Generation of reactive cobalt oxo oxamate radical species for biomimetic oxidation of contaminants

Abstract

The non-heme oxamate anionic cobalt(III) complexes [Co^III(opbaX)]⁻ (opbaX = 4-X-o-phenylenebis(oxamate), X = H, NO₂, CH₃) with different substituents were synthesized and applied to targeted micropollutant degradation. Typical radical scavengers (isopropanol and chlorine anions) have no negative effect on the catalytic oxidation of substrates, and no DMPO–˙OH or DMPO–˙OOH (DMPO = 5,5-dimethyl-pyrroline-oxide) signal was detected by electron paramagnetic resonance spin-trap technique in [Co^III(opbaX)]⁻/H₂O₂ system, suggesting that the non-hydroxyl radical biomimetic catalytic mechanism was dominant in the oxidation process. The results of high-definition ESI-MS pronounced the presence of cobalt-oxo intermediates which played a key role in the catalytic oxidation of substrates. Furthermore, density functional theory calculations were used to evaluate the viability of such cobalt-oxo species and it demonstrated an optimizing electromer with a formulation of [Co^IVO˙]⁻ or [Co^III–OH]˙. The calculations explained that the catalytic activity of [Co^III(opbaX)]⁻ was significantly enhanced by introducing an electron-withdrawing substituent which could change the coordination environment of cobalt to generate more electron-deficient cobalt-oxo species with stronger oxidizing power. This paper made a progress in verifying bio-mimic high oxidation state species and applied it to water purification, which provided deep insight into the properties of transition-metal centers in aqueous catalysis.