Mononuclear Ni(II)-thiolate complexes with pendant thiol and dinuclear Ni(III/II)-thiolate complexes with Ni...Ni interaction regulated by the oxidation levels of nickels and the coordinated ligands

Article abstract of DOI:10.1021/ic700719h

Compared to [Ni^II(SePh)(P(o-C₆H₃-3- SiMe₃-2-S)₂(o-C₆H₃-3-SiMe ₃-2-SH))]^- (1a) and [Ni^II(Cl)(P(o-C ₆H₃-3-SiMe₃-2-S)₂-(o-C ₆H₃-3-SiMe₃-2-SH))]^- (3a) with a combination of the intramolecular [Ni...H-S] and [Ni-S...H-S] interactions, complexes [Ni^II(SePh)(P(o-C₆H ₃-3-SiMe₃-2-S)₂(o-C₆H ₃-3-SiMe₃-2-SH))]^- (1b) and [Ni ^II(Cl)(P (o-C₆H₃-3-SiMe₃-2-S) ₂-(o-C₆H₃-3-SiMe₃-2-SH))] ^- (3b) with intramolecular [Ni...H-S] interaction exhibit lower ν_S-H stretching frequencies (2137 and 2235 cm^-1 for 1b and 3b vs 2250 and 2287 cm^-1 for 1a and 3a, respectively) and smaller torsion angles (27.2° for 3b vs 58.9 and 59.1° for 1a and 3a, respectively). The pendant thiol interaction modes of 1a, 3a, and 3b in the solid state are controlled by the solvent pairs of crystallization. Oxygen oxidation of dinuclear [Ni^II-(P(o-C₆H₃-3- SiMe₃-2-S)₂(o-C₆H₃-3-SiMe ₃-2-SH))]₂ (4) yielded thermally stable dinuclear [Ni ^III(P(o-C₆H₃-3-SiMe₃-2-S) ₂(o-C₆H₃-3-SiMe₃-2-μ-S))] ₂ (5). The two paramagnetic d⁷ Ni^III cores (S = 1/2) with antiferromagnetic coupling (J = -3.13 cm^-1) rationalize the diamagnetic property of 5. The fully delocalized mixed-valence [Ni(II)-Ni(III)] complexes [Ni₂(P(o-C₆H ₃-3-SiMe₃-2-S)₃)₂]^- (6) and [Ni₂(P(o-C₆H₃-3-SiMe₃-2-S) ₃)(P(o-C₆H₃-3-SiMe₃-2-S) ₂(o-C₆H₃-3-SiMe₃-2-SCH ₃))] (7) were isolated upon the reduction of 5 and the methylation of 6, respectively. The electronic perturbation from the sulfur methylation of 6 triggers the stronger Ni...Ni interaction and the geometrical rearrangement from the diamond shape of the [NiS₂Ni] core to the butterfly structure of [Ni(μ-S)₂Ni] to yield 7 with Ni...Ni distances of 2.6088(1) A. The distinctly different Ni...Ni distances (2.6026(7) for 5 and 2.8289(15) A for 6) and the coordination number of the nickels indicate a balance of geometrical requirements for different oxidation levels of [PS₃Ni-NiPS₃] cores of 5 and 6.

Full text of DOI:10.1021/ic700719h

Inorg. Chem. 2007, 46, 8913−8923
Mononuclear Ni(II)-Thiolate Complexes with Pendant Thiol and Dinuclear
Ni(III/II)-Thiolate Complexes with Ni‚‚‚Ni Interaction Regulated by the
Oxidation Levels of Nickels and the Coordinated Ligands
Chien-Ming Lee,† Tzung-Wen Chiou,† Hsin-Hung Chen,† Chao-Yi Chiang,† Ting-Shen Kuo,‡ and
Wen-Feng Liaw*,†
Department of Chemistry, National Tsing Hua UniVersity, Hsinchu 30043, Taiwan,
Instrumentation Center, National Taiwan Normal UniVersity, Taipei, Taiwan
Received April 16, 2007
Compared to [NiII(SePh)(P(o-C6H3-3-SiMe3-2-S)2(o-C6H3-3-SiMe3-2-SH))]− (1a) and [NiII(Cl)(P(o-C6H3-3-SiMe3-2-S)2-
(o-C6H3-3-SiMe3-2-SH))]− (3a) with a combination of the intramolecular [Ni‚‚‚
S] and [Ni S] interactions,
Hs
−S‚‚‚H−
complexes [NiII(SePh)(P(o-C6H3-3-SiMe3-2-S)2(o-C6H3-3-SiMe3-2-SH))]− (1b) and [NiII(Cl)(P (o-C6H3-3-SiMe3-2-S)2-
(o-C6H3-3-SiMe3-2-SH))]− (3b) with intramolecular [Ni‚‚‚
H
−
S] interaction exhibit lower νS stretching frequencies
-
H
(2137 and 2235 cm- for 1b and 3b vs 2250 and 2287 cm- for 1a and 3a, respectively) and smaller torsion
1
1
angles (27.2° for 3b vs 58.9 and 59.1° for 1a and 3a, respectively). The pendant thiol interaction modes of 1a, 3a,
and 3b in the solid state are controlled by the solvent pairs of crystallization. Oxygen oxidation of dinuclear [NiII-
(P(o-C6H3-3-SiMe3-2-S)2(o-C6H3-3-SiMe3-2-SH))]2 (4) yielded thermally stable dinuclear [NiIII(P(o-C6H3-3-SiMe3-2-
S)2(o-C6H3-3-SiMe3-2-
µ
-S))]2 (5). The two paramagnetic d7 NiIII cores (S
)
1/2) with antiferromagnetic coupling (J
Ni(III)] complexes
) −3.13 cm-1) rationalize the diamagnetic property of 5. The fully delocalized mixed-valence [Ni(II)
−
[Ni2(P(o-C6H3-3-SiMe3-2-S)3)2]− (6) and [Ni2(P(o-C6H3-3-SiMe3-2-S)3)(P(o-C6H3-3-SiMe3-2-S)2(o-C6H3-3-SiMe3-2-SCH3))]
(7) were isolated upon the reduction of 5 and the methylation of 6, respectively. The electronic perturbation from
the sulfur methylation of 6 triggers the stronger Ni‚‚‚Ni interaction and the geometrical rearrangement from the
diamond shape of the [NiS2Ni] core to the butterfly structure of [Ni(µ-S)2Ni] to yield 7 with Ni‚‚‚Ni distances of
2.6088(1) Å. The distinctly different Ni‚‚‚Ni distances (2.6026(7) for 5 and 2.8289(15) Å for 6) and the coordination
number of the nickels indicate a balance of geometrical requirements for different oxidation levels of [PS3Ni
−
NiPS3] cores of 5 and 6.
Introduction
widely.1 The X-ray crystallographic studies of the active-
site structure of [NiFe] hydrogenases isolated from D. gigas,
D. Vulgaris, D. fructosoVorans, and D. desulfuricans
ATCC27774 in combination with infrared spectroscopy have
revealed an active site comprised of a heterobimetallic (Scys)2-
Hydrogenases catalyze a reversible two-electron oxidation
of H2 in aerobic and anaerobic microorganisms.1 Two classes
of hydrogenases, [Fe]-only hydrogenases ([Fe]-only H2ases)
and [NiFe] hydrogenases ([NiFe] H2ases), have been studied
-
Ni(µ-Scys)2(µ-X)Fe(CO)(CN)2 (X ) O2-, HO2 , OH-) cluster
(Figure 1).2-5 The bridging ligand X was proposed
* To whom correspondence should be addressed. E-mail: wfliaw@
mx.nthu.edu.tw.
† National Tsing Hua University.
(2) (a) Volbeda, A.; Charon, M. H.; Piras, C.; Hatchikian, E. C.; Frey,
M.; Fontecilla-Camps, J. C. Nature 1995, 373, 580-587. (b) Garcin,
E.; Vernede, X.; Hatchikian, E. C.; Volbeda, A.; Frey, M.; Fontecilla-
Camps, J.-C. Structure 1999, 7, 557-566. (c) Happe, R. P.; Roseboom,
W.; Pierik, A. J.; Albracht, S. P. J Nature 1997, 385, 126. (d) Volbeda,
A.; Garcin, E.; Piras, C.; De Lacey, A. L.; Fernandez, V. M.;
Hatchikian, E. C.; Frey, M.; Fontecilla-Camps, J. C. J. Am. Chem.
Soc. 1996, 118, 12989-12996. (e) Volbeda, A.; Martin, L.; Cavazza,
C.; Matho, M.; Faber, B. W.; Roseboom, W.; Albracht, S. P. J.; Garcin,
E.; Rousset, M.; Fontecilla-Camps, J. C. J. Biol. Inorg. Chem. 2005,
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‡ National Taiwan Normal University.
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Pamplona, A.; Telser, J.; Moura, I.; Moura, J. J. G.; Hoffman, B. M.
J. Am. Chem. Soc. 2002, 124, 281-286. (b) Albracht, S. P. J. Biochim.
Biophys. Acta 1994, 1188, 167-204. (c) Adams, M. W. W.; Stiefel,
E. I. Curr. Opin. Chem. Biol. 2000, 4, 214-220. (d) 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. (e) Whitehead,
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10.1021/ic700719h CCC: $37.00
Published on Web 09/15/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 21, 2007 8913

Lee et al.
