Control of Oxo-Group Functionalization and Reduction of the Uranyl Ion

Uranyl complexes of a large, compartmental N8-macrocycle adopt a rigid, “Pacman” geometry that stabilizes the UV oxidation state and promotes chemistry at a single uranyl oxo-group. We present here new and straightforward routes to singly reduced and oxo-silylated uranyl Pacman complexes and propose mechanisms that account for the product formation, and the byproduct distributions that are formed using alternative reagents. Uranyl­(VI) Pacman complexes in which one oxo-group is functionalized by a single metal cation are activated toward single-electron reduction. As such, the addition of a second equivalent of a Lewis acidic metal complex such as MgN″2 (N″ = N­(SiMe3)2) forms a uranyl­(V) complex in which both oxo-groups are Mg functionalized as a result of Mg–N bond homolysis. In contrast, reactions with the less Lewis acidic complex [Zn­(N″)­Cl] favor the formation of weaker U–O–Zn dative interactions, leading to reductive silylation of the uranyl oxo-group in preference to metalation. Spectroscopic, crystallographic, and computational analysis of these reactions and of oxo-metalated products isolated by other routes have allowed us to propose mechanisms that account for pathways to metalation or silylation of the exo-oxo-group.