Transition metal complexes with sulfur ligands. 117. A reaction cycle for nickel mediated thioester formation from alkyl, CO, and thiolate groups modeling the acetyl-coenzyme A synthase function of CO dehydrogenase

Article abstract of DOI:10.1021/ja952598u

In quest of nickel complexes with sulfur ligation that model the acetyl-CoA synthase function of CO dehydrogenase (CODH), [Ni('S₄C₃Me₂')] (1, 'S₄C₃Me₂'^2- = 1,3-bis(2-mercaptophenylthio)-2,2-dimethyl-propane-(2-)) was synthesized by template alkylation of Na₂[Ni('S₂')₂] ('S₂'^2- = benzene-1,2-dithiolate(2-)) with CMe₂(CH₂Br)₂. Acidic hydrolysis of 1 yielded the thiol "S₄C₃Me₂"-H₂ (2). Reduction of 1 with Na/Hg resulted in cleavage of the 'S₄C₃Me₂'^2- ligand and formation of the thermally stable trinuclear nickel(II) alkyl thiolato complex [Ni('μ-S₂C₃Me₂')]₃ (3, 'S₂C₃Me₂' ^2- = 1-(2-mercaptophenylthio)-2,2-dimethylpropyl(2-)). Treatment of 3 with L = Py, THF, or PMe₃ afforded the mononuclear compounds [Ni('S₂C₃Me₂')(L)] (4, L = Py; 5, L = PMe₃). The stoichiometric reaction of [Ni('S₂C₃Me₂')(L)] with CO led to the cyclic thioester 'S₂C₃Me₂CO' (6, 'S₂C₃Me₂CO' = 2,3-benzo-6,6-dimethyl-8-oxo-1,4-dithia-cyclooctane) and Ni(CO)₄. In the analogous reaction of 5 with CO the intermediate nickel(II) acyl thiolato complex [Ni('S₂C₃Me₂CO')(PMe₃)] (7, 'S₂C₃Me₂CO'(2-) = 1-(2-mercaptophenylthio)-2,2-dimethyl-3-oxobutyl(2-)) could be intercepted and fully characterized. The reaction of Ni(CO)₄ with the thiol 2 yielded the starting Ni(II) complex 1 and allowed to close the reaction cycle that comprises the CODH sequence: [Ni] → [Ni-alkyl] → [Ni-acyl] → [Ni] + thioester. The net reaction can be formulated as 'S₄C₃Me₂'-H₂ (2) + CO → 'S₂C₃Me₂CO' (6) + 'S₂'-H₂ and represents the first example of nickel mediated thioester formation in a complete reaction cycle. X-ray structure determinations of complexes 1,3,4, and 7 revealed approximately square planar coordination geometry for all Ni centers.

Full text of DOI:10.1021/ja952598u

5368
J. Am. Chem. Soc. 1996, 118, 5368-5374
Transition Metal Complexes with Sulfur Ligands. 117.1 A
Reaction Cycle for Nickel Mediated Thioester Formation from
Alkyl, CO, and Thiolate Groups Modeling the
Acetyl-Coenzyme A Synthase Function of CO Dehydrogenase
Dieter Sellmann,* Daniel Ha1ussinger, Falk Knoch, and Matthias Moll
Contribution from the Institut fu¨r Anorganische Chemie der UniVersita¨t, Egerlandstrasse 1,
D-91058 Erlangen (FRG)
ReceiVed August 2, 1995X
Abstract: In quest of nickel complexes with sulfur ligation that model the acetyl-CoA synthase function of CO
dehydrogenase (CODH), [Ni(‘S4C3Me2’)] (1, ‘S4C3Me2’2- ) 1,3-bis(2-mercaptophenylthio)-2,2-dimethyl-propane-
(2-)) was synthesized by template alkylation of Na2[Ni(‘S2’)2] (‘S2’2- ) benzene-1,2-dithiolate(2-)) with
CMe2(CH2Br)2. Acidic hydrolysis of 1 yielded the thiol “S4C3Me2”-H2 (2). Reduction of 1 with Na/Hg resulted in
cleavage of the ‘S4C3Me2’2- ligand and formation of the thermally stable trinuclear nickel(II) alkyl thiolato complex
[Ni(‘µ-S2C3Me2’)]3 (3, ‘S2C3Me2’ 2- ) 1-(2-mercaptophenylthio)-2,2-dimethylpropyl(2-)). Treatment of 3 with L
) Py, THF, or PMe3 afforded the mononuclear compounds [Ni(‘S2C3Me2’)(L)] (4, L ) Py; 5, L ) PMe3). The
stoichiometric reaction of [Ni(‘S2C3Me2’)(L)] with CO led to the cyclic thioester ‘S2C3Me2CO’ (6, ‘S2C3Me2CO’
) 2,3-benzo-6,6-dimethyl-8-oxo-1,4-dithia-cyclooctane) and Ni(CO)4. In the analogous reaction of 5 with CO the
intermediate nickel(II) acyl thiolato complex [Ni(‘S2C3Me2CO’)(PMe3)] (7, ‘S2C3Me2CO’(2-) ) 1-(2-mercaptophen-
ylthio)-2,2-dimethyl-3-oxobutyl(2-)) could be intercepted and fully characterized. The reaction of Ni(CO)4 with the
thiol 2 yielded the starting Ni(II) complex 1 and allowed to close the reaction cycle that comprises the CODH
sequence: [Ni] f [Ni-alkyl] f [Ni-acyl] f [Ni] + thioester. The net reaction can be formulated as ‘S4C3Me2’-H2
(2) + CO f ‘S2C3Me2CO’ (6) + ‘S2’-H2 and represents the first example of nickel mediated thioester formation in
a complete reaction cycle. X-ray structure determinations of complexes 1, 3, 4, and 7 revealed approximately square
planar coordination geometry for all Ni centers.
Introduction
size, polarizability, number of available lone pairs, etc.4 The
donor set causes specific properties of the resultant metal
complexes which can be expressed in terms of structure-function
relationships. Accordingly, the combination of transition metals
and sulfur donors will result in structure-function relationships
that are characteristic for metal sulfur centers. The elucidation
of intrinsic structural and reactivity features of metal sulfur-
ligand complexes is expected to reveal these basic structure-
function relationships. They may also hold for the active centers
of [MS] oxidoredutases, even if these centers differ in metal
nuclearity and specific aspects from the synthetic complexes
under investigation.
