Novel mixed-ligand Pt(II) complexes: Synthesis, multinuclear magnetic resonance, and crystal structures of cis- and trans-Pt(R2SO) (pyrazine)Cl2

Article abstract of DOI:10.1139/v04-020

A new type of mixed-ligand Pt(II) complexes, Pt(R₂SO)(pyrazine) Cl₂, was synthesized from the aqueous reaction of K[Pt(R ₂SO)Cl₃] with pyrazine. The compounds were characterized mainly by IR and multinuclear mag

Full text of DOI:10.1139/v04-020

649
Novel mixed-ligand Pt(II) complexes: Synthesis,
multinuclear magnetic resonance, and crystal
structures of cis- and trans-Pt(R2SO)(pyrazine)Cl2
Fernande D. Rochon and Julien R.L. Priqueler
Abstract: A new type of mixed-ligand Pt(II) complexes, Pt(R2SO)(pyrazine)Cl2, was synthesized from the aqueous re-
action of K[Pt(R2SO)Cl3] with pyrazine. The compounds were characterized mainly by IR and multinuclear magnetic
resonance spectroscopies (1H, 13C, and 195Pt) and crystallography. Compounds with dimethylsulfoxide, tetramethylene-
sulfoxide, di-n-propylsulfoxide, di-n-butylsulfoxide, dibenzylsulfoxide, and diphenylsulfoxide were studied. IR spectros-
copy suggested a cis geometry for the di-n-propylsulfoxide complex and trans geometry for the other compounds. The
195Pt NMR resonances of the complexes were observed between –3042 and –3121 ppm, with that of the diphenyl-
3
sulfoxide complex being at higher field than those of the others. The pyrazine J(195Pt-1H) coupling constant was ob-
served for the DMSO compound (33 Hz), suggesting a trans geometry. No J(195Pt-13C) coupling could be detected. The
crystal structures of trans-Pt(tetramethylenesulfoxide)(pyrazine)Cl2 and of cis-Pt(di-n-propylsulfoxide)(pyrazine)Cl2 were
determined and confirmed the geometry suggested by IR and NMR spectroscopies. The compound with di-n-
butylsulfoxide could not be isolated, because it rapidly formed the pyrazine-bridged dimer.
Key words: platinum, sulfoxide, pyrazine, crystal structure, NMR, IR.
Résumé : Un nouveau type de complexes du platine(II) contenant des ligands mixtes a été synthétisé lors de la réac-
tion en milieu aqueux de K[Pt(R2SO)Cl3] avec la pyrazine. Les composés Pt(R2SO)(pyrazine)Cl2 ont été caractérisés
par spectroscopie infrarouge, par résonance magnétique multinucléaire (1H, 13C et 195Pt) et par cristallographie. Des
complexes avec les ligands diméthylsulfoxyde, tétraméthylènesulfoxyde, di-n-propylsulfoxyde, di-n-butylsulfoxyde, di-
benzylsulfoxyde et diphénylsulfoxyde ont été étudiés. La spectroscopie IR a suggéré une géométrie cis pour le com-
posé contenant le ligand di-n-propylsulfoxyde et une isomérie trans pour les autres complexes. Les résonances en RMN
du 195Pt ont été observées entre –3042 et –3121 ppm. Le composé contenant diphénylsulfoxyde a été observé à plus
3
haut champ que les autres. La constante de couplage de la pyrazine J(195Pt-1H) a été observé pour le composé conte-
nant le DMSO (33 Hz) suggérant une géométrie trans. Aucun couplage de type J(195Pt-13C) n’a été détecté. Les struc-
tures cristallines des deux composés trans-Pt(tétraméthylènesulfoxyde)(pyrazine)Cl2 et cis-Pt(di-n-propylsulfoxyde)-
(pyrazine)Cl2 ont été déterminées par diffraction des rayons-X et les résultats ont confirmé les géométries suggérées
par les spectroscopies IR et RMN. Le composé avec le ligand di-n-butylsulfoxyde n’a pas été isolé, car le dimère à
pont pyrazine se forme trop rapidement.
Mots clés : platine, sulfoxyde, pyrazine, structures cristallines, RMN, IR.
Rochon and Priqueler 658
Introduction
with neutral ligands containing other donor atoms such as S
have also shown some antitumor properties (4). Several
Pt(II) complexes of the types [Pt(R2SO)(diamine)Cl]NO3 (5)
and cis- and trans-Pt(R2SO)(quinoline)Cl2 (6) have shown
biological activity, depending on the sulfoxide chain. Re-
cently, it was reported that the pyrazine–Pt(II) complex [cis-
{Pt(NH3)2Cl}2(µ-pz)]Cl2 and its derivatives showed good
potential as anticancer compounds, especially because they
largely seem to overcome the cross-resistance of cisplatin
(7). The compounds possess an appropriate flexibility to
provide the cross-links with a minimal distortion of the DNA.
The antitumor activity of Pt(II) complexes of the type cis-
Pt(amine)2Cl2 is well known. Cisplatin (cis-Pt(NH3)2Cl2)
remains the most commonly used anticancer agent in che-
motherapy (1, 2), even though it exhibits numerous and sig-
nificant side effects, but the search for more specific and less
toxic drugs is still continuing.
A few trans Pt(II) complexes with amine ligands have
shown anticancer activity in specific organs (3). Compounds
Our group has been involved in research on Pt–sulfoxide
complexes for many years. Sulfoxides are interesting ligands
because they can accept π electron density from the metallic
center. For these types of ligands, the cis complexes are usu-
ally more stable than the trans compounds (contrary to the
amine systems). We have recently reported a study on
mixed-ligand Pt(II) complexes containing pyrimidine (pm)
Received 7 August 2003. Published on the NRC Research
Press Web site at http://canjchem@nrc.ca on 7 May 2004.
F.D. Rochon1 and J.R.L. Priqueler. Département de chimie,
Université du Québec à Montréal, C. P. 8888, Succ. Centre-
ville, Montréal, QC H3C 3P8, Canada.
1Corresponding author (e-mail: rochon.fernande@uqam.ca).
Can. J. Chem. 82: 649–658 (2004)
doi: 10.1139/V04-020
© 2004 NRC Canada

650
Can. J. Chem. Vol. 82, 2004
and sulfoxide ligands (8, 9). Pyrimidine also has empty π
antibonding orbitals that could accept electron density from
Pt. The aqueous reaction of K[Pt(R2SO)Cl3] with pyrimidine
(1:1 ratio) produced first trans-Pt(R2SO)(pm)Cl3, which
could then isomerize to the cis compound (8).
