Synthesis and characterization of a uranyl(vi) complex with 2,6-pyridine-bis(methylaminophenolato) and its ligand-centred aerobic oxidation mechanism to a diimino derivative

Abstract

A uranyl(VI) complex with 2,6-bis(3,5-di-tert-butyl-o-phenolateaminomethyl)pyridine (UO₂(tBu-pdaop), 1) was synthesized and thoroughly characterized by ¹H NMR, IR, elemental analysis, and single-crystal XRD. Right after the dissolution of complex 1 in pyridine or DMSO, the solution was pale red, whereas it gradually turned to dark purple under an ambient atmosphere. ¹H NMR spectra at the initial and final states suggested that both of the two aminomethyl groups in 1 were converted to azomethine ones through aerobic oxidation. Indeed, a uranyl(VI) complex with 2,6-bis(3,5-di-tert-butyl-o-phenolateiminomethyl)pyridine (UO₂(tBu-pdiop), 2) was obtained from the concentrated solution once the reaction was completed, and was characterized by IR, and single-crystal XRD. Kinetic analyses as well as mechanistic studies based on quantum chemical calculations suggested that hydrogen atom transfer from one of the amino groups in complex 1 to nearby O₂ initiates the stepwise oxidation processes to finally afford 2. The present findings demonstrate the novel reactivity of a uranyl(VI) complex, and provide new insights to construct thermally-driven molecular conversion systems by a UO₂²⁺ complex catalyst.