In our quest of complexes that model the reactivity of
oxidoreductases containing metal sulfur centers as active sites,
we have been investigating also nickel complexes with thioether
thiolate ligands.2 The ultimate goal of these investigations are
complexes that show enzyme-like catalytic activity but are
capable of existing in the absence of proteins. It is evident that
such complexes cannot be exact structural reduplications of the
active sites of [MS] oxidoreductases, in particular in those cases
in which the protein of the respective enzyme recognizably
warrants the structural integrity and/or the reactivity of the active
sites. A striking example in this respect in the FeMo cofactor
of nitrogenases. The isolated FeMo cofactor does not catalyze
the reduction of N2, and it is relatively short-lived in aqueous
media (τ1/2 ∼ 2 h).3
Nickel sulfur centers have been recognized as constituents
of hydrogenases and CO dehydrogenases (CODH).5,6 The exact
number of sulfur donors and the coordination geometries of the
nickel atoms in these enzymes remain open to discussion,7 and
they possibly vary depending on the enzyme source. Recent
X-ray crystallographic studies on the hydrogenase isolated from
DesulfoVibrio gigas proved that the active site contains a nickel
atom that is coordinated by four cystein sulfur donors and linked
to a second metal of uncertain nature, possibly iron.8 The exact
structure of the active site(s) of CODH is still unknown, but it
However, the most basic structural feature of any metal
complex is the type of ligating donor atoms which can differ in
* Author to whom correspondence should be addressed.
X Abstract published in AdVance ACS Abstracts, May 1, 1996.
(1) Part 116: Sellmann, D.; Kremitzl, H.-J.; Knoch, F.; Moll, M. Z.
Naturforsch., in press.
(2) (a) Sellmann, D.; Hofmann, T.; Knoch, F. Z. Naturforsch. 1994, 49b,
821-826. (b) Sellmann, D.; Prechtel, W.; Knoch, F.; Moll, M. Z.
Naturforsch. 1992, 47b, 1411-1423. (c) Sellmann, D.; Schillinger, H.;
Knoch, F. Z. Naturforsch. 1992, 47b, 748-753. (d) Sellmann, D.;
Schillinger, H.; Knoch, F. Z. Naturforsch. 1992, 47b, 645-655. (e)
Sellmann, D.; Fu¨nfgelder, S.; Knoch, F. Z. Naturforsch. 1991, 46b, 1593-
1600. (f) Sellmann, D.; Fu¨nfgelder, S.; Knoch, F.; Moll, M. Z. Naturforsch.
1991, 46b, 1601-1608. (g) Sellmann, D.; Fu¨nfgelder, S.; Po¨hlmann, G.;
Knoch, F.; Moll, M. Inorg. Chem. 1990, 29, 4772-4778.
(4) Burger, K. Biocoordination Chemistry; Ellis Horwood: New York,
1990; Chapter 1.
(5) Eidsness, M. K.; Sullivan, R. J.; Scott, R. A. In The Bioinorganic
Chemistry of Nickel; Lancaster, J. R., Jr., Ed.; VCH: New York, 1988; pp
83-90.
(6) (a) Halcrow, M. A.; Christou, G. Chem. ReV. 1994, 94, 2421-2481.
(b) Kolodziej, A. F. In Progress in Inorganic Chemistry; Karlin, K. D.,
Ed.; Wiley: New York, 1994; Vol. 41, pp 493-597.
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S0002-7863(95)02598-4 CCC: $12.00 © 1996 American Chemical Society

Transition Metal Complexes with Sulfur Ligands
J. Am. Chem. Soc., Vol. 118, No. 23, 1996 5369
Chart 1
In order to prevent these reductive CnH2n eliminations, we
have now synthesized complex 1 and investigated its reactivity.
Complex 1 contains a dimethylpropylene bridge between the
C6H4S2 units. This bridge has no â-hydrogen atoms at the
central C atom and is also found in the tetradentate crown
thioether Me8[16]aneS4 (Me8[16]aneS4 ) 3,3,7,7,11,11,15,15-
octamethyl-1,5,9,13-tetrathia-cyclohexadecane) which is re-
markably stable toward cleavage even under strongly reducing
conditions.17
Chart 1 summarizes ligands and abbreviations. Preliminary
results have been published elsewhere.18
Experimental Section
General Methods. Unless noted otherwise, all reactions and
operations were carried out under nitrogen by using standard Schlenk
19
techniques. Solvents were dried and distilled before use. ‘S2’-H2
and BrCH2CMe2CH2Br20 were prepared as described in the literature.
Instruments used for physical measurements were as follows: IR, Zeiss
IMR 25 and Perkin-Elmer 983; NMR, JNM-GX 270 and JNM-EX 270;
and mass spectra, Varian MAT 212. Cyclic voltammetry (CV) was
carried out on an EG&G Potentiostat PAR Model 264A with a Rotel
A glassy carbon working electrode, Pt counter electrode, and Ag/AgCl
reference electrode. MeCN was used as solvent, NBu4PF6 (0.1 M) as
supporting electrolyte and ferrocene/ferrocenium as internal standard
is taken for granted that the catalysis of acetyl-CoA synthesis
being one of the key functions of CODH takes place at a nickel
iron site.9 The nickel atom has been shown to have a sulfur
rich coordination sphere7 and to be spin-coupled to an [FexSy]
cluster.10 EXAFS investigations on CODH from Clostridium
thermoaceticum suggest that a distorted square planar [NiS4]
unit is bound to the [FexSy] cluster.7a,11
21
+
(EFc/Fc (MeCN) ) +680 mV vs NHE). All potentials are referred to
NHE.
CODH catalyses the syntheses of acetyl-CoA from CO, a
CH3 group, and CoA.6,12 Recent mechanistic studies indicate
that CO initially coordinates to the [FexSy] cluster,13 while the
CH3 group binds to the nickel atom.14 Two alternative pathways
are discussed for the subsequent acetyl formation either at the
iron center or at the nickel center.15,16 In both cases, acetyl-
CoA is assumed to form from metal bound acetyl groups and
CoA.
In our search for well defined nickel sulfur complexes that
allow a stepwise combination of CO, alkyl, and thiol groups to
give thioesters, we have previously observed that the CnH2n
bridges of complexes such as [Ni(‘S4C3’)], ([1,3-bis(2-mercap-
tophenylthio)propanenickel(II)]), and [Ni(‘S5’)], ([2,2′-bis(2-
mercaptophenylthio)diethylsulfidenickel(II)]) are easily removed
by reduction to yield the anion [Ni(‘S2’)2]2-, ([bis(benzene-1,2-
dithiolato)nickelate(II)])2f (cf. Chart 1).
X-rayStructureDeterminationsof[Ni(‘S4C3Me2’)](1),[Ni(‘µ-S2C3-
Me2’)]3 (3), [Ni(‘S2C3Me2’)(py)] (4), and [Ni(‘S2C3Me2CO’)(PMe3)]
(7). Black plates of 1 were obtained by slowly cooling a hot (100 °C)
saturated solution of 1 in DMF to room temperature. Deep-red cubes
of 3 formed by layering a THF solution of 3 with MeOH, and red
plates of 4 were formed by layering a pyridine solution of 4 with hexane.