To determine the influence of the pyrimidine ligand in
these reactions, we have started a new study with pyrazine
(pz) instead of pyrimidine. Unlike the latter, pyrazine is a
symmetric molecule, but it too has empty π* orbitals that
could form π bonds with Pt. We have recently published the
results of the first part of this study, on the pyrazine-bridged
dinuclear compounds {Pt(R2SO)Cl2}2(µ-pz) (10), which were
found to have the trans-trans geometry. Pyrazine can form
bridges between two metallic centers quite easily, even when
an excess of pyrazine is used. Most reported works in the lit-
erature involve pyrazine-bridged oligomers.
There is no reported research on Pt compounds containing
both sulfoxides and pyrazine except our recent publication
of the pyrazine-bridged dimers (10). Only four papers on Pt
compounds with nonsubstituted pyrazine were found in the
literature. Among them, only two papers involve Pt(II)
monomeric compounds. Albinati et al. (11) synthesized and
characterized trans-Pt(phosphine)(pz)Cl2, while Siedle et al.
(12) prepared trans-Pt(C2H4)(pz)Cl2 and its derivatives and
used them as catalysts in hydrosilation reactions. No cis
mixed-ligand Pt monomeric compounds with pyrazine have
been reported. The nature of the Pt–pyrazine bond has not
been investigated yet.
In the present publication, we report the results of our
study on the aqueous reactions of K[Pt(R2SO)Cl3] with py-
razine. Six sulfoxides that differ in steric hindrance were
studied: dimethylsulfoxide (DMSO), tetramethylenesulf-
oxide (TMSO), di-n-propylsulfoxide (DPrSO), di-n-butyl-
sulfoxide (DBuSO), dibenzylsulfoxide (DBzSO), and
diphenylsulfoxide (DPhSO). The products Pt(R2SO)(pz)Cl2
were characterized by IR and multinuclear magnetic reso-
nance (1H, 13C, and 195Pt) spectroscopies. The crystal struc-
tures of two compounds with different geometies were
determined.
The TMSO compound (crystal I) was crystallized from a
dichloromethane–acetone (1:1) mixture, and the DPrSO
crystal (crystal II) was obtained from a chloroform–
acetone–methanol (3:1:1) solution. The crystallographic
measurements were done on a Siemens P4 diffractometer
using graphite-monochromatized Mo Kα (λ = 0.71073 Å
(1 Å = 0.1 nm) radiation. The crystals were selected after
examination under a polarizing microscope for homogeneity.
The cell dimensions were determined at room temperature,
from a least-squares refinement of the angles 2θ, ω, and χ
obtained for well-centered reflections. The data collections
were made by the 2θ/ω scan technique using the XSCANS
(13) program. The coordinates of the Pt atoms were deter-
mined from Patterson map calculations. All the other non-
hydrogen atoms were found by the usual Fourier methods.
The refinement of the structures was done on F2 by full ma-
trix least-squares analysis. The hydrogen atoms were fixed
in their calculated positions with Ueq = 1.2 x Ueq of the car-
bon to which they are bonded (or 1.5 x Ueq for methyl
groups). Corrections were made for absorption (integration
for trans-Pt(TMSO)(pz)Cl2 and semiempirical for cis-
Pt(DPrSO)(pz)Cl2), Lorentz, and polarization effects. Crystal
II did not diffract very well. It was also measured with Cu Kα
radiation, but the results did not improve. Attempts to pre-
pare crystals of better quality were not successful. The resid-
ual peaks were located in the close vicinity of the Pt atoms.
The calculations were done using the Siemens SHELXTL
(13) system. The pertinent crystal data and the experimental
details are summarized in Table 1.
The K[Pt(R2SO)Cl3] complexes were synthesized accord-
ing to the method described by Kukushkin et al. (14). The
DBzSO and DPhSO complexes were obtained in small quan-
tities because of the aqueous insolubility of the ligands and
the very favorable formation of the insoluble disubstituted
compound Pt(R2SO)2Cl2.
Preparation of trans-Pt(R2SO)(pz)Cl2
Pyrazine was dissolved in a minimum amount of water.
The K[Pt(R2SO)Cl3] compound, dissolved also in a mini-
mum amount of water, was added gently to the pyrazine so-
lution in a 1:1.1 proportion at room temperature. A pale
yellow to yellow-orange precipitate appeared rapidly, but the
stirring of the solution was continued until precipitation was
complete. After filtration, the precipitate was dried, washed
with ether, and dried in vacuum. For the DBzSO complex,
the reaction was done in a 2:1 MeOH–H2O mixture, while
for the DPhSO complex, the reaction was done in a 3:1
EtOH–H2O mixture.
Experimental
K2[PtCl4] was obtained from Johnson Matthey Inc. and
was purified by recrystallization from water before use.
CDCl3 was purchased from CDN Isotopes. Pyrazine and the
sulfoxide ligands were purchased from Aldrich except
DMSO, which was bought from Anachemia Chemicals Ltd.,
and DPrSO, acquired from Phillips Petroleum Company.
The latter was purified by distillation before use.
trans-Pt(DMSO)(pz)Cl2
The decomposition points were measured on a Fischer-
Johns instrument and were not corrected. The IR spectra
were recorded in the solid state (KBr pellets) on a
PerkinElmer 783 spectrometer between 4000 and 280 cm–1.
All NMR spectra were measured in CDCl3 on a Varian
Gemini 300BB spectrometer operating at 300.069, 75.460,
Yield 89%; dec. 220–235 °C. IR (cm–1): pz (vibration
numbers assigned using the notation in ref. 15): 3117 m (ν13
(B1u)), 1417 s (ν19b (B3u)), 1147 s (ν18a (B1u)), 1112 w (ν3
(B2g)), 1074 m (ν14 (B3u)), 1029 s (ν1 (Ag)), 807 s (ν11
(B2u)); ν(S-O) 1122 w, ν(Pt-N) 517 m, ν(Pt-S) 444 s, ν(Pt-
Cl) 350 s. 1H NMR (ppm): Hpz 8.780 (d, 3.0), 8.770 (d, 3.0);
Hα 3.48 (s+d, 23). 13C NMR (ppm): Cpz 147.01, 145.33; Cα
44.46.