Figure 1. Schematic drawing of the active site of [NiFe] hydrogenases
as deduced from crystallographic studies.2
Figure 2. Schematic drawing of [PPN][NiII(ER)(P(o-C6H3S)2(o-C6H3-
SH))].11
to be an oxide, hydroxide, or hydro-peroxide in the oxidized
state and was found to be absent in the reduced state. The
coordination environment about nickel in the [NiFe] H2ases
is pseudo-tetrahedral in the reduced state and pseudo-square
pyramidal in the oxidized state. The nickel site has been
proposed to be redox active and changes between Ni(III)
and Ni(II), whereas the iron site remains as Fe(II) in all of
the spectrally defined redox states of the enzyme.2-5 The
active center of [NiFe] H2ase exhibits various redox states
in the hydrogen catalytic cycle. The EXAFS/EPR studies
indicate that the formal oxidation state of the nickel center
is paramagnetic Ni(III) in Ni-A, Ni-B, and Ni-C states.2-5
The Ni-A and Ni-B states can be converted into the
reduced forms Ni-SU and Ni-SI via one-electron reduction,
respectively. The Ni-SI state transforms into the EPR-silent,
active state Ni-SIa, which can then be reduced to the EPR-
detectable Ni-C state. Actually, the active form Ni-C (the
paramagnetic Ni-C intermediate) of [NiFe] H2ase was
proposed to exist as the [(Scys-H)NiIII-H-Fe] intermediates
after an active state Ni-SIa (silent-active [(Scys-H)NiII-
(Scys)3]) is passed. Ni-R/Ni-SIa states were proposed to
exist as [(Scys-H)(Scys)NiII(µ-Scys)2Fe(CO)(CN)2] with a
Cys-SH interacting directly with the nickel center (a
[Ni‚‚‚H-Scys] interaction).2-5 In particular, recent X-ray
absorption spectroscopy shows that the nickel site of the
regulatory hydrogenase (RH) in the presence of hydrogen
(RH+H2), proposed as the Ni-C state, isolated from Ralstonia
eutropha is a six-coordinated [NiIII-S2(O/N)3(H)].6
(anη2-EtS-Hcomplex)wasproposed.7NiII(BmMe)2(BmMe)bis-
(2-mercapto-1-methyl-imidazolyl) borate) with a [NiS4H2]
core and the presence of the Ni‚‚‚H-B interaction may
provide a structural model of the nickel site of [NiFe]H2-
ase.8 The EPR and single-crystal X-ray structure provided
evidence for the formation of dinuclear [Ni(II)Ni(III)]-thiolate
complexes generated by one-electron oxidation of a dinuclear
Ni(II)-macrocyclic complex [Bu4N]2[Ni2(E)] (E ) macro-
cyclic ligand)9a and [Bu4N]2[Ni2{P(o-C6H4S)3}2],9b respec-
tively. Recently, a number of [NiII-Fe] model compounds
were reported.10 However, the investigation of the relation-
ship between the core geometry of the bimetallic [Ni-Fe]/
[Ni-Ni] center and the oxidation levels of nickel is limited.
In the previous study, we reported the syntheses and
characterizations of the mononuclear [PPN][NiII(L)(P-(o-
C6H4S)2(o-C6H4SH))] and [PPN][NiIII(L)(P(o- C6H4S)3)] (L
) SePh, SEt, Cl) (Figure 2).11 In this article, the thermally
stable complexes [PPN][NiII(L)(P(o-C6H3-3-SiMe3-2-S)2(o-
C6H3-3-SiMe3-2-SH))] (L ) SePh (1a, 1b), Cl (3a, 3b)) were
synthesized to elucidate the modes of the intramolecular
[Ni-S‚‚‚H-S]/[Ni‚‚‚H-S] interactions regulated by the
solvent pairs of crystallization. A series of dinuclear nickel
complexes [NiII(P(o-C6H3-3-SiMe3-2-S)2(o-C6H3-3-SiMe3-2-
SH))]2 (4), [NiIII(P(C6H3-3-SiMe3- 2-S)2(C6H3-3-SiMe3-2-
µ-S))]2 (5), and the mixed-valence [Ni(II)-Ni(III)] com-
plexes [Na-18-crown-6-ether][Ni2(P(o-C6H3-3-SiMe3-2-S)3)2]
(6) and [Ni2(P(o-C6H3-3- SiMe3-2-S)3)(P(o-C6H3-3-SiMe3-
2-S)2(o-C6H3-3-SiMe3-2-SCH3))] (7), and the dianionic di-
In model compounds, the kinetics studies of the protona-
tion of complex [BPh4] [Ni(SEt)(triphos)] (triphos ) (Ph2-
PCH2CH2)2PPh) revealed that the sulfur atom is the initial
site for the protonation of [BPh4][Ni(SEt)(triphos)], and the
interaction of the proton with both the nickel and sulfur sites
nuclear [K-18-crown-6-ether]2[NiII (P(o-C6H3-3-SiMe3-2-
2
S)3)2] (8) were isolated and characterized to study the
correlation among the presence/absence of Ni‚‚‚Ni interac-
(3) (a) Higuchi, Y.; Yagi, T.; Yasuoka, N. Structure 1997, 5, 1671-1680.
(b) Higuchi, Y.; Ogata, H.; Miki, K.; Yasuoka, N.; Yagi, T. Structure
1999, 7, 549-556. (c) Ogata, H.; Mizoguchi, Y.; Mizuno, N.; Miki,
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8914 Inorganic Chemistry, Vol. 46, No. 21, 2007

Mononuclear Ni(II)-Thiolate Complexes
Scheme 1
Figure 3. ORTEP drawing and labeling scheme of 1a with thermal
ellipsoids drawn at 30% probability level. Selected bond distances (Å) and
angles (°): Ni-Se 2.3213(6), Ni(1)-S(1) 2.2096(11), Ni(1)-S(2) 2.1755-
(11), Ni(1)‚‚‚S(3) 3.690(1), S(2)‚‚‚S(3) 3.681(1), Ni-P(1) 2.0972(11), S(1)-
Ni(1)-S(2) 162.26(4), S(1)-Ni(1)-Se 89.10(3), S(1)-Ni(1)-P(1) 84.99(4),
S(2)-Ni(1)-Se 96.43(3), S(2)-Ni(1)-P(1) 89.55(4), Se-Ni-P(1) 174.00-
(4).
1
Table 1. IR νS-H Stretching Frequencies and H NMR Chemical Shifts
(δS-H) for [P(o-C6H3-3-SiMe3-2-SH)3] (P(S′H)3), 1a, 1b, 3a, 3b, and 4
complexes/
spectroscopic data
ν
S-H, cm-1 (KBr)
δ
S-H, ppm (CDCl3)
P(S′H)3
1a
1b
3a
3b
2466
2250
2137
2287
2235
2385
4.58(br)
8.59(d)
8.59(d)
8.54(d)
8.54(d)
6.24(d)
tion, the geometry of [PS3Ni-NiS3P] core, and the electronic
states (electronic density) of [PS3Ni-NiS3P] cores of com-
plexes.
4
Results and Discussion
8.59 (br) ppm under an N2 atmosphere (Table 1). This study
suggests that 1a and 1b are in dynamic equilibrium in
solution and supports the existence of intramolecular
[Ni‚‚‚H-S]/[Ni-S‚‚‚H-S] interactions, resulting in the 1H
NMR chemical shift of the SH group, from δ 4.58 (br)
(CDCl3) in free ligand P(o-C6H3-3-SiMe3-2-SH)3 to δ 8.59
(br) (SH) (C4D8O) in 1 (Table 1).
The Pendant Thiol Interaction Modes of Complex
[PPN][Ni(SePh)(P(o-C6H3-3-SiMe3-2-S)2(o-C6H3-3-SiMe3-
2-SH))] (1a/1b). As reported in the previous study, the
reaction of [PPN][Ni(CO)(SePh)3] and P(o-C6H3-3-SiMe3-
2-SH)3 led to the formation of [PPN][Ni(SePh)(P(o-C6H3-
3-SiMe3-2-S)2(o-C6H3-3-SiMe3-2-SH))] (1), isolated as a dark
red-brown solid (85% yield) (Scheme 1a).11c In contrast to
the analogue [PPN][Ni(SePh)(P(o-C6H4-2-S)2(o-C6H4-2-
SH))] (2),11 1 coordinated with the trimethylsilyl-substituted
[P(o-C6H3-3-SiMe3-2-S)3]3- ligand was soluble in THF. Two
forms of crystalline products, 1a (plate-shaped, red-brown
crystals) and 1b (block-shaped, dark-red crystals), were
obtained when 1 was recrystallized from THF-diethyl ether
and THF-hexane, respectively, at room temperature, as
shown in parts b and b′ of Scheme 1′. The IR spectrum of
1a in the solid-state reveals one stretching band (2250 cm-1
(KBr)) in the νS-H region, compared to 1b, which displays
a lower-energy νS-H stretching band (2137 cm-1 (KBr)). In
1a and 1b were found to be the same monoclinic P21/c
space group but in the different unit cell dimensions. As
shown in Figure 3, the acute C(24)-S(3)-H(3S) bond
angle of 96(2)° in 1a is believed to result from an intramo-
lecular [Ni‚‚‚H-S]/[Ni-S‚‚‚H-S] interaction (Scheme 1,
Type I). The pendant thiol interaction modes in the solid
state are ascribed to solvent effects during recrystallization.