Yellow plates of 7 separated from a THF/MeOH solution of 7 which
was cooled to -78 °C. Suitable single crystals were sealed under N2
in glass capillaries. The structures were solved by direct methods
(SHELXTL-PLUS). Non-hydrogen atoms were refined anisotropically,
the positions of the hydrogen atoms of 1 and 3 were calculated for
ideal geometry, and the positions of the hydrogen atoms of 4 and 7
were taken from difference Fourier maps. All hydrogen atom positions
were restricted during refinement with common isotropic thermal
parameters. Crystal and data collection parameters for 1, 3, 4, and 7
are summarized in Table 1.
Syntheses. [Ni(‘S4C3Me2’)] (1). (a) Template Alkylation of
Na2[Ni(‘S2’)2] with BrCH2CMe2CH2Br. ‘S2’-H2 (6.29 g, 44.2 mmol)
and subsequently Ni(ac)2‚4H2O (5.50 g, 22.1 mmol) in 60 mL of MeOH
were added to a solution which had been obtained by reaction of
metallic sodium (2.03 g, 88.3 mmol) with 100 mL of MeOH. The
resultant dark-brown solution was stirred for 20 min and evaporated
to dryness, and the remaining residue was dissolved in 150 mL of DMF
and combined with BrCH2CMe2CH2Br (9.15 g, 39.8 mmol) yielding a
green-brown suspension. It was stirred for 18 h at 135 °C in the course
of which a finely divided brown precipitate formed. The solvent was
removed at 100 °C, and the black residue was digested with 150 mL
of MeOH, separated, rinsed with MeOH (125 mL), THF (10 mL), and
Et2O (10 mL), and recrystallized from DMF (100 °C/20 °C). Yield:
7.33 g (81%). Anal. Calcd for C17H18NiS4 (409.30): C, 49.89; H, 4.43;
S, 31.34. Found: C, 50.01; H, 4.48; S, 31.14. MS(FD): m/z ) 408
(M+). 1H NMR (270 MHz, DMF-d7) δ 7.68 (d, 2, C6H4), 7.40 (d, 2,
C6H4), 7.20 (t, 2, C6H4), 7.05 (t, 2, C6H4), 3.57 (s, 4, CH2), 1.40 (s, 6,
(7) (a) Bastian, N. R.; Diekert, G.; Niederhoffer, E. C.; Teo, B.-K.; Walsh,
C. T.; Orme-Johnson, W. H. J. Am. Chem. Soc. 1988, 110, 5581-5582.
(b) Yamamura, T.; Nakamura, N.; Yasui, A.; Sasaki, A.; Arai, H. Chem.
Lett. 1991, 875-878. (c) Scott, R. A. Physica B (Amsterdam) 1989, 158,
84-86. (d) Tan, G. O.; Ensign, S. A.; Ciurli, S.; Scott, M. J.; Holm, R.
H.; Luden, P. W. Proc. Natl. Acad. Sci. USA 1992, 89, 4427-4431. (e)
Cammack, R.; Fernandez, V. M.; Schneider, K. In The Bioinorganic
Chemistry of Nickel; Lancaster, J. R., Jr., Ed.; VCH: New York, 1988; p
180.
(8) (a) Volbeda, A.; Charon, M. H.; Piras, C.; Hatchikian, E. C.; Frey,
M.; Fontecilla-Camps, J. C. Nature 1995, 373, 580-587. (b) Volbeda,
A.; Piras, C.; Charon, M. H.; Hatchikian, E. C.; Frey, M.; Fontecilla-Camps,
J. C. ESF/CCP4 Newsletter on Protein Crystallogr. 1993, 28, 30-34.
(9) Ragsdale, S. W. CRC Crit. ReV. Biochem. Mol. Biol. 1991, 26, 261-
300.
(10) Fan, C.; Gorst, C. M.; Ragsdale, S. W.; Hoffman, B. M. Biochemistry
1991, 30, 431-435.
(11) Cramer, S. P.; Eidsness, M. K.; Pan, W.-H.; Morton, T. A.; Ragsdale,
S. W.; DerVartanian, D. V.; Ljungdale, L. G.; Scott, R. A. Inorg. Chem.
1987, 26, 2477-2479.
(17) Yoshida, T.; Adachi, T.; Ueda, T.; Kaminaka, M.; Higuchi, T. J.
Am. Chem. Soc. 1988, 110, 4872-4873.
(18) Sellmann, D.; Ha¨ussinger, D.; Knoch, F.; Moll, M. J. Bioinorg.
Chem. 1993, 51, 190.
(12) Ragsdale, S. W.; Wood, H. G.; Morton, T. A.; Ljungdahl, L. G.;
DerVartanian, D. V. In The Bioinorganic Chemistry of Nickel; Lancaster,
J. R., Jr., Ed.; VCH: New York, 1988; Chapter 14.
(13) (a) Qiu, D.; Kumar, M.; Ragsdale, S. W.; Spiro, T. G. Science 1994,
264, 817-819. (b) Qiu, D.; Kumar, M.; Ragsdale, S. W.; Spiro, T. G. J.
Am. Chem. Soc. 1995, 117, 2653-2654.
(19) Degani, J.; Fochi, R. Synthesis 1976, 7, 471-472.
(20) (a) Whitmore, F. C.; Popkin, A. H.; Bernstein, H. I.; Wilkins, J. P.
J. Am. Chem. Soc. 1941, 63, 124-127. (b) Shortridge, R. W.; Craig, R.
A.; Greenlee, K. W.; Derfer, J. M.; Boord, C. E. J. Am. Chem. Soc. 1948,
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5370 J. Am. Chem. Soc., Vol. 118, No. 23, 1996
Sellmann et al.
Table 1. Crystal and Data Collectiona Parameters of [Ni(‘S4C3Me2’)] (1), [Ni(‘µ-S2C3Me2’)]3 (3), [Ni(‘S2C3Me2’)(py)] (4), and
[Ni(‘S2C3Me2CO’)(PMe3)] (7)
compd
[Ni(‘S4C3Me2’)]
C17H18NiS4
409.30
orthorhombic
Pbca
1354.8(5)
1182.4(5)
2161.8(8)
[Ni(‘µ-S2C3Me2’)]3
C33H42Ni3S6
807.22
cubic
Pa3h
1926.6(3)
[Ni(‘S2C3Me2’)(py)]
C16H19NNiS2
348.17
monoclinic
P21/c
1073.4(3)
972.1(2)
1614.8(2)
93.33(1)
1.682(1)
4
[Ni(‘S2C3Me2CO’)(PMe3)]
C15H23NiOPS2
373.16
monoclinic
P21/c
1101.1(2)
1545.8(2)
1109.2(2)
110.67(1)
1.766(1)
4
formula
formula weight
crystal system
space group
a [pm]
b [pm]
c [pm]
â [deg]
V [nm3]
Z
3.462(2)
8
1.57
15.78
293
6.1; 5.6
7.153(3)
8
1.50
19.31
293
5.4; 4.3
F
cald [gcm-1
]
1.37
13.91
293
4.3; 4.3
1.40
14.14
293
3.6; 3.3
µ [cm-1
]
T[K]
c
R;b Rw [%]
c
a All data collected with graphite-monochromatized Mo KR radiation (λ ) 71.073 pm). b R ) ∑||F0| - |Fc||/∑|F0|. Rw ) ∑[w(|F0| - |Fc|)]/
∑[w|F0|].
