Mononuclear nickel(III) complexes [NiIII(OR)(P(C 6H3-3-SiMe3-2-S)3)]- (R = Me, Ph) containing the terminal alkoxide ligand: Relevance to the nickel site of oxidized-form [NiFe] hydrogenases

Article abstract of DOI:10.1021/ic801069t

The unprecedented nickel(III) thiolate [Ni^III(OR)(P(C ₆H₃-3-SiMe₃-2-S)₃)^- [R = Ph (1), Me (3)] containing the terminal Ni^III-OR bond, characterized by UV-vis, electron paramagnetic resonance, cyclic voltammetry, and single-crystal X-ray diffraction, were isolated from the reaction of [Ni ^III(Cl)(P(C₆H₃-3-SiMe₃-2-S) ₃)]^_ with 3 equiv of [Na][OPh] in tetrahydrofuran (THF)-CH₃CN and the reaction of complex 1 with 1 equiv of [Bu ₄N][OMe] in THF-CH₃OH, respectively. Interestingly, the addition of complex 1 into the THF-CH₃OH solution of [Me ₄N][OH] also yielded complex 3. In contrast to the inertness of complex [Ni^III(Cl)(P(C₆H₃-3-SiMe ₃-2-S)₃)]^- toward 1 equiv of [Na][OPh], the addition of 1 equiv of [Na][OMe] into a THF-CH₃CN solution of [Ni^III(Cl)(P(C₆H₃-3-SiMe₃-2-S) ₃)]^- yielded the known [Ni^III(CH ₂CN)(P(C₆H₃-3-SiMe₃-2-S) ₃)]^- (4). At 77 K, complexes 1 and 3 exhibit a rhombic signal with g values of 2.31, 2.09, and 2.00 and of 2.28, 2.04, and 2.00, respectively, the characteristic g values of the known trigonal-bipyramidal Ni^III [Ni^III(L)(P(C₆H₃-3-SiMe ₃-2-S)₃)]^- (L = SePh, SEt, Cl) complexes. Compared to complexes [Ni^III(EPh)(P(C₆H ₃-3-SiMe₃-2-S)₃)]^- [E = S (2), Se] dominated by one intense absorption band at 592 and 590 nm, respectively, the electronic spectrum of complex 1 coordinated by the less electron-donating phenoxide ligand displays a red shift to 603 nm. In a comparison of the Ni ^III-OMe bond length of 1.885(2) A found in complex 3, the longer Ni^III-OPh bond distance of 1.910(3) A found in complex 1 may be attributed to the absence of σ and π donation from the [OPh]-coordinated ligand to the Ni^III center.

Full text of DOI:10.1021/ic801069t

Inorg. Chem. 2008, 47, 7908-7913
Mononuclear Nickel(III) Complexes [NiIII(OR)(P(C6H3-3-SiMe3-2-S)3)]- (R )
Me, Ph) Containing the Terminal Alkoxide Ligand: Relevance to the
Nickel Site of Oxidized-Form [NiFe] Hydrogenases
Tzung-Wen Chiou and Wen-Feng Liaw*
Department of Chemistry, National Tsing Hua UniVersity, Hsinchu 30043, Taiwan
Received March 31, 2008
The unprecedented nickel(III) thiolate [NiIII(OR)(P(C6H3-3-SiMe3-2-S)3)]- [R ) Ph (1), Me (3)] containing the terminal
NiIII-OR bond, characterized by UV-vis, electron paramagnetic resonance, cyclic voltammetry, and single-crystal
X-ray diffraction, were isolated from the reaction of [NiIII(Cl)(P(C6H3-3-SiMe3-2-S)3)]- with 3 equiv of [Na][OPh] in
tetrahydrofuran (THF)-CH3CN and the reaction of complex 1 with 1 equiv of [Bu4N][OMe] in THF-CH3OH,
respectively. Interestingly, the addition of complex 1 into the THF-CH3OH solution of [Me4N][OH] also yielded
complex 3. In contrast to the inertness of complex [NiIII(Cl)(P(C6H3-3-SiMe3-2-S)3)]- toward 1 equiv of [Na][OPh],
the addition of 1 equiv of [Na][OMe] into a THF-CH3CN solution of [NiIII(Cl)(P(C6H3-3-SiMe3-2-S)3)]- yielded the
known [NiIII(CH2CN)(P(C6H3-3-SiMe3-2-S)3)]- (4). At 77 K, complexes 1 and 3 exhibit a rhombic signal with g
values of 2.31, 2.09, and 2.00 and of 2.28, 2.04, and 2.00, respectively, the characteristic g values of the known
trigonal-bipyramidal NiIII [NiIII(L)(P(C6H3-3-SiMe3-2-S)3)]- (L ) SePh, SEt, Cl) complexes. Compared to complexes
[NiIII(EPh)(P(C6H3-3-SiMe3-2-S)3)]- [E ) S (2), Se] dominated by one intense absorption band at 592 and 590 nm,
respectively, the electronic spectrum of complex 1 coordinated by the less electron-donating phenoxide ligand
displays a red shift to 603 nm. In a comparison of the NiIII-OMe bond length of 1.885(2) Å found in complex 3,
the longer NiIII-OPh bond distance of 1.910(3) Å found in complex 1 may be attributed to the absence of σ and
π donation from the [OPh]-coordinated ligand to the NiIII center.
