Radical decomposition of hydrogen peroxide catalyzed by aqua complexes [M(H2O)n]2+ (M = Be, Zn, Cd)

Abstract

The catalytic activity of the aqua complexes [M(H2O)n]2+ (1) (M=Be, n=4; M=Zn, Cd, n=6) toward the radical decomposition of H2O2 and generation of the HO radicals was investigated in detail by theoretical (DFT) methods. It was predicted and then confirmed by preliminary experiments that complexes 1 are able to catalyze this process despite the metals have only one stable non-zero oxidation state and, hence, cannot change it during the reaction as proposed in the classical Fenton chemistry. The mechanism of the H2O2 decomposition includes the substitution of a water ligand for H2O2, protolysis of the coordinated H2O2, second H2O-for-H2O2 substitution, elimination of one ligated water molecule (for Zn and Cd), and the homolytic HO–OH bond cleavage in complexes [M(H2O)p(H2O2)(OOH)]+. The principal factors affecting the HO formation are lability of the complexes, their acidity, and their ability to activate H2O2 toward the homolytic HO–OH bond cleavage. The participation of two H2O2 molecules is necessary for an efficient HO generation. The water substitution steps occur via concerted Ia (Be, Cd) or dissociative D (Zn, Cd) mechanisms.