CH3). 13C{1H} NMR (67.8 MHz, DMSO-d6) δ 153.0, 131.0, 130.0,
129.5, 128.0, 123.0 (C6H4), 50.3 (CH2), 34.5 (CH3), 29 (Cq).
(d, 1, C6H4), 7.17 (d, 1, C6H4), 7.08 (t, 1, C6H4), 6.91 (t, 1, C6H4), 8.76
(s, C5H5N), 7.54 (s, C5H5N), 7.19 (s, C5H5N), 3.03 (s, br, 1, SCH2),
2.73 (s, br, 1, SCH2), 1.70 (s, br, 1, NiCH2), 1.11 (s, br, 7, NiCH2,
CH3).
(b) From Ni(CO)4 and ‘S4C3Me2’-H2 (2). Ni(CO)4 (120 µL, 153
mg, 0.91 mmol) was added to ‘S4C3Me2’-H2 (320 mg, 0.91 mmol) in
20 mL of THF. The colorless solution slowly turned brown. In the
course of 20 days dark-brown microcrystals of 1 separated, which were
isolated and characterized by IR and NMR spectroscopy. Yield: 260
mg (70%).
[Ni(‘S2C3Me2CO’)(PMe3)] (7). PMe3 (150 mg, 1.99 mmol) was
added to [Ni(‘µ-S2C3Me2’)]3 (535 mg, 0.663 mmol) in 25 mL of THF
yielding a red-brown solution of [Ni(‘S2C3Me2’)(PMe3)]. CO (50 mL,
2.2 mmol) was injected via a syringe in several portions in the course
of 4 h. The color of the solution changed to yellow-brown, MeOH
(30 mL) was added, and the resultant solution was cooled to -78 °C.
Yellow microcrystals precipitated which were separated after 16 h were
rinsed with Et2O and dried for 1 day in vacuo. Yield: 700 mg (94%).
Anal. Calcd for C15H23NiOPS2 (373.16): C, 48.28; H, 6.21. Found:
C, 48.09; H, 6.15. IR(KBr) νj ) 1630 cm-1 (νCO). 1H NMR (270
MHz, THF-d8) δ 7.47 (d, 1, C6H4), 7.41 (d, 1, C6H4), 6.98 (t, 1, C6H4),
6.83 (t, 1, C6H4), 3.31 (s, 2, SCH2), 2.45 (s, br, 2, COCH2), 1.33 (d,
2JPH ) 9.7 Hz, 9, P(CH3)3), 1.10 (s, 6, CH3). 13C{1H} NMR (67.8
MHz, THF-d8) δ 262.3 (d, 3JPC ) 23.5 Hz, CO) 155.8 (d, 3JPC ) 13.4
‘S4C3Me2’-H2 (2). [Ni(‘S4C3Me2’)] (1) (2.20 g, 5.38 mmol) was
suspended in 50 mL of CH2Cl2 and treated with 25 mL of concentrated
hydrochloric acid for 5 h. The CH2Cl2 phase was separated, and the
remaining green-brown aqueous phase was extracted with CH2Cl2 (50
mL). The CH2Cl2 extracts were combined, washed with water, dried
over Na2SO4, and evaporated yielding a brownish oil, which was
redissolved in CCl4 and filtered over SiO2. Removal of the solvent
yielded ‘S4C3Me2’-H2 as a colorless viscous oil, which was dried for 2
days in vacuo. Yield: 1.45 g (76%). Anal. Calcd for C17H20S4
(352.60): C, 57.91; H, 5.72; S, 36.37. Found: C, 58.38; H, 5.69; S,
34.76. MS(FD): m/z ) 352 (M+). IR(film): νj ) 2520 cm-1 (νSH).
1H NMR (270 MHz, CDCl3) δ 7.49-7.40 (m, 2, C6H4), 7.36-7.27
(m, 2, C6H4), 7.17-7.02 (m, 4, C6H4), 4.31 (s, 2, SH), 3.05 (s, 4, CH2),
1.20 (s, 6, CH3). 13C{1H} NMR (67.8 MHz, CDCl3) δ 136.4, 134.9,
133.1, 129.7, 128.1, 126.5 (C6H4), 47.5 (CH3), 37.7 (CMe2), 27.4 (CH2).
[Ni(‘µ-S2C3Me2’)]3 (3). [Ni(‘S4C3Me2’)] (1.215 g, 2.97 mmol) in
15 mL of THF was added under vigorous stirring to sodium amalgam,
which had been freshly prepared from mercury (200 g) and sodium
metal (1.5 g, 65 mmol). In the course of 1 h the color of the organic
phase changed from brown to deep red. The organic layer was
separated, centrifugated under an atmosphere of argon in order to
remove any undissolved particles, reduced in volume to 3 mL, and
mixed with 25 mL of MeOH. Red crystals of [Ni(‘µ-S2C3Me2’)]3
formed that were separated after 4 h, rinsed with MeOH, and dried.
Yield: 330 mg (41%). Anal. Calcd for C33H42Ni3S6 (807.22): C,
49.10; H, 5.24; Ni, 21.82; S 23.83. Found: C, 49.05; H, 5.35; Ni,
21.72; S, 23.23. 1H NMR (270 MHz, THF-d8) δ 7.4-7.0 (m, 12, C6H4),
3.30 (dd, 4JHH ) 2.4 Hz, 2JHH ) 9.5 Hz, 3, SCH2), 3.05 (d, 2JHH ) 9.5
Hz, 3, SCH2), 1.20 (dd, 4JHH ) 2.4 Hz, 2JHH ) 9.7 Hz, 3, NiCH2), 1.08
Hz) 136.3, 131.0, 130.6, 128.6, 121.5 (C6H4), 61.1 (d, 3JPC ) 4.3 Hz),
1
53.1 (SCH2, CH2CO), 33.7 (CMe2), 28 (br, C(CH3)2), 14.2 (d, JPC
29.5 Hz, P(CH3)3).
)
‘S2C3Me2CO’ (6). (a) From [Ni(‘µ-S2C3Me2’)]3 and CO. In
several portions, CO (200 mL, 8.93 mmol) was injected via a syringe
into a red suspension of [Ni(‘S2C3Me2’)]3 (250 mg, 0.31 mmol) in 40
mL of THF. A colorless solution resulted which was stirred for 1 h
and evaporated to dryness, volatile materials being condensed in a trap
at -196 °C. ‘S2C3Me2CO’ remained as colorless oil, and in the trap
Ni(CO)4 was identified by its νCO IR band at 2042 cm-1. Yield of 6:
210 mg (95%). MS(FD): m/z ) 238 (M+). IR(CHCl3) νj ) 1671
cm-1 (νCO). 1H NMR (270 MHz, CDCl3) δ 7.90 (dd, 1, C6H4), 7.62
(dd, 1, C6H4), 7.48-7.33 (m, 2, C6H4), 2.81 (s, 2, CH2), 2.41 (s, 2,
CH2), 1.10 (s, 6, CH3). 13C{1H} NMR (67, 8 MHz, CDCl3) δ 200.5
(CO), 141.3, 138.2, 137.5, 133.2, 130.7, 129.3 (C6H4) 51.9, 50.3 (CH2),
28 (br, CH3).