Introduction
oxide, hydroxide, or hydroperoxide in the oxidized state and
was found to be absent in the reduced state. Extended X-ray
absorption fine structure (EXAFS) studies of Chromatium
Hydrogenase, a metalloenzyme, catalyzes a reversible two-
electron oxidation of H2 in aerobic and anaerobic microor-
ganisms.1–3 Two classes of hydrogenases, [Fe]-only hydro-
genases ([Fe]-only H2ases) and [NiFe] hydrogenases ([NiFe]
H2ases), have been studied widely. The X-ray crystal-
lographic studies of the active-site structure of [NiFe] H2ases
isolated from DesulfoVibrio gigas, DesulfoVibrio Vulgaris,
DesulfoVibrio fructosoVorans, and DesulfoVibrio desulfuri-
cans ATCC27774 in combination with infrared spectroscopy
have revealed an active site comprised of a heterobimetallic
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M.; Fontecilla-Camps, J. C. Nature 1995, 373, 580–587. (b) Garcin,
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(Scys)2Ni(µ-Scys)2(µ-X)Fe(CO)(CN)2 (X ) O2-, HO2 , OH-)
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cluster.2,3 The bridging ligand X was proposed to be an
* To whom correspondence should be addressed. E-mail: wfliaw@
mx.nthu.edu.tw.
(1) (a) Carepo, M.; Tierney, D. L.; Brondino, C. D.; Yang, T. C.;
Pamplona, A.; Telser, J.; Moura, I.; Moura, J. J. G.; Hoffman, B. M.
J. Am. Chem. Soc. 2002, 124, 281–286. (b) Whitehead, J. P.; Gurbiel,
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7908 Inorganic Chemistry, Vol. 47, No. 17, 2008
10.1021/ic801069t CCC: $40.75  2008 American Chemical Society
Published on Web 08/02/2008

Complexes [NiIII(OR)(P(C6H3-3-SiMe3-2-S)3)]- (R ) Me, Ph)
Scheme 1
Vinosum showed that the Ni-O distance of 1.91 Å is
consistent with a hydroxy bridge (Ni-A state).2 The
coordination environment about Ni in the [NiFe] H2ases is
pseudo square pyramidal in the oxidized state. The Ni site
has been proposed to be redox-active and changes between
NiIII and NiII, while the Fe site remains as FeII in all spectrally
defined redox states of the enzyme.2–5 The EXAFS/electron
paramagnetic resonance (EPR) studies indicate that the
formal oxidation state of the Ni center is paramagnetic NiIII
in Ni-A, Ni-B, and Ni-C states.2–5 Also, the ENDOR
experiments showed that the Ni-A form exhibits an 17O
signal from a solvent-derived (H217O) species, and the signal
disappears upon redox cycling to the Ni-C state.2g In
particular, recent X-ray absorption spectroscopy shows that
the Ni site of the regulatory hydrogenase (RH) in the
presence of hydrogen (RH+H ), proposed as the Ni-C state,
2
isolated from Ralstonia eutropha is a six-coordinate
[NiIIIS2(O/N)3(H)].4
Despite a number of well-characterized high-valent di-
nuclear bis(µ-oxo)nickel(III) complexes,5 no mononuclear
nickel(III) thiolate complexes containing the terminal alkox-
ide ligand were reported.6 Recently, we reported the isolation
and characterization of the mononuclear [NiIII(L)(P(C6H3-
3-SiMe3-2-S)3)]- (L ) 2-S-C4H3S, SePh, SEt, Cl).7 The
increased electron density of the Ni center of complexes
[NiIII(L)(P(C6H3-3-SiMe3-2-S)3)]- modulated by the mono-
dentate ligand L and the substituted groups of the phenylthi-
olate rings promotes the stability of the nickel(III) thiolate
complexes. The similarity of the synthesized [NiIII(SR)(P(C6H3-
3-SiMe3-2-S)3)]- complexes to the nickel active-site structure
of [NiFe] H2ases has inspired the preparation of derivatives
to mimic the features of the Ni-A and Ni-B states of the
catalytic cycle of [NiFe] H2ase. Herein, we report the
synthesis of [NiIII(OR)(P(C6H3-3-SiMe3-2-S)3)]- [R ) Ph (1),
Me (3)] containing the terminal NiIII-OPh and NiIII-OMe
bonds, respectively.
