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

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.