as 0.56,14 predicts a 0(Nb-I) mode
ca. 194 cm-1. A peak at 190 cm-1 is present in this compound.
Due to peaks at 203 cm—1 and 175 cm-1 the intensity of this
vibration may have been lessened. This factor, coupled with
the presence of a similar peak in the chloride, prevents
this band from being positively assigned as v(Nb-I).

The information gained by this far ir study can be used
in conjunction with the analytical data found for the com-
pounds to confirm the nature of bonding between the metal,
ligand, and halogen. Interaction between the ligand's sulfur
atoms and niobium has been maintained. Evidence that some
of the halogen has remained coordinated to the metal is
supported by the appearance of bands that are due to niobium-

halogen vibrations.

Electronic Spectra

Visible and near infrared spectra were studied for
these species in benzene solutions as described earlier.
The spectra and wave number maxima are presented in
Figures 10, 11, and 12 and Table 11. Four bands are
observed for the chloride and iodide containing species and

iodide containing species and five bands for the bromide

49

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Table 11

Electronic Spectral Data for Haloqen Complexes

Cl Br I
11.3 (7.51) 11.4 (59) 11.3 (11.4)
13.5 (13.1) 13.4 (75) 12.4 (12.5)
22.4 (337) 19.7 (400) 21.3 (314)
24.4 (840) 22.8 (525) 24.8 (308)

27.8 (520)

. . . 3 -1
Wave number max1ma are 1n units x 10 cm .

Extinction coefficients are in parentheses.

compound. All of the complexes exhibit bands corresponding
to those found in the spectrum of Nb(pipdtc)4. In addition,
a band ca. 24.0 x 103 cm-1 is not present in Nb(pipdtc)4.
The broad bands observed for these species prevent
rigorous assignment of these transitions as being d-d
transitions. Fowles16 has 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. The yellowish-brown
color of the solutions would also influence transitions in
the visible region as has been reported for the complex
Cu(dmdtc)2.56
If the structure for the chloro species is assumed to

be pseudo D4h symmetry, one would expect three transitions

from the splitting of the d-manifold. The extinction

53

coefficients for the bands at 11.3 and 13.5 x 103 cm.1 permit

the assignment of these bands as d-d transitions. It is not
unreasonable to assign the other band at 22.4 x 103 cm.1 as a
d-d transition although the extinction coefficient is somewhat
high. Other workers5 have reported d-d transitions over

1 to 23,000 cmul. Hence it is con-

the range of 15,000 cm-
ceivable that a similar situation exists in this system.
The iodide species would fall into the series as expected.
The bromo—compound poses problems in that the bands appear
to be out of order. The stoichiometry of this compound

which contains four bromide atoms may account for this

anomaly.

Nuclear Magnetic Resonance Spectra

Solutions of these species for nmr study were prepared by
concentrating the original filtrates from the reaction mixtures
by removal of the solvent in mumo. These viscous solutions were
then diluted in the nmr tubes by the addition of solvent. The
only resonance observed over the range of the instrument
was the solvent peak which showed a shift of 5 ppm upfield
from TMS. Paramagnetic complexes typically shift solvent
peaks corresponding to the amount of interaction between
the solvent and the paramagnetic center. The small shift
observed here is indicative of little interaction between
the solvent and the complex and the low paramagnetic

character of the complexes.

54

Electron Spin Resonance Spectra

The filtrates from the reactions and the solid species
dissolved into benzene gave esr spectra which consisted
of a single unresolved broad peak at ambient temperature.
At 77°K, this peak was lowered in intensity. A representative
spectrum is presented in Figure 13.

Thus far, infrared, far infrared, and electronic
spectra have supported the formation of species in which:
(a) the ligand is bidentate; (b) the halogens are still
attached to the metal; and (c) the chloride and iodide com-
pounds are similar in structure. One can not rule out the
presence in solution of a monomeric six coordinate species
which has as contaminants other organic species in the
outer coordination Sphere of the metal ion.

Because the nature of the species could not be deter-
mined well for any of the complexes except for the chloro-
compound, this complex was diluted into a solution containing

ZrC12(pipdtc) and was handled in the manner described in

2:
the Experimental Section. A dark-brown powder was isolated.
At ambient temperature, no esr signal was detected. On
cooling the sample to 139°K, a spectrum, Figure 14, was
observed that had twenty major lines.

The gross features of the spectrum suggest several
possibilities for the stoichiometry of the complex. The

intense lines in the middle of the spectrum are the type of

lines found for monomeric Nb(IV) complexes. The other lines

55

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57

are similar to the exchange-coupled dimer, Nb(dmtp)4I4,
found by McGinnis.8 From this information, the possible
stoichiometries are: (a) [NbC12(pipdtc)2]2°[NbC12(pipdtc)2]
and (b) [NbC12(pipdtc)2]°[ZrC12(pipdtc)2]'[NbC12(pipdtc)2].
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 line-
widths small.57 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, I, and A is
the normal 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, 8. 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

58

evidence for an exchange-coupled system. If dimer

formation is incomplete, as is frequently the case, dimer
0M5 = i 1 spectra are often considerably obscured by
residual monomeric spectra in the g z 2 region of the
spectrum. This situation is often the case in powders
containing small concentrations of the paramagnetic metal

. 6
ions.

