Highly Efficient Disproportionation of Dihydrogen Peroxide: Synthesis, Structure, and Catalase Activity of Manganese Complexes of the Salicylimidate Ligand

Abstract

AbstractTwo complexes, [Mn2(etsalim)4(Hetsalim)2](ClO4)2 (1), and [Mn(mesalim)2(OAc)(MeOH)]·MeOH (2), in which Hmesalim and Hetsalim are methyl and ethyl salicylimidate, respectively, have been synthesized, fully characterized by X‐ray analyses, magnetic susceptibility, UV/Vis and IR spectroscopy, and their catalase activity has been studied. Complex 1 is dinuclear and shows an Mn−Mn distance of 3.37 Å while complex 2 is mononuclear. Both complexes catalyze the disproportionation of hydrogen peroxide into water and dioxygen; they show very high catalase activity exhibiting saturation kinetics in the presence of NaOH. The rate and turnover number of the catalyzed reaction increase dramatically when a few equivalents of base (NaOH) are added to the reaction mixture. The turnover numbers for hydrogen peroxide disproportionation increase from approximately 200 to more than 1500 per manganese ion in less than 2 min for both complexes after addition of a few equivalents of NaOH. Kinetic studies of the catalyzed reaction performed on the dinuclear complex 1 in the presence of 5 equiv. of NaOH, gives kcat = 807 s−1 (± 16) and KM = 0.091 M (± 0.003). The effective catalytic efficiency is kcat/KM = 8900 M−1s−1. The kinetic parameters of the mononuclear complex 2 in the presence of 5 equiv. of NaOH, are kcat = 190 s−1 (± 4) and KM = 0.022 M (± 0.001). The effective catalytic efficiency is kcat/KM = 8600 M−1s−1. The catalase activity of the mononuclear complex, [Mn(mesalim)2Cl] (3) has also been studied, and it shows turnover numbers that are comparable to those of complex 2. ESI‐MS analyses in deuterated solvents have been used to understand the nature of the active species formed on addition of 5 equiv. of NaOH. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)