Characteristics of nickel(0), nickel(I), and nickel(II) in phosphino thioether complexes: Molecular structure and S-dealkylation of (Ph2P(o-C6H4)SCH3)2Ni0

Article abstract of DOI:10.1021/ja953686h

The molecular structure of a nickel(0) complex with P,S-donor atom ligands has been characterized by X-ray crystallography. Complex 1, (Ph₂P(o-C₆H₄)SCH₃)₂Ni⁰, prepared by Na/Hg amalgam redu

Full text of DOI:10.1021/ja953686h

J. Am. Chem. Soc. 1996, 118, 4115-4123
4115
Characteristics of Nickel(0), Nickel(I), and Nickel(II) in
Phosphino Thioether Complexes: Molecular Structure and
S-Dealkylation of (Ph2P(o-C6H4)SCH3)2Ni0
Jang Sub Kim, Joseph H. Reibenspies, and Marcetta Y. Darensbourg*
Contribution from the Department of Chemistry, Texas A&M UniVersity,
College Station, Texas 77843
ReceiVed NoVember 1, 1995X
Abstract: The molecular structure of a nickel(0) complex with P,S-donor atom ligands has been characterized by
X-ray crystallography. Complex 1, (Ph2P(o-C6H4)SCH3)2Ni0, prepared by Na/Hg amalgam reduction of the NiII
complex [(Ph2P(o-C6H4)SCH3)2Ni](BF4)2 ([2](BF4)2), degrades photochemically with loss of CH3 radicals to yield
complex 3, (Ph2P(o-C6H4)S)2NiII. Crystallographic parameters for the three compounds are as follows: 1, monoclinic
space group C2/c with a ) 11.467(2) Å, b ) 17.613(3) Å, c ) 15.733(2) Å, â ) 96.450(10)°, V ) 3157.5(9) Å3,
and Z ) 4; [2](BF4)2, monoclinic space group P21/c with a ) 9.417(4) Å, b ) 14.822(9) Å, c ) 13.773(2) Å, â )
98.55(3)°, V ) 9101(14) Å3, and Z ) 2; and 3, monoclinic space group P21/c with a ) 9.651(2) Å, b ) 12.971(8)
Å, c ) 12.540(2) Å, â ) 110.46(2)°, V ) 1470.7(11) Å3, and Z ) 2. While complex 1 has a distorted tetrahedral
geometry, complexes [2](BF4)2 and 3 are square planar with trans stereochemistry. The cyclic voltammogram of
[2](BF4)2 in CH3CN shows two redox events assigned to NiII/I and NiI/0, whereas the thiolate 3 reveals only one
reversible wave assigned to NiII/III. The chemical reduction of [2](BF4)2 with Cp2Co provided a NiI species, [(Ph2P(o-
C6H4)SCH3)2NiI]+, characterized at 100 K by an axial EPR signal with gx ) gy ) 2.10 and gz) 1.96. Hyperfine
spectral features resulting from coupling to two 31P nuclei suggests a retention of substantially square planar geometry.
In contrast the isotropic character of the EPR signal of [(Ph2P(o-C6H4)SCH3)(Ph2P(o-C6H4)S)NiI], presumed to be
the first product of the photochemical demethylation of 1 ultimately yielding the doubly demethylated complex 3,
suggested the intervening thioether/thiolate NiI species to be pseudotetrahedral. Protonation of the nickel(0) species
1 produced a five-coordinate nickel hydride complex, [(H)(arom-PSMe)2Ni]BF4, 4.
Introduction
Group VIII metal complexes of N,S, P,S, and As,S donor
chelates have been extensively studied for their ability to transfer
alkyl groups from sulfur to suitable nucleophiles.1-3 Such alkyl
transfers from nickel(II) and palladium(II) complexes of the
o-(diphenylphosphino)thioanisole ligand (Ph2P-o-C6H4SMe, des-
ignated hereafter as arom-PSMe) to amines have been charac-
terized as Menschutkin type SN2 reactions.3b,4 Roundhill and
Benefiel have provided kinetic monitors of reaction (eq 1),
demonstrating bimolecular kinetic behavior with ∆Hq in the
range of +13 to +19 kcal mol-1 and ∆Sq in the range of -14
Roundhill and Benefiel.5 In contrast the two known structures
of bidentate phosphino thioether derivatives of NiII are trans.).6
With this history of nucleophilic displacement of S-alkyls,
the instability of the iodide salt of [(arom-PSMe)2Ni](I)2, [2](I)2,
was readily rationalized according to eq 2, and our goal of
isolating and chemically characterizing a nickel(0) complex, 1,
[(arom-PSMe)2Ni0], necessitated the synthesis of the BF4- salt,
described by eq 3. This salt was successfully used in the
to -30 cal K-1 mol-1 5
(Note: The representation of cis
.
stereochemistry for (Ph2P-o-C6H4SEt)2PtII is as suggested by
X Abstract published in AdVance ACS Abstracts, April 15, 1996.
(1) (a) Hay, R. W.; Galyer, A. L.; Lawrance, G. A. J. Chem. Soc., Dalton
Trans. 1976, 939. (b) Sacconi, L.; Speroni, G. P. Inorg. Chem. 1968, 7,
295. (c) Lindoy, L. F.; Livingstone, S. E.; Locker, T. N. Aust. J. Chem.
1967, 20, 471.
(2) (a) Lindoy, L. F.; Livingstone, S. E.; Locker, T. N. Inorg. Nucl. Chem.
Lett. 1967, 3, 35. (b) Eller, P. G.; Riker, J. M.; Meek, D. W. J. Am. Chem.
Soc. 1973, 95, 3540. (c) Roundhill, D. M.; Beaulieu, W. B.; Bagchi, U. J.
Am. Chem. Soc. 1979, 101, 5428. (d) Roundhill, D. M.; Roundhill, S. G.
N.; Beaulieu, W. B.; Bagchi, U. Inorg. Chem. 1980, 19, 3365. (e) Benefiel,
A.; Roundhill, D. M.; Fultz, W. C.; Rheingold, A. L. Inorg. Chem. 1984,
23, 3316.
(3) (a) McAuliffe, C. A. Inorg. Chem. 1973, 12, 2477. (b) Dutta, R. L.;
Meek, D. W.; Busch, D. H. Inorg. Chem. 1970, 9, 1215. (c) Dutta, R. L.;
Meek, D. W.; Busch, D. H. Inorg. Chem. 1970, 9, 2098. (d) Lindoy, L. F.;
Livingstone, S. E.; Locker, T. N. Inorg. Chem. 1967, 6, 652. (e) Dutta, R.
L.; Meek, D. W.; Busch, D. H. Inorg. Chem. 1971, 10, 1820.
(4) (a) Menschutkin, N. Z. Phys. Chem., Stoechim. Verwandtschaftsl.
1890, 5, 589. (b) Ingold, C. K. Structure and Mechanism in Organic
Chemistry; Cornell University Press: Ithaca, NY, 1953; pp 349, 412. (c)
Abraham, M. H.; Nasehzadeh, A. J. Chem. Soc., Chem. Commun. 1981,
905. (d) Kevill, D. N. J. Chem. Soc., Chem. Commun. 1981, 421.