(b) From [Ni(‘S2C3Me2’)(PMe3)] and CO. [Ni(‘S2C3Me2’)(PMe3)]
was synthesized in situ from [Ni(‘µ-S2C3Me2’)]3 (620 mg, 0.77 mmol)
and PMe3 (175 mg, 2.3 mmol) in 50 mL of THF. CO (52 mL, 2.3
mmol) was injected via a syringe, and the formation of [Ni(‘S2C3Me2-
CO’)(PMe3)] was monitored by IR spectroscopy. A gentle stream of
CO was bubbled through the resultant yellow solution, and the
temperature was raised to 60 °C in the course of which the solution
became colorless. IR spectroscopic monitoring proved the formation
of Ni(CO)4 and ‘S2C3Me2CO’.
2
(s, 9, CH3), 1.06 (s, 9, CH3) 0.80 (d, JHH ) 9.7 Hz, 3, NiCH2). 13C-
{1H} NMR (67.8 MHz, THF-d8) δ 144.4, 142.9, 136.5, 129.3, 128.0,
125.8 (C6H4), 56.4 (SCH2), 46.4, 45.0 (CH3), 30.9, 28.8 (NiCH2, CMe2).
A second set of signals was observed that was assigned to the solvent
complex [Ni(‘S2C3Me2’)(THF)]: 1H NMR (270 MHz, THF-d8) δ 7.4-
2
7.0 (m, 4, C6H4), 3.22 (d, br, JHH ) 8.7 Hz, 1, SCH2), 2.88 (d, br,
2JHH ) 8.7 Hz, 1, SCH2), 1.15 (br, 1, NiCH2), 1.05 (s, br, 3, CH3),
2
0.93 (s, 3, CH3) 0.19 (d, JHH ) 9.1 Hz, 1, NiCH2). 13C{1H} NMR
Results
(67.8 MHz, THF-d8) δ 134.5, 131.3, 129.8, 127.8, 127.5, 126.2 (C6H4),
55.0 (SCH2), 45.6, 43.0 (CH3), 30.1, 29.1 (NiCH2, CMe2).
Syntheses and Reactions. [Ni(‘S4C3Me2’)] (1) was obtained
by template alkylation of Na2[Ni(‘S2’)2] according to eq 1.
In contrast to the template syntheses of related complexes
such as the parent compund [Ni(‘S4C3’)] or [Ni(‘NHS4’)] which
occur at room temperature,2g the synthesis of 1 required drastic
reaction conditions. Brown 1 is stable toward air, diamagnetic,
and well soluble in DMF and DMSO. Number and splitting of
[Ni(‘S2C3Me2’)(py)] (4). [Ni(‘S2C3Me2’)]3 (245 mg, 0.304 mmol)
was dissolved in 10 mL of pyridine yielding an orange solution. When
30 mL of n-hexane was added under stirring, a dark-orange powder
precipitated. It was separated after 12 h, rinsed with n-hexane, and
dried in vacuo for 5 h. Yield: 270 mg (85%). Anal. Calcd for C16H19-
NNiS2 (348.17): C, 55.20; H, 5.50; N, 4.02; S, 18.42. Found: C, 55.30;
H, 5.97; N, 4.06; S, 18.47. 1H NMR (270 MHz, pyridine-d5) δ 7.74

Transition Metal Complexes with Sulfur Ligands
J. Am. Chem. Soc., Vol. 118, No. 23, 1996 5371
the signals in the 1H NMR and 13C{1H} NMR spectra indicated
that 1 contains a twofold element of symmetry in solution.
Decoordination of the ligand and synthesis of the neutral thiol
‘S4C3Me2’-H2 (2) was achieved by hydrolysis of 1 with
hydrochloric acid according to eq 2.
Figure 1. Cyclic voltammogram of [Ni(‘S4C3Me2’)] (10-4 M in MeCN,
10-1 M NBu4PF6, ν ) 100 mVs-1, 298 K, E vs NHE).
mononuclear [Ni(‘S2C3Me2’)(L)] complexes was proved by the
reaction of 3 with pyridine or PMe3 yielding [Ni(‘S2C3-
Me2’)(py)] (4) and [Ni(‘S2C3Me2’)(PMe3)] (5) according to eq
4.
2 is a colorless oil, exhibits a characteristic, sharp νSH band
at 2520 cm-1 in its IR spectrum, and, according to its NMR
spectra, also contains a twofold element of symmetry.
Cyclic voltammetry indicated reversible reducibility of 1
(Figure 1).
On account of this, we tried to reduce 1 also chemically by
lithium butyl or sodium naphthalene. In both cases, however,
desalkylation of 1 took place leading to removal of the C3Me2
Yellow-red 4 dissolves only in pyridine without decomposi-
tion. In solvents such as THF, DMF, or CH2Cl2, the pyridine
ligand dissociated and trinuclear 3 regenerated. The molecular
structure of 4 was determined by X-ray structure analysis. The
bridge between the ‘S2’ units and formation of [Ni(‘S2’)2]2-
.
In a further experiment we used sodium amalgam in THF in
order to reduce 1. Again, no Ni(I) or Ni(0) species could be
isolated, but cleavage of S-C bonds took place. In this case,
however, not the C3Me2 bridge but one of the ‘S2’ units of 1
was eliminated, and trinuclear [Ni(‘µ-S2C3Me2’)]3 (3) formed
according to eq 3.
1
number of signals in the H NMR spectrum showed that 4 is
C1 symmetric also in solution because the protons of the CH2
groups give rise to four separate signals. Complex 5 was
prepared in situ only and characterized by its reaction product
with CO (see below).
In a “side-step”, which later on proved important, we tried
to add oxidatively SH bonds of the neutral ‘S4C3Me2’-H2, (2)
to Ni(CO)4 in order to obtain nickel sulfur hydride complexes.
Such complexes could not be obtained; however, at ambient
temperatures the Ni(II) complex 1 formed according to eq 5 in
good yields. The fate of electrons, protons, and CO which must
be released in this reaction was not elucidated.
Formation of 1,2-benzenedithiolate, ‘S2’2-, was detected by
1H NMR spectroscopy, and 3 was isolated as red microcrystals.