1
and 64.326 MHz for H, 13C, and 195Pt, respectively. The
chloroform peaks were used as an internal standard for the
1H (7.24 ppm) and 13C (77.00 ppm) NMR spectra. For 195Pt,
the external reference was K[Pt(DMSO)Cl3] (in D2O, ad-
justed to –2998 ppm from K2[PtCl6]). The 195Pt NMR spec-
tra were measured between –2500 and –4000 ppm.
trans-Pt(TMSO)(pz)Cl2 (I)
Yield 84%; dec. 156–195 °C. IR (cm–1): pz (vibration
© 2004 NRC Canada

Rochon and Priqueler
Table 1. Crystallographic data for the compounds Pt(R2SO)(pz)Cl2.
651
I
II
trans-Pt(TMSO)(pz)Cl2
cis-Pt(DPrSO)(pz)Cl2
C10H18Cl2N2OPtS
Compound
Chemical formula
C8H12Cl2N2OPtS
MW
450.25
480.31
Space group
Orthorhombic, Pbca
Monoclinic, P21/c
a (Å)
b (Å)
c (Å)
11.931(3)
10.434(2)
20.490(4)
8.911(7)
17.534(12)
9.843(9)
90.69(7)
1538(2)
4
β (°)
Volume (Å3)
Z
2550.8(9)
8
2.345
ρcalcd (g cm–3)
2.075
µ(Mo Kα) (cm–1)
F(000)
11.560
1680
9.594
912
Measured reflections
Independent reflections
Obs. reflections (I > 2σ(I))
R1 (F > 2σ(I))a
18 659
2506
1398
10 597
2715
1543
0.0494
0.0791
wR2 (all data)b
0.1009
1.052
0.1358
1.059
S
aR1 = Σ (|Fo – Fc|) / Σ |Fo|.
2
2
bwR2 = [Σ (w(Fo – Fc2)2) / Σ (w(Fo )2)]1/2
.
numbers assigned using the notation in ref. 15): 3100 w (ν13
(B1u)), 1415 s (ν19b (B3u)), 1394 w (ν19b (B3u)), 1143 s (ν18a
(B1u)), 1119 s (ν3 (B2g)), 1081 s (ν14 (B3u)), 1060 s (ν1 (Ag)),
807 s (ν11 (B2u)); ν(S-O) 1160 m, ν(Pt-S) 479 m, ν(Pt-Cl)
chloroform, filtered, washed with ether, and dried in vac-
uum.
Yield 68%; dec. 183–220 °C. IR (cm–1): (vibration num-
bers assigned using the notation in ref. 15): 3086 w (ν13
(B1u)), 1457 m (ν19a (B1u)), 1412 s (ν19b (B3u)), 1379 m (ν19b
(B3u)), 1130 s (ν18a (B1u)), 1111 w (ν3 (B2g)), 1069 s (ν14
(B3u)), 807 m (ν11 (B2u)); ν(S-O) 1152 s, ν(Pt-N) 515 w,
ν(Pt-S) 448 m, br, ν(Pt-Cl) 345 m, 341 m. 1H NMR (ppm):
Hpz 8.707 (d, 3.3), 8.685 (d, 3.3); Hα 3.76 (m), 3.12 (m); Hβ
2.36 (m), 2.03 (m); Hγ 1.18 (t, 7.5). 13C NMR (ppm): Cpz
147.59, 147.40; Cα 56.51, Cβ 16.68, Cγ 12.75.
1
348 s. H NMR (ppm): Hpz 8.811 (dd, 3.0, 1.2), 8.764 (dd,
3.0, 1.2); Hα 4.00 (m), 3.65 (m); Hβ 2.39 (m), 2.23 (m). 13C
NMR (ppm): Cpz 147.42, 145.70; Cα 57.05, Cβ 24.67.
trans-Pt(DBzSO)(pz)Cl2
Yield 89%; dec. 121–197 °C. IR (cm–1): pz (vibration
numbers assigned using the notation in ref. 15): 3123 w (ν13
(B1u)), 1495 m (ν19a (B1u)), 1422 s (ν19b (B3u)), 1381 m (ν19b
(B3u)), 1130 s (ν18a (B1u)), 1114 s (ν3 (B2g)), 1070 m (ν14
(B3u)), 1030 m (ν1 (Ag)), 805 m (ν11 (B2u)); ν(S-O) 1167 s,
ν(Pt-N) 516 vw, ν(Pt-S) 483 s, ν(Pt-Cl) 350 m.
Results and discussion
Synthesis of the complexes
The aqueous reaction of K[Pt(R2SO)Cl3], where R2SO =
DMSO, TMSO, DBzSO, and DPhSO, with pyrazine in a
1:1.1 proportion produced the monomeric complexes trans-
Pt(R2SO)(pz)Cl2 (eq. [1]). The K[Pt(R2SO)Cl3] solution must
be added to the pyrazine solution to have an excess of pyra-
zine throughout the reaction because an excess of the Pt(II)
salt will produce the pyrazine-bridged dimer recently re-
ported (10), prepared by a similar reaction using a Pt:pz
ratio of 2:1. For the DBzSO complex, the reaction was per-
formed in a 1:2 water–methanol mixture, while a 1:3 water–
ethanol solution was preferred for the DPhSO compound.
trans-Pt(DPhSO)(pz)Cl2:
Yield 85%; dec. 195–250 °C. IR (cm–1): pz (vibration
numbers assigned using the notation in ref. 15): 3094 w (ν13
(B1u)), 1470 w (ν19a (B1u)), 1410 s (ν19b (B3u)), 1381 w (ν19b
(B3u)), 1137 s (ν18a (B1u)), 1112 m (ν3 (B2g)), 1071 m (ν14
(B3u)), 1062 m (ν1 (Ag)), 806 s (ν11 (B2u)); ν(S-O) 1153 m,
1
ν(Pt-N) 507 vw, ν(Pt-S) 444 w, ν(Pt-Cl) 349 m. H NMR
(ppm): Hpz 8.896 (m); Hortho 8.29 (t, 7.8), 7.93 (t, 7.4);
Hmeta, para 7.58 (m). 13C NMR (ppm): Cpz 147.32, 145.93;
Cphenyl 138.42, 127.43, 128.76, 132.95.
[1]
K[Pt(R2SO)Cl3] + pz → trans-Pt(R2SO)(pz)Cl2
+ KCl
Preparation of cis-Pt(DPrSO)(pz)Cl2 (II)
Pyrazine was dissolved in a minimum amount of water.