(5) Benefiel, A.; Roundhill, D. M. Inorg. Chem. 1986, 25, 4027.
preparation and isolation of 1. Its molecular structure, presented
below, is the first nickel(0) complex ligated by phosphorus and
sulfur donor atoms whose structure has been determined by
X-ray crystallography. (The assumption of tetrahedral geometry
for a similar P2S2Ni complex was recently expressed in work
which similarly detailed nickel redox properties with complex
(6) Hsiao, Y. M.; Chojnacki, S. S.; Hinton, P.; Reibenspies, J. H.;
Darensbourg, M. Y. Organometallics 1993, 12, 870.
S0002-7863(95)03686-9 CCC: $12.00 © 1996 American Chemical Society

4116 J. Am. Chem. Soc., Vol. 118, No. 17, 1996
Kim et al.
geometry.)7 Interestingly, in the presence of light, the Ni0
complex 1 also demonstrates instability with respect to methyl
group loss, generating the dithiolate complex 3.
EDTA. Photochemical reactions were conducted with a Conrad-
Hanovia 450 W Hg vapor lamp with Pyrex sleeves (>360 nm) in a
reaction vessel designed for photochemical reaction, all purchased from
the Ace Glass Co.
Interest in S-alkylation/S-dealkylation is currently high as
mechanistic knowledge is appropriate both to desulfurization
technology8 and to biological pathways such as methyl transfers
from S-adenosylmethionine, a natural carbonium ion source, or
the methylation dependent functional switch mechanism in the
Escherichia coli Ada protein.9,10 That metals might serve to
promote bond making as well as bond breaking S-C reactivity
is mechanistically challenging, and explorations of first-row
metals as candidates for this process provide the impetus for
our work. Homolytic S-C bond cleavage reactions are
intimately associated with accessibility of low oxidation states,11,12
and the subsequent partial back-donation of metal electrons into
an S-C π* orbital. The work described below combines studies
of redox properties, geometry changes, and homolytic S-C
activation.
Mass spectra of gas samples generated during the photolysis of 1
were recorded using a VG Analytical 70S high-resolution, double
focusing, and sectored mass spectrometer at ionizing energies of 70
eV at the TAMU Center for Chemical Characterization and Analysis.
Data were collected by a VG Analytical 11/250J data system. GC/
MS of organic samples with high molecular weight were obtained on
a Hewlett-Packard, HP, 5890 gas chromatograph with a HP 5971 mass
selective detector, and a HP cross-linked methyl silicone capillary
column (20 m; 0.20 mm, film thickness 0.33 mm). Elemental analyses
were performed by Galbraith Laboratories, Inc., Knoxville, TN.
C. Preparation of Compounds. o-(Diphenylphosphino)thioani-
sole, (Ph2P(o-C6H4)SCH3, arom-PSMe). o-Aminothioanisole was
prepared from the reaction of o-mercaptoaniline with sodium followed
by iodomethane in methanol.5 o-Iodothioanisole, I(o-C6H4)SCH3, and
o-(diphenylphosphino)thioanisole were prepared by slightly modified
literature procedures, and details are given as supporting information.14-16
Likewise procedures for the preparation of o-iodothioanisole-methyl-
d3 (I(o-C6H4)SCD3) and o-(diphenylphosphino)thioanisole-methyl-d3
(Ph2P(o-C6H4)SCD3, arom-PSCD3) are deposited as supporting infor-
mation.
Bis[o-(diphenylphosphino)thioanisole]nickel(0) ((arom-PSMe)2Ni0,
1). Method a. Under an Ar atmosphere, 0.30 g (0.354 mmol) of
2(BF4)2 in 30 mL of CH3CN was added to a Na/Hg amalgam (24.0
mg of Na in 1 mL of Hg) in a Schlenk flask and the mixture vigorously
stirred for 7 min. The resulting deep red solution was filtered through
degassed Celite to remove a precipitated green solid, the amalgam,
and NaBF4. Since the solubility of the (arom-PSMe)2Ni0 product is
low in CH3CN, the Celite was washed with 4 mL of benzene which
was also filtered and combined with the CH3CN filtrate. Methanol
(15 mL) was layered on top of the red solution, and the flask was placed
in a -5 °C freezer, resulting in the formation of red crystals over the
course of 2 days. The red crystals were washed twice with 5 mL of
anhydrous diethyl ether and dried under vacuum for 8 h to yield 0.15
g (62%) of 1. 1H NMR (C6D6): δ 7.80-6.80 (m, phenyl, 20 H), 2.18
(s, CH3, 6H). 31P{1H} NMR (C6D6): δ 43.8. Elemental analyses were
unreliable presumably due to demethylation, Vide infra.
Method b. A 50 mL Schlenk flask was charged with 0.10 g (0.364
mmol) of Ni(COD)2 and 0.224 g (0.720 mmol) of arom-PSMe ligand
in the Ar-filled glove box. Addition of 15 mL of CH3CN with vigorous
stirring (10 min) gave a red solid which was filtered, washed twice
with 15 mL of anhydrous diethyl ether, and dried under vacuum to
yield 0.160 g (65%) of 1. This procedure was also used to produce
bis[o-(diphenylphosphino)thioanisole-methyl-d3]nickel(0) ((arom-PSCD3)2-
Ni0, 1-d6), using 0.10 g (36.4 mmol) of Ni(COD)2 and 0.224 g (72.0
mmol) of arom-PSCD3 ligand, yielding 0.20 g (80.7%) of (arom-
PSCD3)2Ni0. 1H NMR (C6D6): δ 7.80-6.80 (m, phenyl, 20 H).
31P{1H} NMR (C6D6): δ 43.8.
Bis[o-(diphenylphosphino)thioanisole]nickel(II) Bistetrafluoro-
borate ([(arom-PSMe)2Ni](BF4)2, [2](BF4)2). To a solution of 0.39 g
(1.14 mmol) of Ni(BF4)2‚6H2O in 20.0 mL of 2-propanol was added
0.70 g (2.27 mmol) of the arom-PSMe ligand dissolved in 4.0 mL of
THF. The yellow precipitate which formed immediatly was filtered,
washed with 10.0 mL of 2-propanol followed by 10.0 mL of THF,
and dried under vacuum to yield 0.92 g (95%) of [2](BF4)2. 1H NMR
(CD3COCD3): δ 8.15-7.10 (m, phenyl, 28H), 2.5 (s, CH3, 6H).
31P{1H} NMR (CDCl3): δ -13.8.
Experimental Section
A. Methods and Materials. All reactions, sample transfers, and
sample manipulations were carried out using standard Schlenk tech-
niques (Ar atmosphere) and/or in an argon atmosphere glovebox. 13CO
(99% enriched) was purchased from Cambridge Isotope Laboratory,
Inc. Nickel precursors Ni(BF4)2‚6H2O and Ni(COD)2 (COD ) 1,5-
cyclooctadiene) were obtained from Strem Chemicals, Inc. All other
reagents were commercial products and used as received without further
purification. Dichloromethane was distilled over phosphorus pentoxide
under nitrogen. Acetone was dried using NaI or dried over molecular
sieves (4 Å). Acetonitrile was distilled once from CaH2, once from
P2O5, and freshly distilled from CaH2 and immediately before use.
Toluene, benzene, tetrahydrofuran, diethyl ether, and hexane were
distilled from sodium/benzophenone ketyl under nitrogen.