Complex 3 contains one nickel alkyl bond per [Ni(‘µ-S2C3Me2’)]
unit and possesses C3 symmetry in solid state. It is thermally
stable up to +60 °C and thus exhibits a remarkably high stability
for nickel(II) alkyl complexes.22 Complex 3 dissolves sparingly
only in polar solvents such as DMF or THF. NMR spectra of
such solutions indicate that 3 partially dissociates forming
mononuclear solvent complexes such as [Ni(‘S2C3Me2’)(L)], L
One of the key reactions catalyzed by CO dehydrogenase is
the formation of C-C bonds between CH3 and CO groups.23
Such reactions could be observed when 3, 4, and 5 reacted with
CO. For example, the reaction of CO with 3 led to instanta-
neous formation of Ni(CO)4 and of the thioester ‘S2C3Me2CO’
(6) according to eq 6.
1
) DMF or THF. For instance, the H NMR spectrum of 3 in
THF exhibits one set of four sharp CH2 and two sharp CH3
signals, which can be assigned to [Ni(‘µ-S2C3Me2’)]3, and, in
addition, a second and less intensive set of slightly broadened
CH2 and CH3 signals that indicate another species [Ni(‘S2C3-
Me2’)(L)]. In accordance, the 13C{1H} NMR spectrum of 3 in
THF exhibited one set of 11 intensive signals and a second set
of 11 weak signals. Dissociation of 3 and formation of
Monitoring this reaction by IR spectroscopy showed that the
reaction was complete after addition of 15 equiv of CO. (15
CO is the stoichiometric amount of CO that is required in order
to convert 1 mol of 3 into Ni(CO)4 and the thioester 6). Ni(CO)4
(22) Wilke, G. Angew. Chem., Int. Ed. Engl. 1988, 27, 185-206.
(23) Kumar, M.; Ragsdale, S. W. J. Inorg. Biochem. 1993, 51, 233.

5372 J. Am. Chem. Soc., Vol. 118, No. 23, 1996
Sellmann et al.
Figure 2. Molecular structures (H atoms omitted) of (a) [Ni(‘S4C3Me2’)] (1), (b) [Ni(‘µ-S2C3Me2’)]3 (3), (c) [Ni(‘S2C3Me2’)(py)] (4), and (d)
[Ni(‘S2C3Me2CO’)(PMe3)] (7).
was identified by its νCO IR band at 2042 cm-1; 6 was isolated
as a colorless oil in nearly quantitative yield (95%) and
characterized by NMR and mass spectroscopy and its IR band
at 1678 cm-1 in THF solution.
unaffected by CO at room temperature it reacts at 60 °C with
an excess of CO yielding again the thioester 6, Ni(CO)4, and
PMe3 according to eq 8.
In order to trap potential intermediates the reaction according
to eq 6 was also carried out with a highly concentrated
suspension of 3 in THF. In this case, in addition of 6 and
Ni(CO)4 an orange powder formed. The IR spectrum (KBr) of
the orange powder exhibited strong νCO bands at 1658 and 1641
cm-1 indicating the formation of acyl complexes. However,
1H-NMR spectra of solutions of the orange powder showed it
to be a mixture of products which so far could neither be
separated nor characterized unambiguously.
X-ray Structure Analyses of [Ni(‘S4C3Me2’)] (1), [Ni(‘µ-
S2C3Me2’)]3 (3), [Ni(‘S2C3Me2’)(py)] (4), and [Ni(‘S2C3Me2-
CO’)(PMe3)] (7). Figure 2 shows the molecular structures of
1, 3, 4, and 7; Table 2 lists selected distances and angles.
In all four complexes, the nickel centers exhibit approximately
planar coordination geometries. The donor sets comprise four
sulfur (1), three sulfur and one carbon (2), two sulfur, one
carbon, and one nitrogen (4), or two sulfur, one carbon, and
one phosphorus atom (7). The structural parameters show that
NiS distances can cover a wide range from 209-229 pm. This
may be traced back to particular steric constraints as well as
bonding situations. 1 containing only thiolate and thioether S
donors revealed no anomalies. The Ni-S(thiolate) and Ni-
S(thioether) distances lie in the expected range of 217-220
pm,2g,26,27 and their nearly identical values can be traced back
to the σ-donor-π-donor and σ-donor-π-acceptor character of
the respective bonds.28 Less common is the molecular structure
In contrast, a fully characterizable acyl complex could be
isolated in pure form when the PMe3 complex 5 was treated
with 1 equiv of CO according to eq 7.
[Ni(‘S2C3Me2CO’)(PMe3)] (7) was obtained as yellow crystals
which proved thermally stable in solid state and in THF solutions
up to 60 °C. Complex 7 exhibits a νCO IR band at 1630 cm-1
which compares to the νCO bands of other Ni(II) acyl complexes
such as [Ni(COCH3)(PMe3)2(acac)] (1634 cm-1 24
and
)
(26) Sellmann, D.; Schillinger, H.; Knoch, F. Inorg. Chim. Acta 1992,
198, 351-357.
(27) (a) Desper, J. M.; Gellman, S. H.; Wolf, R. E., Jr.; Cooper, S. R. J.
Am. Chem. Soc. 1991, 113, 8663-8671. (b) Baidya, N.; Mascharak, P.
K.; Stephan, D. W.; Campagna, C. F. Inorg. Chim. Acta 1990, 177, 233-
238.
[Ni(COCH3)(N(SiMe2CH2PPh2)2)] (1615 cm-1).25 Complex 7
was further characterized by NMR spectroscopy and X-ray
structure analysis proving its mononuclearity. Although 7 is
(24) Klein, H.-F.; Karsch, H. H. Chem. Ber. 1976, 109, 2524-2532.
(25) Fryzuk, M. D.; MacNeil, P. A.; Rettig, S. J. J. Organomet. Chem.
1987, 332, 345-360.
(28) Sellmann, D.; Neuner, H.-P.; Knoch, F. Inorg. Chim. Acta 1991,
190, 61-69.