(THF)-CH3CN (3:1 volume ratio) at room temperature,
[NiIII(Cl)(P(C6H3-3-SiMe3-2-S)3)]- and 3 equiv of [Na][OPh]
dissolved in THF-CH3CN (3:1 volume ratio) were stirred
at ambient temperature for 12 h to yield the mononuclear
[NiIII(OPh)(P(C6H3-3-SiMe3-2-S)3)]- (1; yield 73%) after
separation of the insoluble NaCl and the excess [Na][OPh]
by filtration (Scheme 1a).7 Complex 1 is air-stable in the
solid state and exhibits a diagnostic 1H NMR spectrum with
phenyl proton resonances well removed from the diamagnetic
region. The proton resonances [δ 15.8 (br), 10.4 (br), 9.4
(br), -5.3 (br) ppm] were assigned to [OPh]- and [P(C6H3-
3-SiMe3-2-S)3]3- ligands. Compared to the rhombic signal
with g values of 2.31, 2.09, and 2.00 (4.2 K) observed in
[NiIII(SePh)(P(o-C6H3-2-S)3)]-,7a complex 1 displays a rhom-
bic signal with g values of 2.31, 2.04, and 1.99
[THF-CH3CN (3:1 volume ratio)] at 77 K and g values of
2.28, 2.09, and 2.02 in a powdered sample EPR spectrum
(Figure 1). The effective magnetic moment was 1.73 µB for
complex 1. These results are consistent with the central NiIII
possessing a d7 electronic configuration in a trigonal-
bipyramidal ligand field.7
Results and Discussion
In contrast to the inertness of [NiIII(Cl)(P(C6H3-3-SiMe3-
2-S)3)]- toward 1 equiv of [Na][OPh] in tetrahydrofuran
(4) (a) Haumann, M.; Porthum, A.; Buhrke, T.; Liebisch, P.; Meyer-
Klaucke, W.; Friedrich, B.; Dau, H. Biochemistry 2003, 42, 11004–
11015. (b) Brecht, M.; Gastel, M. V.; Buhrke, T.; Friedrich, B.; Lubitz,
W. J. Am. Chem. Soc. 2003, 125, 13075–13083.
In contrast, upon the addition of 1 equiv of a [Na][SPh]
to the THF-CH3CN (3:1 volume ratio) solution of [NiIII(Cl)-
(P(C6H3-3-SiMe3-2-S)3)]-, a pronounced color change from
dark green to blue green occurs at ambient temperature. The
UV-vis, EPR, and single-crystal X-ray diffraction studies
confirmed the formation of [NiIII(SPh)(P(C6H3-3-SiMe3-2-
S)3)]- (2; Figure 2). Presumably, the stronger σ- and
π-electron-donating nature of phenylthiolate [SPh]-, com-
pared to phenoxide [OPh]-, rendering the NiIII center of the
[NiIII(P(C6H3-3-SiMe3-2-S)3)]- motif in the more electron-
rich functionality to promote the stability of complex 2 is
responsible for the facile formation of complex 2 when
[NiIII(Cl)(P(C6H3-3-SiMe3-2-S)3)]- was reacted with 1 equiv
of [Na][SPh] in THF-CH3CN. In comparison with complex
(5) (a) Hikichi, S.; Yoshizawa, M.; Sasakura, Y.; Akita, M.; Moro-oka,
Y. J. Am. Chem. Soc. 1998, 120, 10567–10568. (b) Shiren, K.; Ogo,
S.; Fujinami, S.; Hayashi, H.; Suzuki, M.; Uehara, A.; Watanabe, Y.;
Moro-oka, Y. J. Am. Chem. Soc. 2000, 122, 254–262. (c) Mandimut-
sira, B. S.; Yamarik, J. L.; Brunold, T. C.; Gu, W.; Cramer, S. P.;
Riordan, C. G. J. Am. Chem. Soc. 2001, 123, 9194–9195.
(6) (a) Li, Z.; Ohki, Y.; Tatsumi, K. J. Am. Chem. Soc. 2005, 127, 8950–
8951. (b) Zhu, W.; Marr, A. C.; Wang, Q.; Neese, F.; Spencer, D. J. E.;
Blake, A. J.; Cooke, P. A.; Wilson, C.; Schro¨der, M. Proc. Natl. Acad.
Sci. U.S.A. 2005, 102, 18280–18285. (c) Ogo, S.; Kabe, R.; Uehara,
K.; Kure, B.; Nishimura, T.; Menon, S. C.; Harada, R.; Fukuzumi,
S.; Higuchi, Y.; Ohhara, T.; Tamada, T.; Kuroki, R. Science 2007,
316, 585–587.
(7) (a) Lee, C.-M.; Chen, C.-H.; Ke, S.-C.; Lee, G.-H.; Liaw, W.-F. J. Am.
Chem. Soc. 2004, 126, 8406–8412. (b) Chen, C.-H.; Lee, G.-H.; Liaw,
W.-F. Inorg. Chem. 2006, 45, 2307–2316. (c) Lee, C.-M.; Chuang,
Y.-L.; Chiang, C.-Y.; Lee, G.-H.; Liaw, W.-F. Inorg. Chem. 2006,
45, 10895–10904. (d) Lee, C.-M.; Chiou, T.-W.; Chen, H.-H.; Chiang,
C.-Y.; Kuo, T.-S.; Liaw, W.-F. Inorg. Chem. 2007, 46, 8913–8923.
