Mononuclear nickel(III) and nickel(II) thiolate complexes with intramolecular S-H proton interacting with both sulfur and nickel: Relevance to the [NiFe]/[NiFeSe] hydrogenases

Article abstract of DOI:10.1021/ja048568l

Mononuclear, distorted square planar [Ni^II(ER)(P(o-C ₆H₄S)₂(o-C₆H₄SH))] ^- (ER = SePh (1), 2-S-C₄H₃S (2)) with a S-H proton directly interacting with both nickel and sulfur atoms were prepared by reaction of [Ni(CO)(SePh)₃]^-/[Ni(CO)(2-S-C ₄H₃S)₃]^- and P(o-C₆H ₄SH)₃, individually. The presence of combinations of intramolecular [Ni-S...H-SR]/[Ni...H-SR] interactions was verified in the solid state by the observation of an IR ν_SH stretching band (2273 and 2283 cm^-1 (KBr) for complexes 1 and 2, individually) and ¹H NMR spectra (δ 8.079 (d) (CD₂Cl₂) and 8.39 (d) (C₄D₈O) ppm (-SH) for complexes 1 and 2, respectively) and subsequently confirmed by X-ray diffraction study. The exo-thiol proton (o-C₆H₄SH) in complexes 1 and 2 was identified as a D₂O exchangeable proton from NMR and IR studies and was quantitatively removed by Lewis base Et₃N to yield Ni(II) dimer [Ni^II(P(o-C₆H₄S)₃)]₂ ^2- (5). Instead of the ligand-based oxidation to form dinuclear Ni(II) complexes and dichalcogenide, oxidation of THF-CH₃CN solution of complexes 1 and 2 by O₂ resulted in the formation of the mononuclear, distorted trigonal bipyramidal [Ni^III(ER)(P(o-C ₆H₄S)₃)]^- (ER = SePh (3), 2-S-C ₄H₃S (4)) accompanied by byproduct H₂O identified by ¹H NMR, respectively. The 4.2 K EPR spectra of complexes 3 and 4 exhibiting high rhombicities with three principal gvalues of 2.304, 2.091, and 2.0 are consonant with Ni(III) with the odd electron in the d_z2 orbital. Complex 3 undergoes a reversible Ni^III/II process at E_1/2 = -0.67 V vs Ag/AgCl in MeCN.

Full text of DOI:10.1021/ja048568l

Published on Web 06/17/2004
Mononuclear Nickel(III) and Nickel(II) Thiolate Complexes with
Intramolecular S-H Proton Interacting with Both Sulfur and
Nickel: Relevance to the [NiFe]/[NiFeSe] Hydrogenases
Chien-Ming Lee,† Chien-Hong Chen,† Shyue-Chu Ke,*,‡ Gene-Hsiang Lee,§ and
Wen-Feng Liaw*,†
Contribution from the Department of Chemistry, National Tsing Hua UniVersity,
Hsinchu 30043, Taiwan; Department of Physics, National Dong Hwa UniVersity,
Hualien, Taiwan; and Instrumentation Center, National Taiwan UniVersity, Taipei, Taiwan
Received March 11, 2004; E-mail: wfliaw@mx.nthu.edu.tw
Abstract: Mononuclear, distorted square planar [NiII(ER)(P(o-C6H4S)2(o-C6H4SH))]- (ER ) SePh (1), 2-S-
C4H3S (2)) with a S-H proton directly interacting with both nickel and sulfur atoms were prepared by reaction
of [Ni(CO)(SePh)3]-/[Ni(CO)(2-S-C4H3S)3]- and P(o-C6H4SH)3, individually. The presence of combinations
of intramolecular [Ni-S‚‚‚H-SR]/[Ni‚‚‚H-SR] interactions was verified in the solid state by the observation
of an IR νSH stretching band (2273 and 2283 cm-1 (KBr) for complexes 1 and 2, individually) and 1H NMR
spectra (δ 8.079 (d) (CD2Cl2) and 8.39 (d) (C4D8O) ppm (-SH) for complexes 1 and 2, respectively) and
subsequently confirmed by X-ray diffraction study. The exo-thiol proton (o-C6H4SH) in complexes 1 and 2
was identified as a D2O exchangeable proton from NMR and IR studies and was quantitatively removed
by Lewis base Et3N to yield Ni(II) dimer [NiII(P(o-C6H4S)3)]2 (5). Instead of the ligand-based oxidation to
2-
form dinuclear Ni(II) complexes and dichalcogenide, oxidation of THF-CH3CN solution of complexes 1
and 2 by O2 resulted in the formation of the mononuclear, distorted trigonal bipyramidal [NiIII(ER)(P(o-
C6H4S)3)]- (ER ) SePh (3), 2-S-C4H3S (4)) accompanied by byproduct H2O identified by 1H NMR,
respectively. The 4.2 K EPR spectra of complexes 3 and 4 exhibiting high rhombicities with three principal
2
g values of 2.304, 2.091, and 2.0 are consonant with Ni(III) with the odd electron in the dz orbital. Complex
3 undergoes a reversible NiIII/II process at E1/2 ) -0.67 V vs Ag/AgCl in MeCN.
Introduction
of a heterobimetallic (Scys)2Ni(µ-Scys)2(µ-X)Fe(CO)(CN)2 (X )
O, OH) cluster.1-5 The bridging ligand X was proposed to be
Hydrogenases are enzymes that catalyze the reversible
heterolytic dissociation of molecular hydrogen.1-4 [NiFe] hy-
drogenase containing a [Ni-Fe] center that is believed to be
the catalytic site for hydrogen activation and functions more
effectively in the direction of H2 oxidation.1-4 The X-ray
crystallographic studies of the active-site structure of [NiFe]
hydrogenases isolated from D. gigas, D. Vulgaris, D. fructoso-
Vorans, and D. desulfuricans ATCC27774 in combination with
infrared spectroscopy have revealed an active site comprised
an oxide or hydroxide in the oxidized state and was found to
be absent in the reduced state.1-5 Three nonprotein diatomic
groups (two cyanide and one carbonyl ligands) ligate the iron
center. In the conversion between oxidized and reduced states,
the iron remains low-spin Fe(II), while the Ni changes between
Ni(III) and Ni(II). EXAFS/EPR studies indicate that the formal
oxidation state of the Ni center is paramagnetic Ni(III) in Ni-
A, Ni-B, and Ni-C states.5-15 The catalytically active form
(5) Matias, P. M.; Soares, C. M.; Saraiva, L. M.; Coelho, R.; Morais, J.; Le
Gall, J.; Carrondo, M. A. J. Biol. Inorg. Chem. 2001, 6, 63-81.