Diffusion of the less-polar solvent hexane into the THF
solution of 1 yielded 1b. Diffusion of n-hexane into a THF-
H2O solution (10:0.2 mL) of 1, bearing comparison with
diffusion of diethyl ether into THF solution of 1, gave the
layered, red-brown crystals identified as 1a. The pendant thiol
proton of 1a is partially polarized and approaches the second
sulfur atom. It can be interpreted in terms of the thiol proton
interacting with both sulfur and nickel atoms, where the
proton is bound to sulfur (S(3)) and attracted by Ni(II) and
sulfur (S(2)) atoms, that is, a combination of [Ni‚‚‚H-S]
and [Ni-S‚‚‚H-S] interactions (Scheme 1, Type I). How-
comparison with free ligand P(o-C6H3-3-SiMe3-2-SH)3 (νS-H
)
2466 cm-1 (KBr))(Table 1), the shifts of νS-H stretching
frequencies in 1a and 1b imply the specific intramolecular
[Ni‚‚‚H-S]/[Ni-S‚‚‚H-S] interactions.11c
In THF-d8 solution, however, 1a and 1b exhibit identical
1H NMR spectra at 25 °C, with thiol proton resonance at δ
Inorganic Chemistry, Vol. 46, No. 21, 2007 8915

Lee et al.
Scheme 2
ever, the X-ray structure of 1b is not of suitable quality for
this publication.
The Pendant Thiol Interaction Modes of Mononuclear
[PPN][NiII(Cl) (P(o-C6H3-3-SiMe3-2-S)2(o-C6H3-3-SiMe3-
2-SH))] (3a and 3b) in the Solid State. When P(o-C6H3-
3-SiMe3-2-SH)3 (0.1 mmol) and [PPN]2[NiCl4] (0.1 mmol)
were dissolved in THF-CH3CN (5:0.5 mL volume ratio) at
room temperature, a reaction ensued over the course of 30
min to give the mononuclear [PPN][Ni(Cl)(P(o-C6H3-3-
SiMe3-2-S)2(o-C6H3-3-SiMe3-2-SH))] (3) (yield 65%) after
separation of the neutral [Ni(P(o-C6H3-3-SiMe3-2-S)2(o-
C6H3-3-SiMe3-2-SH))]2 (4) (yield 25%) by hexane (Scheme
2). 3 is soluble in CH2Cl2 and is extremely O2-sensitive in
solution. Two types of crystalline products, layered red-
brown (3a) and chunky red-brown (3b) crystals, were isolated
upon diffusion of diethyl ether into the CH2Cl2 solution of
3 and diffusion of hexane into the CH2Cl2 solution of 3, res-
pectively, at room temperature (parts b and b′ of Scheme 2′).
Molecular structures of 3a and 3b are shown in parts a
and b of Figure 4, respectively, and selected bond lengths
and bond angles are collected in figure captions. The
geometry of the nickel center in 3a containing 0.5 THF
solvent in crystal lattice is distorted square planar (ClPS2)
with the pendant thiol proton attracted by Ni(II) and sulfur,
that is, a combination of intramolecular [Ni‚‚‚H-S] and [Ni-
S‚‚‚H-S] interactions (Type I, as shown in Scheme 2 and
the graphic). The Ni-P(1)-C(14)-C(13) torsion angle of
59.1° in 3a is significantly larger than that of Ni(1)-P(1)-
C(19)-C(20) (27.2°) in 3b. In particular, the S(3)‚‚‚Ni
distance of 3.666(1) Å in 3a is longer than that of 3.287(1)
Å in 3b (types I and II, as shown in Scheme 2 and the
graphic).
Figure 4. ORTEP drawing and labeling scheme of (a) 3a, and (b) 3b
with thermal ellipsoids drawn at 30% probability level. Selected bond
distances (Å) and angles (°): 3a: Ni-Cl(1) 2.217(14), Ni(1)-S(1) 2.190-
(13), Ni(1)-S(2) 2.204(13), Ni(1)‚‚‚S(3) 3.66(1), S(1)‚‚‚S(3) 3.68(1), Ni-
P(1) 2.087(14); S(1)-Ni-S(2) 163.3(6), S(1)-Ni-Cl(1) 93.6(5), S(1)-
Ni-P(1) 89.3(5), S(2)-Ni-Cl(1) 93.7(5), S(2)-Ni-P(1) 84.6(5), Cl(1)-
Ni-P(1) 175.0(6). 3b: Ni-Cl(1) 2.2243(9), Ni(1)-S(1) 2.1570(10), Ni(1)-
S(2) 2.1649(10), Ni(1)‚‚‚S(3) 3.287(1), S(1)‚‚‚S(3) 3.955(1), Ni-P(1)
2.0962(10); S(1)-Ni-S(2) 154.94(4), S(1)-Ni-Cl(1) 93.96(4), S(1)-Ni-
P(1) 89.97(4), S(2)-Ni-Cl(1) 94.45(4), S(2)-Ni-P(1) 88.07(4), Cl(1)-
Ni-P(1) 164.77(4).
As shown in Table 1, the IR νS-H stretching frequencies
of 1a, 1b, 3a, and 3b are significantly disturbed by the
monodentate coordinated ligands, [SePh]-/[Cl]-, and the
pendant thiol proton interaction modes. The νS-H stretching
frequencies (2250 and 2137 cm-1, respectively) of 1a and
1b with the [SePh]- coordinated ligand are lower than those
of 3a (2287 cm-1) and 3b (2235 cm-1), ligated by [Cl]-. Of
importance, the νS-H difference of 113 cm-1 between 1a and
1b is significantly larger than that of 52 cm-1 between 3a
and 3b. These results support that an increase in the nickel
electronic density modulated by the monodentate ligand
attracts the proton of the pendant thiol effectively and causes
the weaker S-H bond. This result is consistent with the
higher pKa of HSePh (pKa ) 7.1) than HCl (pKa ) 1.8) in
dimethyl sulfoxide.12
(12) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456-463.
8916 Inorganic Chemistry, Vol. 46, No. 21, 2007

Mononuclear Ni(II)-Thiolate Complexes
Scheme 3
1
Both 3a and 3b exhibit identical H NMR spectra, with
and -0.11), derived from the protons of three trimethylsilyl
groups, indicate that trimethylsilyl groups of 4 are not
equivalent. The 2H NMR spectrum (δ 6.37 (br) (C4H8O) (S-
D)) and IR νS-D spectrum (1748 (br) cm-1 (VS-D, KBr))
demonstrated that the pendant thiol proton of 4 is D2O
exchangeable.
the thiol protons resonances at δ 8.54 (br) (C4D8O) under a
N2 atmosphere (Table 1). It is noticed that the upfield
chemical shift of the pendant thiol proton from δ 8.59 (1) to
8.54 (3) is consistent with νS-H stretching frequencies shifted
from 2250/2137 (1a/1b) to 2287/2235 cm-1 (3a/3b) (KBr).
Neutral Dinuclear [NiII(P(o-C6H3-3-SiMe3-2-S)2(o-C6H3-
3-SiMe3-2-SH))]2 (4) and [NiIII(P(o-C6H3-3-SiMe3-2-S)2-
(o-C6H3-3-SiMe3-2-µ-S))]2 (5). Compared to the reaction of
P(o-C6H3-3-SiMe3-2-SH)3 and [NiCl4]2-, reaction of P(o-
C6H3-3-SiMe3-2-SH)3 (0.1 mmol) and NiCl2‚6H2O (0.1
mmol) yielded the neutral dinuclear Ni(II) complex [Ni(P(o-
C6H3-3-SiMe3-2-S)2(o-C6H3-3-SiMe3-2-SH))]2 (4) in THF at
ambient temperature. Alternatively, treatment of 1 (0.133 g,
0.1 mmol) with [Et3O][BF4] (0.1 mmol) in THF at 5 °C also
led to the formation of 4 (yield 70%), accompanied by
insoluble [PPN][BF4] solid (part a of Scheme 3). Presumably,
alkylation by electrophile ([Et3O][BF4]) occurs at the more
accessible [SePh]- site, followed by coordinative association
of two neutral [NiII(P(o-C6H3-3-SiMe3-2-S)2(o-C6H3-3-SiMe3-
2-SH))] units to produce 4. 4 exhibits extreme air sensitivity
in solution (THF, diethyl ether). Compared to complexes 1a/
1b (IR VS-H 2250 and 2137 cm-1 (KBr)) and 3a/3b (IR νS-H
2287 and 2235 cm-1 (KBr)), the higher νS-H stretching
frequency (2385 cm-1 (KBr)) of 4 implies the absence of
intramolecular [Ni-S‚‚‚H-S] interactions (Table 1). The 1H
NMR spectrum of 4 in CDCl3 shows the thiol proton
resonances at δ 6.24(d), a ∼1.66 ppm downfield shift
compared to the free ligand. Three singlets (at δ 0.51, 0.33,
The single-crystal X-ray structure of 4 is shown in Figure
5, and selected bond distances and angles are collected in
the figure caption. The elongation of Ni(1)‚‚‚S(3)H(3A) and
Ni(2)‚‚‚S(6)H(6A) distances (3.947(1) and 3.868(1) Å,
respectively) in 4, compared to the Ni‚‚‚S(3)H distance of
3.690(1) and 3.305(1) Å in 1a and 1b, respectively, also
implicates the absence of the intramolecular [Ni-S‚‚‚H-S]
interactions. Formation of 4 via the coordinative association
of two [NiIIP(o-C6H3-3-SiMe3-2-S)2(o-C6H3-3-SiMe3-2-SH)]
units can lend support to the previous proposal that the
distinct electron-donating ability of the coordinated ligands
may serve to regulate the intramolecular [Ni-S‚‚‚H-S]/
[Ni‚‚‚H-S] interactions.11 The geometry about both of the
central Ni(II) can be regarded as slightly distorted square
planar. The dihedral angle between the two planes (Ni(1)S-
(1)S(2) and Ni(2)S(1)S(2)) is 78.1° (Supporting Information
Table S1), and the Ni(1)‚‚‚Ni(2) distance of 2.5808(8) Å is
short enough to suggest a certain degree of interaction
between the two Ni(II) centers of 4. Compared to the known
nickel dimer (Supporting Information Table S1),13 the short
Ni(1)‚‚‚Ni(2) distance found in 4 may be attributed to the
electron deficiency of Ni(II) centers derived from the less
electron-donating [P(o-C6H3-3-SiMe3-2-S)2(o-C6H3-3-SiMe3-
Inorganic Chemistry, Vol. 46, No. 21, 2007 8917

Lee et al.