Inorganic Chemistry, Vol. 47, No. 17, 2008 7909

Chiou and Liaw
Figure 3. UV-vis spectrum (in THF) of complex 1.
reactivity of [NiIII(OR)(P(C6H3-3-SiMe3-2-S)3)]- complexes,
complex [NiIII(OMe)(P(C6H3-3-SiMe3-2-S)3)]- (3) containing
the terminal methoxide ligand was synthesized by adopting
complex 1 serving as a precursor. In contrast to the reaction
of [NiIII(Cl)(P(C6H3-3-SiMe3-2-S)3)]- and [Na][OPh] yielding
complex 1 as shown in Scheme 1a, the addition of 1 equiv
of[Na][OMe]intoaTHF-CH3CNsolutionof[NiIII(Cl)(P(C6H3-
3-SiMe3-2-S)3)]- produced the known NiIII complex [NiIII(CH2-
CN)(P(C6H3-3-SiMe3-2-S)3)]- (4) characterized by the UV-vis
spectrum [characteristic absorptions (543 and 648 nm) for
(CH3CN); Scheme 1b]. These results implicate the electron-
deficient [NiPS3] core induced by the elimination of a NaCl
salt from the reaction of [NiIII(Cl)(P(C6H3-3-SiMe3-2-S)3)]-
and [Na][OMe] may promote coordination of CH3CN to the
[NiPS3] core to yield the proposed [NiIII(NCCH3)(P(C6H3-
3-SiMe3-2-S)3)] intermediate. The stronger base [OMe]-,
compared to [OPh]-, may then trigger deprotonation of the
coordinated CH3CN of the proposed [NiIII(NCCH3)(P(C6H3-
3-SiMe3-2-S)3)] intermediate, and the subsequent coordina-
tion of the [CH2CN]- carbanion to NiIII yields the thermally
stable complex 4.7
Figure 1. (a) X-band frozen-solution [THF-CH3CN (3:1 volume ratio)]
EPR spectrum of complex 1 with g values of 2.31, 2.04, and 1.99 recorded
at 77 K and (b) X-band powdered sample EPR spectrum of complex 1
with g values of 2.28, 2.09, and 2.02.
Upon slow injection of 1 equiv of [n-Bu4N][OMe] in
MeOH into the THF-MeOH solution of complex 1 and after
stirring overnight at room temperature, the anionic 3 was
isolated as a dark green solid and characterized by single-
crystal X-ray diffraction (Scheme 1c). The conversion of
complex 1 to complex 3 was monitored by UV-vis
spectrometry; the intense absorption bands at 603 and 743
nm disappeared, accompanied by the formation of absorption
bands at 599 and 749 nm. The frozen-solution EPR spectrum
of complex 3 in THF-CH3OH (3:1 volume ratio) at 77 K,
essentially indistinguishable from those of the 77 K EPR
spectra of complexes 1 and 2 (g ) 2.31, 2.04, and 1.99 in
THF-CH3CN (3:1 volume ratio) at 77 K), exhibits high
rhombicities with three principal g values of 2.28, 2.04, and
2.00 (Figure 4). Compared to complex 2 (E1/2 ) -1.09 V)
and [NiIII(ER)(P(C6H3-3-SiMe3-2-S)3)]- (E1/2 ) -1.32 V for
ER ) SEt and E1/2 ) -1.26 V for ER ) SePh), complexes
1 and 3 with the coordinated [OPh]- and [OMe]- ligands
reveal one reversible NiIII/II redox process at -1.08 V [E1/2
(CH3CN)] and -0.96 V [E1/2 (MeOH)] (vs Cp2Fe/Cp2Fe+),
respectively (Figure 5). The more anodic redox potential of
complexes 1 and 3, compared to those of complexes 2 and
[NiIII(SEt)(P(C6H3-3-SiMe3-2-S)3)]-, is ascribed to the weaker
electron-donating ability of the coordinated [OR]- (R ) Ph,
Figure 2. ORTEP drawing and labeling scheme of complex 2 with thermal
ellipsoids drawn at the 50% probability level. Selected bond distances (Å)
and angles (deg): Ni(1)-S(4) 2.2459(9), Ni(1)-S(1) 2.2986(8), Ni(1)-S(2)
2.2698(9), Ni(1)-S(3) 2.2133(8), Ni(1)-P(1) 2.1375(8); S(4)-Ni(1)-P(1)
176.82(3),S(4)-Ni(1)-S(1)93.16(3),S(4)-Ni(1)-S(2)90.38(4),S(4)-Ni(1)-S(3)
96.11(3),S(1)-Ni(1)-S(2)1.6.92(3),S(2)-Ni(1)-S(3)123.94(4),S(1)-Ni(1)-S(3)
128.06(4).
2 and [NiIII(SePh)(P(C6H3-3-SiMe3-2-S)3)]- dominated by
one intense absorption band at 592 and 590 nm, respectively,
the electronic spectrum of complex 1 coordinated by the less
electron-donating phenoxide ligand displays a red shift to
603 nm (Figure 3).