B. Instrumentation. Infrared spectra were recorded on a Mattson
Galaxy 6021 or an IBM IR/32 using 0.1 mm NaCl sealed cells or KBr
pellets. 1H, 2H, 13C, and 31P NMR spectra were obtained on a Varian
XL200 spectrometer; all 31P NMR spectra were referenced to PPh3
dissolved in THF and in a sealed capillary tube, which shows a singlet
resonance peak at -5.0 ppm referenced to external H3PO4. UV-vis
spectra were recorded on a Hewlett-Packard 8452A diode array
spectrometer. Solution spectra were obtained using 10 mm path length
quartz cells.
Cyclic voltammograms were recorded on a BAS-100A electrochemi-
cal analyzer using a Ag/AgNO3 reference and glassy carbon working
electrodes with 0.1 M [n-Bu4N]PF6 electrolyte. All redox potentials
were calibrated against ferrocenium [Cp2Fe]PF6 (E1/2 ) 400 mV)13 and
referenced to NHE. EPR spectra were recorded on a Bruker ESP 300
equipped with an Oxford ER910A cryostat operating at 10 or 100 K.
An NMR gaussmeter (Bruker ERO35M) and Hewlett-Packard fre-
quency counter (HP5352B) were used to calibrate the field and
microwave frequency. Samples were first frozen in liquid N2, and then
cooled to a low temperature for analyses. The concentration of
paramagnetic nickel(I) species was calculated and compared to the
double integral of the EPR spectrum of 1.0 mM Cu(II) in 10.0 mM
(7) James, T. L.; Smith, D. M.; Holm, R. H. Inorg. Chem. 1994, 33,
4869.
(8) (a) Riaz, U.; Curnow, O. J.; Curtis, D. J. Am. Chem. Soc. 1994, 116,
4357. (b) Robertson, M. J.; Day, C. L.; Jacobson, R. A.; Angelici. R. J.
Organometallics 1994, 13, 179. (c) Benson, J. W.; Angelici, R. J. Inorg.
Chem. 1993, 32, 1871. (d) Benson, J. W.; Angelici, R. J. Organometallics
1993, 12, 680. (e) Birnbaum, J.; Dubois, M. R. Organometallics 1994, 13,
1014. (f) Dietz, J. G; Bernatis, P.; Dubois, M. R. Organometallics 1993,
12, 3630.
Bis[o-(diphenylphosphino)benzenethiolato]nickel(II) ((arom-
PS)2NiII, 3). A mixture of 0.590 g (1.89 mmol) of Ni(I)2 and 1.16 g
(3.78 mmol) of arom-PSMe in 30 mL of ethanol (90%) was refluxed
for 2 h, resulting in a color change from red-brown to green. The
green precipitate which settled out was filtered, washed with 20 mL of
CH3CN followed by 20 mL of diethyl ether, and dried under vacuum
to yield 0.70 g (57%) of 3. 1H NMR (CDCl3): δ 6.85-7.70 (m, C6H4,
(9) Myers, L. C.; Terranova, M. P.; Ferentz, A. E.; Wagner, G.; Verdine,
G. L. Science 1993, 261, 1164.
(10) Ohkubo, T.; Sakashita, H.; Sakuma, T.; Kainosho, M,; Sekiguchi,
M.; Morikawa, K. J. Am. Chem. Soc. 1994, 116, 6035.
(11) Cha, M.; Shoner, S. C.; Kovacs, J. A. Inorg. Chem. 1993, 32, 1860.
(12) Sellmann, D.; Reisser, W. J. Organomet. Chem. 1985, 297, 319.
(13) Gagne, R. R.; Koval, C. A.; Lisensky, G. C. Inorg. Chem. 1980,
19, 2854.
(14) Workman, M. O.; Dyer, G.; Meek, D. W. Inorg. Chem. 1967, 6,
1543.
(15) Dyer, G.; Meek, D. W. Inorg. Chem. 1967, 6, 3983.
(16) Heaney, H.; Millar, I. T. Org. Synth. 1973, 5, 1120.

Ni0, NiI, and NiII in Phosphino Thioether Complexes
J. Am. Chem. Soc., Vol. 118, No. 17, 1996 4117
Table 1. Experimental Data for the X-ray Crystal Structures of (arom-PSMe)2Ni0 (1), [(arom-PSMe)2Ni](BF4)2 ([2](BF4)2), and (arom-PS)2NiII
(3).
complex
1
[2](BF4)2
C38H34B2F8P2S2Ni
849.04
monoclinic
P21/c
3
chemical formula
formula weight (g/mol)
space group
C38H34P2S2Ni
675.4
monoclinic
C2/c
C36H28P2S2Ni
645.4
monoclinic
P21/c
a (Å)
b (Å)
c (Å)
â (deg)
V (Å3)
Z
11.467(2)
17.613(3)
15.733(2)
96.450(10)
3157.5(9)
4
9.417(4)
14.822(9)
13.773(2)
98.55(3)
9101(14)
2
9.651(2)
12.971(8)
12.540(2)
110.46(2)
1470.7(11)
2
F(calcd) (g cm-3
temp (K)
)
1.421
193
1.483
293
1.457
293
radiation
Mo KR (λ ) 0.710 73 Å)
abs coeff (C m-1
)
8.70
0.9579/0.9857
6.50
7.72
9.30
difabs
7.46
8.96
min/max trans coeff
0.637/0.999
5.18
12.57
R (%)a
wR (%)a
5.60
a Residuals: R ) ∑|Fo - Fc|/∑Fo; wR ) {[∑w(Fo - Fc)2]/[∑w(Fo)2]}1/2
.
8H). 31P{1H} NMR (CDCl3): δ 56.1. Anal. Calcd for NiP2S2-
C36H28B2F8 (found): C, 67.0 (66.0); H, 4.34 (4.26).
D. X-ray Structure Determination. X-ray quality crystals of 1
were grown by diffusion of methanol into acetonitrile and benzene
(method b) or from 1 produced in acetonitrile solution by deprotonation
of [(H)(arom-PSMe)2](BF4) with triethylamine. Orange crystals of
[2](BF4)2 were crystallized from acetone/diethyl ether and green crystals
of 3 from CH2Cl2/methanol. Preliminary examination and data
collection for 1 and [2](BF4)2 were performed on a Siemens R3m/V
X-ray diffractometer with an oriented graphite monochromator; for 3
a Rigaku AFC5R X-ray diffractometer with an oriented graphite
monochromator was used. Both instruments used Mo KR radiation (λ
) 0.710 73 Å). Diffractometer control software P3VAX 3.42 was
supplied by Siemens Analytical Instruments, Inc. All crystallographic
calculations were performed with use of the Siemens SHELXTL-PLUS
program package. The structures were solved by direct methods and
refined using a full-matrix least-squares anisotropic refinement for all
non-hydrogen atoms. Carbon-bound hydrogen atoms were placed in
idealized positions with isotropic thermal parameters fixed at 0.08 Å.
Summaries of the X-ray crystallographic experimental data are given
in Table 1.
