Distinguishing Desirable and Undesirable Reactions in Multicomponent Systems for Redox Activation of the Uranyl Ion

Although it has been established that covalent functionalization of the U-O bonds in the uranyl dication (UO₂²⁺) generally requires use of strong reductants and electrophiles, little work has examined how interactions between the individual reaction components could affect final outcomes in solution. Here, the patterns of such reactivity have been studied in a UO₂²⁺-containing model system supported by a workhorse pentadentate ligand, 2,2'-[(methylimino)bis(2,1-ethanediylnitrilomethylidyne)]bis-phenol. Oxo activation and functionalization have been tested with (i) electrochemical and chemical reduction, and (ii) coordinating and noncoordinating solvents. In acetonitrile, uranyl reduction was achieved cleanly, but treatment of the reduced species with tris(pentafluorophenyl)borane (BCF) resulted in a mixture of products arising from direct electron transfer to BCF. In dichloromethane (CH₂Cl₂), electrochemical reduction of uranyl was achieved cleanly, but clean chemical reactivity was inaccessible. Despite these challenges, one trinuclear and oxo-deficient uranium-containing product was crystallized from CH₂Cl₂ solution and characterized; thus, desirable electrophilic reactivity can proceed to some degree in CH₂Cl₂ with BCF. Computational studies were used to investigate the properties of the trinuclear uranium product and the changes that could be inducible by further reduction. Taken together, the reactivity patterns identified here could inform design of improved systems for actinyl oxo functionalization.