To further examine the effect of electronic modulations
of the monodentate ligand [OR]- on the stability and
7910 Inorganic Chemistry, Vol. 47, No. 17, 2008

Complexes [NiIII(OR)(P(C6H3-3-SiMe3-2-S)3)]- (R ) Me, Ph)
Figure 4. X-band frozen-solution [THF-MeOH (3:1 volume ratio)] EPR
spectrum of complex 3 with g values of 2.28, 2.04, and 2.00 recorded at
77 K.
Figure6.UV-visspectrum(inTHF-CH3CN)oftheproposed[NiII(OPh)(P(C6H3-
7b
3-SiMe3-2-S)3)]2-
.
Figure 7. ORTEP drawing and labeling scheme of complex 1 with thermal
ellipsoids drawn at the 50% probability level. Selected bond distances (Å)
and angles (deg): Ni(1)-O(1) 1.910(3), Ni(1)-S(1) 2.2603(11), Ni(1)-S(2)
2.3191(11), Ni(1)-S(3) 2.2254(11), Ni(1)-P(1) 2.1313(11); O(1)-Ni(1)-P(1)
177.25(10), O(1)-Ni(1)-S(1) 90.39(9), O(1)-Ni(1)-S(2) 97.20(9),
O(1)-Ni(1)-S(3) 94.35(8), S(1)-Ni(1)-S(2) 106.04(4), S(2)-Ni(1)-S(3)
122.29(4), S(1)-Ni(1)-S(3) 130.25(5).
Compared to the reported dinuclear bis(µ-oxo)nickel(III)
complexes [(PhTttBu)Ni]2(µ-O)2, [TpMe3Ni(µ-O)NiTpMe3],
and [Ni2(µ-O)(Me3-tpa)2]2+,5 complexes 1 and 3 are the first
examples of mononuclear nickel(III) thiolate complexes
containing a terminal phenoxide/alkoxide ligand, character-
ized by single-crystal X-ray diffraction. Single-crystal X-ray
structures of complexes 1 and 3 are shown in Figures 7 and
8, respectively, and selected bond lengths and bond angles
are collected in the figure captions. The geometry of the Ni
center in complex 3 is a distorted trigonal bipyramid, with
O(1) and P(1) occupying the axial positions. The Ni-O(R)
distances of 1.910(3) and 1.885(2) Å in complexes 1 and 3,
respectively, are comparable to the Ni-O(H) bond distance
(1.91 Å) observed in the active-site structure of the oxidized
form [NiFe] H2ase,2e–g and similar to the corresponding
metrics for dinuclear bis(µ-oxo)nickel(III) complexes estab-
lished by single-crystal X-ray analysis.5 The shortening of
the Ni-OMe bond distance [1.885(2) Å] in complex 3,
compared to the Ni-OPh bond length of 1.910(3) Å in
complex 1, is presumably caused by the stronger electron-
donating methoxide ligand coordinating to the electron-
deficient NiIII core to stabilize complex 3. Of importance, in
contrast to the Ni-OPh and Ni-OMe bond distances of
1.910(3) and 1.885(2) Å found in 1 and 3, respectively, the
shorter Ni-SPh distance of 2.246(1) Å observed in complex
Figure 5. Cyclic voltammograms of (a) a 2.0 mM CH3CN solution of
complex 2, (b) a 2.0 mM CH3CN solution of complex 1, and (c) a 2.0 mM
MeOH solution of complex 3 in 0.1 M [n-Bu4N][PF6] with a glassy carbon
working electrode. [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 (CH2Cl2)].
Me) ligand compared to [SEt]- and implies the existence of
the metastable [NiII(OR)(P(C6H3-3-SiMe3-2-S)3)]2-. Reduc-
tion of complex 1 by 1 equiv of [PPN][BH4] in THF-
CH3CN under N2 at ambient temperature yielded the
proposed [NiII(OPh)(P(C6H3-3-SiMe3-2-S)3)]2- [UV-vis:
447 and 792 nm (Figure 6)], the analogue of the known
[NiII(PPh3)(P(C6H4-2-S)3)]-,7b and then the thermally un-
stable [NiII(OPh)(P(C6H3-3-SiMe3-2-S)3)]2- converted into
the known [NiII (P(C6H3-3-SiMe3-2-S)3)2]2- characterized by
2
UV-vis and single-crystal X-ray diffraction.7d
It has been known that the reactivity of [NiIII(L)(P(C6H3-
3-SiMe3-2-S)3)]- may be tailored by ligand L.7 To further
explore the nickel(III) thiolate complexes containing the
terminal NiIII-OH bond, we repeated the reaction of complex
1 and [Me4N][OH] in THF-CH3OH. On the basis of the
UV-vis spectrum and single-crystal X-ray diffraction,
complex 3 was isolated after the reaction solution of complex
1 and 1 equiv of [Me4N][OH] was stirred for 3 h and the
insoluble solid was removed.
Inorganic Chemistry, Vol. 47, No. 17, 2008 7911

Chiou and Liaw
yielding complex 4 (Scheme 1b)] upon the reaction of
[NiIII(Cl)(P(C6H3-3-SiMe3-2-S)3)]- and nucleophiles ([Na][OR],
R ) Ph, Me) in THF-CH3CN may be regulated by
nucleophiles.
