Nickel-promoted reductive carbon-sulfur bond cleavage: a model for the first step in the reaction promoted by methyl coenzyme M reductase

Abstract

Sulfur-ligated Ni complexes are of interest as active-site models of the Ni-containing enzymes hydrogenase (H[sub 2]-ase), methyl coenzyme M reductase (Me-CoM reductase), and Co-dehydrogenase (CODH). Me-CoM reductase catalyzes the last step in the reaction sequence which converts CO[sub 2] to CH[sub 4] in methanogenic bacteria. This step involves the cleavage of the methyl thioether C-S bond of the substrate methyl coenzyme M (Me-S-CoM). Ni(I) is believed to be the active species responsible for promoting this reaction. However, it has been difficult to probe the mechanism by which this occurs, since Me-CoM reductase rapidly loses activity upon breakage of whole cells during purification. Only one Ni complex, Ni[sup II](dioxo[16]aneN[sub 5]), has been shown to induce stoichiometric methane formation from Me-S-CoM; however, the mechanism proposed to explain this reaction is difficult to understand, and involves a Ni(II/III), as opposed to a Ni(I/II), redox couple. Other nickel complexes have been shown to desulfurize thioethers in the presence of reducing agents (e.g., LiAlH[sub 4]); however, the reactive Ni species have not been characterized. As part of a systematic study aimed at determining the influence of the local Ni coordination environment on reactivity, the authors have synthesized a series of structurally related sulfur-ligated Ni(II) complexes,more » including Ni[sup II]L[sub S5] (1). Since redox changes (Ni(III) [yields] Ni(II) and/or Ni(II) [yields] Ni(I)) appear to play an important role in the mechanisms of the enzymes mentioned above, the authors have also examined the redox chemistry of these molecules. Herein, they report a Ni-promoted reductive C-S bond cleavage reaction that represents a plausible chemical model for the first step in the reaction promoted by Ni in Me-CoM reductase. 30 refs., 4 figs.« less