Syntheses, electrochemistry and reactivities of pyridine amide complexes of chromium(III) and manganese(III)

Abstract

Reactions of anhydrous CrCl₃ with H₂bpb and H₂bpc [H₂bpb = 1,2-bis(pyridine-2-carboxamido)benzene; H₂bpc = 4,5-dichloro-1,2-bis(pyridine-2-carboxamido)benzene] in dimethylformamide and manganese(III) acetate with H₂bpc in methanol, yielded the respective [Cr^III(bpb)Cl]·xH₂O, [Cr^III(bpc)Cl]·H₂O and [Mn^III(bpc)(O₂CMe)] complexes. The electrochemistry of the chromium(III) amide complexes has been studied by cyclic voltammetry. They exhibit reversible oxidation couples with E° values ranging from 0.28 to 0.80 V vs. ferrocenium–ferrocene. Except for [Cr^III(bpb)(CN)₂]^– which has a quasireversible reduction couple at –1.97 V, the electrochemical reductions are irreversible. The complexes [Cr^III(bpb)(H₂O)₂]ClO₄, [Cr^III(bpb)Cl(MeOH)] and [Mn^III(bpc)(O₂CMe)] have been shown to catalyse olefin epoxidation by iodosobenzene. With [Mn(bpc)(O₂CMe)], catalytic oxidation of alkanes by PhIO has also been observed. An oxygen-rebound mechanism involving a Mn^vO or a Cr^vO intermediate is proposed for the PhIO oxidation reactions.