Does Reduction-Induced Isomerization of a Uranium(III) Aryl Complex Proceed via C–H Oxidative Addition and Reductive Elimination across the Uranium(II/IV) Redox Couple?

Reaction of the uranium­(III) bis­(amidinate) aryl complex {TerphC­(N i Pr)2}2U­(Terph) (2, where Terph = 4,4″-di-tert-butyl-m-terphenyl-2′-yl) with a strong reductant enabled isolation of isomeric uranium­(III) bis­(amidinate) aryl product {TerphC­(N i Pr)2}2U­(Terph*) (3, where Terph* = 4,4″-di-tert-butyl-m-terphenyl-4′-yl). In terms of connectivity, 3 differs from 2 only in the positions of the U–C and C–H bonds on the central aryl ring of the m-terphenyl-based ligand. A deuterium labeling study ruled out mechanisms for this isomerization involving intermolecular abstraction or deprotonation of the ligand C–H bonds activated during the reaction. Due to the complexity of this rapid, heterogeneous reaction, experimental studies could not further distinguish between two different intramolecular C–H activation mechanisms. However, high-level computational studies were consistent with a mechanism that included two sets of unimolecular, mononuclear C–H oxidative addition and reductive elimination steps involving uranium­(II/IV).