Bis[o-(diphenylphosphino)thioanisole]hydridonickel(II) Tetraflu-
oroborate ([(H)(arom-PSMe)2Ni](BF4), 4). To a flask containing 0.2
g (0.24 mmol) of [2](BF4)2 and a Na/Hg amalgam (15.0 mg of Na in
1 mL of Hg) was added 25 mL of benzene. The solution was
vigorously stirred until the nickel complex was reduced, resulting in a
color change from yellow to deep red. The red solution was filtered
through degassed Celite and placed in an ice bath. A stoichiometric
amount of HBF4‚Et2O in 10.0 mL of diethyl ether was added to the
solution, resulting in a color change from red to clear at the end of
addition concomitant with the precipitation of an orange solid. The
orange solid was filtered and washed with 20.0 mL of benzene followed
by 2 × 15.0 mL of diethyl ether and dried under vacuum to yield 0.10
g (94%) of 4. 1H NMR (CD3COCD3): δ 8.00-7.45 (m, phenyl, 28
H), 2.15 (s, SCH3, 3 H), -16.19 (t, JP-H ) 45.0 Hz, Ni-H, 1 H).
31P{1H} NMR (CD3CN): δ 44.3.
Photolysis of 1. Under an Ar atmosphere, 40.0 mg (0.060 mol) of
1, obtained from chemical reduction of [2](BF4)2, in 25 mL of THF
was transferred via cannula to a 100 mL water-jacketed photochemical
reaction vessel. Photolysis (15 min with a 450 W mercury vapor lamp)
of the solution resulted in a color change from deep red to green, from
which 30.0 mg (78%) of (arom-PS)2NiII, 3, was recovered. The1H
NMR spectrum of the photolyzed solution of 1 in C6D6 showed the
growth of a single resonance of ethane at 0.85 ppm with a decrease of
resonance for complex 1.
Results and Discussion
Synthesis and Properties of the Free Ligand and NiII and
Ni0 Complexes. The bidentate ligand arom-PSMe was first
prepared and used in the synthesis of transition metal complexes
by Livingstone and co-workers, and later by others.3a,c-e,5 We
find that the use of KI instead of CuBr in the synthesis of
o-halothioanisole in the first step16 of the ligand synthesis and
the subsequent use of o-iodothioanisole for introduction of the
PPh2 group leads to products of higher purity and yield, eq 4.
Photolysis of
1 with 2,2,6,6-Tetramethyl-1-piperidinyloxy
(TEMPO). To a 1H NMR tube was added a solution of 14.0 mg (0.021
mmol) of 1 and 7.0 mg (0.044 mmol) of TEMPO in 1 mL of benzene-
d6. The red solution was photolyzed for 15 min. The resultant yellow-
green solution was examined with 1H NMR which showed the formation
of 2,2,6,6-tetramethyl-1-piperidinylmethoxide (TEMPO-Me) and (arom-
PS)2NiII, 3. The solution was filtered through silica gel, dried, and
1
extracted with THF. The organic product was identified by H NMR
and GC/MS. 1H NMR (C6D6) of TEMPO-Me: δ 3.60 (s, OCH3,
3
H), 1.55-1.25 (m, CH2CH2CH2, 6 H), 1.20 (s, CH3 12 H). Mass
spectrum: m/z (relative intensity) 171 (9) [M]+, 156 (100) [M - CH3]+,
125 (7), 109 (32), 100 (20), 97 (17), 88 (60), 82 (20), 69 (80), 56 (95),
55 (99).
The thioether complex [2](BF4)2, prepared according to eq
3,6,17 precipitated from a mixture of 2-propanol and THF as a
yellow solid. X-ray quality crystals were obtained from acetone/
diethyl ether as orange parallelepipeds that are soluble in CH3CN
Photolysis of a Mixture of 1 and 1-d6. To a 2H NMR tube (i.d. )
10 mm) was added a solution of 10.0 mg (14.8 mmol) of 1 and 10.0
mg (14.7 mmol) of 1-d6 in 3 mL of benzene-d6. This red solution was
irradiated under He atmosphere for 15 min which produced a green
1
and acetone. The H and 13P{1H} NMR and UV/vis spectral
data for [2](BF4)2 are summarized in Table 2. These spectro-
scopic parameters are solvent dependent, probably indicative
of solvent or counterion interactions at nickel, the most dramatic
of which is shown by the 31P{1H} NMR spectra of [2](BF4)2.
In CD2Cl2 solution the 31P{1H} chemical shift of 51.8 ppm is
typical of other four-coordinate NiII complexes with P,S-donor
2
solution. The H NMR spectrum due to CH3CD3 (s, 0.73 ppm) and
CD3CD3 (s, 0.69 ppm) was observed. These gases were flushed by
bubbling the reaction solution with He, passed through a glass tube in
a dry ice/acetone bath, and finally entrapped in a trap immersed in
liquid N2. Immediately, the collected gases were analyzed by a VG
Analytical 70S mass spectrometer. The green solution was filtered
and dried under vacuum to give 18.0 mg (95%) of a green solid of 3.
(17) Rigo, P.; Bressan, M.; Basato, M. Inorg. Chem. 1979, 18, 860.

4118 J. Am. Chem. Soc., Vol. 118, No. 17, 1996
Kim et al.
1
Table 2. 31P and H NMR Data (ppm) and UV/Vis Band Positions (nm) for Solutions of arom-PSMe Ligand and Nickel(0) and Nickel(II)
derivatives.
complex
1H NMR
31P NMR
UV/Visd,e
arom-PSMea
2.43 (s, 3H), 6.73∼6.81 (m, 1H), 6.99∼7.13 (m, 1H), 7.22∼7.40 (m, 12H)
2.18 (s, 6H), 6.80∼7.80 (m, 20H)
-14.4
298 (3580)
284 (2320)
394 (1120, sh)
308 (5090)
370 (1430, sh)
298 (25 560)
424 (2850)
(arom-PSMe)2Ni0 (1)b
43.8
[(arom-PSMe)2Ni](BF4)2 ([2](BF4)2)c
(arom-PS)2NiII (3)a
2.48(s, 6H), 6.70∼8.14 (m, 20H)
-13.0
56.1
6.85∼7.70 (m, 28H)
[(H)(arom-PSMe)2Ni](BF4) (4)c
-16.20 (t, JP-H ) 45 Hz, 1H), 2.17 (s, 6H), 7.45∼8.00 (m, 20H)
44.3
284 (1540)
346 (840, sh)
a In CDCl3. b In C6D6. c In CD3CN. d Data taken in corresponding proton solvents., i.e., CHCl3, C6H6, and CH3CN. e ꢀ is given in parentheses.
Scheme 1
a suitable base for deprotonation of 4,22 also leading to 1.
Methods A and B have provided the nickel(0) complex 1 in
good yield and high purity, while method C was more successful
in the production of X-ray quality crystals.
Complex 1 is somewhat air sensitive as a solid and very air
sensitive in solution, decomposing to uncharacterizable products.
It dissolves in benzene and toluene to form red solutions, but
is insoluble in polar solvents such as methanol and acetonitrile.
1
The H NMR and 31P{1H} NMR spectra recorded in CDCl3
show a singlet for the methyl protons at 2.18 ppm and for
phosphorus at 43.8 ppm, respectively, Table 2. Protonation of
the solution of the nickel(0) complex by HBF4 leads to formation
of the hydride complex [(arom-PSMe)2(H)Ni]BF4 (4).23
Crystal Structures of (arom-PSMe)2Ni0 (1), [(arom-
PSMe)2Ni](BF4)2 ([2](BF4)2), and (arom-PS)2NiII (3). The
molecular structures of 1, [2]2+, and 3 are shown in Figures
1-3, respectively, and selected bond distances and angles are
compared in Table 3. As in crystal structures of four-coordinate
nickel(0) complexes ligated by mixed ligand sets of CO, PR3,
and olefins,25 1 can best be described in terms of distorted
tetrahedral geometry. The S-Ni-P bite angle of 1 is 90.5(1)°,
whereas the S-Ni-P(1a) angle between ligands is 118.0(1)°.
