Mechanistic Studies on Oxygen-Insertion into Osmium(III)–Carbon Bond via High-Valent Oxido-Osmium(V) Complex

An osmium(III)-metalacycle complex consisting of 2-phenylpyridine is converted to the 2-phenoxidopyridine-osmium(III) complex upon treatment with a tertiary amine N-oxide, where a high-valent oxido-osmium(V) complex is proposed as a key reactive intermediate (H. Sugimoto et al. Organometallics, 2021, 40, 102–106). To gain insight into the reaction mechanism, substituent effects of 2-phenylpyridine moiety on the oxygen atom insertion reaction are examined using a series of 2-(4-X-phenyl)pyridines. All the cyclometalated complexes react with the tertiary amine N-oxide to produce the phenoxido-osmium(III) complexes. Kinetic analyses are conducted on the adduct formation process and the oxygen insertion reaction. In the former process, the binding constant of the amine N-oxide to the osmium(III) center is not influenced by electron withdrawing nature of the substituent X, but decrease of the reaction rate is observed in the latter process as electron withdrawing nature of X increases. Furthermore, the electronic effects of the amine N-oxides are examined using p-substituted N,N-dimethylaniline N-oxide (p-Y-C6H4N(O)(Me)2) to find that reaction rates increase as the electron withdrawing nature of Y increases. These results are consistent with the proposed mechanism in which the C–O bond formation involves conversion of the osmium(III)-N-oxide adduct to its oxido-osmium(V) species as the active oxidant. An osmium(III)-metalacycle complex consisting of 2-phenylpyridine is converted to the 2-phenoxidopyridine-osmium(III) complex upon treating with tertiary amine N-oxide. Substituent effects of 2-phenylpyridine moiety and the amine N-oxide on the C–O bond formation are examined. The results are consistent with the proposed mechanism in which the C–O bond formation involves conversion of the osmium(III)-N-oxide adduct to its oxido-osmium(V) species.