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.