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.