Transition Metal Complexes with Sulfur Ligands
J. Am. Chem. Soc., Vol. 118, No. 23, 1996 5373
Table 2. Selected Distances (pm) and Angles (deg) of 1, 3, 4, and
7
Scheme 1
[Ni(‘S4C3Me2’)] (1)
Ni(1)-S(1)
Ni(1)-S(2)
Ni(1)-S(3)
Ni(1)-S(4)
217.3(2)
219.1(2)
217.9(2)
217.7(3)
S(1)-Ni(1)-S(2)
S(1)-Ni(1)-S(3)
S(2)-Ni(1)-S(3)
S(1)-Ni(1)-S(4)
S(2)-Ni(1)-S(4)
S(3)-Ni(1)-S(4)
91.8(1)
176.6(1)
87.7(1)
88.6(1)
171.3(1)
91.4(1)
[Ni(‘µ-S2C3Me2’)]3 (3)
Ni(1)-S(1)
Ni(1)-S(2)
Ni(1)-C(3)
Ni(1)-S(1A)
228.9(2)
213.4(2)
195.0(6)
219.1(2)
S(1)-Ni(1)-S(2)
S(1)-Ni(1)-C(3)
S(2)-Ni(1)-C(3)
S(1)-Ni(1)-S(1A)
S(2)-Ni(1)-S(1A)
C(3)-Ni(1)-S(1A)
87.8(1)
159.9(2)
86.7(2)
91.6(1)
172.1(1)
96.3(2)
[Ni(‘S2C3Me2’)(py)] (4)
Ni(1)-S(1)
Ni(1)-S(2)
Ni(1)-N(1)
Ni(1)-C(18)
224.2(3)
209.7(3)
191.9(7)
194.8(10)
S(1)-Ni(1)-S(2)
S(1)-Ni(1)-N(1)
S(2)-Ni(1)-N(1)
S(1)-Ni(1)-C(18)
S(2)-Ni(1)-C(18)
N(1)-Ni(1)-C(18)
89.9(1)
96.6(2)
166.6(2)
160.4(3)
85.7(3)
91.8(4)
Complex 1 also results from the reaction of the neutral
thioether-thiol ‘S4C3Me2’-H2 (2) and Ni(CO)4. This reaction
possibly involves oxidative addition of S-H bonds to the
nickel(0) center leading to an intermediary nickel sulfur hydride
species. This species, however, could not be intercepted.
[Ni(‘S2C3Me2CO’)(PMe3)] (7)
In contrast to the parent complex [Ni(‘S4C3’)],2f the dimethyl
derivative 1 is redoxactive and exhibits a quasi-reversible
cathodic wave in the cyclic voltammogram. This wave can
tentatively be assigned to the formation of an Ni(I) species.
Efforts to synthesize this Ni(I) species by chemical reduction
of 1 remained unsuccessful. However, reduction of 1 by sodium
amalgam yielded the trinuclear Ni(II) alkyl complex 3. Inves-
tigation of the reactivity of this trinuclear complex 3 toward
two electron donors including CO has led to a reaction cycle
that relates to the acetyl-CoA formation catalyzed by CODH.
The cycle models the stepwise formation of nickel bound alkyl
and acyl groups, the nickel mediated formation of thioesters,
and the final regeneration of the starting complex. The cycle
is outlined in Scheme 1, and it shows only those key intermedi-
ates that have been completely characterized.
Ni(1)-S(1)
Ni(1)-S(2)
Ni(1)-P(1)
Ni(1)-C(1)
C(1)-O(1)
221.2(3)
217.4(2)
216.6(3)
187.3(6)
121.2(5)
S(1)-Ni(1)-S(2)
S(1)-Ni(1)-P(1)
S(2)-Ni(1)-P(1)
S(1)-Ni(1)-C(1)
S(2)-Ni(1)-C(1)
P(1)-Ni(1)-C(1)
91.5(1)
94.7(1)
170.7(1)
173.3(2)
86.5(2)
88.0(2)
of 3. Chiral 3 contains a C3 axis and consists of three identical
subunits which are linked to each other via bridging thiolate
donors. The Ni-S distances distinctly differ and cover the wide
range of 213-229 pm. Only the Ni-S(1A) distance (219.1(2)
pm) in the Ni-S(thiolate) bridge lies in the expected range.
The Ni-S(2)(thioether) distance trans to this bond is relatively
short (213.4(2) pm), and the Ni-S(1) distance (228.9(2) pm)
trans to the alkyl ligand is strongly elongated, probably due to
the trans influence of the alkyl group.29 The central [Ni3S3]
ring exhibits a chair conformation like cyclohexane such that
the square planar coordination of each Ni center becomes
tetrahedrally distorted. The Ni-C(1) distance of 3 compares
with those in other Ni alkyl complexes.26,30 4, too, exhibits a
significant elongation of the Ni-S(1) bond (224.2(3) pm) that
is trans to the alkyl ligand. In contrast, the Ni-S(2)thioether
distance (209.7(3) pm) is extraordinarily short, while Ni-C and
Ni-N distances are in the expected range. The Ni-C(1)
distance (187.3(6) pm) of the acyl complex 7 is rather similar
to that of other square planar nickel acyl complexes,30,31 the
Ni-S distances (221.2(3) and 217.4(2) pm) are in the expected
range.
A two electron reduction of 1 yields 3. In the presence of
donor ligands L (L ) Py, THF, DMF, PMe3), complex 3
reversibly dissociates into mononuclear four coordinate nickel
alkyl complexes [Ni(‘S2C3Me2’)(L)] of which 4 (L ) py) has
been isolated. The mononuclear complexes react with CO in
order to give the acyl complexes [Ni(‘S2C3Me2CO’)(L)] of
which 7 (L ) PMe3) could be trapped. In the presence of an
excess of CO the acyl complexes give the cyclic thioester ‘S2C3-
Me2CO’ (6) and Ni(CO)4. Finally, the reaction of Ni(CO)4 with
the thioether-thiol 2 regenerates 1 and closes the reaction cycle.
Thus, the net reaction is the nickel mediated formation of a
thioester from alkyl, CO, and thiol groups and can be expressed
by eq 9.
Discussion
In the search for nickel complexes that possess sulfur rich
coordination spheres and are able to model reactions catalyzed
by CO dehydrogenase, the new sulfur ligand ‘S4C3Me2’-H2 (2)
and its nickel complex [Ni(‘S4C3Me2’)] (1) have been synthe-
sized. Complex 1 contains nickel in a square planar coordina-
tion of sulfur donors and does not exhibit biologically irrelevant
ligands such as phosphines or cyclopentadienyl.
Furthermore, a comparison of eq 9 and the reaction sequence
in Scheme 1 demonstrates that the nickel center performs two
functions. Firstly, it acts as mediator of the two electron transfer
reactions during which S-C bonds are cleaved and formed (1
f 3 and 7 f 6). Secondly, it facilitates the formation of an
acyl group from an alkyl group and CO (4 f 7). The formation
of acyl groups from metal bound alkyl groups and gaseous CO
is well established and presumably requires prior coordination
(29) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry, 5th
ed.; Wiley: New York, 1988; p 1299.
(30) (a) Carmona, E.; Gonza´lez, F.; Poveda, M. L.; Atwood, J. L.; Rogers,
R. D. J. Chem. Soc., Dalton Trans. 1980, 2108-2115. (b) Carmona, E.;
Gonza´lez, F.; Poveda, M. L.; Atwood, J. L.; Rogers, R. D. J. Chem. Soc.,
Dalton Trans. 1981, 777-782.
(31) Huttner, G.; Orama, O.; Bejenke, V. Chem. Ber. 1976, 2533-2536.

5374 J. Am. Chem. Soc., Vol. 118, No. 23, 1996
Sellmann et al.
of CO in cis position to the alkyl group.32,33 Such a coordination
of CO can be expected to be favored if the metal center is
coordinatively unsaturated. Trinuclear 3 and all complexes
[Ni(‘S2C3Me2’)(L)] are four-coordinate Ni(II) alkyl species. The
highest (common) coordination number of Ni(II) is six and
numerous four coordinate Ni(II) complexes are known to readily
add at least one additional fifth ligand. In this respect complex
3 and its mononuclear [Ni(‘S2C3Me2’)(L)] derivatives can be
considered coordinatively unsaturated such that additional
coordination of CO is favored. Due to the planar nickel
coordination, addition of CO may take place cis to the alkyl
group.
