A New Tripodal Ligand System with Steric and Electronic Modularity for Uranium Coordination Chemistry

The synthesis of a potentially redox active tripodal ligand containing a tris(aryloxide) functionalized mesitylene anchor, ((<sup><i>t</i>Bu</sup>ArOH)<sub>3</sub>mes) (<b>1</b>), and its metalation with low-valent uranium to form [((<sup><i>t</i>Bu</sup>ArO)<sub>3</sub>mes)U] (<b>1-U</b>) is reported. The results from characterization by X-ray crystallography, spectroscopic studies, and computational analysis, as well as initial reactivity studies, support a +3 uranium oxidation state. Comparison to the previously synthesized complex, [((<sup><i>t</i>Bu</sup>ArO)<sub>3</sub>tacn)U] (<b>2-U</b>), featuring the redox-innocent triazacyclononane anchor reveals that changing the anchor from the flexible triazacyclononane to a rigid mesityl fragment increases the structural flexibility of the aryloxide substituents in complexes of <b>1</b>. The synthesis and crystal structures of uranium(IV) amide complexes of <b>1-U</b> and <b>2-U</b> are discussed.