Coordination of Coenzyme M and Its Derivatives on NiII(tetraazacycle) Complexes: A Model for the Active Site of Methyl Coenzyme M Reductase

A series of tetraazamacrocyclic nickel(II) complexes coordinated by methyl coenzyme M (MeSCoM), coenzyme M (HSCoM), and 3-methylthiopropionate (Metp) have been synthesized as structural models of the active site of methyl coenzyme M reductase in the oxidized MCRsilent state. They include RSRS-[Ni(tmc)(L)](OTf) {L = MeSCoM (2), HSCoM (4), and Metp (5)} (tmc = 1,4,8,11-tetramethyl- 1,4,8,11-tetraazacyclotetradecane) and [Ni(pyc)(L)](OTf) (L = MeSCoM (6), HSCoM (7), Metp (8), pyc = 5-oxo-7-(2-pyridyl)-1,4,8,11-tetraazacyclotetradecane). The X-ray crystal analysis revealed that MeSCoM, HSCoM, and Metp are bound to Ni in an eta(1) manner through interactions with one O atom of each ligand. The tmc complexes assume a pentacoordinate geometry, which is in between a square pyramid and a trigonal bipyramid, while the pyc complexes are octahedral with coordination of the pendant pyridine at the site trans to the sulfonate or carboxylate ligand.