Five-coordinate nickel complexes of this type can be expected
to be fluxional2c and to have elongated (labilized) nickel-ligand
bonds. The elongation of nickel ligand bonds in such species
is demonstrated, for example, by the comparison of the
molecular structures of the square-planar [Ni(‘S4C3’)]2g and its
five-coordinate square-pyramidal PMe3 adduct [Ni(PMe3)-
(‘S4C3’)].2f The labilization of all nickel-ligand bonds and the
potential cis configuration of thiolate and acyl groups in 8 can
be expected to favor the reductive elimination of the thiolactone
6.
Another question of interest is why the trinuclear complex 3
reacts with CO to give instantaneously Ni(CO)4 and the thioester
6. This very rapid reaction contrasts the reaction between the
mononuclear 5 and CO which allows to intercept the acyl
intermediate 7. It could be speculated that trinuclear 3 contains
one nickel center which mediates the thioester synthesis, while
the other two [NiS] centers assist this reaction by transferring
CO and/or electrons.35 In this respect, the arrangement of a
“reactive nickel site” linked to additional metal sulfur centers
would resemble the conditions in CODH, where a [FexSy] cluster
is linked to the sulfur-rich nickel site. However, this assumption
does not explain the rapid conversion of all three nickel centers
of 3 into Ni(CO)4 and 6. Thus, the different reactivity of
trinuclear 3 toward CO in comparison to mononuclear 5 may
be traced back rather to the high reactivity of the mononuclear
solvent complexes such as [Ni(‘S2C3Me2’)(THF)] which result
from partial dissociation of 3 in THF.
In contrast to the well established formation of metal bound
acyl groups from metal alkyl species and CO, nickel acyl
complexes with physiologically acceptable coordination spheres
are extremely rare. Likewise rare are the examples for the nickel
mediated formation of thioesters. Only three model systems
are known that undergo the reaction sequence Ni f Ni alkyl
f Ni acyl f thioester. Holm et al. showed that [NiII(N(C2H4-
-
SR)3)]2+ (R ) i-Pr, t-Bu) fragments can add CH3 and
subsequently CO yielding the acyl derivative [NiII(COMe)-
(N(C2H4SR)3)]+. This acyl complex slowly reacts with RSH
to form the thioester RSC(O)Me, nickel metal, and the proto-
+ 32a,b
nated free ligand HN(C2H4SR)3)3
.
Recently, Tucci and
Holm described the reaction of [Ni(bpy)(R′)(SR)] with CO that
yields thioesters RSCOR′ and [Ni(bpy)(CO)2] (R′ ) CH3, C2H5;
various aryl substituents, e.g., 2,6-C6H3(CH3)2 or 2,6-C6H3-
Cl2).32c Matsunaga and Hillhouse34 demonstrated that thietan,
C3H6S, oxidatively adds to [Ni(bpy)(COD)] yielding the Ni(II)
alkyl thiolato complex [Ni(bpy)(C3H6S)] that further reacts with
CO to give the thiolactone (CH2)3SCO in 28% yield. IR
spectroscopic evidence suggests formation of a thioester in the
reaction between [Ni(CH3)(‘Me-S2’)2]- and CO.26
In conclusion, the principal findings of this investigation are
that the nickel sulfur centers of the complexes described here
provide sites for the nickel mediated thioester formation from
complex bound thiolate, alkyl, and CO groups. The absence
of an [FexSy] cluster, which is found in the nickel iron site of
CODH, requires the nickel center(s) to act as electron relays
via formation of Ni(CO)4. For the first time, the overall reaction
that models the acetyl-CoA synthesis catalyzed by CODH could
be carried out in a cyclic reaction sequence. The results lend
support to a pathway of acetyl-CoA formation that comprises
CO insertion into a Ni-CH3 bond and an intramolecular S-C
bond formation between nickel bound acyl groups and thiolate
ligands.
The reaction sequence outlined in Scheme 1 contrasts these
systems in several regards. First of all, Scheme 1 demonstrates
that the nickel mediated thioester synthesis can be achieved in
a cyclic reaction sequence at nickel centers exhibiting sulfur
rich coordination spheres. Secondly, with the exception of 7,
the nickel centers in Scheme 1 exhibit sulfur-only coordination
spheres. Finally, Scheme 1 is also of interest with regard to
the question whether the thioester formation proceeds via
intramolecular or intermolecular pathways. Isolation and
reactivity of the complexes 3, 4, and 7 and above all the cyclic
structure of the thioester 6 imply an intramolecular mechanism.
However, simultaneously the important question must be raised
in which way the final step of the thioester S-C bond formation
takes place.
Acknowledgment. Support of these investigations by Deut-
sche Forschungsgemeinschaft, Bundesministerium fu¨r For-
schung und Technologie and Fonds der Chemischen Industrie
is gratefully acknowledged.
The last isolable intermediate before release of the thiolactone
6 is the acyl complex 7. Complex 7 contains thiolate and acyl
C donors in trans positions. This trans arrangement is unfavor-
able for the thioester S-C bond formation. It may also explain
why additional CO is necessary in order to induce the final step
of thioester formation. The reaction of 7 with CO could yield
a four-coordinate species by substitution of L through CO, but
this substitution is expected to leave the acyl group and thiolate
donor in the unfavorable trans positions. More likely is the
addition of CO leading to a five-coordinate species such as
[Ni(‘S2C3Me2CO’)(CO)(L)], (8).
Supporting Information Available: Listings of complete
crystal and data collection parameters, atomic coordinates,
isotropic thermal parameters, anisotropic displacement param-
eters, and interatomic distances and bond angles of 1, 3, 4, and
7 (11 pages). For ordering information see any current masthead
page. Further details of the X-ray crystal structure analyses
have been deposited and can be obtained from the Fachinfor-
mationszentrum Karlsruhe GmbH, D-76344 Eggenstein-Leo-
poldshafen by citing the depository Nos. CSD 404007 [Ni(‘S4C3-
Me2’)] (1), CSD 404008 [Ni(‘µ-S2C3Me2’)]3 (3), CSD 404004
[Ni(‘S2C3Me2’)(py)] (4), CSD 404005 [Ni(‘S2C3Me2CO’)-
(PMe3)] (7), the authors, and the reference.
(32) (a) Stavropoulos, P.; Muetterties, M. C.; Carrie´, M.; Holm, R. H. J.
Am. Chem. Soc. 1991, 113, 8485-8492. (b) Stavropoulos, P.; Carrie´, M.;
Muetterties, M. C.; Holm, R. H. J. Am. Chem. Soc. 1990, 112, 5385-
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