Coordination of Methyl Coenzyme M and Coenzyme M at Divalent and Trivalent Nickel Cyclams: Model Studies of Methyl Coenzyme M Reductase Active Site

Divalent and trivalent nickel complexes of 1,4,8,11-tetraazacyclotetradecane, denoted as cyclam hereafter, coordinated by methyl coenzyme M (MeSCoM–) and coenzyme M (HSCoM–) have been synthesized in the course our model studies of methyl coenzyme M reductase (MCR). The divalent nickel complexes Ni­(cyclam)­(RSCoM)2 (R = Me, H) have two trans-disposed RSCoM– ligands at the nickel­(II) center as sulfonates, and thus, the nickels have an octahedral coordination. The SCoM2– adduct Ni­(cyclam)­(SCoM) was also synthesized, in which the SCoM2– ligand chelates the nickel via the thiolate sulfur and a sulfonate oxygen. The trivalent MeSCoM adduct [Ni­(cyclam)­(MeSCoM)2]­(OTf) was synthesized by treatment of [Ni­(cyclam)­(NCCH3)2]­(OTf)3 with (nBu4N)­[MeSCoM]. A similar reaction with (nBu4N)­[HSCoM] did not afford the corresponding trivalent HSCoM– adduct, but rather the divalent nickel complex polymer [−NiII(cyclam)­(CoMSSCoM)−]n was obtained, in which the terminal thiol of HSCoM– was oxidized to the disulfide (CoMSSCoM)2– by the Ni­(III) center.