Figure 5. ORTEP drawing and labeling scheme of 4 with thermal
ellipsoids drawn at 30% probability level (Me groups of SiMe3 were omitted
for clarity). Selected bond distances (Å) and angles (°): Ni(1)-S(1) 2.2078-
(13), Ni(1)-S(4) 2.2696(12), Ni(1)-S(2) 2.1557(13), Ni(1)-S(3) 3.947-
(1), Ni(1)-P(1) 2.1062(13), Ni(1)‚‚‚Ni(2) 2.5808(8); S(1)-Ni(1)-S(2)
158.14(6), S(1)-Ni(1)-S(4) 84.01(4), S(2)-Ni(1)-S(4) 98.74(5), S(1)-
Ni(1)-P(1) 87.45(5), S(2)-Ni(1)-P(1) 89.42(5), S(4)-Ni(1)-P(1) 171.43-
(5), Ni(2)-S(4)-Ni(1) 70.33(4), Ni(1)-S(1)-Ni(2) 70.59(4).
Figure 6. ORTEP drawing and labeling scheme of 5 with thermal
ellipsoids drawn at 30% probability level (Me groups of SiMe3 were
omitted). Selected bond distances (Å) and angles (°): Ni(1)-S(1) 2.2529-
(11), Ni(1)-S(4) 2.2837(11), Ni(1)-S(5) 2.2143(11), Ni(1)-S(6) 2.2722-
(11), Ni(1)-P(2) 2.1271(11), Ni(1)‚‚‚Ni(2) 2.6026(7); S(1)-Ni(1)-S(4)
87.3(4), S(1)-Ni(1)-S(5) 95.40(4), S(1)-Ni(1)-S(6) 98.32(4), S(1)-Ni-
(1)-P(2) 172.11(4), S(4)-Ni(1)-P(2) 86.27(4), S(4)-Ni(1)-S(6) 102.73-
(4), S(1)-Ni(1)-S(6) 98.32(4), S(5)-Ni(1)-S(6) 115.07(5), S(5)-Ni(1)-
P(2) 86.78(4), S(6)-Ni(1)-P(2) 87.57(4), Ni(1)-S(1)-Ni(2) 70.05(3),
Ni(1)-S(4)-Ni(2) 70.18(3).
2-SH)]2- (one phosphine, one terminal thiolate, and two
bridging thiolates) coordinated to each Ni(II) center and the
acute dihedral angles (Ni(1)S(1)S(2) and Ni(2)S(1)S(2)).
As shown in part b of Scheme 3, upon injecting O2 (3
mL, 14 psi, at 290 K) into a THF solution of 4 (0.126 g, 0.1
mmol) at ambient temperature, the reaction ensued over 2 h
to give dark-green, thermally stable dinuclear Ni(III) [NiIII-
(P(o-C6H3-3-SiMe3-2-S)2(o-C6H3-3-SiMe3-2-µ-S))]2 (5) char-
acterized by single-crystal X-ray diffraction and UV-vis
spectroscopy, and byproduct H2O identified 1H NMR.11 The
1H NMR spectrum of 5 in CD2Cl2 exhibits the signals for
the trimethylsilyl protons at δ 0.49 (s), 0.23 (s), -0.59 (s),
and the expected signals for the benzene protons at 6.85 (t),
6.89 (t), 7.33 (dd), 7.39 (t), 7.41 (t), 7.81 (dd), 8.12 (d). 5 in
frozen CH2Cl2 shows EPR silent at 77 K. On the basis of
1H NMR, EPR, and magnetic measurements, 5 was charac-
terized as a diamagnetic species. Thus, the electronic
structure of 5 can be best described as two paramagnetic d7
NiIII cores with antiferromagnetic coupling (J ) -3.13 cm-1),
that is, both centers have S ) 1/2 spins magnetically coupled
to each other possibly via the direct d-d orbital overlap (Ni-
(1)‚‚‚Ni(2) ) 2.6026(7) Å) and the bridging sulfurs. Both
complexes show the butterfly structure of the Ni(µ-S)2Ni unit
on going from Ni(II) 4 to Ni(III) 5 with little Ni-S-Ni bond
angles and a Ni‚‚‚Ni distance change (Ni-S-Ni bond angles
of 70.59(4) and 70.33(4)° and a Ni‚‚‚Ni distance of 2.5808-
(8) Å for 4, and 70.05(3) and 70.18(3)°, and 2.6026(7) Å
for 5). Figure 6 displays a thermal ellipsoid plot of 5
containing 0.5 THF solvent in crystal lattice, and selected
bond distances and angles are given in the figure caption.
Analysis of the bond angles of 5 reveals that Ni(1) and Ni-
(2) are best described as existing in a distorted geometry
midway between trigonal bipyramidal and tetragonal pyra-
midal. The mean Ni-S(bridging)/Ni-S(terminal) lengths of 2.268-
(1)/2.243(1) Å observed in 5 are longer than those of 2.239(1)
and 2.156(1) Å found in 4, respectively. The lengthening of
the average Ni-S bonds of 5 can be ascribed to the strain
effect of the chelating ligands in the coordination sphere from
tridentate to tetradentate, which overwhelms the contracting
effect caused by the metal-centered oxidation.
The electrochemistry of 5, measured in THF with 0.1 M
[n-Bu4N][PF6] supporting electrolyte at ambient temperature
(scan rate 100 mV/s), reveals two reversible oxidation-
reduction processes at -0.775 and -1.492 V (E1/2) (vs Cp2-
Fe/Cp2Fe+), respectively (Figure 7). For E1/2 ) -0.775 V,
5 undergoes one-electron reduction (∆Ep ) 167 mV, ipa/ipc
) 0.59) to a mixed-valence [Ni(III)Ni(II)] species, which
can be further reduced to a [Ni(II)Ni(II)] species at E1/2
)
-1.492 V (∆Ep ) 222 mV, ipa/ipc ) 0.84). It is noticed that
the value of ipa/ipc at -0.775 V (E1/2 value of the Ni3+-
Ni3+/Ni3+-Ni2+ couple) is dramatically different from the
value of 1. However, when the scan range (potential) was
switched from 0.0 to -1.0 V, the value of ipa/ipc at -0.775
V is close to 1 (0.83) and the value of ipc at -0.389 V
becomes smaller. These results may implicate that the mixed-
valence [Ni(III)-Ni(II)] complex is unstable.
Mixed-Valence [Ni(II)-Ni(III)] [Na-18-crown-6-ether]-
[Ni2(P(o-C6H3-3- SiMe3-2-S)3)2] (6), [Ni2(P(o-C6H3-3-SiMe3-
2-S)3)(P(o-C6H3-3-SiMe3-2-S)2(o-C6H3- 3-SiMe3-2-SCH3))]
(7) and [Ni(II)-Ni(II)] [K-18-crown-6-ether]2[Ni2(P(o-
C6H3-3- SiMe3-2-S)3)2] (8). When a THF solution of 5, [Na]-
(13) (a) Barclay, G. A.; McPartlin, E. M.; Stephenson, N. C. Acta
Crystallogr. 1969, B25, 1262-1273. (b) Nicholson, J. R.; Christou,
G.; Huffman, J. C.; Folting, K. Polyhedron 1987, 6, 863-870. (c)
Snyder, B. S.; Rao, Ch. P.; Holm, R. H. Aust. J. Chem. 1986, 39,
963-974. (d) Villa, A. C.; Manfredotti, A. G.; Nardelli. M.; Pelizzi,
C. Chem. Commun. 1970, 1322-1323. (e) Fackler, J. P., Jr.; Zegarski,
W. J. J. Am. Chem. Soc. 1973, 95, 8566-8574. (f) Schwarzenbach,
G. Chem. ZVesti 1965, 19, 200-208. (g) Vance, T. B., Jr.; Warner,
L. G.; Seff, K. Inorg. Chem. 1977, 16, 2106-2110. (h) Handa, M.;
Mikuriya, M.; Okawa, H.; Kida, S. Chem. Lett. 1988, 1555-1558. (i)
Colpas, G. J.; Kumar, M.; Day, R. O.; Maroney, M. J. Inorg. Chem.