The K[Pt(R2SO)Cl3] compound, dissolved also in a mini-
mum amount of water, was added gently to the pyrazine so-
lution in a 1:1 proportion at room temperature. A yellow
sticky paste formed immediately and stuck to the magnetic
agitator. The paste was dissolved in chloroform and the solu-
tion evaporated to dryness. The residue was recrystallized in
A pale yellow to yellow-orange precipitate was formed.
The time at which precipitation was observed varied with
the bulkiness of the sulfoxide. The yields varied between
84% and 89%.
The characterization of the products has shown that these
compounds have the trans configuration, as expected be-
© 2004 NRC Canada

652
Can. J. Chem. Vol. 82, 2004
cause of the large trans effect of sulfoxides. Sulfoxide com-
plexes having the cis configuration are usually more stable
than the trans compounds, because the π bonding is more ef-
fective in the cis configuration. For example, the aqueous re-
action of K[Pt(R2SO)Cl3] with R2SO produces first the trans
isomer, which rapidly isomerizes in water to the cis com-
pound except in the case of very sterically demanding
sulfoxides. Similarly, the reaction of K[Pt(R2SO)Cl3] with
pyrimidine produces first trans-Pt(R2SO)(pm)Cl2, which can
isomerize to the cis compound (8). Since pyrazine is a very
similar ligand to pyrimidine, both of them having empty π*
orbitals that can accept electron density from Pt, the trans-
Pt(R2SO)(pz)Cl2 compounds were expected to undergo iso-
merization, at least in organic solvents. However, several
attempts in different solvents to isomerize the four trans-
Pt(R2SO)(pz)Cl2 compounds were without success. There-
fore, it appears that the reactivity of the R2SO–pyrazine
complexes is quite different from that of the R2SO–
pyrimidine compounds (8, 9). The pyrazine-bridged dimers
recently published were identified as the trans-trans isomers
(10), and various attempts to isomerize them were also not
successful.
Pt(DBzSO)(pz)Cl2 could not be analyzed by NMR spectros-
copy, being insoluble in all the solvents tested (H2O, MeOH,
EtOH, CHCl3, CH2Cl2, CH3COCH3, and DMF).
The synthesized complexes were pure, since only one res-
onance was observed in their 195Pt NMR spectra (confirmed
by 1H and 13C NMR). The complexes trans-Pt(TMSO)(pz)Cl2
and cis-Pt(DPrSO)(pz)Cl2 were also studied by X-ray dif-
fraction methods, which confirmed the geometry of the com-
plexes.
IR spectroscopy
The IR spectrum of pyrazine has been reported (15), and
our IR band assignments (Experimental section) are based
on that study. The vibrations of coordinated pyrazine in the
Pt complexes were observed at energies higher than or iden-
tical to those reported for the free ligand. The ν(S-O) vibra-
tions absorb at higher energy than in the free sulfoxides
(avg. +120 cm–1), since the ligands are bonded to Pt by the S
atom. The pπ–dπ bond is strengthened upon coordination in
the complexes. The ν(S-O) energies (avg. 1151 cm–1) are
identical with those observed in the pyrazine-bridged
dimeric compounds (avg. 1157 cm–1 (10)) and can be com-
pared with the values reported for Pt(DMSO)(NH3)Cl2 and
Pt(DMSO)(2-Me-py)Cl2 (1128 and 1138 cm–1, respectively
(16)).
The aqueous reaction of K[Pt(DPrSO)Cl3] with pyrazine
under the same conditions as for the other K[Pt(R2SO)Cl3]
complexes was quite different. With a Pt:pz ratio of 1:1 to
1:1.1, a bright yellow sticky paste formed immediately
upon mixing. After the purification process in CHCl3, the
product was identified as the mononuclear complex cis-
Pt(DPrSO)(pz)Cl2.
For all the complexes except Pt(DPrSO)(pz)Cl2, only one
ν(Pt-Cl) vibration was observed, between 348 and 350 cm–1.
This suggests a trans geometry because, although two bands
are predicted by group theory for both cis (Cs skeletal sym-
metry) and trans isomers (C2v) of this type, two bands are
usually observed for the cis compounds and only one band
for the trans isomers (as reported, for example, for the
complexes cis- and trans-Pt(DMSO)(L)Cl2, where L = 2,6-
lutidine and dimethylamine (17) and pyrimidine (8)). How-
ever, for Pt(DPrSO)(pz)Cl2, two ν(Pt-Cl) bands located at
341 and 345 cm–1 were observed, suggesting a cis geometry.
An absorption band observed around 460 cm–1 was as-
signed to a ν(Pt-S) vibration as suggested in the literature
(8–10, 16, 18–21), while a band around 515 cm–1 was as-
signed to the ν(Pt-N) vibration, based also on some pub-
lished results (8–10, 22, 23).
[2]
K[Pt(DPrSO)Cl3] + pz → cis-Pt(DPrSO)(pz)Cl2
+ KCl
The lower yield of this reaction (68%) can be attributed to
the oily nature of the product. When the Pt:pz ratio was
greater than 1:1, the pyrazine-bridged dimeric species trans-
{Pt(DPrSO)Cl2}2(µ-pz) was formed as reported earlier. The
fact that the final dinuclear product exhibits the trans-trans
geometry (10) suggests that the mononuclear intermediate is
trans-Pt(DPrSO)(pz)Cl2. However, with a Pt:pz ratio of
1:1.1, the trans monomeric compound, which should first be
produced, could not be isolated, suggesting that the com-
pound trans-Pt(DPrSO)(pz)Cl2 isomerized rapidly. This was
quite surprising since all the complexes formed with the
other ligands are believed to have the trans configuration and
did not isomerize even upon heating in organic solvents.
