N2 O Reductase Activity of a [Cu4 S] Cluster in the 4CuI Redox State Modulated by Hydrogen Bond Donors and Proton Relays in the Secondary Coordination Sphere.

The model complex [Cu4 (μ4 -S)(dppa)4 ]2+ (1, dppa=μ2 -(Ph2 P)2 NH) has N2 O reductase activity in methanol solvent, mediating 2 H+ /2 e- reduction of N2 O to N2 +H2 O in the presence of an exogenous electron donor (CoCp2 ). A stoichiometric product with two deprotonated dppa ligands was characterized, indicating a key role of second-sphere N-H residues as proton donors during N2 O reduction. The activity of 1 towards N2 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 [Cu4 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 CuZ catalytic site of nitrous oxide reductase: activity in the 4CuI :1S redox state, use of a second-sphere proton donor, and reactivity dependence on both primary and secondary sphere effects.