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, E ⁰(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 E ⁰(TiOMnII/MnIII) with the aid of 2,2′-azino-bis(3-ethylbenz-thiazoline-6-sulfonic acid (ABTS ^2-) 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 ABTS ^2-/ABTS ^•- reference redox pair yields E ⁰(TiOMnII/MnIII) = 0.57 V at pH 7, increasing to 0.62 V at pH 4. The Cu(NC) ₂ ⁺/Cu(NC) ₂ ²⁺ reference system yields E ⁰(TiOMnII/MnIII) = 0.62 and 0.69 at pH 7 and 6, respectively. The difference of ∼0.1 V is interpreted in terms of the local charge effect of the positive Cu(NC) ₂ ²⁺ ions. The moderate redox potential of Mn(III) produced by slow heating of Mn(II) and measured in aqueous suspensions is in contrast with the high oxidative reactivity of Mn(III) produced by photolysis of solid Ti(IV)OMnII-SBA under gas atmosphere. This apparent discrepancy is attributed to the formation of excited Mn(III) in contrast to ground state Mn(III) produced by heating at 305 °C and to partial hydration of the manganese in the TiOMn-SBA suspensions. The titanium in TiOMn-SBA remained T(IV) under all conditions of the present work.