EPR Spectroscopic Detection of the Elusive Fe^V═O Intermediates in Selective Catalytic Oxofunctionalizations of Hydrocarbons Mediated by Biomimetic Ferric Complexes

Abstract

Herein, we report the EPR spectroscopic study of the bioinspired catalyst systems for selective hydrocarbon oxofunctionalizations based on dinuclear ferric complexes with TPA* and PDP* aminopyridine ligands, hydrogen peroxide, and acetic acid (TPA* = tris(3,5-dimethyl-4-methoxypyridyl-2-methyl)amine, PDP* = bis(3,5-dimethyl-4-methoxypyridyl-2-methyl)-(S,S)-2,2′-bipyrrolidine). Using very low temperatures, −75 to −85 °C, the extremely unstable and reactive iron–oxygen intermediates, directly reacting with olefins even at −85 °C, have been detected for the first time. Their EPR parameters (g1 = 2.070–2.071, g2 = 2.005–2.008, g3 = 1.956–1.960) were very similar to those of the known oxoiron(V) complex [(TMC)FeV═O(NC(O)CH3)]+ (g1 = 2.053, g2 = 2.010, g3 = 1.971, TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). On the basis of EPR and reactivity data, the detected intermediates were assigned to the FeV═O active oxidizing species of the catalyst systems studied.