The aqueous reaction of K[Pt(DBuSO)Cl3] with pyrazine
under the same conditions (Pt:pz ratio of 1:1) produced the
pyrazine-bridged dinuclear species trans-{Pt(DBuSO)Cl2}2(µ-
pz). No monomeric compound was obtained with the ligand
DBuSO. The experimental conditions were varied (different
solvents) and the Pt:pz ratio was varied from 1:1 to 1:15, but
no monomeric mixed-ligand complex could be detected. It
seems that in the case of K[Pt(DBuSO)Cl3], unlike the other
K[Pt(R2SO)Cl3] complexes, the binding of the first N atom
of pyrazine to Pt activates the second N atom to react more
rapidly than the first
195Pt NMR spectroscopy
The 195Pt NMR spectra of the monomeric compounds
Pt(R2SO)(pz)Cl2 (except the DBzSO complex) were mea-
sured in CDCl3, and the results were compared with those
observed for the pyrimidine analogues Pt(R2SO)(pm)Cl2 (8)
and for the pyrazine-bridged dimeric species trans-
{Pt(R2SO)Cl2}2(µ-pz) (10). The chemical shifts are summa-
rized in Table 2. All the synthesized complexes were pure as
1
shown by multinuclear (195Pt, H, and 13C) magnetic reso-
nance spectroscopy. The compound containing a DBzSO
ligand was found to be insoluble in all the classical organic
solvents.
Except for the DPhSO complex, the 195Pt NMR chemical
shifts vary between –3042 and –3046 ppm. The δ(195Pt) val-
ues of these monomers are very close to those determined
for the pyrazine-bridged dimeric species (Table 2). In the py-
rimidine series, the signals of the monomers were observed
at slightly lower fields (avg. –3065 ppm) than those of the
pyrimidine-bridged dimers (avg. –3075 ppm) (8, 9). Our re-
All the mixed-ligand monomeric species were character-
ized by IR and multinuclear magnetic resonance spectro-
scopies. The low solubility of these complexes was a
major problem in this project. The compound trans-
© 2004 NRC Canada

Rochon and Priqueler
653
Table 2. 195Pt chemical shifts (ppm) of the complexes Pt(R2SO)(pz)Cl2 (in CDCl3) and related com-
plexes in the literature.
{Pt(R2SO)Cl2}2(µ-pz)a
Pt(R2SO)(pm)Cl2
b
R2SO
Geometry
Pt(R2SO)(pz)Cl2
DMSO
TMSO
DPrSO
trans
trans
cis
trans
trans
trans
–3045
–3042
–3046
–3049c
–3041
–3070
–3056
–2951
–3059
–3060
–3148
–3048
–3051
–3113
d
DBuSO
DPhSO
—
–3121
aReference 10; in CDCl3 except where otherwise indicated.
bReference 8; in CD2Cl2.
cIn DMF-d7.
dNot prepared.
sults are also in agreement with the published work on
analogues with phosphine ligands (11), where the 195Pt
NMR chemical shift of trans-Pt(PR3)(pz)Cl2 was reported at
–3589 ppm, and that of the dinuclear coompound trans-
{Pt(PR3)Cl2}2(µ-pz) at –3587 ppm. These results seem to in-
dicate that the electron density on the Pt atom is the same in
the monomers as in the pyrazine-bridged dimers.
ally has a deshielding effect. For Pt–amine complexes, δ(Pt)
was found at lower fields for amines having bulky substitu-
ents on the nitrogen atom (27, 28). For K[Pt(R2SO)Cl3] (29)
and the corresponding pyrazine-bridged dimeric complexes
(10), we have suggested that the difference was caused by
inverse polarization of the π electrons of the S=O bond, and
this concept might be an interesting interpretation of our re-
sults in the present study as well. This phenomenon has been
suggested in the literature to explain some 13C NMR results
for Pt–carbonyl (30) and Pt–carboxylato complexes (31).
Such an effect would be present in all Pt–R2SO complexes
but would be more important for sulfoxide ligands contain-
ing electron-attracting aromatic groups attached directly to
the S atom. This effect would increase the electron density
on the atoms in close proximity to the S atom, especially the
Pt atom, which would result in a higher field δ(Pt) signal.
The value of δ(195Pt) for the cis-DPrSO complex was
within the range observed for the trans monomers (except
the DPhSO compound), which was not expected. In the py-
rimidine series (8), the signals of the cis compounds were
reported at much lower fields than those of the trans isomers
(avg. ∆δ (δcis – δtrans) = 132 ppm). For sulfoxide Pt(II) com-
plexes, the cis isomers are more stable than the trans com-
pounds since the π(d-d) bonding is more efficient in the cis
geometry, resulting in a lower electron density on the Pt cen-
ter. The δ(Pt) are therefore observed at lower field for the cis
1H and 13C NMR spectroscopies
isomers. For mixed-ligand complexes, the value of ∆δ (δcis
–
δ
trans) will also depend on the second ligand. If the latter can
The pyrazine signals of the complexes will first be dis-
cussed. Pyrazine is a symmetric molecule, and only a single
form π bonds, the difference between the two isomers
should increase, while an amine ligand should reduce the ∆δ
(δcis – δtrans) values. In Pt(DMSO)(NH3)Cl2, the signal of the
cis isomer was reported at –3045 ppm, whereas that of its
trans analogue was observed at –3067 ppm (16, 24). In our
R2SO-pz compounds, only one cis compound (with DPrSO)
was prepared, and its signal was observed at the same field
as those of the trans isomers of the other R2SO-pz com-
pounds. We do not have enough data at the moment to reach
a sound conclusion and will continue our attempts, which
have not been successful up to now, to prepare other cis
compounds and the trans DPrSO complex.
1
resonance is observed in the H and 13C spectra at 8.59 and
145.07 ppm, respectively, in CDCl3. When pyrazine is
bonded to a metal, the molecule is no longer symmetric, as
shown in Scheme 1. Two signals should now be observed for
1
the coordinated molecule in the H NMR (Ha and Hb) and
13C NMR (Ca and Cb) spectra. The atoms Ha and Ca should
be more affected by coordination to the metal than Hb and
Cb.
The chemical shifts and the coupling constants of the
complexes are presented in the Experimental section, and
the values ∆δ (δcomplex – δpyrazine) are listed in Table 3. Coor-
dination of pyrazine to the Pt atom causes a deshielding of
all the pyrazine atoms. The chemical shifts of Ha and Ca are
more deshielded than those of Hb and Cb as expected, result-
ing in larger ∆δ values for Ha and Ca. For the DPhSO com-
plex, the two multiplets were not separated. The reduction of
electron density in bonded pyrazine is a consequence of the
The 195Pt NMR signal of the DPhSO complex was ob-
served at much higher fields than those of the other com-
pounds, as also reported for the other DPhSO complexes in
Table 2 and for a series of compounds of the types
Pt(R2SO)(R-CN)Cl2 (25) and K[Pt(R2SO)Cl3] (25, 26).