At 77 K, complexes 1 and 3 exhibit a rhombic signal with
g values of 2.31, 2.09, and 2.00 and of 2.28, 2.04, and 2.00,
respectively, the characteristic g values of the trigonal-
bipyramidal low-spin d7 NiIII complexes [NiIII(L)(P(C6H3-
3-SiMe3-2-S)3)]- (L ) SEt, SePh, Cl, CH2CN).7 In contrast
to the NiIII-SPh and NiIII-SEt bond distances of 2.246(1)
and 2.273(1) Å found in [NiIII(SR)(P(C6H3-3-SiMe3-2-S)3)]-
(R ) Ph, Et), respectively, the longer terminal NiIII-OPh
bond distance of 1.910(3) Å observed in complex 1,
compared to the NiIII-OMe bond length of 1.885(2) Å found
in complex 3, may implicate the absence of the significant
σ and π donation from [OPh] to NiIII.8 Presumably, the
electron-rich functionalities of the NiIII center derived from
[P(C6H3-3-SiMe3-2-S)3]3- acting as a strong σ- and π-donat-
ing group in complexes 1 and 3 are responsible for the
stabilization of the NiIII state to prevent the reduction of NiIII
by the monodentate [OR]- (R ) Ph, Me) ligand. This study
unambiguously illustrates the aspect of how the coordinated
ligands ([OR]- and [P(C6H3-3-SiMe3-2-S)3]3-) of the mono-
nuclear [NiIII(OR)(P(C6H3-3-SiMe3-2-S)3)]- function coop-
eratively to reach an optimum electronic condition to stabilize
the nickel(III) thiolate alkoxide complexes.
Figure 8. ORTEP drawing and labeling scheme of complex 3 with thermal
ellipsoids drawn at the 50% probability level. Selected bond distances (Å)
and angles (deg): Ni(1)-O(1) 1.885(2), Ni(1)-S(1) 2.2591(10), Ni(1)-S(2)
2.2988(10), Ni(1)-S(3) 2.2794(10), Ni(1)-P(1) 2.1162(10); O(1)-Ni(1)-P(1)
176.65(8), O(1)-Ni(1)-S(1) 91.35(8), O(1)-Ni(1)-S(2) 100.97(9),
O(1)-Ni(1)-S(3) 96.18(9), S(1)-Ni(1)-S(2) 136.15(4), S(2)-Ni(1)-S(3)
105.84(4), S(1)-Ni(1)-S(3) 114.51(4).
Table 1. Selected Bond Length (Å) and Angle (deg) for Complexes
1-3, [NiIII(SePh)(P(C6H3-3-SiMe3-2-S)3)]-, and
[NiIII(SEt)(P(C6H3-3-SiMe3-2-S)3)]-
Ni-ER (E )
O, S, Se; R
compound
) Ph, Et)
Ni-Save
Ni-P
1
3
2
1.910(3)
1.885(2)
2.246(1)
2.273(1)
2.347(1)
2.268(1) 2.131(1)
2.279(1) 2.116(1)
2.261(1) 2.138(1)
2.253(1) 2.135(1)
2.263(1) 2.132(1)
Experimental Section
[NiIII(SEt)(P(C6H3-3-SiMe3-2-S)3)]-
[NiIII(SePh)(P(C6H3-3-SiMe3-2-S)3)]-
Manipulations, reactions, and transfers were conducted under N2
according to Schlenk techniques or in a glovebox. Solvents were
distilled under N2 from appropriate drying agents (diethyl ether from
CaH2; acetonitrile from CaH2-P2O5; methylene chloride from
CaH2; hexane and THF from sodium benzophenone) and stored in
dried, N2-filled flasks over 4 Å molecular sieves. N2 was purged
through these solvents before use. Solvent was transferred to the
reaction vessel via a stainless cannula under a positive pressure of
N2. The reagents [Na][OC6H5], [n-Bu4N][OCH3], bis(triphenylphos-
phoranylidene)ammonium chloride ([PPN][Cl]; Fluka), diphenyl
disulfide, nickel(II) dichloride, [(CH3)4N][OH], and [Na][OMe]
were used as received. Compound [PPN][Ni(Cl)(P(o-C6H3-3-SiMe3-
2-S)3)] was synthesized by published procedures.7c UV-vis spectra
were recorded on a GBC Cintra 10e. 1H NMR spectra were obtained
on a Varian Unity-500 spectrometer. Electrochemical measurements
were performed with CHI model 421 potentionstat (CH Instrument)
instrumentation. Cyclic voltammograms were obtained from 2.0
mM analyte concentration in THF using 0.1 M [n-Bu4N][PF6] as
the 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 (CH2Cl2). Analyses of C,
H, and N were obtained with a CHN analyzer (Heraeus).
2, compared to the Ni-SEt distance of 2.273(1) Å observed
in [NiIII(SEt)(P(C6H3-3-SiMe3-2-S)3)]-,7 may implicate the
significant σ and π donation from [SPh] to NiIII (Table 1).8
Presumably, the tunable electron-donating functionalities of
the tetradentate ligand [P(C6H3-3-SiMe3-2-S)3]3- of [NiIII(L)-
(P(C6H3-3-SiMe3-2-S)3)]- modulated by the monodentate
ligand L may reimburse the electronic deficiency of the Ni
center induced by the less electron-donating ability of the
[OR]- ligand to stabilize the [NiIII-OR] motif.
