Similarities and differences in properties and behavior of two H2O2-activated manganese catalysts having structures differing only by methyl and ethyl substituents

Abstract

The complex [Mn(IV)(Me2EBC)(OH)2](PF6)2, in which Me2EBC is 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane, is a remarkably selective H2O2 oxidation catalyst that has been shown to be useful in removing stains from fabrics without affecting their colors. Mn(IV) is the highest oxidation state detected and the dihydroxo complex forms a peroxyhydroxy derivative that is responsible for catalytic oxidations. Study of the diethyl homolog of this catalyst has revealed surprising differences in chemical behavior. Oxidation of this new manganese complex, Mn(Et2EBC)Cl2, using aqueous H2O2, at −30°C following removal of chloride ion, yields [Mn(Et2EBC)(OH)2](PF6)2. Above 0°C, H2O2 oxidation of Mn(Et2EBC)Cl2 oxidizes the ethyl substituents. X-ray structure determinations of Et2EBC complexes with Mn(II), Mn(III), and Mn(IV) are reported. The complex [Mn(Et2EBC)(OH)2](PF6)2 displays a surprisingly mild oxidizing potential of +0.556 V for the Mn4+/Mn3+ couple; however, its hydrogen abstraction ability for selected substrates is limited by the BDECH value of 82 kcal mol−1, the same as reported for [Mn(Me2EBC)(OH)2](PF6)2. However, unlike the methyl derivative, electrochemical results indicate a 5+/4+ couple, in addition to the expected 4+/3+ and 3+/2+ couples. The significance of these differences in behavior is discussed. Mass spectral studies have identified some products of ethyl group oxidations.