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† Department of Chemistry, National Tsing Hua University.
‡ Department of Physics, National Dong Hwa University.
§ National Taiwan University.
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8406
J. AM. CHEM. SOC. 2004, 126, 8406-8412
10.1021/ja048568l CCC: $27.50 © 2004 American Chemical Society

Mononuclear Ni(III)/Ni(II) Thiolate Complexes
A R T I C L E S
Scheme 1
Ni-C of [NiFe] hydrogenase was proposed to exist as the
[(Scys-H)(Scys)NiIII-(µ-H)(µ-Scys)2Fe(CO)(CN)2] with a Scys-H
proton interacting with nickel.8-13 Also, the architecture of the
silent-active Ni-SIa is proposed as a Cys-SH proton directly
interacting with nickel, [(Scys-H)(Scys)NiII(µ-Scys)2-Fe(CO)-
(CN)2].11,12
Interestingly, the recent X-ray crystal structures of CO-
inhibited forms and single-crystal EPR studies of the reduced
active site of [NiFe] H2ase isolated from D. Vulgaris Miyazaki
F implicate that the Ni-C intermediate is a formal Ni(III)
oxidation state with a hydride (H-) bridging between the Ni
and the Fe atoms and the sulfur atom of Cys 546 hydrogenated
for the catalytic reaction of the enzyme.14,15 Upon illumination,
the Ni-C state is transformed into a fourth paramagnetic Ni-L
state. These conversions are considered to correspond to
photodissociation of proton species from the [Ni-Fe] center.8-15
In addition, the active form Ni-C has also been shown to
contain a solvent exchangeable proton, assigned as a Cys-SH
proton directly interacting with nickel from ENDOR studies.16
Very recently, a particularly interesting new type of hydrogenase
isolated from Ralstonia eutropha, the so-called regulatory
hydrogenase (RH), was found to belong to the subclass of H2-
sensing [NiFe] hydrogenase. The oxidation state and coordina-
tion ligands of the Ni site of RH that were investigated by X-ray
absorption spectroscopy imply that a five-coordinate [NiII-S3(O/
N)2] for RHox and a six-coordinate [NiIII-S2(O/N)3(H)] for
RH+H2 17,18
.
In model compounds,19 an electrochemical study provided
evidence for such a Ni(III)-H species generated by one-electron
reduction of a nickel(II) macrocyclic complex accompanied by
protonation.20 The similarity of the CO-inhibited nickel active-
site structure of [NiFe]/[NiFeSe] hydrogenases to the recently
synthesized [Ni(CO)(SPh)n(SePh)3-n]- (n ) 0, 1, 2)21 complexes
has inspired the preparation of derivatives modified in such a
way to better mimic the features of the Ni-A/Ni-B, and Ni-
SIa/Ni-R states of the catalytic cycle of [NiFe] hydrogenase.
Our premise is that information on the structures, stabilities,
and reactivity of Ni(II)-exo-thiol and Ni(III)-thiolate complexes
could be useful in interpreting the catalytic cycle of H2 activation
in [NiFe] hydrogenase. Herein, we report the synthesis and
reactivity of [Ni(ER)(P(o-C6H4S)2(o-C6H4SH))]- (ER ) SePh
(1), 2-S-C4H3S (2)) containing an intramolecular S-H proton
directly interacting with both nickel and sulfur atoms by reaction
of [Ni(CO)(ER)3]- and P(o-C6H4SH)3, respectively. In addition,
this report also details our successful efforts toward synthesis
of the mononuclear Ni(III) complexes in a sulfur-rich ligation
sphere, [NiIII(ER)(P(o-C6H4S)3)]- (ER ) SePh (3), 2-S-C4H3S
(4)), isolated upon exposure of complexes 1 and 2 to O2,
respectively.
Results and Discussion
Reaction of the [PPN][Ni(CO)(SePh)3] with tris(2-phenylthi-
ol)phosphine (P(o-C6H4SH)3) in THF at ambient temperature
produced the four-coordinate [PPN][Ni(SePh)(P-(o-C6H4S)2-
(o-C6H4SH))] (1) as a dark red-brown solid isolated in 90%
yield.22 Complex 1 is soluble in THF-CH3CN (3:1 volume
ratio) and displays extremely air-sensitive in solution but is
stable to air for minutes in the solid state. Complex 1 exhibits
a diagnostic 31P/1H spectra with the [P(o-C6H4S)2(o-C6H4SH)]
phosphine resonance at δ 68.98 (s) ppm (vs H3PO4) (CH2Cl2)
and the benzene protons resonances at δ 6.92-6.99 (m), 7.59-
7.62 (m) ppm (CD2Cl2) which are consistent with the Ni(II)
having a low-spin d8 electronic configuration in a square planar
ligand field. The magnetic measurement of complex 1 is in
accord with the diamagnetic forms. The sequences of reaction
given in Scheme 1 reasonably account for the observation. The
reductive elimination of diphenyl diselenide accompanied by
oxidative addition of S-H bond to [Ni0] unit (Scheme 1a)21
and reductive elimination of H2 from the [NiII-H] intermediate
results in the formation of Ni(I)-exo-thiol intermediate A
(Scheme 1b). This highly reactive Ni(I) species A then triggers
the cleavage of the second S-H bond affording the reactive
(14) Ogata, H.; Mizoguchi, Y.; Mizuno, N.; Miki, K.; Adachi, S.-I.; Yasuoka,
N.; Yagi, T.; Yamauchi, O.; Hirota, S.; Higuchi, Y. J. Am. Chem. Soc.
2002, 124, 11628-11635.
(15) Foerster, S.; Stein, M.; Brecht, M.; Ogata, H.; Higuchi, Y.; Lubitz, W. J.
Am. Chem. Soc. 2003, 125, 83-93.
(16) (a) Fan, C.; Teixeira, M.; Moura, J.; Moura, I.; Huynh, B. H.; Le Gall, J.;
Peck, H. D., Jr.; Hoffman, B. M. J. Am. Chem. Soc. 1991, 113, 20-24. (b)
Whitehead, J. P.; Gurbiel, R. J.; Bagyinka, C.; Hoffman, B. M.; Maroney,
M. J. J. Am. Chem. Soc. 1993, 115, 5629-5635.
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12479. (b) Kleihues, L.; Lenz, O.; Bernhard, M.; Buhrke, T. Friedrich, B.