DPhSO is not only the most sterically hindered ligand, but
also it is different from the other sulfoxides in that it con-
tains aromatic groups directly attached to the sulfur atom.
These groups are probably responsible for the different elec-
tronic effects observed for this compound. The electron den-
sity on the Pt atom is increased in the DPhSO complex
compared to the complexes containing the other sulfoxide
ligands. The shielded resonance is probably not caused by
the bulkiness of the substituents on the sulfur, because the
presence of bulky substituents on the atom bonded to Pt usu-
1
σ bond (N→Pt), which causes a deshielding effect in H and
13C NMR. In the literature, for a few complexes of the type
trans-Pt(PR3)(pz)Cl2 (11), the reported ∆δa values in 1H
NMR (avg. 0.38 ppm) were larger than ours, while their ∆δb
value was 0.15 ppm. The difference might be caused by the
larger trans effect of phosphine ligands compared to that of
sulfoxides (10). For the trans-trans pyrazine-bridged dimers
{Pt(R2SO)Cl2}2(µ-pz) (in which the pyrazine is symmetric),
the average ∆δ value was around 0.52 ppm (10). It seems
© 2004 NRC Canada

654
Can. J. Chem. Vol. 82, 2004
1
Table 3. H and 13C NMR ∆δ (δcomplex – δpyrazine) (ppm) of pyrazine resonances for the complexes
Pt(R2SO)(pz)Cl2.
Compound
(1H)∆δa
(1H)∆δb
(13C)∆δa
(13C)∆δb
trans-Pt(DMSO)(pz)Cl2
trans-Pt(TMSO)(pz)Cl2
cis-Pt(DPrSO)(pz)Cl2
trans-Pt(DPhSO)(pz)Cl2
0.195
0.226
0.122
0.311
0.185
0.179
0.100
0.311
1.94
2.35
2.52
2.25
0.26
0.63
2.33
0.86
cis-Pt(DPrSO)(pz)Cl2 (∆δb = 2.33 ppm) than for the other
complexes (∆δb = 0.26–0.86 ppm). The difference is proba-
bly due to the cis geometry of the DPrSO compound. The
latter ligand is very sterically demanding, and the rotation
around the Pt—N bond is restricted. In solution, the
pyrazine plane would probably be almost perpendicular to
the Pt(II) coordination plane to reduce the steric hindrance
to a minimum. The 13C ∆δavg of cis-Pt(DPrSO)(pz)Cl2 is
larger (2.44 ppm) than the corresponding values of the trans
compounds (avg. 1.38 ppm). A similar observation was pre-
viously reported for Pt(R2SO)(pm)Cl2, where the value ∆δavg
was 2.13 ppm for the cis isomers and 1.51 ppm for the trans
complexes (8). For trans-Pt(PR3)(pz)Cl2, the Ca and Cb sig-
nals were observed at 147.2 and 145.0 ppm, respectively
(11). We have not observed any J(195Pt-13C) couplings for
the pyrazine carbon atoms.
Scheme 1.
that the formation of two σ bonds (N→Pt) by pyrazine de-
creases further the electron density on pyrazine.
1
The H ∆δ values for cis-Pt(DPrSO)(pz)Cl2 are smaller
than those of the trans complexes (Table 3). These differ-
ences might be due to the different geometry, but more cis
compounds are needed before a conclusion can be reached.
Steric hindrance in the cis complex is an important factor
that must be considered. In the trans compounds, the rota-
tion around the Pt—N bond is not very restricted because
the π component of the bond is probably not very important.
For the cis isomers, the rotation of the pyrazine ring around
the Pt—N bond is more hindered, and the planar ligand is
probably close to being perpendicular to the Pt(II) coordina-
tion plane to reduce the steric hindrance. The rotation will
be more restricted with more bulky sulfoxides.
The chemical shifts and the coupling constants J(1H-1H)
of the sulfoxide ligands are presented in the Experimental
section, and the values of ∆δ (δcomplex – δsulfoxide) are listed in
1
Tables 4 (1H NMR) and 5 (13C NMR). In H NMR, the ∆δ
values are positive, indicating that the sulfoxide protons are
more deshielded in the ligands than in the free molecules.
The protons located in positions α to the binding atom are
the most affected by coordination to the metallic center. For
the DMSO complex, only one signal at 3.48 ppm was
observed; this chemical shift is close to that reported for
trans-Pt(DMSO)(pm)Cl2 (3.45 ppm (8)) and for trans-
Pt(DMSO)(thiazole)Cl2 (3.48 ppm (34)). For the other com-
plexes, two series of signals were observed. For the TMSO
complex, the two geminal protons of each CH2 group are in
different environments. Those closer to the O atom will be
more deshielded than the others. For the other ligands, the
two series of signals result from restricted rotation around
the S—C bonds. The difference between the two series of
resonances decreases as the distance from the S atom in-
creases.
The chemical shifts of the pyrazine protons in trans-
Pt(DPhSO)(pz)Cl2 were at lower fields (∆δavg = 0.311 ppm)
than those of the other trans compounds (∆δavg = 0.20 ppm).
This is in agreement with the observation of the 195Pt NMR
signal of this compound at higher field than those of the
other complexes, since an increase of electron density on the
Pt atom (upfield shift in 195Pt NMR) corresponds to a reduc-
1
tion of electron density on the ligand (downfield shift in H
and 13C NMR).
The coupling constants J(1Ha-1Hb) are 3.0 Hz for trans-
3
Pt(DMSO)(pz)Cl2 and trans-Pt(TMSO)(pz)Cl2 and 3.3 Hz
for cis-Pt(DPrSO)(pz)Cl2. A coupling constant with the
3
A coupling constant J(195Pt-1H) = 23 Hz was observed
Pt(II) nucleus, J(195Pt-1H) = 33 Hz, was also observed for
3
for trans-Pt(DMSO)(pz)Cl2. This is in agreement with
results reported for the similar trans complexes
Pt(DMSO)(thiazole)Cl2 (22 Hz) (34) and Pt(R2SO)(pm)Cl2
(22 Hz) (8). The other ligands gave low-intensity multiplets,
and the coupling with 195Pt could not be seen.
the DMSO complex. This value is in agreement with the re-
sults in the literature reported for trans-Pt(DMSO)(Ypy)Cl2
(Ypy = pyridine derivative) (32) and for trans-Pt(Ypy)2X2
(33). The coupling constants J(195Pt-1H) are usually smaller
3
for trans isomers. For example, the values of J(195Pt-1H) for
3
In 13C NMR, the resonances of the C atoms in α positions
were observed at lower fields than those of the correspond-
ing atoms in the free sulfoxides (∆αδ = 2.31–3.76 ppm), ex-
cept for the DPhSO complex (Table 5). These values are
quite small compared to similar values for other types of lig-
ands such as amines. Sulfoxides can accept electron density
from the metal center, and the π (Pt→S) bond will increase
the electron density on the sulfoxide ligand. Furthermore,
the inverse polarization of the π electrons of the δ–S—Oδ+
Pt(DMSO)(pm)Cl2 are 31 and 44 Hz for the trans and cis
compounds, respectively (8).
