Synthesis, structure and properties of the manganese-doped polyoxotitanate cage [Ti18MnO30(OEt)20(MnPhen)3] (Phen = 1,10-phenanthroline)

Abstract

The novel heterometallic polyoxotitanate cage [Ti₁₈MnO₃₀(OEt)₂₀(MnPhen)₃] (1), obtained by solvothermal reaction of Ti(OEt)₄ with Mn(AcO)₃·(H₂O)₂ and 1,10-phenanthroline (Phen) in EtOH, has a C₃ symmetric core structure containing an interstitial tetrahedral Mn^II ion and is surrounded by three Mn^II(Phen) fragments. The molecular structure is retained in thin film electrodes of 1 deposited by solution drop-casting onto fluorinated tin oxide (FTO). Both solid state and solution phase electrochemical measurements show dual redox couples, consistent with the two distinct Mn coordination environments in the cage structure. Sintering of 1 in air at 600 °C produces a black crystalline solid which consists of Mn-doped TiO₂ (mainly in the rutile phase) together with α-Mn₂O₃. Such a composite semiconductor has an optical band gap of ca. 1.80 eV, similar to that of α-Mn₂O₃.