J. Bacteriol. 2000, 182, 2716-2724.
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W.; Friedrich, B.; Dau, H. Biochemistry 2003, 42, 11004-11015. (b) Brecht,
M.; van Gastel, M.; Buhrke, T.; Friedrich, B.; Lubitz, W. J. Am. Chem.
Soc. 2003, 125, 13075-13083.
(19) Darensbourg, M. Y.; Lyon, E. J.; Smee, J. J. Coord. Chem. ReV. 2000,
206, 533-561.
(20) Efros, L. L.; Thorp, H. H.; Brudvig, G. W.; Crabtree, R. H. Inorg. Chem.
1992, 31, 1722-1724.
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S.-M. J. Am. Chem. Soc. 1997, 119, 9299-9300. (b) Liaw, W.-F.; Chen,
C.-H.; Lee, C.-M.; Lee, G.-H.; Peng, S.-M. J. Chem. Soc., Dalton Trans.
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(22) Block, E.; Ofori-Okai, G.; Zubieta, J. J. Am. Chem. Soc. 1989, 111, 2327-
2329.
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A R T I C L E S
Lee et al.
as indicated by IR and 2H NMR. The IR νS-H at 2273 br cm-1
shifting to absorbance at 1676 br cm-1 (νS-D, KBr) is roughly
consistent with the calculated position, based only on the
difference in masses between S-H and S-D. A 2H NMR
resonance that appeared as a broad peak at δ 7.972 (br) (CH2Cl2)
ppm (using natural abundance of D in CH2Cl2 solvent as internal
standard, δ 5.32 ppm) was assigned to the S-D resonance, and
Figure 1. ORTEP drawing and labeling scheme of the [Ni(SePh)(P(o-
C6H4S)2(o-C6H4SH))]- anion. Selected bond distances (Å) and angles
(deg): Ni-S(3) 2.1648(8); Ni-S(1) 2.2127(8); Ni-Se 2.3349(5); S(3)-
Ni-S(1) 162.04(4); P(1)-Ni-Se 174.72(3); S(3)-Ni-Se 93.22(2); S(1)-
Ni-Se 92.21(2).
2
D2O was found at 1.601 (br) ppm in the H NMR spectrum.
Similarly, the reductive elimination/oxidative addition was
also displayed by the reaction of complex [Ni(CO)(2-S-
C4H3S)3]- and P(o-C6H4SH)3.21b When a THF solution of
[Ni(CO)(2-S-C4H3S)3]- was treated with 1 equiv of P(o-C6H4-
SH)3, an immediate change in color of the solution from yellow
brown to dark red-brown was observed. The reaction mixture
led to the isolation of the extremely air-sensitive red-brown
compound [Ni(2-S-C4H3S)(P(o-C6H4S)2(o-C6H4SH))]- (2)
(Scheme 1). In addition to the X-ray analysis, a higher IR νS-H
(2283 cm-1 (KBr)) compared to that of complex 1 (2273 cm-1
(KBr)) also supports the formation of complex 2. The 1H NMR
spectra of complex 2 show the expected signals for the phenyl
and exo-thiol (8.39 (d) (C4D8O) ppm) groups involved and
display characteristics of diamagnetic d8 square-planar Ni(II)
species. In contrast to complex 1, the X-ray crystal structure of
complex 2 shows that the exo-thiol tends to adopt a [Ni‚‚‚H-
S] interaction (Figure 4). We also noticed that the S(3)‚‚‚Ni
distance of 3.609 Å is much shorter than the S(3)‚‚‚S(1) distance
of 3.75 Å in complex 2 (Type II). It seems unlikely that the
thiol proton shuttles between two sulfur atoms or the complete
proton transfer results in a six-coordinate d6 NiIV-H species in
THF solution of complexes 1 and 2 at room temperature. This
evidence shows that electronic modulations of the terminally
coordinated donor ligand are capable of inducing significantly
alternative interaction modes, [Ni-S‚‚‚H-S], [Ni‚‚‚H-S], or
some mixture thereof. These interactions [Ni-S‚‚‚H-S], [Ni‚
‚‚H-S], or some mixture may contribute to the stabilization of
Ni(II)-exo-thiol complexes 1 and 2.
Ni(III)-H intermediate B (Scheme 1c).23 It is presumed that
Ni(I) intermediate A prefers cleavage of the proposed “agostic
NiI‚‚‚H-S bond” resulting in the formation of RS-NiIII-H
species along with the release of CO as opposed to being
stabilized by “η2-[NiI‚‚‚H-S] interaction” (Scheme 1c).24 The
subsequent reductive elimination of H2 from intermediate B led
to the formation of complex 1 (Scheme 1d).23 These results
establish that the same stoichiometric quantities of (PhSe)2 are
formed concurrently with complex 1.
The infrared KBr solid spectrum of complex 1 shows one
broad stretching band (2273 cm-1) in the νS-H region. The lower
energy νS-H band of complex 1 shifted by ∼215 cm-1 from
that of the free ligand P(o-C6H4SH)3 (2488 cm-1) implies the
specific [Ni-S‚‚‚H-S]/[Ni‚‚‚H-S] interactions. Intramolecular
[Ni-S‚‚‚H-S]/[Ni‚‚‚H-S] interactions in complex 1 also result
1
in the H NMR chemical shift of the SH group, shifting from
δ 4.07 (d) ppm (CDCl3) in free ligand P(o-C6H4SH)3 to δ 8.079
(d) ppm (S-H) (CD2Cl2) in complex 1.22 The acute C(7)-S(2)-
H(2S) bond angle of 97.3(18)° observed in single-crystal X-ray
diffraction studies is believed to result from an intramolecular
[Ni-S‚‚‚H-SR] interaction (Figure 1), which supports the
observation of the lower IR νS-H stretching frequency in
complex 1. We noticed that the S(2)‚‚‚S(3) distance of 3.737
Å in complex 1 is longer than the S(2)‚‚‚Ni distance of 3.639
Å (Type I, below). Specifically, the S(2)-H(2S) is partially
polarized in a sense on the approach of the sulfur atom (S(3)).
It is best interpreted in terms of the thiol proton interacting with
both sulfur and nickel atoms, where the proton is bound to sulfur
and “attracted” between a Ni(II) and the second sulfur atoms,
a combination of [Ni-S‚‚‚H-S]/[Ni‚‚‚H-S] interactions (Type
I, below). In particular, a three-center “agostic” η2-[Fe-H-S]
interaction has been proposed for the protonation of [Fe(SR)-
(CO)3L]- (L ) PEt3, P(OEt)3),24 and proton transfer between
sulfur and hydride has been observed in [Ir(H)2(HS(CH2)3SH)-
(Pcy3)2]+.25
Upon addition of dry O2, a pronounced color change from
red-brown to dark green occurs in the CH3CN-THF solution
(1:3 volume ratio) of complexes 1 and 2 at 5 °C, respectively.
