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('S(4)C(3)Me(2)')] (1, 'S(4)C(3)Me(2)'(2-) = 1,3-bis(2-mercaptophenylthio)-2,2-dimethyl-propane-(2-)) was synthesized by template alkylation of Na-2[Ni('S-2')(2)] ('S-2'(2-) = benzene-1,2-dithiolate(2-)) with CMe(2)(CH2Br)(2). Acidic hydrolysis of 1 yielded the thiol ''S(4)C(3)Me(2)''-H-2 (2). Reduction of 1 with Na/Hg resulted in cleavage of the 'S(4)C(3)Me(2)'(2-)ligand and formation of the thermally stable trinuclear nickel(II) alkyl thiolato complex [Ni('mu-S(2)C(3)Me(2)')](3) (3, 'S(2)C(3)Me(2)'(2-) = 1-(2-mercaptophenylthio)-2,2-dimethylpropyl(2-)). Treatment of 3 with L = Py, THF, or PMe(3) afforded the mononuclear compounds [Ni('S(2)C(3)Me(2)')(L)] (4, L = Py; 5, L = PMe(3)). The stoichiometric reaction of [Ni('S(2)C(3)Me(2)')(L)] with CO led to the cyclic thioester 'S(2)C(3)Me(2)CO' (6, 'S(2)C(3)Me(2)CO' = 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('S(2)C(3)Me(2)CO')(PMe(3))] (7 'S(2)C(3)Me(2)CO'(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 'S(4)C(3)Me(2)'-H-2 (2) + CO --> 'S(2)C(3)Me(2)CO' (6) + 'S-2'-H-2 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 al Ni centers.