Likewise the P-Ni-P(1a) angle of 126.9(1)° and S-Ni-S(1a)
angle of 115.4(1)° in complex 1 are irregular; however, the angle
atom ligands (typically around 50 ppm),6,17,18 whereas in CD3CN
solution, a very broad resonance is barely discernible at -13.0
ppm (indicative of free ligand). Incremental additions of CD2Cl2
to the CD3CN solution of [2](BF4)2 resulted in the broadening
of the resonance at 52 ppm, with growth of the high-field -13.0
ppm resonance. Broad resonances are expected to arise from
complex/ligand exchange; however, the presence of line-
broadening hexacoordinate paramagnetic solvates cannot be
ruled out. Similar indications of ligand loss or paramagnetic
species in CD3CN are indicated by the H NMR spectrum of
[2](BF4)2.
The synthesis of the thiolate complex 3 was best achieved
by thermal demethylation of the thioether nickel iodide complex,
eq 5. Such nucleophilic demethylation is the reverse of the
1
(21) Gordon, A. J. The Chemist's companion; John Wiley & Sons: New
York, 1972; p 60.
(22) (a) Pearson, R. G. Chem. ReV. 1985, 85, 41. (b) Norton, J. R.;
Krisjansdottir, S. S. Acidity of Hydrido Transition Metal Complexes in
Solution. In Transition Metal Hydrides; Dedieu, A., Ed.; VCH: New York,
1991; p 309.
(23) Upon addition of a stoichiometric amount of HBF4 (diluted in diethyl
ether) to a red solution of 1, prepared from either the chemical reduction
of Ni2+ derivatives with Na/Hg amalgam or ligand replacement of Ni(COD)2
in benzene solution, an immediate color change occurred from deep red to
colorless, precipitating an orange-yellow solid (eq 6).
route used to prepare nickel-bound thioethers in N2S2Ni
complexes.19 The green solid obtained from reaction 5 is
soluble in CH2Cl2 and CH3Cl, and is surprisingly stable in both
acids and bases. Characteristic 1H and 13P{1H} NMR and UV/
vis spectral features for 3 are listed in Table 2.
Three synthetic methods (Scheme 1) were used for the
preparation of nickel(0) complexes with P,S-donor ligands.
Chemical reduction of NiII by Na/Hg amalgam has been
successful for [(PSEt)2Ni](BF4)2 (PSEt ) Ph2P(CH2)2SCH2CH3)
and [(PSSP)Ni](BF4)2 (PSSP ) Ph2P(CH2)2S(CH2)3S(CH2)2-
Ph2), producing (PSEt)2Ni0 and (PSSP)Ni0, respectively.6 The
Ni(COD)2 has been extensively used for the synthesis of a
variety of nickel(0) complexes since the 1,5-cyclooctadiene
ligands are easily replaced.20 Triethylamine (pKa ) 12.0)21 is
Efforts to grow a single crystal of this hydride complex in solution were
fruitless even at low temperature. However, spectroscopic results are almost
identical to those structurally characterized [(H)(PSSP)Ni]+ complex.6,24
The 13P{1H} NMR spectrum recorded in CD3CN exhibits a singlet at 44.3
1
ppm, while in the H NMR spectrum the hydride complex has a triplet at
-16.2 ppm (JP-H ) 45 Hz) attributable to a metal hydride coupled to two
phosphorus atoms and a singlet of methyl protons at 2.17 ppm (Table 2).
1
The H NMR spectral resonances of 4 are temperature independent down
to -78 °C.
(24) (a) Chojnacki, S. S. Ph.D. Thesis, University of Texas A&M, 1993;
p 60. (b) Chojnacki, S. S.; Kim, J. S.; Hsiao, Y. M.; Reibenspies, J. H.;
Darensbourg, M. Y. To be submitted to Inorg. Chem. for publication.
(25) (a) Sacconi, L.; Ghilardi, C. A.; Mealli, C; Zanobini, F. Inorg. Chem.
1975, 14, 1380. (b) Sheldrick, W. S. Acta Crystallogr., Sect. B 1975, 31,
305. (c) Kru¨ger, C.; Tsay, Y. H. Acta Crystallogr., Sect. B 1972, 28, 1941.
(d) Brauer, D. J.; Kru¨ger, C. J. Organomet. Chem. 1974, 77, 423. (e)
Bennett, M. A.; Chiraratvatana, C.; Robertson, G. B.; Tooptakong, U.
Organometallics 1988, 7, 1394. (f) Nickel, T.; Goddard, R.; Kru¨ger, C.;
Po¨rschke, K.-R. Angew. Chem., Int. Ed. Engl. 1994, 33, 879. (g) Maekawa,
M.; Munakata, M.; Kuroda-Sowa, T.; Hachiya, K. Inorg. Chim. Acta 1994,
227, 137. (h) White, S. G.; Stephan, D. W. Organometallics 1988, 7, 903.
(18) Kyba, E. P.; Davis, R. E.; Fox, M. A.; Clubb, C. N.; Liu, S.-T.;
Reitz, G. A.; Scheuler, V. J.; Kashyap, R. P. Inorg. Chem. 1987, 26, 1647.
(19) (a) Mills, D. K.; Darensbourg, M. Y.; Reibenspies, J. H. Inorg.
Chem. 1990, 29, 4364. (b) Farmer, P. J.; Reibenspies, J. H.; Lindahl, P. A.;
Darensbourg, M. Y. J. Am. Chem. Soc. 1993, 115, 4665.
(20) Ittel, S. D. Inorg. Synth. 1990, 28, 95.

Ni0, NiI, and NiII in Phosphino Thioether Complexes
J. Am. Chem. Soc., Vol. 118, No. 17, 1996 4119
Table 3. Selected Bond Distances and (Å) and Bond Angles (deg)
for (arom-PSMe)2Ni0 (1), [(arom-PSMe)2Ni](BF4)2 ([2](BF4)2), and
(arom-PS)2NiII (3)
1
2
3
Bond Distances
2.205(2)
2.143(2)
1.803(6)
1.814(7)
1.841(6)
1.857(6)
1.827(6)
1.397(8)
Ni-S
2.180(2)
2.217(2)
1.778(5)
1.820(5)
1.803(5)
1.813(4)
1.814(4)
1.398(6)
2.166(3)
2.173(3)
1.78(1)
Ni-P
S(1)-C(1)
S(1)-C(5)
P(1)-C(2)
P(1)-C(3)
P(1)-C(4)
C(1)-C(2)
1.809(9)
1.811(9)
1.80(1)
1.36(1)
Bond Angles
90.5(1)
Figure 1. Ball and stick drawing of the molecular structure of (arom-
PSMe)2Ni0 (1).