1
The UV-vis, EPR, H NMR, SQUID, and X-ray diffraction
studies confirmed the formation of the mononuclear [NiIII(ER)-
(P(o-C6H4S)3)]- (ER ) SePh (3), 2-S-C4H3S (4)) accompanied
by byproduct H2O identified by 1H NMR (Scheme 2). To
unambiguously corroborate the formation of byproduct H2O,
we treated the CH2Cl2 solution of [Ni(SePh)(P(o-C6H4S)2(o-
C6H4SD))] with dry O2 at ambient temperature. UV-vis spectra
are consistent with the formation of complex 3 (also identified
by X-ray diffraction) along with the byproduct D2O found at
1.601 (br) ppm (CH2Cl2) in the 2H NMR spectra (using natural
abundance of D in CH2Cl2 solvent as internal standard, δ 5.32
ppm). The 4.2 K EPR spectra of complexes 3 and 4 are identical
and exhibit high rhombicities with three principal g values of
2.304, 2.091, and 2.0. There is no 31P hyperfine coupling
The H/D exchange reaction between complex 1 and D2O
occurred in CH3CN-THF solution (1:3 volume ratio) at 5 °C
(23) (a) Allan, C. B.; Davidson, G.; Choudhury, S. B.; Gu, Z.; Bose, K.; Day,
R. O.; Maroney, M. J. Inorg. Chem. 1998, 37, 4166-4167. (b) James, T.
L.; Cai, L.; Mutterties, M. C.; Holm, R. H. Inorg. Chem. 1996, 35, 4148-
4161. (c) Cha, M.; Shoner, S. C.; Kovacs, J. A. Inorg. Chem. 1993, 32,
1860-1863.
(24) Darensbourg, M. Y.; Liaw, W.-F.; Riordan, C. G. J. Am. Chem. Soc. 1989,
111, 8051-8052.
(25) Jessop, P. G.; Morris, R. H. Inorg. Chem. 1993, 32, 2236-2237.
9
8408 J. AM. CHEM. SOC. VOL. 126, NO. 27, 2004

Mononuclear Ni(III)/Ni(II) Thiolate Complexes
A R T I C L E S
Figure 2. ORTEP drawing and labeling scheme of the [Ni(SePh)(P(o-
C6H4S)3)]- anion. Selected bond distances (Å) and angles (deg): Ni-P(1)
2.1371(13); Ni-S(1) 2.1944(14); Ni-S(3) 2.2626(14); Ni-S(2) 2.2905(14);
Ni-Se 2.3714(8); P(1)-Ni-S(1) 87.41(5); S(1)-Ni-S(3) 129.16(6); S(1)-
Ni-S(2) 128.43(6); S(3)-Ni-S(2) 101.88(6); P(1)-Ni-Se 176.60(5);
S(1)-Ni-Se 90.51(4).
Scheme 2
Figure 3. EPR spectra of complexes 3 (a) and 4 (b) frozen in CH2Cl2; g
values are indicated. Conditions: microwave frequency 9.516 22 GHz,
microwave power 2 mW, field modulation 100 kHz, modulation amplitude
4mT, temperature 4.2 K.
based reduction with the NiIII/II redox potential E1/2 ) -0.67 V
vs Ag/AgCl (∆Ep ) 0.07 V, scan rate ) 0.1 V/s) in MeCN,
which is comparable to the NiIII/II redox potentials, ca. -390 to
-640 mV vs SCE of [NiFe] hydrogenases.1 In addition to the
X-ray structural analysis, the red-shift absorption bands (595,
954 nm) for complex 4 in the UV-vis spectrum also support
the formation of complex 4.
Results show that complex 2 is much more O2-sensitive than
the corresponding complex 1.21 A mechanism proposed to
explain the observed formation of complexes 3 and 4 is shown
in Scheme 2. Upon contact with dry O2, the O2 oxidant has
available only the ability of converting the exo-thiol to a thiyl
radical (intermediate C, Scheme 2) accompanied by byproduct
H2O (Scheme 2a).27a,b The rapidly intramolecular addition of a
generated thiyl radical to the adjacent sulfur atom of the thiolate
ligand yields a Ni(II)-disulfide radical intermediate D (Scheme
2b) which rapidly transfers an electron to Ni(II) to produce the
Ni(I)-disulfide intermediate E (Scheme 2c).27c This highly
reactive 19-electron Ni(I) species E is then driven by the
formation of complexes 3 and 4 via an intramolecular oxidative
addition of disulfide. Many attempts, based on the EPR monitor
of the reaction in THF-CH3CN (3:1 volume ratio) at low
temperature, that were made to detect the intermediates C/D/E
were unsuccessful. This proposed mechanism is analogous to
the well-known “induced internal electron-transfer reactions”27a,b
and was successfully employed synthetically by Jaun and co-
workers in which cyclization of the photochemically generated
thiyl radical yields a sulfuranyl radical accompanied by transfer
of a methyl group to Ni(I) to produce a methyl-Ni(II)
intermediate.28
observed under various conditions even in ENDOR detection.
Spin quantity of the EPR signal accounts for almost 100% of
complex 3. The average g value (gav ) 2.132) that is much
higher than the spin-only g value 2.0023 indicates large orbital
contribution to the paramagnetism, suggesting that the unpaired
electron is associated primarily with the nickel ion (Figure 3).
These results are consistent with the formation of the Ni(III)
oxidation state. The fact that one of the g values is close to the
free electron g value suggests a dz ground state.26 The 1H NMR
2
spectra of complexes 3 and 4 at 298 K show the expected
paramagnetic properties. The effective magnetic moment in solid
state by SQUID magnetometer was 1.85 and 1.91 µB for
complexes 3 and 4, respectively, which is consistent with the
NiIII having a low-spin d7 electronic configuration in a distorted
trigonal bipyramidal ligand field. The most noteworthy char-
acteristic of complex 3 is the existence of a reversible, metal-
The stable mononuclear Ni(III)-thiolate 3 and 4 are soluble
in CH2Cl2 and THF-CH3CN and form thermally sensitive, air-
(27) (a) Stein, C. A.; Taube, H. Inorg. Chem. 1979, 18, 2212-2216. (b)
Branscombe, N. D. J.; Atkins, A. J.; Marin-Becerra, A.; McInnes, E. J. L.;
Mabbs, F. E.; McMaster, J.; Schro¨der, M. Chem. Commun. 2003, 1098-
1099. (c) Clark, K. A.; George, A.; Brett, T. J.; Ross, C. R.; Shoemaker,
R. K. Inorg. Chem. 2000, 39, 2252-2253.
(28) Signor, L.; Knuppe, C.; Hug, R.; Schweizer, B.; Pfaltz, A.; Jaun, B. Chem.s
Eur. J. 2000, 6, 3508-3516.
