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(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 NiII-H and NiI-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.
P. Stavropoulos et al., "Structural and Reaction Chemistry of Nickel Complexes in Relation to Carbon Monoxide Dehydrogenase: A Reaction System Simulating Acetyl-coenzyme a Synthase Activity," Journal of the American Chemical Society, vol. 113, no. 22, pp. 8485-8492, American Chemical Society (ACS), Oct 1991.
The definitive version is available at http://dx.doi.org/10.1021/ja00022a041
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