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 mu-oxo bridged Mn complex onto mesoporous silica was observed when SBA15 was treated with an acetonitrile solution of [Mn-2(mu-O)(2)Cl(mu-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 [Mn-2(mu-O)(2)Cl(mu-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)(2)Ce(NO3)(6) as stoichiometric oxidant. In contrast, homogenous aqueous solutions of 1 do not evolve oxygen upon treatment with (NH4)(2)Ce(NO3)(6). Labeling studies with (H2O)-O-18 confirm that the oxygen formed in this catalysis is derived from water. Monitoring the O-2 evolution allowed determination of an initial rate for the catalysis (TOFi = 1.1 x 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.