S(1)-Ni-P(1)
S(1)-Ni-P(1a)
S(1)-Ni-S(1a)
P(1)-Ni-P(1a)
Ni-S(1)-C(1)
Ni-S(1)-C(5)
Ni-P(1)-C(2)
Ni-P(1)-C(3)
Ni-P(1)-C(4)
S(1)-C(1)-C(2)
P(1)-C(2)-C(1)
88.49(5)
91.51(5)
180.0
88.7(1)
91.3(1)
180.0(1)
180.0(1)
107.2(3)
118.0(1)
115.4(1)
126.9(1)
104.7(2)
119.6(3)
104.7(2)
125.2(2)
120.3(2)
116.6(4)
118.5(4)
180.0
106.6(2)
105.5(2)
106.5(2)
118.5(2)
112.9(2)
119.8(3)
115.8(3)
107.5(3)
113.6(3)
119.5(3)
119.5(8)
116.0(8)
complex 1; Ni-S < Ni-P in complexes [2]2+ and 3) possibly
reflects the better match of the soft/soft interaction of Ni(0) and
P in 1 vs the borderline hard/borderline soft interactions of Ni(II)
and S in complexes [2]2+ and 3. In complex [2]2+ the Ni-S
distance of 2.180(2) Å is slightly shorter than those of the four-
coordinate Ni(II) complexes with phosphino thioether sites
bridged by CH2CH2: 2.208 Å for [(PSEt)2Ni](BF4)2 and 2.213
Å for [(PSSP)Ni](ClO4)2.6,26 This is consistent with a strength-
ening of the Ni-Sarom bond as compared to the Ni-Salkyl bond.
It should be noted that complex 3 has been synthesized by
other groups as well as ourselves, and isolated as green
crystals.2a,b In fact, an earlier crystal structure of complex 3
was reported by Zubieta and co-workers in a series of structures
of square planar nickel complexes.27 That structure was,
however, taken on a brown crystal and displayed crystal data
different from those of other members of the series as well as
our own complex 3 and known group VIII metal dithiolate
complexes, i.e., (arom-PS)2PdII and (PS)2NiII.2e,6 The source
of this discrepancy is not clear.
Electrochemistry. The cyclic voltammogram of [2](BF4)2
shows two redox couples, Figure 4. On the basis of EPR results,
Vide infra, the first redox couple is assigned to NiII/I and the
second event to NiI/0. As the sweep rate increases from 100 to
500 mV/s, the peak separation of the NiII/I couple increases from
129 to 239 mV, indicative of a quasi-reversible electrochemical
process, discussed further below. In contrast, the NiI/0 couple
is fully reversible.28 Table 4 compares the electrochemical data
for [2](BF4)2 to results of bidentate [(PSEt)2Ni](BF4)2 and
tetradentate [(PSSP)Ni](BF4)2 derivatives, which show the same
CV features.28
Figure 2. Ball and stick drawing of the molecular structure of [(arom-
PSMe)2Ni](BF4)2 ([2](BF4)2).
Figure 3. Ball and stick drawing of the molecular structure of (arom-
PS)2NiII (3).
between the normals of the planes defined by S2Ni and P2Ni is
92°. The S-arom-P unit is substantially planar; the dihedral
angle of S(2)-C(1)-C(2)-P(1) is only 6°. Although other
Ni(0) complexes of bidentate and tetradentate phosphino thio-
ethers are known and, on the basis of solution spectroscopic
data, have been suggested to be of tetrahedral geometry, to our
knowledge this is the first crystal structure of such a com-
plex.6,17,25
As indicated by the S-Ni-S(1a) and P-Ni-P(1a) angles
of 180.0°, both complexes [2]2+ and 3 are strictly square planar
with phosphorus and sulfur atoms trans to each other. Com-
parable thioether and thiolate complexes [(PSEt)2Ni]2+ and
(PS)2NiII (PS ) Ph2PCH2CH2S-) also have square planar
geometries.6,2e In both [2]2+ and 3, the S-Ni-P angles within
the chelate ring are only slightly smaller than the S-Ni-P(1a)
angles between ligands, in contrast to pseudotetrahedral 1. The
Ni-S distances diminish in the order 1 > [2]2+ > 3. Perhaps
surprisingly, the Ni-P distance of complex 1, 2.143(2) Å, is
shorter than that of [2]2+, 2.217(2) Å, or 3, 2.173(3)Å. The
reversal in the order of Ni-L distances (Ni-S > Ni-P in
Scheme 2 delineates the differences in the stabilization of
reduced nickel as dependent on the P-S linking group; E1/2
values are referenced to NHE. The poorer electron donating
ability of arom-PSMe ligand provides easier access to NiI and
(26) Aurivillius, K.; Bertinsson, G. -I. Acta Crystallogr. 1981, B37, 72.
(27) Block, E.; Ofori-Okai, G.; Kang, H.; Zubieta, J. Inorg. Chim. Acta
1991, 188, 7.
(28) The redox potentials for NiII complexes which were reported in ref
6 have been calibrated to ferrocenium (Cp2FePF6, E1/2 ) 400 mV) in order
to compare with the potentials reported here: +400 mV for (PS)2NiII; -280
mV (∆E ) 117 mV) and -901 mV (∆E ) 101 mV) for NiII/I and NiI/0
,
respectively, of [(PSEt)2Ni](BF4)2; -347 mV (∆E ) 125 mV) and -894
mV (∆E ) 63 mV) for NiII/I and NiI/0, respectively, of [(PSSP)Ni](BF4)2.

4120 J. Am. Chem. Soc., Vol. 118, No. 17, 1996
Kim et al.
Figure 4. Cyclic voltammograms of a 1.5 mM solution of [(arom-
PSMe)2Ni](BF4)2 ([2](BF4)2). Scans toward negative potential using a
glassy carbon working electrode at (a) 100, (b) 200, (c) 300, (d) 400,
and (e) 500 mV/s (scale referenced to Ag/AgNO3 and calibrated for
ferrocene).
Table 4. Electrochemical Dataa
E
1/2 (mV)
complex
Epc (mV) Epa (mV) (∆Ep (mV))
1b
NiII/I
NiI/0
NiII/I
NiI/0
NiII/III
NiII/I
NiI/0
NiII/I
NiI/0
78
-859
-138
-831
702
-221
-850
-284
-862
-770
-815 (89)
-93 (89)
Figure 5. Cyclic voltammograms of (a) a 1.0 mM solution of (arom-
PSMe)2Ni0 (1) in 0.1 M [(n-Bu)4N][PF6]/CH3CN and (b) a 1.0 mM
solution of (arom-PS)2NiII (3) in 0.1 M [(n-Bu)4N][PF6]/CH2Cl2. Scans
toward negative potential using a glassy carbon working electrode at
200 mV/s (scale referenced to Ag/AgNO3 and calibrated for ferrocene).
b
[2](BF4)2
-49
-738
638
-784 (93)
670 (64)
3c
d
[(PSEt)2Ni](BF4)2
-339
-952
-410
-926
-280 (118)
-901 (102)
-347 (126)
-894 (63)
d
[(PSSP)Ni](BF4)2
NiI/0couple is greater by a factor of 2 for the P to S aromatic
linkage relative to CH2CH2. A further destabilization is realized
on linking the thioether donors; this is attributed to steric
restriction or impedance to the rearrangement needed as the
geometrical preference of reduced nickel changes from square
planar to tetrahedral.
a All potentials calibrated for ferrocene are recorded with a vitreous
carbon disk working electrode and 0.1 M [n-Bu4N][PF6] as a supporting
electrolyte vs the Ag/0.01 M AgNO3 reference electrode. The potential
scan rate is 200 mV/s. b CH3CN. c CH2Cl2. d Reference 24.
