Biological systems have evolved enzymes that rely on multimetallic active sites of abundant transition metals with low symmetry, for example in [NiFe] or [CuMo] carbon monoxide dehydrogenases, to perform efficient conversions of small molecules. Multinuclear metal complexes have been of long-standing interest due to their ability to mediate multi-step redox reactions and provide enhanced catalytic properties. However, the nature of metal-metal cooperative effects that enable substrate activations remain largely ill-defined. To characterize electronic properties and chemical transformations that result from interactions between two metal centers, we have developed binucleating, naphthyridine-based ligands that place the metal centers in close contact. For dicopper systems, the roles of the metals in binding various organic fragments have been explored, and a bimetallic mechanism for substrate conversions in the catalytic alkyne-azide cycloaddition (click) reaction has been identified. For investigations of heterobimetallic complexes, unsymmetrical ligands that allow selective binding of two different metal centers have been developed. One such ligand features a dipyridyl binding pocket on one side of the naphthyridine core, and a single phosphine donor on the other side. This ligand allows the highly selective assembly of various isostructural heterobimetallic complexes of abundant first-row transition metals, involving combinations of Cu<sup>I</sup> and M<sup>II</sup> (M = Mn, Fe, Co, Ni, Cu, Zn).  Complexes supported by these unsymmetrical ligands also enable studies on the impact of symmetry and coordination number on reactivity at homobimetallic active sites, as the unsymmetrical environment allows for differentiated electronic environments. This presentation will describe new results along these lines, as well as new bond activations and chemical transformations mediated by bimetallic sites.

Metal-metal cooperativity in small molecule activations at bimetallic reaction centers

2019 Fall National Meeting

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