In 13C NMR spectroscopy, similar results were observed.
The two pyrazine signals in the spectra of the complexes
were observed at lower fields than the single 13C resonance
of free pyrazine, with Ca being more affected (∆δa ~ 2 ppm)
by coordination to the metal than Cb (Table 3). The forma-
tion of the N→Pt σ bond reduces the electron density on the
pyrazine ligand. Surprisingly, ∆δ for Cb is much larger for
© 2004 NRC Canada

Rochon and Priqueler
Table 4.1H NMR ∆δ (δcomplex – δsulfoxide) (ppm) for the complexes Pt(R2SO)(pz)Cl2.
655
Complex
Hα
Hβ
Hγ
Hδ
trans-Pt(DMSO)(pz)Cl2
1.00
J(195Pt-1H)=23 Hz
trans-Pt(TMSO)(pz)Cl2
cis-Pt(DPrSO)(pz)Cl2
trans-Pt(DPhSO)(pz)Cl2
1.17, 0.82
0.25, –0.01
1.20, 0.56
0.62, 0.29
0.17
0.66, 0.30 (ortho)
0.14 (meta)
0.14 (para)
Note: Hα is located on the C atom closest to the S atom, Hβ is on the second C from S, etc.
Table 5.13C NMR ∆δ (δcomplex – δsulfoxide) (ppm) for the complexes Pt(R2SO)(pz)Cl2.
Complex
Cα
3.76
Cβ
Cγ
Cδ
trans-Pt(DMSO)(pz)Cl2
trans-Pt(TMSO)(pz)Cl2
cis-Pt(DPrSO)(pz)Cl2
trans-Pt(DPhSO)(pz)Cl2
2.66
2.31
–0.75
0.54
–0.52
–6.99
2.86 (ortho)
–0.34 (meta)
2.12 (para)
Note: Cα is the C atom closest to the S atom, Cβ is the second C from S, etc.
bond would increase the electron density on the S atom and
its neighbouring atoms, especially the α carbon atoms. This
inverse polarization of the S—O π bond was first suggested
by Cooper and Powell (30) to explain the 13C NMR spectra
of some Pt–carbonyl complexes and was later put forward in
the interpretation of the 13C NMR spectra of some Pt–
carboxylate complexes (31). The chemical shifts in 13C
NMR spectroscopy are particularly dependent on such
mesomeric effects.
For trans-Pt(DPhSO)(pz)Cl2, the signals of the α carbon
atoms were observed at much higher field than those of the
free ligand (∆αδ = –6.99 ppm). Similar behavior was reported
for the pyrazine-bridged dimer trans-{Pt(DPhSO)Cl2}2(µ-pz),
with ∆αδ = –6.61 ppm (10). We suggest that the inverse po-
larization of the π electrons in the S=O bond would be
greater in this ligand because of the presence of two strongly
electron-attracting phenyl groups located directly on the S
atom. This phenomenon would increase the electron density
on the C atoms located in positions α to the S atom. The π
component of the Pt—S bond (back-donation) might also be
more important for this ligand because of the presence of the
two electron-attracting groups on the ligand.
The NMR results for these complexes are very similar in
general to those reported for the pyrazine-bridged dimers
(10). This behavior is different from that reported for the
R2SO-pm complexes (8, 9). Since the two bridging atoms in
pyrazine are far from each other, the presence of a second Pt
atom does not influence the chemical shifts of the
sulfoxides. There are seven bonds between Cα and the sec-
ond Pt atom in the dinuclear species. In the pyrimidine se-
ries, the distances are shorter, and the influence of the
second metal center is more important.
has π* orbitals that could form π bonds with Pt, but to a
much reduced extent as compared to sulfoxides. Some pub-
lished work on pyridine (33) and pyrimidine (8, 9) com-
plexes seems to indicate the presence of some π bonding in
the Pt—N bonds, although this is not easy to study.
Crystal structures
The crystal structures of one trans complex,
Pt(TMSO)(pz)Cl2 (I), and the one cis compound produced,
Pt(DPrSO)(pz)Cl2 (II), were studied by X-ray diffraction
methods. The results confirmed the geometry of the com-
plexes suggested by IR and NMR spectroscopies. Figures 1
and 2 show the labeled diagrams of the two complexes. The
bond distances and angles are listed in Table 6. The CIF ta-
bles of the two crystal structure determinations have been
deposited as supplementary material.2
The coordination around the Pt atoms is square planar as
expected. The bond angles around the Pt atom are close to
the ideal values. The largest deviation can be seen in the cis
compound (II), where the N-Pt-Cl angle is reduced to
87.7(4)° while the N-Pt-S angle is increased to 92.0(4)°, to
reduce the steric hindrance caused by the bulky sulfoxide
ligand. In Pt–sulfoxide complexes, the O atom of the
sulfoxide is often in the Pt coordination plane, which re-
duces the steric hindrance caused by the presence of the
sulfoxide. In the cis compound (II), where this factor is very
important, the O is almost in the coordination plane, with a
deviation of 0.12(3) Å, while in the trans compound (I), the
deviation is 1.41(1) Å.
The pyrazine ring is planar in both I and II. The dihedral
angle between the pyrazine plane and the Pt coordination
plane is 54.6(4)° and 60.3(5)° for the trans (I) and cis (II)
complexes, respectively. This angle is usually larger in cis
The results of this study seem to indicate that the Pt–
pyrazine bond is more complicated than expected. Pyrazine
2 Supplementary data may be purchased from the Directory of Unpublished Data, Document Delivery, CISTI, National Research Council
Canada, Ottawa, ON K1A 0S2, Canada (http://www.nrc.ca/cisti/irm/unpub_e.shtml for information on ordering electronically). CCDC
216747 and 216748 contain the crystallographic data for this manuscript. These data can be obtained, free of charge, via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
U.K.; fax +44 1223 336033; or deposit@ccdc.cam.ac.uk).
