Water oxidation catalysis via immobilization of the dimanganese complex [Mn2(μ-O)2Cl(μ-O2CCH3)(bpy)2(H2O)](NO3)2 onto silica

Adsorption of a dinuclear μ-oxo bridged Mn complex onto mesoporous silica was observed when SBA15 was treated with an acetonitrile solution of [Mn2(μ-O)2Cl(μ-O2CCH3)(H2O)(bpy)2](NO3)2 (1). This complex was immobilized via the displacement of NO3(-) into solution, and characterization by spectroscopic (DRIFTS and DRUV-vis) and magnetic data indicates that the intact dication is electrostatically bound to the silica surface. Loadings of up to 4.1% by weight of [Mn2(μ-O)2Cl(μ-O2CCH3)(H2O)(bpy)2](2+) were achieved. TEM images of the grafted material revealed retention of the mesoporous structure of SBA15, and no clusters of manganese greater than ca. 10 nm were observed. The SBA15-supported dimanganese complex functions as a catalyst for the oxidation of H2O with (NH4)2Ce(NO3)6 as stoichiometric oxidant. In contrast, homogenous aqueous solutions of 1 do not evolve oxygen upon treatment with (NH4)2Ce(NO3)6. Labeling studies with H2(18)O confirm that the oxygen formed in this catalysis is derived from water. Monitoring the O2 evolution allowed determination of an initial rate for the catalysis (TOFi = 1.1 × 10(-3) s(-1)). These studies also reveal a first order dependence on manganese surface concentration, and a zero order rate dependence for (NH4)Ce(NO3)6. Spectroscopic investigations were employed to investigate the difference in activities between dissolved and supported dimanganese complexes.