Synthesis, characterization and reactivity of non-heme 1st row transition metal-superoxo intermediates

Abstract

Metalloenzymes activate dioxygen to generate metal-oxygen adducts that perform a wide range of biological functions. Among the metal-dioxygen intermediates, highly reactive metal-superoxo species are implicated as key intermediates in the catalytic cycle of various enzymatic reactions. Thus, extensive research on model compounds as well as on enzymatic systems has been performed for several decades to understand nature of the metal-superoxo species. In this review, we focus on the synthetic mononuclear metal-superoxo complexes employing copper, iron, nickel and manganese, which are known to exist as metal centers in enzymatic systems. The synthesis, characterization and reactivity studies using synthetic model compounds are investigated to provide mechanistic insights into the catalytic reactions of metalloenzymes. Two different geometries of the mononuclear metal-superoxo intermediates, with end-on and side-on binding modes, are observed that have different spectroscopic features and electronic configurations confirmed by various physicochemical methods. Furthermore, the factors affecting dioxygen activation and reactivity toward organic substrates are revealed by modifying supporting ligand to investigate steric, electronic, hydrogen bonding, solvent and donor atom effects. In the reactivity studies, most of the metal-superoxo species undergo electrophilic reactions including C-H activation, phenol oxidation and oxygen atom transfer. There are a few examples of nucleophilic reactivity of metal-superoxo species, such as aldehyde deformylation. The experimental and theoretical results presented in this review provide us with a better understanding of dioxygen activation and of synthetic strategies using model compounds that can be used to develop efficient bioinspired catalysts.