Chromium(v)-oxo and chromium(iii)-superoxo complexes bearing a macrocyclic TMC ligand in hydrogen atom abstraction reactions

A Cr(v)-oxo complex bearing a macrocyclic TMC ligand, [Cr ^V(TMC)(O)(OCH₃)]²⁺, was synthesized, isolated, and characterized by various physicochemical methods, including UV-vis, ESI-MS, resonance Raman, EPR and X-ray analysis. The reactivity of the Cr(v)-oxo complex was investigated in C-H and O-H bond activation reactions. The reactivity of a Cr(iii)-superoxo complex, [Cr^III(TMC)(O₂)(Cl)] ⁺, was investigated in O-H bond activation reactions as well. By comparing reactivities of the Cr(iii)-superoxo and Cr(v)-oxo complexes under the identical reaction conditions, we were able to demonstrate that the Cr(iii)-superoxo complex is more reactive than the Cr(v)-oxo complex in the activation of C-H and O-H bonds. The present results provide strong evidence that under certain circumstances, metal-superoxo species can be an alternative oxidant for high-valent metal-oxo complexes in oxygenation reactions.