(26) Grove, D. M.; van Koten, F.; Zoet, R. J. Am. Chem. Soc. 1983, 105, 1379-
1380.
9
J. AM. CHEM. SOC. VOL. 126, NO. 27, 2004 8409

A R T I C L E S
Lee et al.
sensitive solutions in organic solvents. On the basis of this
investigation and related observations, it is presumed that the
polarizable anionic thiolate/selenolate donor ligand and a
relatively rigid tetradentate ligand in complexes 3 and 4 place
Ni(III) in an optimum electronic condition to minimize the
possibility of autoreduction of Ni(III) and prevent formation of
polynuclear nickel thiolate species. In the deprotonation study,
10 equiv of triethylamine were required to quantitatively remove
the H+ from anionic complex 1 and transform it into the known
Ni(II) dimer [NiII(P(o-C6H4S)3)]2 (5) by removing [SePh]-
2-
ligand in THF-CH3CN at 5 °C.29
Figures 1 and 2 display the thermal ellipsoid plot of the
anionic complexes 1 and 3, respectively, and selected bond
distances and angles are given in the figure captions. The
constraint of the [P(o-C6H4S)2(o-C6H4SH)] ligand generates
162.04(4)° S(1)-Ni-S(3) angles, enforcing a slight distortion
from a square planar at the four-coodinate nickel site ([NiS2-
SeP] core) in complex 1. The geometry and coordination
environment around Ni respond rapidly to the oxidation state
change of Ni; the geometry of Ni(II) in complex 1 adopts a
distorted square planar. In comparison, the geometry of Ni(III)
in complex 3 is a distorted trigonal bipyramidal with S(3)-
Ni-S(2), S(1)-Ni-S(3), and S(1)-Ni-S(2) bond angles of
101.88(6)°, 129.16(6)°, and 128.43(6)°, individually. The large
C(8)-P(1)-Ni angle of 117.14(10)° dramatically differs from
the C(6)-P(1)-Ni and C(14)-P(1)-Ni angles of 106.25(11)°
and 109.97(11)° found in complex 1. The significant difference
(∆Ni-S ) 0.0479(8) Å) of Ni-S bond distances between Nis
S(1) (2.2127(8) Å) and Ni-S(3) (2.1648(8) Å) in complex 1
may be attributed to the “intramolecular [S(2)-H(2S)‚‚‚S(3)]
interaction”. The Ni-Se bond length of 2.3349(5) Å in complex
1 is comparable to the Ni(II)-Se bond length of 2.317(2) Å
(average) observed in [Ni(CO)(SePh)3]-.21 Relative to complex
1, the mean Ni-Se and Ni-S distances in complex 3 have
increased by 0.036 and 0.06 Å, respectively. The strain effect
of the chelate ligand in the coordination sphere from complex
1 to 3 (or complex 2 to 4; from tridentate to tetradentate)
overwhelming the contraction effect expected as a consequence
of metal-centered oxidation may explain the observed lengthen-
ing of the average Ni-S/Ni-Se bond lengths of complex 3.
ORTEP diagrams of complexes 2 and 4 are shown in Figures
4 and 5, individually. Selected interatomic distances and angles
are collected in the figure captions. Bond angles (S(1)-Ni-
S(4) 94.43(3)°, S(2)-Ni-S(4) 92.08(3)°, P(1)-Ni-S(1)
89.41(3)°, and P(1)-Ni-S(2) 85.97(3)°) around the Ni(II) of
complex 2 define a distorted square planar coordination sphere.
The mean Ni-S distance of 2.2001(9) Å in complex 2 falls
into the range observed for four-coordinate square planar Ni-
thiolate complexes (2.14-2.20 Å)30 and are within the range
of Ni-Scys bond distances observed in the structure of [NiFe]
hydrogenase (2.2-2.6 Å).8b The geometry around the Ni(III)
center of the mononuclear Ni(III) complex 4 is a distorted
trigonal bipyramidal. The Ni-S distances (2.215(1), 2.265(1),
Figure 4. ORTEP drawing and labeling scheme of the [Ni(2-S-C4H3S)P-
((o-C6H4S)2(o-C6H4SH))]- anion. Selected bond distances (Å) and angles
(deg): Ni-P(1) 2.1080(8); Ni-S(1) 2.1637(9); Ni-S(2) 2.2137(9); Ni-
S(4) 2.2230(9); P(1)-Ni-S(1) 89.41(3); P(1)-Ni-S(2) 85.97(3); S(1)-
Ni-S(2) 160.95(4); P(1)-Ni-S(4) 173.32(4); S(1)-Ni-S(4) 94.43(3);
S(2)-Ni-S(4) 92.08(3).
Figure 5. ORTEP drawing and labeling scheme of the [Ni(2-S-C4H3S)-
(P(o-C6H4S)3)]- anion. Selected bond distances (Å) and angles (deg): Ni-
P(1) 2.1598(11); Ni-S(1) 2.2147(11); Ni-S(4) 2.2650(12); Ni-S(3)
2.2697(13); Ni-S(2) 2.3069(11); P(1)-Ni-S(1) 86.07(4); P(1)-Ni-S(4)
177.02(5); S(1)-Ni-S(4) 90.98(4); S(1)-Ni-S(3) 133.20(5); S(1)-Ni-
S(2) 121.10(5); S(3)-Ni-S(2) 104.76(5).
2.270(1), and 2.307(1) Å) in this five-coordinate complex 4 are
at the upper end of the 2.12-2.28 Å range for oxidized D. gigas
hydrogenases and have increased by 0.06 Å (average) from
complex 2. The most striking feature of these structures is that
complexes 3 and 4 are authentic mononuclear Ni(III) complexes
despite the lack of bulky substituents near the sulfur/selenium
atoms. The five-coordinate complexes 3 and 4 have the feature
of containing four sulfurs (or combinations of three sulfurs and
one selenium) and one phosphorus atom. It is these electron-
rich functionalities that are primarily responsible for the relative
stabilization of the Ni(III) state and the rigidity of the chelating
ligand preventing expansion of the coordination sphere to
accommodate a sixth, bridging sulfur ligand.
