Model Studies of Methyl CoM Reductase: Methane Formation via CH3–S Bond Cleavage of Ni(I) Tetraazacyclic Complexes Having Intramolecular Methyl Sulfide Pendants

The Ni­(I) tetraazacycles [Ni­(dmmtc)]+ and [Ni­(mtc)]+, which have methylthioethyl pendants, were synthesized as models of the reduced state of the active site of methyl coenzyme M reductase (MCR), and their structures and redox properties were elucidated (dmmtc, 1,8-dimethyl-4,11-bis­{(2-methylthio)­ethyl}-1,4,8,11-tetraaza-1,4,8,11-cyclotetradecane; mtc, 1,8-{bis­(2-methylthio)­ethyl}-1,4,8,11-tetraaza-1,4,8,11-cyclotetradecane). The intramolecular CH3–S bond of the thioether pendant of [NiI(dmmtc)]­(OTf) was cleaved in THF at 75 °C in the presence of the bulky thiol DmpSH, which acts as a proton source, and methane was formed in 31% yield and a Ni­(II) thiolate complex was concomitantly obtained (Dmp = 2,6-dimesityphenyl). The CH3–S bond cleavage of [NiI(mtc)]+ also proceeded similarly, but under milder conditions probably due to the lower potential of the [NiI(mtc)]+ complex. These results indicate that the robust CH3–S bond can be homolytically cleaved by the Ni­(I) center when they are properly arranged, which highlights the significance of the F430 Ni environment in the active site of the MCR protein.