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-ophenolateaminomethyl) pyridine (UO2(tBu-pdaop), 1) was synthesized and thoroughly characterized by 1H NMR, IR, elemental analysis, and single-crystal XRD. Right after dissolution of complex 1 in pyridine or DMSO, the solutions were pale red, whereas it gradually turned to dark purple under 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-ophenolateiminomethyl) pyridine (UO2(tBu-pdiop), 2) was obtained from the concentrated solution once the reaction is completed, which was characterized by IR, elemental analysis, and singlecrystal XRD. Kinetic analyses as well as mechanistic study 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. To the best of our knowledge, this is the first report on an air-oxidizable uranyl(VI) complex. The present findings demonstrate the novel reactivity of a uranyl(VI) complex, and provide new insights to construct thermally-driven molecular conversion system by a UO22+ complex catalyst.