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 III -superoxo species, Mn(BDPP)(O 2 •- ) ( 2 , H 2 BDPP = 2,6-bis((2-( S )-diphenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) and Mn(BDP Br P)(O 2 •- ) ( 2' , H 2 BDP Br P = 2,6-bis((2-( S )-di(4-bromo)phenylhydroxyl-methyl-1-pyrrolidinyl)methyl)pyridine), synthesized by bubbling O 2 into solutions of their Mn II precursors, Mn(BDPP) ( 1 ) and Mn(BDP Br P) ( 1' ), 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 III center ( S Mn = 2) antiferromagnetically coupled to a superoxo radical ligand ( S OO• = 1/2), yielding an overall S = 3/2 ground state. Complexes 2 and 2' were shown to be capable of abstracting a H atom from 2,2,6,6-tetramethyl-1-hydroxypiperidine (TEMPO-H) to form Mn III -hydroperoxo species, Mn(BDPP)(OOH) ( 5 ) and Mn(BDP Br P)(OOH) ( 5' ). Complexes 5 and 5' can be independently prepared by the reactions of the isolated Mn III -aqua complexes, [Mn(BDPP)(H 2 O)]OTf ( 6 ) and [Mn(BDP Br P)(H 2 O)]OTf ( 6' ), with H 2 O 2 in the presence of NEt 3 . The parallel-mode EPR measurements established a high-spin S = 2 ground state for 5 and 5' .