Probing the electronic and mechanistic roles of the μ-sulfur atom in a synthetic Cu model system.

Nitrous oxide (NO) contributes significantly to ozone layer depletion and is a potent greenhouse agent, motivating interest in the chemical details of biological NO fixation by nitrous oxide reductase (NOR) during bacterial denitrification. In this study, we report a combined experimental/computational study of a synthetic [4Cu:1S] cluster supported by N-donor ligands that can be considered the closest structural and functional mimic of the Cu catalytic site in NOR reported to date. Quantitative N measurements during synthetic NO reduction were used to determine reaction stoichiometry, which in turn was used as the basis for density functional theory (DFT) modeling of hypothetical reaction intermediates. The mechanism for NO reduction emerging from this computational modeling involves cooperative activation of NO across a Cu/S cluster edge. Direct interaction of the μ-S ligand with the NO substrate during coordination and N-O bond cleavage represents an unconventional mechanistic paradigm to be considered for the chemistry of Cu and related metal-sulfur clusters. Consistent with hypothetical participation of the μ-S unit in two-electron reduction of NO, Cu K-edge and S K-edge X-ray absorption spectroscopy (XAS) reveal a high degree of participation by the μ-S in redox changes, with approximately 21% S 3p contribution to the redox-active molecular orbital in the highly covalent [4Cu:1S] core, compared to approximately 14% Cu 3d contribution per copper. The XAS data included in this study represent the first spectroscopic interrogation of multiple redox levels of a [4Cu:1S] cluster and show high fidelity to the biological Cu site.