Triazenide-supported [Cu4S] structural mimics of CuZ that mediate N2O disproportionation rather than reduction

Abstract

As part of the nitrogen cycle, environmental nitrous oxide (N₂O) undergoes the N₂O reduction reaction (N₂ORR) catalyzed by nitrous oxide reductase, a metalloenzyme whose catalytic active site is a tetranuclear copper–sulfide cluster (Cu_Z). On the other hand, heterogeneous Cu catalysts on oxide supports are known to mediate decomposition of N₂O (deN₂O) by disproportionation. In this study, a Cu_Z model system supported by triazenide ligands is characterized by X-ray crystallography, NMR and EPR spectroscopies, and electronic structure calculations. Although the triazenide-ligated Cu₄(μ₄-S) clusters are closely related to previous formamidinate derivatives, which differ only in replacement of a remote N atom for a CH group, divergent reactivity with N₂O is observed. Whereas the formamidinate-ligated clusters were previously shown to mediate single-turnover N₂ORR, the triazenide-ligated clusters are found to mediate deN₂O, behavior that was previously unknown to natural or synthetic copper–sulfide clusters. The reaction pathway for deN₂O by this model system, including previously unidentified transition state models for N₂O activation in N–O cleavage and O–O coupling steps, are included. The divergent reactivity of these two related but subtly different systems point to key factors influencing behavior of Cu-based catalysts for N₂ORR (i.e., Cu_Z) and deN₂O (e.g., CuO/CeO₂).