1990, 29, 4779-4788. (j) Watson, A. D.; Rao, Ch. P.; Dorfman, J.
R.; Holm, R. H. Inorg. Chem. 1985, 24, 2820-2826. (k) Allan, C.
B.; Davidson, G.; Choudhury, S. B.; Gu, Z.; Bose, K.; Day, R. O.;
Maroney, M. J. Inorg. Chem. 1998, 37, 4166-4167. (l) Choudhury,
S. B.; Pressler, M. A.; Mirza, S. A.; Day, R. O.; Maroney, M. J. Inorg.
Chem. 1994, 33, 4831-4839.
8918 Inorganic Chemistry, Vol. 46, No. 21, 2007

Mononuclear Ni(II)-Thiolate Complexes
Figure 7. Cyclic voltammograms of 5 in the scan range (a) 0 to -2.5 V
(solid line) and (b) 0 to -1.0 V (dash line). The rest potential was measured
at -0.47 V.
Figure 9. ORTEP drawing and labeling scheme of 6 with thermal
ellipsoids drawn at 30% probability level. Selected bond distances (Å) and
angles (°): Ni(1)-S(1A) 2.1540(17), Ni(1)-S(2A) 2.2118(15), Ni(1)-S(3)
2.2418(15), Ni(1)‚‚‚S(1) 2.680(2), Ni(1)‚‚‚S(3A) 3.905(2), Ni(1)-P(1A)
2.1546(15), Ni(1)‚‚‚Ni(1A) 2.8290(13); P(1A)-Ni(1)-S(1A) 88.50(6),
P(1A)-Ni(1)-S(2A) 86.27(6), S(1A)-Ni(1)-S(2A) 148.99(8), P(1A)-
Ni(1)-S(3) 169.99(6), S(1A)-Ni(1)-S(3) 94.70(6), S(2A)-Ni(1)-S(3)
86.12(6), P(1A)-Ni(1)-S(1) 91.03(5), S(1A)-Ni(1)-S(1) 109.32(6),
P(1A)-Ni(1)-Ni(1A) 89.83(5), S(1A)-Ni(1)-Ni(1A) 63.39(6), S(2A)-
Ni(1)-Ni(1A) 146.99(7).
The single-crystal X-ray structure of 6 is depicted in Figure
9, and selected bond dimensions for 6 are presented in the
figure caption. 6 consists of two four-coordinated nickel
centers in a distorted-square-planar geometry, with three
thiolate sulfur atoms and one phosphorus atom. The two
nickel atoms and two sulfur atoms (S(1) and S(1A)) of 6
are almost constrained to a coplanar arrangement. Two
groups, the coplanar [S(1A)Ni(1)Ni(1A)S(1)] and the co-
planar [Ni(1)S(3)P(1)Ni(1A)S(3A)P(1A)], are in a perpen-
dicular conformation with an angle of 89.82°. Compared to
the bridging Ni(1)-S(1) and Ni(1)-S(4) bond distances of
2.253(1) and 2.284(1) Å, respectively, found in 5, the longer
Ni(1)‚‚‚S(1) distance of 2.680(2) Å (Ni(1)‚‚‚S(3A) 3.905-
(2) Å) observed in 6 indicates that reduction of 5 yielding 6
(reduction of [NiIIINiIII] to mixed-valence [NiIIINiII]) results
in the elongation of Ni(1)‚‚‚S(1) distance and the relief of
the strained tetradentate bonding mode. The geometrical
rearrangement from the butterfly structure of [Ni(µ-S)2Ni]
unit of 5 to the diamond shape of [Ni(S)2Ni] core of 6 reflects
the effects caused by the addition of one electron. Upon one-
electron reduction of 5 yielding 6, the Ni‚‚‚Ni distance is
lengthened by as much as 0.22 Å. This result implicates,
qualitatively, that the SOMO in 6 can be identified as having
antibonding character in Ni‚‚‚Ni interaction. Compared to
the Ni‚‚‚Ni distance of 2.6026(7)/2.501 Å and the ligation
mode of the coordinated [P(o-C6H3-3-SiMe3-2-S)3]3-/[P(o-
C6H4-2-S)3]3- ligands observed in 5 and [Ni(P(o-C6H4-2-
S)2(o-C6H4-2-µ-S))]2- (10), respectively, the longer Ni‚‚‚Ni
distance (2.829(1) Å) and the ligation mode of the coordi-
nated [P(o-C6H3-3-SiMe3-2-S)3]3- ligands of 6 are attributed
to the more electron-rich functionalities of the mixed-valence
[Ni(II)Ni(III)] core and the electronic/steric perturbations
Figure 8. EPR spectrum of 6 with g values of 2.11, 2.07, and 2.03 at 77
K (* ) impurity) (an isotropic signal with g values of 2.077 at 298 K).
[BEt3H] (or KC8), and 18-crown-6-ether (1:1:2 molar ratio)
were stirred at ambient temperature for 1 h (part c of Scheme
3), a reduction occurred to yield the mixed-valence [Ni(II)-
Ni(III)] [Na-18-crown-6-ether][Ni2(P(o-C6H3-3-SiMe3-2-
S)3)2] (6), identified by UV-vis, EPR, and single-crystal
X-ray diffraction. Compared to the rhombic signal with g
values of 2.304, 2.091, and 2.00 (4.2 K) observed in complex
[NiIII(SePh)(P(o-C6H3-2-S)3)]- (11), the 77 K EPR spectrum
of 6 exhibits high rhombicities with three principal g values
of 2.113, 2.073, and 2.033 (an isotropic signal with a g value
of 2.077 at 298 K) (Figure 8).14 The UV-vis spectrum of 6
exhibits an intense absorption around 1124 nm with an
extinction coefficient of >2000 L mol-1 cm-1, which may
be ascribed to the intervalence transition of the fully
delocalized mixed-valence complexes (Supporting Informa-
tion Figure S1).14 These results support that 6 adopts a fully
delocalized mixed-valence [Ni(III)-Ni(II)] electronic con-
figuration in a distorted-square-planar ligand field.
(14) (a) Stebler, A.; Ammeter, J. H.; Fuerholz, U.; Ludi, A. Inorg. Chem.
1984, 23, 2764-2767. (b) Ondrechan, M. J.; Ko, J.; Zhang, L.-T. J.
Am. Chem. Soc. 1987, 109, 1672-1676.
Inorganic Chemistry, Vol. 46, No. 21, 2007 8919

Lee et al.
Figure 11. X-band frozen solution EPR spectrum of 7 with g values of
2.17, 2.05, and 2.02, recorded at 77 K (an isotropic signal with g value of
2.08 at 298 K).
Figure 10. ORTEP drawing and labeling scheme of 7 with thermal
ellipsoids drawn at 30% probability level. Selected bond distances (Å) and
angles (°): Ni(1)-S(1) 2.3544(18), Ni(1)-S(2) 2.1885(15), Ni(1)-S(3)
2.2551(15), Ni(1)-S(6) 2.2699(16), Ni(1)-P(1) 2.1184(16), Ni(1)‚‚‚Ni(2)
2.6088(11), Ni(2)-S(3) 2.2686(15), Ni(2)-S(4) 2.4036(18), Ni(2)-S(5)
2.1986(15), Ni(2)-S(6) 2.2600(16), Ni(2)-P(2) 2.1120(16); P(1)-Ni(1)-
S(2) 86.95(6), P(1)-Ni(1)-S(3) 171.96(6), S(2)-Ni(1)-S(3) 94.48(6),
P(1)-Ni(1)-S(6) 86.56(6), S(2)-Ni(1)-S(6) 145.32(6), S(3)-Ni(1)-S(6)
87.79(6), P(1)-Ni(1)-S(1) 87.84(6), S(2)-Ni(1)-S(1) 113.02(6), P(2)-
Ni(2)-S(5) 87.50(6), P(2)-Ni(2)-S(6) 172.42(6), S(5)-Ni(2)-S(6) 95.01-
(6), P(2)-Ni(2)-S(3) 86.08(6), S(5)-Ni(2)-S(3) 143.92(6), S(5)-Ni(2)-
S(4) 112.02(6), Ni(1)-S(6)-Ni(2) 70.32(5), Ni(1)-S(3)-Ni(2) 70.44(5),
Ni(2)-S(4)-C(55) 104.3(5).
from the substituted trimethylsilyl groups (the strongly
σ-donating SiMe3 group).