Conclusion and Comments
Studies on the mononuclear Ni(II)-exo-thiol and Ni(III)-
thiolate complexes (1-4) have led to the following results
related to the structure, reactivity, and spectroscopic properties
of the nickel site of bimetallic Ni-Fe active site of [NiFe]
hydrogenase.
(1) NiII-exo-thiol complexes 1 and 2 stabilized by an
intramolecular S-H proton directly interacting with both nickel
and sulfur atoms were synthesized and characterized by IR, 1H
(29) Franolic, J. D.; Wang, W. Y.; Millar, M. J. Am. Chem. Soc. 1992, 114,
6587-6588.
(30) (a) Cha, M.; Gatlin, C. L.; Critchlow, S. C.; Kovacs, J. A. Inorg. Chem.
1993, 32, 5868-5877. (b) Kruger, H.-J.; Holm, R. H. J. Am. Chem. Soc.
1990, 112, 2955-2963. (c) Mills, D. K.; Reibenspies, J. H.; Darensbourg,
M. Y. Inorg. Chem. 1990, 29, 4364-4366. (d) Baidya, M.; Olmstead, M.;
Mascharak, P. K. Inorg. Chem. 1991, 30, 929-937. (e) Kumar, M.; Colpas,
G. J.; Day, R. O.; Maroney, M. J. Am. Chem. Soc. 1989, 111, 8323-8325.
(f) Fox, S.; Wang, Y.; Silver, A.; Millar, M. J. Am. Chem. Soc. 1990, 112,
3218-3220.
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8410 J. AM. CHEM. SOC. VOL. 126, NO. 27, 2004

Mononuclear Ni(III)/Ni(II) Thiolate Complexes
A R T I C L E S
NMR, UV-vis, and X-ray diffraction. The exo-thiol proton in
complexes 1 and 2 was identified as a D2O exchangeable proton
from H NMR and IR νS-D studies and was quantitatively
investigation also supports that the [NiFe] hydrogenases are
reversibly inactivated upon exposure to O2,32,33 as observed in
complexes 1 and 2 immediately converting to the Ni(III) state
(complexes 3 and 4) upon exposure to O2.
2
removed by Lewis base Et3N (10 equiv) to yield the known
Ni(II) dimmer, complex 5.
Experimental Section
(2) The distinct electron-donating ability of the terminally
coordinated ligands [SePh]- and [S-C4H3S]- in complexes 1
and 2, respectively, may serve to regulate the intramolecular
[Ni-S‚‚‚H-S]/[Ni‚‚‚H-S] interactions or some mixture thereof.
(3) The mononuclear Ni(III) complexes 3 and 4 in a
coordination environment containing largely chalcogenolate
ligands were obtained upon addition of dry O2 to 1 and 2 and
Manipulations, reactions, and transfers were conducted under
nitrogen according to Schlenk techniques or in a glovebox (argon gas).
Solvents were distilled under nitrogen from appropriate drying agents
(diethyl ether from CaH2; acetonitrile from CaH2-P2O5; methylene
chloride from CaH2; hexane and tetrahydrofuran (THF) from sodium
benzophenone) and stored in dried, N2-filled flasks over 4 Å molecular
sieves. Nitrogen was purged through these solvents before use. Solvent
was transferred to the reaction vessel via stainless cannula under a
positive pressure of N2. The reagents bis(triphenylphosphoranylidene)-
ammonium chloride ([PPN][Cl]) (Fluka), iron pentacarbonyl, diphenyl
diselenide, cyclopentadienylnickel(I) carbonyl dimer, di(2-thienyl)
disulfide, deuterium oxide, 99.9 atom % D (Aldrich), and triethylamine
(TCI) were used as received. Compounds tris(2-thiophenyl)phosphine
(P-(o-C6H4SH)3)22 and [PPN][Ni(CO)(SePh)3]/[PPN][Ni(CO)(2-S-
C4H3S)3] were synthesized according to published procedures.21 Infrared
spectra of the ν(CO) and ν(SH) stretching frequencies were recorded
on a PerkinElmer model spectrum one B spectrophotometer with sealed
solution cells (0.1 mm, KBr windows) or KBr solid. UV-vis spectra
were recorded on a GBC Cintra 10e. H and H NMR spectra were
obtained on a Varian Unity-500 spectrometer. Electrochemical mea-
surements were performed with CHI model 421 potentionstat (CH
Instrument) instrumentation. Cyclic voltammograms were obtained from
2.5 mM analyte concentration in MeCN using 0.1 M [n-Bu4N][PF6] as
a supporting electrolyte. Potentials were measured at 298 K vs a Ag/
AgCl reference electrode by using a glassy carbon working electrode.
Under the conditions employed, the potential (V) of the ferrocinium/
ferrocene couple was 0.39 (MeCN). Analyses of carbon, hydrogen, and
nitrogen were obtained with a CHN analyzer (Heraeus).
1
characterized by EPR, UV-vis, H NMR, SQUID, and X-ray
diffraction. Factors contributing to the stability of the distorted
trigonal bipyramidal complexes 3 and 4 are attributed to a
relatively rigid multidentate ligand and a polarizable anionic
chalcogenolate donor ligand sufficient to place Ni(III) in an
electron-rich (optimum electronic) condition but insufficient to
cause autoreduction of the Ni(III) state and polymerization.
(4) The 4.2 K EPR signals of complexes 3 and 4 with three
principal g values of 2.304, 2.091, and 2.0 are compatible with
the g values near 2.31, 2.24, and 2.01 (for Ni-A) and 2.33,
2.16, and 2.01 (for Ni-B) ascribed to Ni(III) observed in the
oxidized Ni-A as well as Ni-B states of [NiFe] hydrogenases,
1
2
1
12b,31
2
a formal Ni(III) in a 3(dz ) electronic configuration.
The
redox potential E1/2 ) -0.67 V (vs Ag/AgCl) in MeCN is
comparable to the NiIII/II redox potentials, ca. -390 to -640
mV (vs SCE) of [NiFe] hydrogenases.1
(5) Both complexes 1 and 2 are extremely O2-sensitive, and
replacemeant of selenolate with thiolate ligand in complex 1
has a significant effect on its sensitivity toward O2. Compared
to complex 1, the less electron-donating thiolate ligand ([2-S-
C4H3S]-) coordinated to Ni(II) in complex 2 accelerated the
oxidation of complex 2 to yield complex 4 which may implicate
that oxidation of complexes 1/2 by O2 occurred, initially, at
the exo-thiol instead of the Ni(II) center.
