Structural and reaction chemistry of nickel complexes in relation to carbon monoxide dehydrogenase: A reaction system simulating acetyl-coenzyme A synthase activity

Article abstract of DOI:10.1021/ja00022a041

A series of nickel complexes [Ni(NS₃^R)L]⁺ derived from the tripodal ligand NS₃^R = 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(NS₃^R)Cl]⁺ with MeMgCl affords [Ni(NS₃^R)Me]⁺, which with CO yields [Ni(NS₃^R)COMe]⁺. Reaction of [Ni(NS₃^tBu)Cl]⁺ with NaBH₄ gives [Ni(NS₃^tBu)H]⁺ and the Ni(I) species [Ni(NS₃^tBu)]⁺. The hydride complex was obtained with minimal Ni(I) contamination by removal of ethylene from the equilibrium system [Ni(NS₃^tBu)Et]⁺/[Ni(NS₃ ^tBu)H]⁺/C₂H₄. Reaction of [Ni(NS₃^tBu)]⁺ with CO affords [Ni(NS₃^tBu)CO]⁺, whereas [Ni(NS₃^tBu)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(NS₃^R)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.