To further corroborate the electronic effect (oxidation
states of nickel) modulating the Ni‚‚‚Ni interaction of
dinuclear 5 and 6, the neutral dinuclear [Ni2(P(o-C6H3-3-
SiMe3-2-S)3)(P(o-C6H3-3-SiMe3-2-S)2(o-C6H3-3-SiMe3-2-
SCH3))] (7) was synthesized. As shown in part d of Scheme
3, treatment of 6 with [Me3O][BF4] in THF-CH3CN solution
yielded 7, identified by UV-vis and single-crystal X-ray
diffraction. X-ray crystal structure of 7 is depicted in Figure
10, and selected bond coordinates are presented in the figure
captions. The apparently shorter Ni(1)‚‚‚Ni(2) distance
(2.6088(11) Å) of 7, compared to the Ni‚‚‚Ni distance of
2.829(1) Å in 6, indicates a greater extent of Ni(1)‚‚‚Ni(2)
interaction in 7. Obviously, the electronic perturbation caused
by the sulfur methylation of 6 triggers the stronger Ni(1)‚‚
‚Ni(2) interaction accompanied by the geometrical rear-
rangement from the diamond shape of the [Ni(S)2Ni] core
of 6 to the butterfly structure of the [Ni(µ-S)2Ni] unit of 7
to reimburse the deficiency of electron density surrounding
[Ni(II)Ni(III)] centers. 7 exhibited the disorder of S(1) and
S(4) atoms. The combined scattering factors, 0.65S(1)(thiolate)
Figure 12. ORTEP drawing and labeling scheme of 8 with thermal
ellipsoids drawn at 30% probability level. Selected bond distances (Å) and
angles (°): Ni(1)-P(1A) 2.1097(14), Ni(1)-S(1) 2.2377(14), Ni(1)-S(2A)
2.2171(16), Ni(1)-S(3A) 2.1678(15), Ni(1)‚‚‚S(3) 3.146, Ni(1)‚‚‚Ni(1A)
3.186; P(1A)-Ni(1)-S(3A) 88.78(6), P(1A)-Ni(1)-S(2A) 86.62(6), S(3A)-
Ni(1)-S(2A) 154.26(7), P(1A)-Ni(1)-S(1) 175.87(6), S(3A)-Ni(1)-S(1)
95.35(6), S(2A)-Ni(1)-S(1) 89.58(6).
which are different from those of the mononuclear d7 Ni-
(III) complex [NiIII(SePh)(P(o-C6H4S)3)]-.11a,11c These results
implicate that 7 may exist as the delocalized mixed-valence
[Ni(II)-Ni(III)] complex, owing to the strong interaction
between the nickel centers and between nickel and sulfur,14
although the valence-trapped case cannot be unambiguously
excluded from this result alone.
Therefore, the lengthening of the Ni‚‚‚Ni distance was
expected to be driven by reduction of 6; the reduction of 6
by 1 equiv of KC8 in THF under N2 at ambient temperature
yielded a dianionic [Ni2(P(o-C6H3-3-SiMe3-2-S)3)2]2- (8) with
a Ni‚‚‚Ni distance of 3.186 Å (part e of Scheme 3). Figure
12 displays the ORTEP plot of 8. The nickel center is best
described as existing in a distorted-square-planar coordination
environment surrounded by three thiolate sulfur atoms and
one phosphorus atom. In comparison with the butterfly
structure of the [Ni(µ-S)2Ni] core of 4 with a Ni(1)‚‚‚Ni(2)
distance of 2.5808(8) Å, the two coplanar [NiS3P]
+ 0.35S(1)(thioether) and 0.35S(4)(thiolate) + 0.65S(4)(thioether)
,
respectively, were used in the refinement. There is weak
interaction between Ni(1)/Ni(2) and the thioether sulfur atom
(Ni(1)‚‚‚S(1) distance of 2.354(2) and Ni(2)‚‚‚S(4) distance
of 2.404(2) Å). Interestingly, the frozen-solution EPR
spectrum of 7 (77 K) in diethyl ether (Figure 11), essentially
indistinguishable from those of the 77 K EPR spectrum of
the fully delocalized mixed-valence 6, exhibits high rhom-
bicities with three principal g values of 2.17, 2.05, and 2.02,
8920 Inorganic Chemistry, Vol. 46, No. 21, 2007

Mononuclear Ni(II)-Thiolate Complexes
Scheme 4
([Ni(1)S(1)S(3A)S(2A)P(1A)] and [Ni(1A)S(1A)S(3)S(2)P-
(1)]) with an Ni‚‚‚Ni distance of 3.186 Å in 8 are almost
parallel. The longer Ni‚‚‚Ni distance, compared to that of 4,
is presumably ascribed to the more electron-rich [Ni(II)Ni-
(II)] centers of 8, deriving from the stronger electron-donating
thiolate-coordinated ligands.
Conclusion and Comments
Studies on the mononuclear/dinuclear Ni(III)-/Ni(II)-
thiolate 1-5, 8, and the mixed-valence [Ni(II)-Ni(III)]
thiolate complexes 6-7 have led to the following results,
including certain results from earlier studies.
(1) In 3b, the presence of intramolecular [Ni‚‚‚H-S]
interaction was verified in the solid state by the observation
of a lower IR νS-H stretching frequency compared to those
of 1a and 3a and subsequently was confirmed by single-
crystal X-ray diffraction, respectively. The pendant thiol
interaction modes ([Ni‚‚‚H-S] vs [Ni-S‚‚‚H-S]) in the
solid state (1a/3a vs 3b) are controlled by the solvent pairs
of crystallization. Obviously, the results obtained from this
work show that the IR νS-H stretching frequency in combina-
tion with the torsion angle Ni-P(1)-C(14)-C(13) may
serve as an efficient tool for the discrimination of the
existence of the intramolecular [Ni‚‚‚H-S]/[Ni-S‚‚‚H-S]
interactions in complexes [NiII(L)(P(o-C6H3-3-SiMe3-2-S)2-
(o-C6H3-3-SiMe3-2-SH))]- (L ) SePh, Cl).
(2) In contrast to 1a and 3a/3b, neutral dinuclear complex
4 containing the less-basic nickel center (the less electronic
density) regulated by the coordinated ligands does not show
the intramolecular [Ni‚‚‚H-S]/[Ni-S‚‚‚H-S] interactions.
(3) Oxidation of dinuclear complex 4 by O2 yielded stable
dinuclear complex 5 with a Ni‚‚‚Ni distance of 2.6026(7)
Å. From the viewpoint of the absence of paramagnetism
(SQUID) and the EPR signal, 5 can be considered as d7 Ni-
(III) and d7 Ni(III) units antiferromagnetically coupled to
each other (J value of -3.13 cm-1 in 5), where thiolate
bridges may also mediate the antiferromagnetic interactions.
The electrochemistry of 5 exhibits two reversible oxidation-
reduction processes at -0.775 and -1.492 V (E1/2) (vs Cp2-
Fe/Cp2Fe+), respectively.
(4) The reduction of 5 yielded the mixed-valence [Ni(II)-
Ni(III)] complex [Ni2(P(o-C6H3-3-SiMe3-2-S)3)2]- (6), ex-
hibiting an intense absorption band at 1124 nm, ascribed to
the intervalence transition of the fully delocalized mixed-
valence complexes. Rhombic EPR signals (g1 ) 2.11, g2 )
2.07, and g3 ) 2.03 at 77 K) confirmed the existence of the
unpaired electron in 6.
(5) As shown in Scheme 4, the geometrical (coordination
environment) change from distorted square pyramidal to
distorted square planar (i.e., from butterfly structure of the
[Ni(µ-S)2Ni] core to the diamond shape of the [NiS2Ni] core)
occurs upon going from a [d7 Ni(III)-d7 Ni(III)] electronic
structure of 5 to a fully delocalized mixed-valence [d8 Ni-
(II)-d7 Ni(III)] electronic structure of 6. In particular, the
different oxidation levels of the [PS3Ni-NiPS3] units
between 5 and 6 reflect the distinctly different Ni‚‚‚Ni
distances, 2.6026(7) for 5 and 2.8290(13) Å for 6, to reach
the optimum electron density of the [PS3Ni-NiPS3] core of
5 and 6. The electronic perturbation from the sulfur methy-
lation of 6 triggers the stronger Ni‚‚‚Ni interaction and the
geometrical rearrangement from the diamond shape of the
[NiS2Ni] core to the butterfly structure of [Ni(µ-S)2Ni],
resulting in the formation of 7 (Scheme 4). Presumably, the
shortening of the Ni‚‚‚Ni distance of 7 (vs 6) was employed
to reimburse the electron deficiency induced by methylation,
neutralizing the thiolate negative charge. Also, the geo-
metrical change of the nickel centers from distorted tetra-
hedral to square planar and the lengthening of the Ni‚‚‚Ni
distance (from 2.581 to 3.186 Å) occur upon going from a
neutral [Ni(II)Ni(II)] electronic structure of 4 to a dianionic
[Ni(II)Ni(II)] electronic structure of 8. Such geometrical
differences as determined by the nickel oxidation state in 5
versus that in 6 and determined by the coordinated ligands
in 6 versus those in 7 (or in 4 versus those in 8) demand
ligands capable of matching the geometrical requirement of
the [Ni(SR)2Ni] unit as well as maintaining the preferred
coordination number of the nickels (Scheme 4). Obviously,
the presence/absence of Ni‚‚‚Ni interaction in cooperation
with the geometrical rearrangement was employed by di-
nuclear complexes 4-8 as an efficient tool to reach an
optimum electronic condition to stabilize the dinuclear
complexes.
These results unambiguously illustrate the aspects of how
a coordinated ligand and the electronic states (oxidation
states) of two nickel centers function to trigger the geo-
metrical rearrangement, to promote the stability of the
dinuclear nickel complexes via the Ni‚‚‚Ni interaction. This
result may provide some clues to rationalize the oxidized-
form active-site structure of [NiFe] hydrogenases, a hetero-
bimetallic (Scys)2Ni(µ-Scys)2(µ-X)Fe(CO)(CN)2 (X ) O2-,
-
HO2 , OH-) with the butterfly structure of the [Ni(µ-Scys)2-
Fe] core and a Ni‚‚‚Fe distance of 2.6 Å.
