Single-Source Materials for Metal-Doped Titanium Oxide: Syntheses, Structures, and Properties of a Series of Heterometallic Transition-Metal Titanium Oxo Cages

Titanium dioxide (TiO2) doped with transition-metal ions (M) has potentially broad applications in photocatalysis, photovoltaics, and photosensors. One approach to these materials is through controlled hydrolysis of well-defined transition-metal titanium oxo cage compounds. However, to date very few such cages have been unequivocally characterized, a situation which we have sought to address here with the development of a simple synthetic approach which allows the incorporation of a range of metal ions into titanium oxo cage arrangements. The solvothermal reactions of Ti(OEt)(4) with transition-metal dichlorides ((MCl2)-Cl-II, M = Co, Zn, Fe, Cu) give the heterometallic transition-metal titanium oxo cages [Ti4O(OEt)(15)(MCl)] [M = Co (2), Zn (3), Fe (4), Cu (5)], having similar MTi4(mu(4)-O) structural arrangements involving ion pairing of [Ti4O(OEt)(15)](-) anion units with MCl+ fragments. In the case of the reaction of MnCl2, however, two Mn-II ions are incorporated into this framework, giving the hexanuclear Mn2Ti4(mu(4)-O) cage [Ti4O(OEt)(15)(Mn2Cl3)] (6) in which the MCl+ fragments in 2-5 are replaced by a [ClMn(mu-Cl)MnCl](+) unit. Emphasizing that the nature of the heterometallic cage is dependent on the metal ion (M) present, the reaction of Ti(OEt)(4) with NiCl2 gives [Ti-2(OEt)(9)(NiCl)](2) (7), which has a dimeric Ni(mu-Cl)(2)Ni bridged arrangement arising from the association of [Ti-2(OEt)(9)](-) ions with NiCl+ units. The syntheses, solid-state structures, spectroscopic and magnetic properties of 2-7 are presented, a first step toward their applications as precursor materials.