Scheme 2
The electrochemical data found in Table 4 show some
irreversibility of the NiII/I couple as compared to the typically
reversible NiI/0 couple in the nickel(II) complexes [2](BF4)2,
[(PSEt)2Ni](BF4)2, and [(PSSP)Ni](BF4)2. The current ratio of
the NiII/I couple (ipa/ipc ≈ 1.4) of [2](BF4)2 is assumed to reflect
the fact that electron transfer is coupled to a structural change
of the NiI species. Such structural changes of complexes that
accompany electron transfer have been reviewed by Geiger,30
and general results are consistent with those seen here. That
is, consumption of one electron by the nickel(II) complex
[2](BF4)2 produces a nickel(I) species of structure close to that
of the original square planar nickel(II) complex, and subse-
quently, a tetrahedral twist of this intermediate yields a nickel(I)
species close in structure to the tetrahedral nickel(0) complex
1. The distorted tetrahedral structure of the NiI complex has
been previously suggested for bidentate phosphine and macro-
cyclic tetradentate P,S donor ligands from similar electrochemi-
cal data.31,32 Interestingly, the second reduction, the NiI/0 couple
of [2](BF4)2, is scan rate independent and fully reversible (ipa/
ipc ≈ 1.0), indicating that the major structural change has
occurred following addition of the first electron.
Shown in Figure 5a is the cyclic voltammogram of complex
1 recorded in CH3CN. Electrochemically, the cyclic voltam-
mogram features of the nickel(0) complex should be the same
as those of the nickel(II) complex, [2](BF4)2. However, the low
(29) Matschmer, V. H.; Krause, L.; Krech, F. Z. Allg. Chem. 1970, 373,
1.
(30) Geiger, W. E. Prog. Inorg. Chem. 1985, 33, 275.
(31) Martelli, M.; Pilloni, G.; Zotti, G.; Daolio, S. Inorg. Chim. Acta
1974, 11, 155.
(32) Kyba, E. P.; Davis, R. E.; Fox, M. A.; Clubb, C. N.; Liu, S.-T.;
Reitz, G. A.; Scheuler, V. J.; Kashyap, R. P. Inorg. Chem. 1987, 26, 1647.
Ni0 species relative to the PSEt and PSSP ligands. (This
generalization is also demonstrated in the electrochemistry of
free phosphines. The E1/2 for the oxidation of the free phosphine
ligand increases with increasing phenyl substitution.)29 Scheme
2 also shows that stabilization of the NiII/I as compared to the

Ni0, NiI, and NiII in Phosphino Thioether Complexes
J. Am. Chem. Soc., Vol. 118, No. 17, 1996 4121
Scheme 3
solubility of 1 in CH3CN leads to more irreversible behavior
as well as small current readings. In CH2Cl2, complex 1 is more
soluble; however, solution degradation is rapid under electrolytic
conditions.
The cyclic voltammogram of 3 is presented in Figure 5b,
and displays a reversible one-electron oxidation at 670 mV.
Because of its good reversibility, the assignment to NiII/III (rather
than the usual oxidation of thiolate S to the thiyl radical)6 is
tentatively made. No reduction is seen within this solvent
window. A similar thiolate nickel complex, (PS)2NiII, shows
an irreversible peak at 400 mV which is presumed to be due to
the oxidation of thiolate S to the thiyl radical.6
Figure 6. EPR spectrum (s) measured at 100 K for the frozen CH3CN
solution of (arom-PSMe)2NiI prepared by Cp2Co reduction of [2](BF4)2
at -46 °C. Overlaid is the EPR spectrum (- - -) measured at 10 K for
the solution containing (arom-PSMe)(arom-PS)NiI resulting from
S-dealkylation of complex 1.
Scheme 4
Electron Spin Resonance Studies. Corroboration of the
assignment of the single-electron-reduced 2r, intermediate from
NiII to Ni0, to a NiI species was provided by EPR spectroscopy.
Three bulk chemical synthetic approaches to the nickel(I)
complex were used: (a) the reduction of [2](BF4)2 with Cp2Co,
(b) the oxidation of 1 with CeIV, and, (c) the comproportionation
of [2](BF4)2 and 1 at low temperature (Scheme 3). All three
methods yielded a yellow paramagnetic solution which showed
the same EPR spectral characteristics. Shown in Figure 6, for
example, is the EPR spectrum obtained upon adding 1.1 equiv
of Cp2Co to a red solution of [2](BF4)2 in CH3CN at -46 °C
and further cooling to 100 K for the analysis.
The spectrum exhibits an axial signal, from which g values
gx ) gy ) 2.10 and gz ) 1.96 are calculated. Each peak is
split into three lines of intensity ratio 1:2:1, indicating coupling
of the unpaired electron with two equivalent 31P nuclei. The
peak to peak widths of the 31P hyperfine coupling on the gx )
gy and gz lines are 42 and 45 G, respectively. The concentration
of this nickel(I) complex was calculated and compared to the
double integral of the EPR spectrum of 1.0 mM CuII in 10 mM
EDTA and shows 61% conversion of [2](BF4)2 to (arom-
PSMe)2NiI. The (arom-PSMe)2NiI complex is stable at low
temperatures (-46 °C or lower) but decomposes on warming
to 22 °C.
A similar EPR spectrum of a nickel(I) complex produced by
Cp2Co reduction of [Ni(pspyr)2]+ (where pspyr ) 4-(diphen-
ylphosphino)-1-(2-pyridyl)-2-thiabutane) at -55 °C was reported
by Holm et al.7 This ligand achieves four-coordinate nickel(I)
diphosphino dithioether from six-coordinate nickel(II) on reduc-
tion and deligation of the axial nitrogens. The 31P hyperfine
coupling is on the order of 40-60 G. In contrast, the EPR
spectrum of a NiI(PMe3)4+ complex, known from X-ray crystal
structure analysis to have a tetrahedral geometry with average
P-Ni-P angles of 109°,33 shows an isotropic, broad resonance
with g ) 2.12 G, and no 31P hyperfine coupling. This suggests
that 31P hyperfine coupling with an unpaired electron of NiI is
highly geometry dependent, a conclusion to be further explored
below. Since the cyclic voltammetry studies, Vide infra,
indicated some structural distortions from the square planar
[(arom-PSMe)2Ni]2+ as it underwent one-electron reduction, we
conclude that the observed 31P coupling is indicative of a
geometry intermediate between square planar and tetrahedral
for the NiI analogue. We do not know the acuteness of the
dependence of tetrahedral twist, Pdπ-Nidπ overlap, and 31P
coupling.
S-Dealkylation of (Ph2P(o-C6H4)SCH3)2Ni0. The nickel(II)
complex [2](BF4)2 in CH3CN solution was stable to photolysis.34
However, red solutions of complex 1, prepared by either
chemical reduction of [2](BF4)2 in benzene (method A) or
deprotonation of hydride complex 4 with Et3N in CH3CN
(method C), are light sensitive. Under ambient lighting, the
red solution turns green within a day, producing dithiolate
complex 3; under photolytic conditions, 15 min is sufficient.
1
The H NMR spectrum of a sample of complex 1, irradiated
for 15 min in C6D6, exhibited resonances for (arom-PS)2NiII,
3, as well as a singlet at 0.85 ppm, consistent with the presence
of ethane and insinuating the generation of methyl radicals
during the decomposition as shown in Scheme 4. No methane
was observed either in the NMR spectrum or by gas chroma-
tography on the head gas.
