Dynamic Clustering of Aqueous Ti(IV) Cations

Titanium (Ti), the ninth-abundant element of the Earth crust, distributes widely in rocks, soils, waters and bodies of almost all organisms. The solution chemistry of Ti(IV), the most stable oxidation state of Ti, is the key of distribution, transformation and utilization of Ti element. Despite its pervasiveness in nature and chemical industry, the speciation of Ti(IV) cations in aqueous solution remains controversial after decades of researches. Herein, aided by combined state-of-the-art characterizations, we discover that hydrated Ti(IV) cations predominantly exist as atomically precise [Ti8O12(H2O)24]8+ clusters at higher concentrations and as [Ti(H2O)n(OH)]3+ (n = 4 or 5) monomers at lower concentrations, instead of commonly believed titanyl ions. The two structures transform into each other spontaneously and reversibly when diluting or concentrating the solution. More importantly, the transformation also governs the hydrothermal growth behavior of rutile-phase TiO2 from solution. Our work addresses a long-standing controversy regarding the structure of Ti(IV) (aq), reveals a highly dynamic clustering and dissociation behavior of Ti(IV) (aq), and benefits fundamental understanding of Ti chemistry.