Cation-Driven Assembly of Hydrated Ti(IV) Cations toward Titanium-Oxo Clusters

In mixed solutions, the interactions between counterions, co-ions, solvent–solvent, and solvent–solutes are interactive and synergistic not only with each other but also in relation to potential chemical changes. Specifically, the speciation in aqueous solutions of metal-oxo clusters is highly sensitive to the presence of codissolved solutes, which governs the behaviors of disassembling, transformation, and hydrolysis toward metal oxides. Herein, we report an unusual cation effect that promotes the self-assembly of cationic titanium-oxo clusters, which is observed with alkali metals, alkaline earth metals, transition metals, and ammonium cations. The significance of such a cation effect is concentration-dependent and follows the periodic trends. We find the hydration of added cations to be responsible for the observed effect by decreasing the activity of water molecules, as confirmed by a direct water activity measurement, a disturbed hydrogen-bond network structure, and an elevated proton activity. Our work enriches the solution chemistry of metal-oxo clusters and provides new perspectives for investigating the physicochemical properties of concentrated electrolyte solutions and mixed solutions.