Preparation of Complex [PPN][Ni(SePh)P((o-C6H4S)2(o-C6H4SH))]
(1). A CH3CN solution (15 mL) of [PPN][Ni(CO)(SePh)3] (0.547 g,
0.5 mmol) was added to a THF solution (10 mL) of P(o-C6H4SH)3
(0.180 g, 0.5 mmol) by cannula under positive N2 at 0 °C. The reaction
mixture was allowed to warm to room temperature after the reaction
solution was stirred for 30 min at 0 °C. The resulting red-brown solution
was reduced to 10 mL under vacuum, and diethyl ether (25 mL) was
added to precipitate the red-brown solid [PPN][Ni(SePh)(P(o-C6H4S)2-
(o-C6H4SH))] (1) (0.53 g, 90%). Diffusion of diethyl ether into a THF-
MeCN (3:1 volume ratio) solution of complex 1 at -15 °C led to dark
red-brown crystals suitable for X-ray crystallography. IR(KBr): 2273
The studies of structures and reactivities of complexes 1-4
may lend support to the proposal that the catalytic cycle centered
on the nickel site and may be useful for taking into consideration
the uptake cycle describing a potential mode of cleaving the
H2 molecule as well as involvement of the cysteine ligand, the
hydride formation step, and the cysteine/selenocysteine serving
as a role in modulating the catalytic properties of [NiFe]/
[NiFeSe] hydrogenases.5-15 The formation of complexes 1 and
2 possessing a S-H proton directly interacting with both nickel
and sulfur atoms via the proposed intermediate B [NiIII(ER)-
(P(o-C6H4S)2(o-C6H4SH))(H)]- (ER ) SePh, 2-S-C4H3S)
(Scheme 1) implicates that the initial H2 activation may take
place at nickel, and Scys can act either as a proton storage site
or as a participant in the promotion of heterolytic H2 cleavage
in [NiFe] hydrogenase.5-16 Isolation of complexes 1/2 suggests
that Ni-R/Ni-SIa states may exist as [(Scys-H)NiII(Scys)3]
geometry with a specific [Scys-H‚‚‚Ni(Scys)3] interaction,5-16
an interaction thought to stabilize higher valence states of nickel
and serve as a proton storage site, and the mononuclear NiIII-
thiolate complexes 3/4 may reflect the existence of a Ni-A/
Ni-B state as a [(Scys)2NiIII(Scys)2(OH)Fe(CN)2(CO)]-coordi-
nation environment in [NiFe] hydrogenase. Additionally, this
1
(νSH) cm-1. H NMR (CD2Cl2): δ 8.079 (d) (SH), 6.820-7.344 (m),
7.67 (d) (P(o-C6H4S)2(o-C6H4SH), SeC6H5). 31P NMR (CH2Cl2): δ
68.98 ppm (vs H3PO4). Absorption spectrum (CH2Cl2) [λmax, nm (ꢀ,
M-1 cm-1)]: 428 (2960), 510 (1250). Anal. Calcd for C60H48NP3S3-
SeNi: C, 64.94; H, 4.36; N, 1.26. Found: C, 64.82; H, 4.44; N, 1.26.
Preparation of Complex [PPN][Ni(2-S-C4H3S)P((o-C6H4S)2(o-
C6H4SH))] (2). Di(2-thienyl) disulfide (0.23 g, 1 mmol) and [PPN]-
[HFe(CO)4] (0.283 g, 0.4 mmol) were loaded into a 20-mL Schlenk
tube and dissolved in THF (5 mL). After being stirred for 15 min at
ambient temperature, the solution was transferred to the Schlenk tube
containing [NiCp(CO)]2 (0.061 g, 0.2 mmol) by cannula under a positive
pressure of N2. The reaction mixture was stirred for 6 h at ambient
temperature, and then hexane (10 mL) was added to precipitate the
brown oily product [PPN][Ni(CO)(2-S-C4H3S)3].21b The brown oily
product was dried under N2 purge and redissolved in THF. The yellow-
brown solution of [PPN][Ni(CO)(2-S-C4H3S)3] was then transferred
to another Schlenk flask containing P(o-C6H4SH)3 (0.144 g, 0.4 mmol)
(32) Jones, A. K.; Lamle, S. E.; Pershad, H. R.; Vincent, K. A.; Albracht, S. P.
J.; Armstrong, F. A. J. Am. Chem. Soc. 2003, 125, 8505-8514.
(33) Roberts, L. M.; Lindahl, P. A. J. Am. Chem. Soc. 1995, 117, 2565-2572.
(31) Pierik, A. J.; Schmelz, M.; Lenz, O.; Friedrich, B.; Albracht, S. P. FEBS
Lett. 1998, 438, 231-235.
9
J. AM. CHEM. SOC. VOL. 126, NO. 27, 2004 8411

A R T I C L E S
Lee et al.
by cannula under positive N2 at 0 °C. After the reaction mixture was
stirred for 10 min at 0 °C under N2, degassed hexane (25 mL) was
added slowly to precipitate the red-brown solid [PPN][Ni(2-S-C4H3S)-
(P(o-C6H4S)2(o-C6H4SH))] (2) (0.201 g, 47%). Diffusion of diethyl ether
into a CH3CN solution of complex 2 at -15 °C for four weeks led to
dark red-brown crystals suitable for X-ray crystallography. IR(KBr):
2283 (νSH) cm-1. 1H NMR (C4D8O): δ 8.39 (d) ppm (S-H). Absorption
spectrum (CH2Cl2) [λmax, nm (ꢀ, M-1 cm-1)]: 440(4320). Anal. Calcd
for C58H46NP3S5Ni: C, 65.17; H, 4.34; N, 1.31. Found: C, 65.04; H,
4.46; N, 1.01.
spectrum (CH2Cl2) [λmax, nm (ꢀ, M-1 cm-1)]: 384 (10617), 430 (8283),
595 (3100), 790 (911), 954 (1061). Anal. Calcd for C58H45NP3S5Ni:
C, 65.23; H, 4.25; N, 1.31. Found: C, 64.95; H, 4.65; N, 1.02.
EPR Measurements. EPR measurements were performed at X-band
using a Bruker EMX spectrometer equipped with a Bruker TE102 cavity
and a Bruker VT2000 temperature control unit (120-300 K). For liquid
helium temperature measurements, an Oxford ESR910 continuous-flow
cryostat (4-200K) was used. The microwave frequency was measured
with a Hewlett-Packard 5246L electronic counter. X-band EPR spectra
of complexes 3 and 4 frozen in CH2Cl2 were obtained with a microwave
power of 2 mW, frequency at 9.516 22 GHz, and modulation amplitude
of 0.4 mT at 100 kHz.
