Effects of organic ammonium cations on the isolation of {Ti4} cyclic clusters from water: an 17O NMR study

Abstract

For the preparation of aqueous Ti-oxo clusters (TOCs) without organic ligands, organic ammonium cations (OACs) are important, but the mechanism is obscure. Here, the Ti⁴⁺/SO₄²⁻ system was chosen as a model to investigate the roles of OACs. Three {Ti₄} clusters were isolated from this system with different cationic additives. Structural analysis revealed a structural role of OACs: the hierarchically ordered array of the OACs in the organic layers was responsible for the stability and formation of crystalline precipitates. ¹⁷O NMR spectroscopy was used to study the interactions of OACs and SO₄²⁻ with the Ti-oxo species in water under highly acidic conditions. SO₄²⁻ enhanced the condensation of Ti⁴⁺ by binding to the TOC species. OACs could not enhance the condensation of Ti⁴⁺ but could change the solution speciation and especially promote the formation of {Ti₁₈O₂₇} clusters. We speculate that OACs interact with the cationic/neutral TOCs through anionic bridges to change the speciation. Under the synthetic conditions of {Ti₄}, {Ti₁₈O₂₇} is the native TOC in solutions but {Ti₄} is not.