Mononuclear Manganese(III) Superoxo Complexes: Synthesis, Characterization, and Reactivity.

Metal-superoxo species are typically proposed as key intermediates in the catalytic cycle of dioxygen activation by metalloenzymes involving different transition metal cofactors. In this regard, while a series of Fe-, Co-, and Ni-superoxo complexes have been reported to date, well-defined Mn-superoxo complexes remain rather rare. Herein, we report two mononuclear Mn-superoxo species, Mn(BDPP)(O) (, HBDPP = 2,6-bis((2-()-diphenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) and Mn(BDPP)(O) (, HBDPP = 2,6-bis((2-()-di(4-bromo)phenylhydroxyl-methyl-1-pyrrolidinyl)methyl)pyridine), synthesized by bubbling O into solutions of their Mn precursors, Mn(BDPP) () and Mn(BDPP) (), at -80 °C. A combined spectroscopic (resonance Raman and electron paramagnetic resonance (EPR) spectroscopy) and computational study evidence that both complexes contain a high-spin Mn center ( = 2) antiferromagnetically coupled to a superoxo radical ligand ( = 1/2), yielding an overall = 3/2 ground state. Complexes and were shown to be capable of abstracting a H atom from 2,2,6,6-tetramethyl-1-hydroxypiperidine (TEMPO-H) to form Mn-hydroperoxo species, Mn(BDPP)(OOH) () and Mn(BDPP)(OOH) (). Complexes and can be independently prepared by the reactions of the isolated Mn-aqua complexes, [Mn(BDPP)(HO)]OTf () and [Mn(BDPP)(HO)]OTf (), with HO in the presence of NEt. The parallel-mode EPR measurements established a high-spin = 2 ground state for and .