From Alcohols to Ketones Efficiently Catalyzed by Osmium Com‐Plexes: An Insight Into the Effect of PNN and NN Ligands

Two new types of osmium complexes, PNN‐osmium complex ( Os1 ) and NN‐osmium complex ( Os2 ) have been developed and proven to be effective catalysts for the transfer‐dehydrogenative (TDH) oxidation of various (hetero)aromatic, cycloalkyl, and aliphatic alcohols to the respective ketones (55 examples). Notably, both complexes have been fully characterized and their comparative performance as catalysts investigated; Os1 has been further proven more effective in TDH oxidation of alcohols with a high turnover number (TON) reaching up to 63,000. Comparative experiments and DFT computations indicate that PNN‐complex Os1 exhibits higher reactivity in transfer‐dehydrogenative reactions than its NN counterpart Os2 due to the steric hindrance of catalysts. A joint theoretical and experimental investigation further shows that the current osmium system follows an outer‐sphere bifunctional mechanism, which involves the transformation between osmium hydride and amido‐osmium species. Furthermore, these osmium catalysts show great potential promise for applications in atom‐economic synthesis of ketonic fine chemicals.