58

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 + ”int (14)

where H1 and H2 are each of the form:

Hl,2 = gHHzSz + gi(HxSx+H sy) + AIISzIz + A1(S I +5 I ) (15)

Y X X Y Y

H representing the interaction energy between two ions,

int'

has the form:

_ 2 2 2 .
Hint — msz -l/3S(S+1)] + E(SX ‘Sy ) - JS/ 52 (16)

where S = Sl + S2 if the ions are in sites of the same
symmetry and D and E are the zero field splitting parameters.
In a complex which contains an even number of electrons the
degeneracy of the ground state may be removed in accordance
with the 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:

:1:
I
{I}
u

20 (17)

59

Since the D value provides a reasonable measurement
of the intermetal distances which may give structural
information 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 of
the copper59 and vanadyl60 tartrates suggest that the error
in the derived esr parameters under the axial symmetry
approximation is not very large. If D is attributed to the
magnetic dipolar interaction between two electron spins, it

. 61
is expressed as

2
D = 3/4ng2<1 Begs 6>max

r12

(19)

where r12 is the interelectronic distance and 615 the angle

between the r12 vector and the magnetic field direction.

Assuming that eequals the angle between the niobium-niobium
axis and the magnetic field and l/<r123> = 33, R being the

R
niobium-niobium distance, one obtains:

0.325g211-3coszejll/3 (20)

(i)=[ _
D(cm 1)

 

Rcalc

60

Taking Hi] to be along the Nb-Nb axis, and 6 = 0°, one

obtains the esr parameters listed in Table 12.

Table 12

Esr Parameters for the Chloride Complex
Diluted into ZrC12(pipdtc)2

gH = 1.7873
A l*= 64.2
D1 = 577.8
R = 3.30 i

* in units of 10"4 cm-1

The data for the chloro complex suggest that the Species
formed by the solid solution is an incomplete dimer. The
All value for the compound is similar to that reported by
McGinnis for Nb(dmtp)4I4,8 while 9)) is low compared to
most Nb(IV) compounds. The fact that no resonance at g ~ 4
was observed down to 77°K does not exclude the formation of
a dimeric species in the solid state. The effect of diluting
a dimeric compound into a diamagnetic host allows for packing
conditions to influence the nature of the complex in the

solid as Oppossed to the solution. Nb2(dmtp)4Cl was reported

4
to be diamagnetic based on magnetic susceptibility measurements
by the Faraday method.8 When a toluene solution of this
compound was diluted into ten-fold concentration of
ZrCl4(dmtp)4, a nineteen line esr spectrum was observed at

both ambient and 77°K temperatures.62 Hence, esr methods

61

are superior to magnetic susceptibility methods in the
determination of exchange-coupled species in which this

interaction is not strong. The approximate Nb-Nb distance

42 but lengthened

from the value of 3.06 A for NbCl43 illustrates the closeness

of the metal atoms, which under dilute conditions can show

which was comparable to that found for a-NbI

electron exchange.

 

SUMMARY AND CONCLUS I ONS

Eight coordinate Nb(pipdtc)4 was isolated from the

reaction of ammonium piperdinyldithiocarbamate (NH pipdtc)

4
with niobium tetrahalides. The infrared spectrum indicates
that pipdtc- is bidentate. The electronic spectrum exhibits
four d-d transitions and supports a D2d dodecahedral con-
figuration for the complex. Esr spectra confirm this
geometry with the parameters <g> = 1.9677, 9" = 1.9155,

gi = 1.9938, and <a> = 0.0110 cm-l. By using molecular

orbital theory developed for D complexes, metal-ligand

2d
bonding parameters were obtained which indicate strong mixing
of metal and ligand orbitals.

When two moles of NH4pipdtc were allowed to react with
the tetrahalides, complexes of varying stoichiometries were
obtained. In each, the pipdtc- was bidentate as determined
from infrared spectra. Far infrared spectra showed bands
which could be assigned as v(Nb-S) and v(Nb-X) (X = Cl, Br,
I). Two bands in the near infrared-visible region were
assigned as d-d transitions. Esr spectra of these species
were unresolved single peaks at both ambient and 77°K tempera-
tures. The chloro compound when diluted into a solution of

the disubstituted zirconium complex, gave a powder esr spec-

trum which was indicative of an exchange-coupled dimer. The esr

62

63

parameters are 9" = 1.7873 and AI) = 0.00642 cm-l. The

zero field splitting is 0.05778 cm-1 which corresponds to a

O
niobium-niobium separation of 3.30 A.

 

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