Determination of the Redox Potential of Immobilized Oxo-Bridged Metals in Porous Supports. The Ti–O–Mn–SBA System

The present work describes for the first time the determination of a redox potential of an oxo-bridged metal unit anchored to a mesoporous surface, which is not accessible to electrode reaction. The redox pair Ti(IV)OMnII/MnIII-SBA has been chosen because of the importance of the oxo-bridged TiOMn MMCT unit for photochemical energy storage research. The only feasible technique for determination of the standard redox potential, E0(MnII/MnIII), segments in the porous Ti(IV)O(MnII/MnIII)-SBA is from equilibrium constants with redox couples of known electrode potential. The present work describes the determination of E0(TiOMnII/MnIII) with the aid of 2,2′-azino-bis(3-ethylbenz-thiazoline-6-sulfonic acid (ABTS2-) and the 2,9-dimethyl-1,10-phenanthroline (Neocuproine or NC) complex with Cu(II). The results show that TiOMnIII is a moderate oxidant with pH dependent redox potential. The ABTS2-/ABTS•- reference redox pair yields E0(TiOMnII/MnIII) = 0.57 V at pH 7, increasing to 0.62 V at pH 4. The Cu(NC)2+/Cu(NC)2...