Experimental Section
Manipulations, reactions, and transfers were conducted under
nitrogen according to Schlenk techniques or in a glovebox. 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
Inorganic Chemistry, Vol. 46, No. 21, 2007 8921

Lee et al.
use. Solvent was transferred to the reaction vessel via a stainless
cannula under a positive pressure of N2. The reagents bis-
(triphenylphosphoranylidene)ammonium chloride ([PPN][Cl]) (Flu-
ka), diphenyl diselenide, nickel(II) dichloride, Deuterium oxide, 99.9
atom % D (Aldrich) were used as received. Compounds [P(o-C6H3-
3-SiMe3-2-SH)3],15 [PPN][Ni(CO)(SePh)3], and [PPN][Ni(SePh)-
(P(o-C6H3-3-SiMe3-2-S)3)] were synthesized by published proce-
dures.11,16 Infrared spectra of the ν(S-H) stretching frequencies
were recorded on a PerkinElmer model spectrum one B spectro-
photometer with sealed solution cells (0.1 mm, KBr windows) or
KBr solid. UV-vis spectra were recorded on a GBC Cintra 10e.
a CH2Cl2 solution of 3 at -15 °C for 4 weeks led to layered red-
brown crystals of 3a (0.04 g, 33%) (Scheme 2). IR: 2287 w (νS-H
)
cm-1 (KBr, pellet). 1H NMR (C4D8O): δ 8.54 (br) (SH), 6.73∼6.74
(m), 6.87 (t), 7.17∼7.38 (m) (P(C6H3-3-SiMe3-2-S)3), 0.29 (s)
(SiMe3) ppm. On the other hand, diffusion of hexane into a CH2-
Cl2 solution of 3 at -15 °C for 4 weeks yielded chunky dark red-
brown crystals of 3b (0.041 g, 33%) (Scheme 2) suitable for X-ray
crystallography. IR: 2235 w (νS-H) cm-1 (KBr). 1H NMR
(C4D8O): δ 8.54 (br) (SH), 6.73∼6.74 (m), 6.87 (t), 7.17∼7.38
(m) (P(C6H3-3-SiMe3-2-S)3); 0.29 (s) (SiMe3). Absorption spectrum
(CH2Cl2) [λmax, nm (ꢀ, M-1 cm-1)]: 430 (2200), 530 (1200). Anal.
Calcd for C63H67ClNiNP3S3Si3: C, 62.76; H, 5.60; N, 1.16.
Found: C, 62.86; H, 5.89, N, 1.24.
2
1H and H NMR spectra were obtained on a Varian Unity-500
spectrometer. Electrochemical measurements were performed with
a 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 a supporting
electrolyte. Potentials were measured at 298 K versus a Ag/AgCl
reference electrode using a glassy-carbon working electrode. Under
the conditions employed, the potential (V) of the ferrocinium/
ferrocene couple was 0.39 (CH2Cl2). Analyses of carbon, hydrogen,
and nitrogen were obtained with a CHN analyzer (Heraeus).
Preparation of [PPN][Ni(SePh)(P(o-C6H3-3-SiMe3-2-S)2(o-
C6H3-3-SiMe3-2- SH))] (1a) and (1b). Complexes [PPN][Ni(CO)-
(SePh)3] (0.2 mmol, 0.218 g)16 and P(C6H3-3-SiMe3-2-SH)3 (0.2
mmol, 0.115 g) were loaded into a 50-mL flask and 15 mL of THF
was added by a cannula under positive N2 pressure. The reaction
mixture was stirred at ambient temperature for 1 h, and then hexane
(20 mL) was added to precipitate the red-brown solid [PPN][Ni-
(SePh)(P(o-C6H3-3-SiMe3-2-S)2(o-C6H3-3- SiMe3-2-SH))] (1) (0.225
g, 85%). Diffusion of diethyl ether into a THF solution of 1 at
room temperature for 1 week yielded layered brown-red crystals
Preparation of [Ni(P(o-C6H3-3-SiMe3-2-S)2(o-C6H3-3-SiMe3-
2-SH))]2 (4). A portion (0.12 mL) of [Et3O][BF4] was slowly added
into a THF (10 mL) solution of 1 (0.133 g, 0.1 mmol) at 5 °C. A
vigorous reaction occurred with the white precipitate [PPN][BF4].
Hexane (10 mL) was added, and the resulting mixture was filtered
through Celite to remove the insoluble [PPN][BF4]. Solvent was
removed under a vacuum to yield complex [Ni(P(o-C6H3-3-SiMe3-
2-S)2(o-C6H3-3-SiMe3- 2-SH))]2 (4) (0.088 g, 70%). Hexane (3
mL) was added to redissolve 4 and recrystallization from hexane
solution at -15 °C for 4 weeks gave red-brown crystals suitable
for X-ray crystallography. IR: 2385 w (νS-H) cm-1 (KBr, pellet).
1H NMR (CDCl3): δ 6.24 (d) (SH), 6.39 (t), 6.83 (t), 6.91 (t),
7.15 (t), 7.25 (t), 7.42 (d), 7.64 (d) (P(C6H3-3-SiMe3-2-S)3), -0.11
(s), 0.33 (s), 0.51 (s) (SiMe3). Absorption spectrum (THF) [λmax
,
nm ꢀ, M-1 cm-1)]: 440 (3080), 5350 (1050). Anal. Calcd for
C54H74Ni2P2S6Si6: C, 51.34; H, 5.90. Found: C, 50.93; H, 6.33.
D/H Exchange for the Reaction of 4 and D2O. To a THF (10
mL) solution of 4 (0.126 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 4 and D2O was stirred for 24 h
at 5 °C. Solvent was removed under a vacuum and hexane (5 mL)
was added to redissolve the red-brown solid. The flask containing
the red-brown solution was tightly sealed and kept at -15 °C for
4 weeks. The crystals [Ni(P(o-C6H3-3-SiMe3-2-S)2(o-C6H3-3-SiMe3-
characterized as 1a (0.212 g, 80%) (Scheme 1). IR: 2250 w (νS-H
)
cm-1 (KBr). 1H NMR (C4D8O): δ 8.59 (br) (SH), 6.65∼6.77 (m),
6.87 (t), 7.15∼7.33 (m) (PhSe, P(o-C6H3-3-SiMe3-2-S)3), 0.20 (s),
0.30 (s) (SiMe3). However, diffusion of hexane into a THF solution
of 1 at ambient temperature for 1 week yielded chunky dark red-
brown crystals characterized as 1b (0.220 g, 83%) (Scheme 1)
suitable for X-ray crystallography. IR: 2137 w (νS-H) cm-1 (KBr).
1H NMR (C4D8O): δ 8.59 (br), 6.65∼6.77 (m), 6.87 (t), 7.15∼7.33
(m) (PhSe, P(o-C6H3-3-SiMe3-2-S)3), 0.20 (s), 0.30 (s) (SiMe3).
Absorption spectrum (THF) [λmax, nm (ꢀ, M-1 cm-1)]: 420 (2700),
500 (1150). Anal. Calcd for C69H72NNiP3S3SeSi3: C, 62.48; H,
5.47; N, 1.06. Found: C, 62.18; H, 5.22; N, 0.98.
2-SD))]2 (4-D) and 4 were isolated. IR (KBr, pellet): 1748 (νS-D
)
cm-1. 2H NMR (C4H8O): δ 6.37 (br) (SD) versus C4D8O (natural
abundance of D in C4H8O solvent, δ 1.73 and 3.58).
Preparation of [Ni(P(o-C6H3-3-SiMe3-2-S)3)]2 (5). Pure oxygen
gas (3 mL, 14 psi, at 290 K) was injected through a red-brown
hexane solution (10 mL) of 4 (0.126 g, 0.1 mmol) at room
temperature. A significant change in color from red brown to dark
green occurred after the reaction mixture was stirred for an
additional 2 h at room temperature. The resulting solution was
filtered, and then the solution was concentrated to 3 mL under a
vacuum. Crystallization from hexane solution of complex [Ni(P(o-
C6H3-3-SiMe3-2-S)3)]2 (5) (0.076 g, 60%) at -15 °C for 4 weeks
Preparation of [PPN][Ni(Cl)(P(o-C6H3-3-SiMe3-2-S)2(o-C6H3-
3-SiMe3-2- SH))] (3a) and (3b). Compound P(C6H3-3-SiMe3-2-
SH)3 (0.058 g, 0.1 mmol) and [PPN]2[NiCl4] (0.128 g, 0.1 mmol),
prepared from reaction of NiCl2 (0.1 mmol) and [PPN][Cl] (0.2
mmol) in MeOH, were dissolved in mixed solvent THF-CH3CN
(10:1 volume ratio). After the reaction mixture was stirred at
ambient temperature for 30 min, hexane was added to separate the
red-brown precipitate and the orange-red solution. The orange-red
solution was then transferred to another flask under positive N2,
and dried under a vacuum to obtain the orange-red solid character-
ized as complex [Ni(P(o-C6H3-3-SiMe3-2-S)2(o-C6H3-3-SiMe3-2-
SH))]2 (4) (0.032 g, 25%). The red-brown solid was then redissolved
in THF (5 mL) and filtered through Celite to remove [PPN][Cl].