© 2004 NRC Canada

656
Can. J. Chem. Vol. 82, 2004
Fig. 1. Labeled diagram of trans-Pt(TMSO)(pz)Cl2 (I). The ellipsoids correspond to 30% probability.
Fig. 2. Labeled diagram of cis-Pt(DPrSO)(pz)Cl2 (II). The ellipsoids correspond to 25% probability.
compounds, where the bulkiness of the ligands is a more im-
portant factor.
Å. In I, the Pt—N bond is trans to the sulfoxide and is sig-
nificantly longer (2.053(11) Å). These results are close to
those reported for similar compounds (8–10, 33, 34, 37, 38).
All these data confirm that the trans influence of the sulf-
oxide ligand is clearly larger than that of the chloro ligand
or pyrazine.
The bond distances and bond angles in the sulfoxide lig-
ands are as expected and agree with published data on Pt–
R2SO complexes (8–10, 16, 39). As observed in other TMSO
compounds, the C-S-C angle in trans-Pt(TMSO)(pz)Cl2 is
small (94.0(7)°) since the S atom is in a five-membered ring.
The conformation of the TMSO ligand is of the envelope
type, as seen in Fig. 1. The four C atoms are planar, and the
The Pt—S bond lengths in the two structures are within
the range reported for other Pt–sulfoxide complexes (8–10,
16, 34–36). In terms of trans influence, pyrazine and chlo-
ride are not very different, and the lengths of the Pt—S
bonds do not seem to be very sensitive to the trans ligand. In
cis-Pt(DPrSO)(pz)Cl2, the Pt—Cl bond trans to the sulfoxide
ligand is longer (2.316(5) Å) than the one trans to pyrazine
(2.292(5) Å). The large trans influence of sulfoxides is well
known. In crystal I, the Pt—Cl bonds are trans to each other,
and their lengths (avg. 2.293(3) Å) are within the normal
range. The lengths of Pt—N bonds are usually more sensi-
tive to the trans ligands. In crystal II, where the trans ligand
is a chloro ligand, the length of the Pt—N bond is 2.004(14)
S atom is 0.75(2)
between this plane and the Pt coordination plane is
out of the plane. The dihedral angle
Å
© 2004 NRC Canada

Rochon and Priqueler
Table 6. Bond distances and angles in crystals I and II.
657
I, trans-Pt(TMSO)(pz)Cl2
II, cis-Pt(DPrSO)(pz)Cl2
Bond lengths (Å)
Pt—Cl
2.290(3)
2.296(3)
2.217(3)
2.053(11)
1.488(10)
1.779(14)
1.34(2)
2.292(5)
2.316(5)a
2.209(6)
2.004(14)
1.45(2)
1.78(2)
1.36(3)
1.36(3)
1.47(3)
Pt—S
Pt—N
S—O
S—C (avg.)
N—C (pz) (avg.)
C—C (pz) (avg.)
C—C (R2SO) (avg.)
1.38(2)
1.54(2)
Bond angles (°)
Cl-Pt-Cl
S-Pt-Cl
177.98(14)
92.13(11)
89.89(12)
89.0(3)
90.3(2)
90.0(2)
179.3(3)
87.7(4)
N-Pt-Cl
88.9(3)
177.2(5)
92.0(4)
N-Pt-S
Pt-S-O
Pt-S-C (avg.)
Pt-N-C (avg.)
O-S-C (avg.)
C-S-C
175.8(4)
112.9(5)
115.7(5)
121.5(10)
108.5(7)
94.0(7)
115.5(7)
111.5(10)
122.3(13)
108.1(13)
100.9(12)
115(2)
C-N-C (pz) (avg.)
C-C-N (pz) (avg.)
S-C-C (R2SO) (avg.)
C-C-C (R2SO) (avg.)
116.4(13)
122(2)
123(2)
104.1(11)
109.1(12)
109(2)
111(3)
atrans to DPrSO.
5. N. Farrell, D.M. Kiley, W. Schmidt, and M.P. Hacker. Inorg.
Chem. 29, 397 (1990).
6. M. van Beusichem and N. Farrell. Inorg. Chem. 31, 634 (1992).
7. S. Komeda, G.V. Kalayda, M. Lutz, A.L. Spek, Y. Yamanaka,
T. Sato, M. Chikuma, and J. Reedjik. J. Med. Chem. 46, 1210
(2003).
8. N. Nédélec and F.D. Rochon. Inorg. Chim. Acta, 319, 95
(2001).
9. N. Nédélec and F.D. Rochon. Inorg. Chem. 40, 5236 (2001).
10. J.R.L. Priqueler and F.D. Rochon. Inorg. Chim. Acta. 357,
2165 (2004).
17.1(10)°. In cis-Pt(DPrSO)(pz)Cl2, the C-S-C angle is
smaller (100.9(12)°) than the other angles around the S
atom, since the multiple S—O bond occupies more space
than the single S—C bonds.
The average pyrazine C-N-C and C-C-N internal angles
are 116.4(13)° and 122(2)°, respectively, for I and 115(2)°
and 123(2)°, respectively, for II. For the free molecule, these
angles were reported to be 115.1° and 122.4°, respectively
(40). Therefore, the structure of pyrazine does not change
upon coordination to Pt(II).
11. A. Albinati, F. Isaia, W. Kaufmann, C. Sorato, and L.M.
Venanzi. Inorg. Chem. 28, 1112 (1989).
Acknowledgement
12. A.R. Siedle, K.R. Mann, D.A. Bohling, G. Filipovich, P.E.
Toren, F.J. Palensky, R.A. Newmark, R.W. Duerst, W.L.
Stebbings, H.E. Mishmash, and K. Melancon. Inorg. Chem.
24, 2216 (1985).
13. XSCANS and SHELXTL (PC, Version 5) [computer programs].
Bruker Analytical X-Ray Systems, Madison, Wis. 1995.
14. Y.N. Kukushkin, Y.E. Vyaz’menskii, and L.I. Zorina. Russ. J.
Inorg. Chem. 13, 1573 (1968).
The authors are grateful to the Natural Sciences and Engi-
neering Research Council of Canada for financial support of
the project.
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