Osmium(ii) complexes for light-driven aerobic oxidation of amines to imines

Abstract

Herein, we describe the synthesis and characterization of three Os(II) complexes (i.e., [Os(fptz)₂(PPhMe₂)₂] (1, fptzH = 3-trifluoromethyl-5-pyridyl-1,2,4-triazole), [Os(fptz)₂(CO)(L₁)] (2, L₁ = PPh₃; 3, L₁ = pyridine)) that have been successfully utilized as good photocatalysts to promote aerobic oxidative coupling of amines to imines with molecular oxygen in air as a green oxidant. Complex 1 is the most effective catalyst for the oxidative coupling of benzylamine with molecular O₂ (air) as the oxidant because of the complex's strong absorption of visible light and long-lived triplet state. The application of a low catalyst loading (0.06 mol%) of complex 1 to the oxidative coupling of a wide range of amines affords the corresponding imines efficiently and selectively in most cases. The reaction mechanism was investigated via relevant control and quenching experiments. The results indicated that the reaction occurs via an active ¹O₂-involved pathway. The ¹O₂-generating ability of complex 1 as a photosensitizer was evaluated using 9,10-dimethylanthracene (DMA) as a chemical trap for ¹O₂.