Conclusion and Comments
Mononuclear 1 containing a terminal [OPh]-coordinated
ligand, characterized by UV-vis, EPR, CV, and single-
crystal X-ray diffraction, was synthesized from the reaction
of [NiIII(Cl)(P(C6H3-3-SiMe3-2-S)3)]- with 3 equiv of
[Na][OPh] in THF-CH3CN (3:1 volume ratio) at room
temperature. In the synthesis of mononuclear [NiIII(OMe)-
(P(C6H3-3-SiMe3-2-S)3)]- containing a terminal [OMe]-
ligand, methoxide [n-Bu4N][OMe] triggers ligand substitution
of complex 1 to yield complex 3, in contrast to the addition
of [NiIII(Cl)(P(C6H3-3-SiMe3-2-S)3)]- into [Na][OMe] pro-
ducing the known 4 in THF-CH3CN. These results show
that the reaction pathways [ligand-displacement reaction
yielding complex 1 (Scheme 1a) vs deprotonation reaction
Preparation of [PPN][Ni(OC6H5)P(C6H3-3-SiMe3-2-S)3] (1).
Compounds [PPN][Ni(Cl)P(C6H3-3-SiMe3-2-S)3] (0.242 g, 0.2
mmol) and [Na][OC6H5] (0.070 g, 0.6 mmol) were dissolved in 5
mL of a THF-CH3CN (volume ratio 3:1) mixture solution in a
Schlenk tube under N2. The reaction solution was stirred for 12 h
at ambient temperature, and a solution color change from yellow
green to deep green was observed. The blue-shift absorptions from
430 and 620 nm ([PPN][Ni(Cl)P(C6H3-3-SiMe3-2-S)3]) to 411 and
603 nm implicated the formation of 1. The mixture solution was
(8) Ashby, M. T.; Enemark, J. H.; Lichtenberger, D. L. Inorg. Chem. 1988,
27, 191.
7912 Inorganic Chemistry, Vol. 47, No. 17, 2008

Complexes [NiIII(OR)(P(C6H3-3-SiMe3-2-S)3)]- (R ) Me, Ph)
filtered through Celite to remove the insoluble NaCl and the excess
[Na][OC6H5]. Diethyl ether (20 mL) was then added to precipitate
the deep-green solid 1 (yield 0.181 g, 73%). Crystals suitable for
X-ray diffraction were obtained by layering of a THF-CH3CN
solution of complex 1 and diethyl ether at ambient temperature for
Reaction of Complex 1 and [(CH3)4N][OH] in THF-CH3OH.
The CH3OH solution of [(CH3)4N][OH] (0.018 g, 0.2 mmol) was
added to the THF-CH3OH solution [5 mL (volume ratio 3:1)] of
complex 1 (0.248 g, 0.2 mmol) under N2. The mixture solution
was stirred for 3 h at ambient temperature, and the solution color
change from deep green to dark green was observed. The reaction
solution was monitored by UV-vis, and the electronic absorptions
(418, 599, and 749 nm) implicated the formation of the complex
3. The resulting dark-green solution was filtered through Celite to
remove the insoluble [(CH3)4N][OPh]. The filtrate was concentrated
under vacuum, and diethyl ether was then added to precipitate the
dark-green solid complex 3 (yield 0.186 g, 72%) characterized by
UV-vis and X-ray diffraction analysis.
Crystallography. Crystallographic data of complexes 1-3 are
summarized in Tables S2-S4 in the Supporting Information,
respectively. The crystals chosen for X-ray diffraction studies
measured 0.30 × 0.18 × 0.07 mm for complex 1, 0.48 × 0.38 ×
0.20 mm for complex 2, and 0.42 × 0.30 × 0.15 mm for complex
3, respectively. Each crystal was mounted on a glass fiber and
quickly coated in an epoxy resin. Unit-cell parameters were obtained
by least-squares refinement. Diffraction measurements for com-
plexes 1-3 were carried out on a SMART CCD (Nonius Kappa
CCD) diffractometer with graphite-monochromated Mo KR radia-
tion (λ ) 0.7107 Å) and between 2.08 and 25.34° for complex 1,
between 2.08 and 25.34° for complex 2, and between 2.08 and
25.36° for complex 3. Least-squares refinement of the positional
and anisotropic thermal parameters of all non-H atoms and fixed
H atoms were based on F2. A SADABS absorption correction was
made.9 The SHELXTL structure refinement program was em-
ployed.10
1
6 days. H NMR (C4D8O): δ 15.8 (br), 10.4 (br), 9.4 (br), -5.3
(br) ppm ([OC6H5]- and [P(C6H3-3-SiMe3-2-S)3]3-). Absorption
spectrum (THF) [λmax, nm (ꢀ, M-1 cm-1)]: 411 (6700), 603 (2900),
743 (1550), 910 (1100). Anal. Calcd for C69H71P3NiOS3Si3: C,
65.65; H, 5.67. Found: C, 65.15; H, 5.70.
