Impact of Electronic and Steric Changes of Ligands on the Assembly, Stability, and Redox Activity of Cu4(μ4-S) Model Compounds of the CuZ Active Site of Nitrous Oxide Reductase (N2OR)

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Model compounds have been widely utilized in understanding the structure and function of the unusual Cu 4 (μ 4 -S) active site (Cu Z ) of nitrous oxide reductase (N 2 OR). However, only a limited number of model compounds that mimic both structural and functional features of Cu Z are available, limiting insights about Cu Z that can be gained from model studies. Our aim has been to construct Cu 4 (μ 4 -S) clusters with tailored redox activity and chemical reactivity via modulating the ligand environment. Our synthetic approach uses dicopper(I) precursor complexes (Cu 2 L 2 ) that assemble into a Cu 4 (μ 4 -S)L 4 cluster with the addition of an appropriate sulfur source. Here, we summarize the features of the ligands L that stabilize precursor and Cu 4 (μ 4 -S) clusters, along with the alternative products that form with inappropriate ligands. The precursors are more likely to rearrange to Cu 4 (μ 4 -S) clusters when the Cu(I) ions are supported by bidentate ligands with 3-atom bridges, but steric and electronic features of the ligand also play crucial roles. Neutral phosphine donors have been found to stabilize Cu 4 (μ 4 -S) clusters in the 4Cu(I) oxidation state, while neutral nitrogen donors could not stabilize Cu 4 (μ 4 -S) clusters. Anionic formamidinate ligands have been found to stabilize Cu 4 (μ 4 -S) clusters in the 2Cu(I):2Cu(II) and 3Cu(I):1Cu(II) states, with both the formation of the dicopper(I) precursors and subsequent assembly of clusters being governed by the steric factor at the ortho positions of the N -aryl substituents. Phosphaamidinates, which combine a neutral phosphine donor and an anionic nitrogen donor in the same ligand, form multinuclear Cu(I) clusters unless the negative charge is valence-trapped on nitrogen, in which case the resulting dicopper precursor is unable to rearrange to a multinuclear cluster. Taken together, the results presented in this study provide design criteria for successful assembly of synthetic model clusters for the Cu Z active site of N 2 OR, which should enable future insights into the chemical behavior of Cu Z .