Magnetic Measurements. The magnetization data were recorded
on a SQUID magnetometer (MPMS5 Quantum Design company) with
an external 0.5 T magnetic field for complex 3 (1 T for complex 4) in
the temperature range 2 to 300 K. The magnetic susceptibility of the
experimental data was corrected for diamagnetism by the tabulated
Pascal’s constants.
Crystallography. Crystallographic data and structure refinement
parameters of complexes 1, 2, 3, and 4 are summarized in the
Supporting Information. The crystals of 1, 2, 3, and 4 chosen for X-ray
diffraction studies measured 0.27 × 0.23 × 0.06 mm3, 0.25 × 0.25 ×
0.10 mm3, 0.25 × 0.12 × 0.06 mm3, and 0.40 × 0.30 × 0.08 mm3,
respectively. Each crystal was mounted on a glass fiber and quickly
coated in epoxy resin. Unit-cell parameters were obtained by least-
squares refinement. Diffraction measurements for complexes 1, 2, 3,
and 4 were carried out on an SMART CCD (Nonius Kappa CCD)
diffractometer with graphite-monochromated Mo KR radiation (λ )
0.7107 Å) and between 1.25° and 27.50° for complex 1, between 1.25°
and 27.50° for complex 2, between 1.45° and 27.50° for complex 3,
and between 1.46° and 27.50° for complex 4. Least-squares refinement
of the positional and anisotropic thermal parameters of all non-hydrogen
atoms and fixed hydrogen atoms (except H(2S) and H(3A) in complex
1 and 2, respectively) was based on F2. An SADABS35 absorption
correction was made. The SHELXTL36 structure refinement program
was employed.
D/H Exchange for Reaction of Complex 1/2 and D2O. To a THF-
MeCN (3:1, 8 mL) solution of complex 1 (0.11 g, 0.1 mmol) at 5 °C,
a 100-fold excess of D2O (0.18 mL, 10 mmol) was added. The reaction
solution containing the mixture solution of complex 1 and D2O was
stirred for 10 min at 5 °C. Diethyl ether (3 mL) was then added slowly
to layer above the mixture solution. The flask was tightly sealed and
kept in the refrigerator at -15 °C for 1 week. Red-brown crystals [PPN]-
[Ni(ER)(P(o-C6H4S)2(o-C6H4SD))] (ER ) SePh (1a), 2-S-C4H3S (2a))
were isolated (0.09 g, 89% (1a)). IR (KBr): 1676 (νSD) cm-1 (1a),
2
1682 (νSD) cm-1 (2a). H NMR (CH2Cl2): δ 7.972 (br) ppm (1a) vs
CHDCl2 (natural abundance of D in CH2Cl2 solvent, δ 5.32 ppm).34
Reaction of Et3N and Complex 1. To a THF-MeCN (3:1 ratio, 8
mL) solution of complex 1 (0.11 g, 0.1 mmol) at 5 °C, a 30-fold excess
of Et3N (0.03 mL, 3 mmol) was added. The mixture solution was stirred
for 10 min at 5 °C. Diethyl ether (2 mL) was added slowly to layer
above the mixture solution. The flask was tightly sealed and kept in
the refrigerator at -15 °C for 1 week. The well-known dark red crystals
[PPN]2[Ni2(P(o-C6H4S)3)2] (5) were isolated (0.1 g, 90%) and character-
ized by single-crystal X-ray diffraction.29
Preparation of Complex [PPN][Ni(SePh)(P(o-C6H4S)3)] (3). Pure
oxygen gas (3 mL) was purged through a red-brown THF-MeCN (3:1
volume ratio, 20 mL) solution of complex 1 (0.11 g, 0.1 mmol) at 0
°C. The reaction solution was allowed to warm to 15 °C slowly and
stirred for an additional 10 min. The resulting deep-green solution was
reduced in volume by N2 purge, and diethyl ether was then added to
precipitate the dark-green solid [PPN][Ni(SePh)(P(o-C6H4S)3)] (3) (0.08
g, 70%). Diffusion of diethyl ether into a THF-MeCN (3:1 volume
ratio) solution of complex 3 at -15 °C yielded dark-green crystals
Acknowledgment. We gratefully acknowledge financial
support from the National Science Council (Taiwan). Authors
thank Prof. Hua-Fen Hsu (National Cheng Kung University)
1
suitable for X-ray crystallography. H NMR (CD3CN): δ -3.70 (br),
5.48 (br), 6.73 (br), 7.75 (br), 10.20 (br), 10.27 (br), 12.21 (br) ppm
(SePh, o-C6H4S). Absorption spectrum (CH3CN) [λmax, nm (ꢀ, M-1
cm-1)]: 594(790), 941(340). Anal. Calcd for C60H48NP3S3SeNi: C,
64.99; H, 4.27; N, 1.26. Found: C, 64.90; H, 4.25; N, 1.07.
2
for helpful discussions and Miss Hui-Chi Tan for H NMR
measurements.
Supporting Information Available: Selected bond distances
and angles of complexes 1-4 (Table S1), crystallographic data
of complexes 1-4 (Tables S2 and S3), and X-ray crystal-
lographic file in CIF format for the structure determinations of
[PPN][Ni(SePh)(P(o-C6H4S)2(o-C6H4SH))], [PPN][Ni(2-S-
C4H3S)(P(o-C6H4S)3), [PPN][Ni(SePh)(P(o-C6H4S)3)], and
[PPN][Ni(2-S-C4H3S)P((o-C6H4S)2(o-C6H4SH))]. This material
is available free of charge via the Internet at http://pubs.acs.org.
Preparation of Complex [PPN][Ni(2-S-C4H3S)(P(o-C6H4S)3)] (4).
Dry oxygen gas (9 mL) was purged through a red-brown THF solution
(10 mL) of complex 2 (0.428 g, 0.4 mmol) at 0 °C. The reaction solution
was stirred at 0 °C for an additional 15 min, and a significant change
in color of the reaction solution from red brown to deep green was
observed. Hexane (25 mL) was then added to precipitate the dark-
green solid [PPN][Ni(2-S-C4H3S)(P(o-C6H4S)3)] (4) (0.30 g, 71%).
Diffusion of hexane into a THF solution of complex 4 at -15 °C for
3 weeks led to dark-green crystals suitable for X-ray crystallography.
1H NMR (CD2Cl2): δ -3.99 (br), 5.48 (br), 8.42 (br), 9.70 (br), 11.82
(br), 12.62 (br), 13.67 (br) ppm (S-C4H3S, o-C6H4S). Absorption
JA048568L
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