Assembly, Structure, and Reactivity of Cu₄S and Cu₃S Models for the Nitrous Oxide Reductase Active Site, Cu_Z*

Abstract

Bridging diphosphine ligands were used to facilitate the assembly of copper clusters with single sulfur atom bridges that model the structure of the Cu(Z)* active site of nitrous oxide reductase. Using bis(diphenylphosphino)amine (dppa), a [Cu(I)4(μ4-S)] cluster with N-H hydrogen bond donors in the secondary coordination sphere was assembled. Solvent and anion guests were found docking to the N-H sites in the solid state and in the solution phase, highlighting a kinetically viable pathway for substrate introduction to the inorganic core. Using bis(dicyclohexylphosphino)methane (dcpm), a [Cu(I)3(μ3-S)] cluster was assembled preferentially. Both complexes exhibited reversible oxidation events in their cyclic voltammograms, making them functionally relevant to the Cu(Z)* active site that is capable of catalyzing a multielectron redox transformation, unlike the previously known [Cu(I)4(μ4-S)] complex from Yam and co-workers supported by bis(diphenylphosphino)methane (dppm). The dppa-supported [Cu(I)4(μ4-S)] cluster reacted with N3(-), a linear triatomic substrate isoelectronic to N2O, in preference to NO2(-), a bent triatomic. This [Cu(I)4(μ4-S)] cluster also bound I(-), a known inhibitor of Cu(Z)*. Consistent with previous observations for nitrous oxide reductase, the tetracopper model complex bound the I(-) inhibitor much more strongly and rapidly than the substrate isoelectronic to N2O, producing unreactive μ3-iodide clusters including a [Cu3(μ3-S)(μ3-I)] complex related to the [Cu4(μ4-S)(μ2-I)] form of the inhibited enzyme.