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.

A uranyl(VI) complex with 2,6-bis(3,5-di- tert -butyl- o -phenolateaminomethyl)pyridine (UO 2 ( t Bu-pdaop), 1) was synthesized and thoroughly characterized by 1 H 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. 1 H 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 (UO 2 ( t Bu-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 O 2 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 UO 2 2+ complex catalyst.