N O Reductase Activity of a [Cu S] Cluster in the 4Cu Redox State Modulated by Hydrogen Bond Donors and Proton Relays in the Secondary Coordination Sphere

The model complex [Cu (μ -S)(dppa) ] (1, dppa=μ -(Ph P) NH) has N O reductase activity in methanol solvent, mediating 2 H /2 e reduction of N O to N +H O in the presence of an exogenous electron donor (CoCp ). A stoichiometric product with two deprotonated dppa ligands was characterized, indicating a key role of second-sphere N-H residues as proton donors during N O reduction. The activity of 1 towards N O was suppressed in solvents that are unable to provide hydrogen bonding to the second-sphere N-H groups. Structural and computational data indicate that second-sphere hydrogen bonding induces structural distortion of the [Cu S] active site, accessing a strained geometry with enhanced reactivity due to localization of electron density along a dicopper edge site. The behavior of 1 mimics aspects of the Cu catalytic site of nitrous oxide reductase: activity in the 4Cu :1S redox state, use of a second-sphere proton donor, and reactivity dependence on both primary and secondary sphere effects.