Impact of Electronic and Steric Changes of Ligands on the Assembly, Stability, and Redox Activity of Cu(μ-S) Model Compounds of the Cu Active Site of Nitrous Oxide Reductase (NOR).

Model compounds have been widely utilized in understanding the structure and function of the unusual Cu(μ-S) active site (Cu) of nitrous oxide reductase (NOR). However, only a limited number of model compounds that mimic both structural and functional features of Cu are available, limiting insights about Cu that can be gained from model studies. Our aim has been to construct Cu(μ-S) clusters with tailored redox activity and chemical reactivity via modulating the ligand environment. Our synthetic approach uses dicopper(I) precursor complexes (CuL) that assemble into a Cu(μ-S)L cluster with the addition of an appropriate sulfur source. Here, we summarize the features of the ligands L that stabilize precursor and Cu(μ-S) clusters, along with the alternative products that form with inappropriate ligands. The precursors are more likely to rearrange to Cu(μ-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(μ-S) clusters in the 4Cu(I) oxidation state, while neutral nitrogen donors could not stabilize Cu(μ-S) clusters. Anionic formamidinate ligands have been found to stabilize Cu(μ-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 positions of the -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 active site of NOR, which should enable future insights into the chemical behavior of Cu.