Preparation of [PPN][Ni(SC6H5)P(C6H3-3-SiMe3-2-S)3] (2).
Compounds [PPN][Ni(Cl)P(C6H3-3-SiMe3-2-S)3] (0.242 g, 0.2
mmol) and [Na][SC6H5] (0.027 g, 0.2 mmol) were loaded into a
20 mL Schlenk tube and dissolved in 5 mL of a THF-CH3CN
(volume ratio 4:1) mixture solution under N2. The reaction solution
was stirred for 4 h at ambient temperature, and then the mixture
solution was filtered through Celite to remove the insoluble NaCl.
Diethyl ether (20 mL) was then added to precipitate the green solid
2 (yield 0.175 g, 70%). Crystals suitable for X-ray diffraction were
obtained by layering of a THF-CH3CN solution of complex 2 and
diethyl ether at ambient temperature for 6 days. 1H NMR (C3D6O):
δ 14.8 (br), 10.5 (br), 8.3 (br), -4.0 (br) ppm ([SC6H5]- and
[P(C6H3-3-SiMe3-2-S)3]3-). Absorption spectrum (MeCN) [λmax, nm
(ꢀ, M-1 cm-1)]: 357 (7350), 592 (1650), 949 (700). Anal. Calcd
for C69H71P3NiS4Si3: C, 64.82; H, 5.60. Found: C, 64.49; H, 5.69.
Preparation of [PPN][Ni(OCH3)P(C6H3-3-SiMe3-2-S)3] (3).
Complex 1 (0.248 g, 0.2 mmol) was loaded into a 30 mL Schlenk
tube, followed by the addition of 5 mL of a THF-CH3OH (volume
ratio 9:1) mixture solvent by a cannula under positive N2 pressure.
A portion of [n-Bu4N][OCH3] [0.32 mL (20% in CH3OH solution)]
was then added into the THF-CH3OH solution of complex 1 by
syringe. The mixture solution was stirred overnight at room
temperature. The mixture solution was filtered through Celite to
remove the insoluble [Bu4N][OC6H5], and diethyl ether (20 mL)
was then added to precipitate the dark-green solid [PPN]-
[Ni(OCH3)P(C6H3-3-SiMe3-2-S)3] (3) (yield 0.127 g, 54%). Dark-
green crystals suitable for X-ray diffraction analysis were obtained
by layering of the THF-CH3OH solution of complex 3 and diethyl
ether at ambient temperature for 6 days. 1H NMR (C3D6O): δ 15.4
(br), 10.4 (br), 9.3 (br), -4.9 (br) ppm ([OCH3]- and [P(C6H3-3-
SiMe3-2-S)3]3-). Absorption spectrum (CH3OH) [λmax, nm (ꢀ, M-1
cm-1)]: 418 (3750), 599 (1400), 749 (800). Anal. Calcd for C64H69-
P3NiOS3Si3: C, 64.80; H, 5.86. Found: C, 64.07; H, 5.76.
EPR Measurements. EPR measurements were performed at the
X band using a Bruker EMX spectrometer equipped with a Bruker
TE102 cavity. The microwave frequency was measured with a
Hewlett-Packard 5246 L electronic counter. X-band EPR spectra
of complex 1 in THF-CH3CN were obtained with a microwave
power of 19.78 mW (20.02 mW for complex 3), a frequency at
9.604 GHz (9.636 GHz for complex 3), and a modulation amplitude
of 0.80 G at 100 kHz.
Acknowledgment. We gratefully acknowledge financial
support from the National Science Council of Taiwan. The
authors thank Ting-Shen Kuo for single-crystal X-ray
structural determinations.
Reaction of [PPN][Ni(Cl)(P(C6H3-3-SiMe3-2-S)3)] and [Na]-
[OCH3]. Complexes [PPN][Ni(Cl)(P(C6H3-3-SiMe3-2-S)3)] (0.242
g, 0.2 mmol) and [Na][OCH3] (0.011 g, 0.2 mmol) were dissolved
in a THF-CH3CN [5 mL (volume ratio 3:1)] mixture solution under
N2, and the solution was stirred for 3 h at ambient temperature.
The solution color changing from yellow-green to purple was
observed. The reaction solution was monitored by UV-vis, and
the electronic absorptions (543, 648, and 925 nm) implicated the
formation of the known 4.7c The resulting purple solution was
filtered through Celite to remove the insoluble NaCl. The filtrate
was concentrated under vacuum, and diethyl ether was then added
to precipitate the known dark-purple solid 4 (yield 0.186 g, 72%)
characterized by UV-vis.7c
Supporting Information Available: X-ray crystallographic file
in CIF format and tables of crystallographic data for the structure
determinations of 1-3. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC801069T
(9) Sheldrick, G. M. SADABS, Siemens Area Detector Absorption Cor-
rection Program; University of Go¨ttingen: Go¨ttingen, Germany, 1996.
(10) Sheldrick, G. M. SHELXTL, Program for Crystal Structure Determi-
nation; Siemens Analytical X-ray Instruments Inc.: Madison, WI, 1994.
Inorganic Chemistry, Vol. 47, No. 17, 2008 7913