STRUCTURAL AND REACTION CHEMISTRY OF NICKEL-COMPLEXES IN RELATION TO CARBON-MONOXIDE DEHYDROGENASE - A REACTION SYSTEM SIMULATING ACETYL-COENZYME-A SYNTHASE ACTIVITY

A series of nickel complexes [Ni(NS3R)L]+ derived from the tripodal ligand NS3R = N(CH2CH2SR)3 (R = i-Pr, t-Bu) has been prepared in order to investigate the stabilities and reactions of certain species potentially relevant to the nickel sites in carbon monoxide dehydrogenase (CODH). Reaction of [Ni(NS3R)Cl]+ with MeMgCl affords [Ni(NS3R)Me]+, which with CO yields [Ni(NS3R)COMe]+. Reaction of [Ni(NS3tBu)Cl]+ with NaBH4 gives [Ni(NS3tBu)H]+ and the Ni(I) species [Ni(NS3tBu)]+. The hydride complex was obtained with minimal Ni(I) contamination by removal of ethylene from the equilibrium system [Ni(NS3tBu)Et]+/[Ni(NS3tBu)H]+/C2H4. Reaction of [Ni(NS3tBu)]+ with CO affords [Ni(NS3tBu)CO]+, whereas [Ni(NS3tBu)H]+ under the same conditions gives Ni(CO)4 and protonated ligand. All reactions were performed in THF, and all complexes were isolated as BPh4-salts. This series includes rare examples of the stabilization of Ni-methyl, -acyl, -hydride, and -carbonyl ligands in the absence of nonphysiological (C/P/As) coligands. Trigonal-bipyramidal stereochemistry has been demonstrated for five complexes with L = Cl, Me, COMe, H, and CO by X-ray crystallography. The methyl/acyl transformations with carbon monoxide and the formation in high yield of the thioesters R'SCOMe (R' = Et, CH2Ph, Ph) upon reaction of [Ni(NS3R)COMe]+ with thiols in THF are two previously undocumented processes mediated at Ni(II) sites lacking non-physiological (C/P/As) ligation. These are relevant to current views of the catalytic reaction cycle of Clostridium thermoaceticum CODH, which is presented. The Ni(II)-H and Ni(I)-CO species may be pertinent to the CO/CO2 activity of CODH. Full details of all preparations, other reactions, and structures are presented. This work is an initial attempt to place the reaction chemistry of CODH on a rational basis, and provides some viability for the present reaction cycle of the enzyme.