Hexane was then added to precipitate the red-brown solid character-
ized as complex [PPN][Ni(Cl)(P(o-C6H3-3-SiMe3-2-S)2(C6H3-3-
SiMe3-2-SH))] (3) (0.072 g, 65%). Diffusion of diethyl ether into
1
led to dark-green crystals suitable for X-ray crystallography. H
NMR (CD2Cl2): δ 6.85 (t), 6.89 (t), 7.33 (d), 7.39 (t), 7.41(t), 7.81
(dd), 8.12 (d) (P(o-C6H3-3-SiMe3-2-S)3), -0.59 (s), 0.23 (s), 0.49
(s) (SiMe3). Absorption spectrum (CH3CN) [λmax, nm (ꢀ, M-1
cm-1)]: 435 (5100), 636 (1750), 758 (550), 1154 (1300). FAB-
mass spectrum: m/z 1260 (([NiP(o-C6H3-3-SiMe3-2-S)3]2)+, 40.6%);
m/z 73 ((SiMe3)+, 100%). Anal. Calcd for C54H72Ni2P2S6Si6: C,
51.42; H, 5.75. Found: C, 51.38; H, 5.90.
Preparation of [Na-18-crown-6-ether][Ni2(P(o-C6H3-3-SiMe3-
2-S)3)2] (6). A 0.126 g of 5 (0.1 mmol) and 0.053 g of 18-crown-
6-ether (0.2 mmol) were dissolved in 5 mL THF in a 30 mL Schlenk
tube under nitrogen. A portion (0.1 mL) of NaBEt3H (0.1 M in
THF solution) was then added into the Schlenk tube by syringe.
The reaction solution was stirred under nitrogen at ambient
(15) Block, E.; Ofori-Okai, G.; Zubieta, J. J. Am. Chem. Soc. 1989, 111,
2327-2329.
(16) Liaw, W.-F.; Horng, Y.-C.; Ou, D.-S.; Chiang, C.-Y.; Lee, G.-H.; Peng,
S.-M. J. Am. Chem. Soc. 1997, 119, 9299-9300.
8922 Inorganic Chemistry, Vol. 46, No. 21, 2007

Mononuclear Ni(II)-Thiolate Complexes
temperature for 1 h and monitored by UV-vis. The color of the
reaction solution changes from green to brown green. Hexane (20
mL) was then added to precipitate the brown-green solid [Na-18-
crown-6-ether][Ni2(P(o-C6H3-3-SiMe3-2-S)3)2] (6) (yield 0.115 g,
65%). Crystals for X-ray diffraction analysis were obtained by a
layer of hexane and THF solution of 6 at -20 °C. Absorption
spectrum (THF) [λmax, nm (ꢀ, M-1cm-1)]: 654 (3667), 1142 (2548).
Anal. Calcd for C66H96P2NaNi2O6S6Si6: C, 51.19; H, 6.25.
Found: C, 51.64; H, 6.48.
filtered through Celite to remove graphite. The volume of the
solution was reduced to 5 mL under a vacuum, and hexane (25
mL) was added to precipitate the red-brown solid [K-18-crown-6-
ether]2[Ni2(P(o-C6H3-3-SiMe3-2-S)3)2] (8) (yield 0.125 g, 67%).
Crystals suitable for X-ray diffraction were obtained by a layer of
hexane and THF-CH3CN (10:1 volume ratio) solution of 8 at -15
°C for 2 weeks.
Crystallography. Crystallographic data of 1a/1b, 3a/3b, 4-5,
6-7, and 8 were summarized in Supporting Information Tables
S1-S5, 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
1a, 1b, 3a, 3b, and 4-8 were carried out on a SMART CCD
(Nonius Kappa CCD) diffractometer with graphite-monochromated
Mo KR radiation (λ ) 0.7107 Å). 7 exhibited a disorder of the
S(1) and S(4) atoms. The combined scattering factors, 0.65S-
A THF solution (10 mL) of 5 (0.126 g, 0.1 mmol) was added to
KC8 solid (0.014 g, 0.1 mmol) in a 30 mL Schlenk tube under
positive nitrogen. The reaction solution was stirred under nitrogen
at ambient temperature for 1 h. The color of the reaction solution
changed from green to brown green. Then a THF solution (5 mL)
of 18-crown-6-ether (0.053 g, 0.2 mmol) was transferred to the
above brown-green solution by cannula. After the reaction solution
was stirred for another 3 h under nitrogen at ambient temperature,
the brown-green solution was filtered through Celite to remove
graphite and was monitored by UV-vis. The volume of solution
was reduced to 5 mL under a vacuum, and then hexane (25 mL)
was added to precipitate the brown-green solid [K-18-crown-6-
ether][Ni2(P(o-C6H3-3-SiMe3-2- S)3)2] (6′) (yield 0.102 g, 65%)
characterized by UV-vis and single-crystal X-ray diffraction.
Preparation of [Ni2(P(o-C6H3-3-SiMe3-2-S)3)(P(o-C6H3-3-
SiMe3-2-S)2(o-C6H3-3-SiMe3-2-SCH3))] (7). A CH3CN solution
(3 mL) of [Me3O][BF4] (0.015 g, 0.1 mmol) was added dropwise
into the THF-CH3CN (3:1 volume ratio) mixed solution of
complex 6′ (0.156 g, 0.1 mmol) at 5 °C under N2 atmosphere. The
mixture solution was stirred for 30 min, then allowed to warm to
room temperature, and stirred for another 2 h. The color of reaction
solution changed from brown green to green. The mixture solution
was dried under a vacuum and then redissolved in hexane. The
green solution was filtered through Celite to remove the insoluble
[K-18-crown-6-ether][BF4]. Solvent was removed under a vacuum
to yield [Ni2(P(o-C6H3-3-SiMe3-2-S)3)(P(o-C6H3-3-SiMe3-2-S)2(o-
C6H3-3-SiMe3-2-SCH3))] (7) (yield 0.091 g, 71%). A hexane
solution of 7 kept at -15 °C for 4 weeks gave dark-green crystals
suitable for X-ray crystallography. Absorption spectrum ((Et)2O)
[λmax, nm (ꢀ, M-1 cm-1)]: 421 (10 100), 495 (5456), 669 (3170),
1144 (2032). C55H75Ni2P2S6Si6: C, 51.75; H, 5.92. Found: C, 51.84;
H, 6.17.
(1)(thiolate) + 0.35S(1)(thioether) and 0.35S(4)(thiolate) + 0.65S(4)(thioether)
,
respectively, were used in the refinement. Least-squares refinement
of the positional and anisotropic thermal parameters of all of the
non-hydrogen atoms and fixed-hydrogen atoms was based on F 2.
A SADABS absorption correction was made.17 The SHELXTL
structure refinement program was employed.18
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 5246L electronic counter. The X-band EPR
spectrum of 6 in THF was obtained with a microwave power of
19.922 mW (19.824 mW for 7), a frequency of 9.602 GHz (9.604
GHz for 7), and a modulation amplitude of 0.80 G at 100 kHz.
Acknowledgment. We gratefully acknowledge financial
support from the National Science Council of Taiwan.
Supporting Information Available: X-ray crystallographic files
in CIF format for the structure determinations of [PPN][Ni(SePh)-
(P(o-C6H3-3-SiMe3-2-S)2(o-C6H3-3-SiMe3-2-SH))], [PPN][Ni(Cl)-
(P(o-C6H3-3-SiMe3-2-S)2(o-C6H3-3-SiMe3-2-SH))], [Ni(P(o-C6H3-
3-SiMe3-2-S)2(o-C6H3-3-SiMe3-2-SH))]2, [Ni(P(o-C6H3-3-SiMe3-
2-S)3)]2, [Na-18-crown-6-ether][Ni2(P(o-C6H3-3-SiMe3-2-S)3)2],
[Ni2(P(o-C6H3-3-SiMe3-2-S)3)(P(o-C6H3-3-SiMe3-2-S)2(o-C6H3-3-
SiMe3-2-SCH3))], and [K-18-crown-6-ether]2[Ni2(P(o-C6H3-3-
SiMe3-2-S)3)2]. This material is available free of charge via the
Internet at http://pubs.acs.org.
Preparation of [K-18-crown-6-ether]2[Ni2(P(o-C6H3-3-SiMe3-
2-S)3)2] (8). A THF solution (10 mL) of 6′ (0.155 g, 0.1 mmol)
was added to KC8 solid (0.014 g, 0.1 mmol) in a 30 mL Schlenk
tube under positive nitrogen. The reaction solution was stirred under
nitrogen at ambient temperature for 1 h. A color change of the
reaction solution from brown green to red brown was observed. A
THF solution (5 mL) of 18-crown-6-ether (0.053 g, 0.2 mmol) was
then added to the above red-brown solution by cannula under
positive nitrogen and stirred for another 3 h. The solution was then
IC700719H
(17) Sheldrick, G. M. SADABS, Siemens Area Detector Absorption Cor-
reaction Program; University of Go¨ttingen: Go¨ttingen, Germany,
1996.
(18) Sheldrick, G. M. SHELXTL, Program for Crystal Structure Determi-
nation; Siemens Analytical X-ray Instruments Inc.: Madison, WI,
1994.
Inorganic Chemistry, Vol. 46, No. 21, 2007 8923