Synthetic W/Cu Mimics of the Mo/Cu Active Site in Aerobic CODH: Nonclassical Isocyanide Coordination and the Role of the Bridging Sulfide

Abstract

Aerobic carbon monoxide dehydrogenase (CODH) uses an unsupported Mo–S–Cu heterobimetallic active site to catalyze CO oxidation, but the mechanistic role of the bridging sulfide remains to be fully resolved. We report a series of W/Cu complexes designed to test whether steric shielding from a bidentate tBuxantphos ligand can suppress unwanted Cu···Sdithiolate and Cu···O═M interactions that plagued the reactivity of a previously reported NHC-ligated model system. Addition of [Cu(tBuxantphos)]+ to [(bdt)WVI(O)(OSiiPr3)(X)]− (bdt = 1,2-benzenedithiolate; X = O, S, Se) unexpectedly favors Cu coordination at the W═O rather than the W═X unit, giving X═W–O–Cu cores as the thermodynamically preferred isomers, opposite to the connectivity predicted by hard–soft acid–base theory and found in CODH itself. The resulting complexes bind isocyanides at CuI with nonclassical (blue-shifted) C≡N stretching frequencies. In contrast, no reactivity is observed toward CO gas, consistent with DFT predictions that CO coordination is endergonic. However, the calculations reveal that the CO adduct of the hypothetical O═W–S–Cu–CO isomer is more stabilized than its S═W–O–Cu–CO counterpart, an effect traced by molecular orbital analysis to delocalization of electron density from the redox-active dithiolate through the bridging sulfide into the Cu–CO π-backbonding manifold. These results provide an electronic rather than purely structural rationale for why the CODH active site retains a bridging sulfide despite an apparent thermodynamic preference for a bridging oxo in the synthetic analogues investigated here.