Further evidence of C-S bond cleavage and the production
of methyl radicals was provided by photolysis of complex 1 in
the presence of the alkyl radical trapping agent TEMPO.35 As
indicated in Scheme 4, complex 3 and 2,2,6,6-tetramethyl-1-
(33) Gleizes, A.; Dartiguenave, M.; Dartiguenave, Y.; Galy, J.; Klein,
H. F.; J. Am. Chem. Soc. 1977, 99, 5187.
(34) No distinctive chemical change was observed during the photolysis
of [2](BF4)2 in CH3CN for 1.5 h.

4122 J. Am. Chem. Soc., Vol. 118, No. 17, 1996
Kim et al.
Scheme 5
Scheme 6
piperidinylmethoxide (TEMPO-Me) were produced.36,37 The
control experiment, i.e., a solution of complex 1 (1.0 × 10-2
M) and TEMPO (2.1 × 10-2 M) in the dark, showed no reaction
under ambient conditions. In the course of the photochemical
reaction of complex 1 with TEMPO, dimerization of methyl
radicals or abstraction of hydrogen by the methyl radical has
not been observed, which is consistent with relatively large rate
constants for nitroxyl radical couplings with primary carbon
radicals (k20 ) 1.2 × 109 M-1 s-1).35a,38
As depicted in Scheme 5, successive homolytic cleavage of
the H3C-Sthioether bonds suggests the possibility of an intermedi-
ate thioether/thiolate nickel(I) species. EPR spectroscopy was
employed as a probe according to the following procedure: A
sample of the red solution of 1, produced by the deprotonation
of complex 4 with Et3N in CH3CN, was exposed to laboratory
lighting for 30 min, and then cooled to 100 K, and further to
10 K. The EPR spectrum was invariant with temperature, and
the same feature was observed repeatedly on additional samples
of light-degraded complex 1. That spectrum, shown in Figure
6, is isotropic with g ) 2.08, in contrast with that of the
anisotropic [(arom-PSMe)2NiI]+, which is given for comparison.
Also dissimilar to [(arom-PSMe)2NiI]+, no 31P hyperfine
coupling was detected for (presumably) [(arom-PSMe)(arom-
PS)NiI]. The structural implications consistent with these EPR
data are as follows: The neutral thioether/thiolate NiI species
has a ground state geometry closer to tetrahedral, affording
weaker interactions between the two phosphorus atoms and the
unpaired electron on nickel; the cationic dithioether NiI species
has a ground state geometry sufficiently square planar-like to
account for the hyperfine 31P-unpaired electron interaction.
2H NMR spectra show the production of three ethane iso-
topomers, CD3CD3, CH3CD3, and CH3CH3, eq 7. The 2H NMR
spectrum found ethane-d6 and ethane-d3 in a 1:2 ratio; while
the 1H NMR spectrum showed ethane and ethane-d3. The three
different ethanes from the mixture were further confirmed by
the mass spectrum of the head gas. The results are consistent
with an intermolecular combination of methyl radicals.
Concluding Comments
Sulfur dealkylation in reduced nickel and iron complexes of
polydentate thioethers has been previously observed by Kovacs
et al. and Sellman et al.;11,12 thus, the photosensitivity of reduced
The light-sensitive feature of C-S bonds of thioether ligands
upon coordination to nickel(II) has been previously noted by
Schrauzer and co-workers.39 Photolysis of S,S′-diaralkyl
nickel(II) derivatives yields neutral thiolate nickel(II) complexes
and organic products derived from carbon radicals. The
formation of nickel metal-based paramagnetic species in this
reaction was subsequently confirmed by EPR studies.40
(36) 2,2,6,6-Tetramethyl-1-piperidinylmethoxide (TEMPO-Me) was ana-
lyzed by NMR and GC/MS as described in the Experimental Section.
Coinjection of TEMPO and tert-butylperoxide into the hot injection port
(250 °C) of the GC/mass spectrometer provides authentic TEMPO-Me for
comparison.37
Organic Products Resulting from Photolytic S-Dealkyla-
tion. The observation of ethane during the photolysis of 1
prompts the question of C-C bond formation via an inter- or
intramolecular mechanism. To this end, the arom-PSMe ligand
was substituted with CD3 and a deuterated nickel(0) derivative,
(arom-PSCD3)2Ni0 (1-d6), was prepared. Formation of CD3CD3
in the course of irradiation of 1-d6 in benzene provides an
assignment of the chemical shift at 0.69 ppm to CD3CD3 in the
2H NMR spectrum. A 1:1 mixture of 1 and 1-d6 was photolyzed
in benzene-d6 or benzene, and the products were analyzed by
1H and 2H NMR and mass spectroscopy. Together the 1H and
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Ni0, NiI, and NiII in Phosphino Thioether Complexes
J. Am. Chem. Soc., Vol. 118, No. 17, 1996 4123
nickel toward S-C bond cleavage exhibited by complex 1 is
not without precedent. An interesting feature of this P2S2 system
however lies in the possibility of interpreting the EPR spectrum
of the intermediate NiI species in terms of geometry. Clearly
d10 Ni0 complexes are expected to be tetrahedral regardless of
the donor ligand; d8 NiII complexes in P2S2 donor environments
are square planar, for S ) a thiolate or thioether donor. Thus,
the geometrical preference of d9 NiI is not clear and, we concur
with the statement of Holm et al., “the structural systematics
of NiI are not extensively developed”.7 The contribution in this
regard from our P2S2Ni studies is summarized in Scheme 6.
The EPR evidence suggests that loss of a single Me• and creation
of a neutral, d9 NiI in diphosphino thioether thiolate ligation
substantially retains the tetrahedral geometry of the parent Ni0
complex 1. In contrast the cationic NiI in diphosphino dithio-
ether coordination, although distorted, is sufficiently square
planar for an anisotropic EPR spectrum and 31P hyperfine
interactions. We can only conclude that delocalization of the
odd electron is favored for the dithioether ligand environment
in the cationic species and dictates a distorted square planar
geometry, whereas electron/thiolate ligand repulsion governs
the geometrical preference of the thioether/thiolate ligands,
species is also tetrahedral,33 the balance of factors is indeed
precarious.
Acknowledgment. Financial support from the National
Science Foundation (Grants CHE 91-09579 and 94-15901) is
gratefully acknowledged, as are contributions from the R. A.
Welch Foundation. Funding for the X-ray diffractometer and
crystallographic computing system were also provided by the
National Science Foundation (Grant CHE 85-13273). Ap-
preciation is expressed to Dr. Lloyd Sumner for invaluable
assistance with the mass spectrometer and to a reviewer for an
enlightening comment.
Supporting Information Available: Figures showing al-
ternate views and packing diagram of complexes 1, 2(BF4)2,
and 3 and tables summarizing crystallographic data and refine-
ment, atomic coordination, bond lengths, bond angles, aniso-
tropic displacement parameters, and H atom coordinates (27
pages). This material is contained in many libraries on
microfiche, immediately follows this article in microfilm version
of the journal, can be ordered from the ACS, and can be
downloaded from the Internet; see any current masthead page
for ordering information and Internet access instructions.
+
encouraging tetrahedral character. Since the NiI(PMe3)4
JA953686H