Ionic Liquid-Based Low-Temperature Synthesis of Crystalline Ti(OH)OF·0.66HO: Elucidating the Molecular Reaction Steps by NMR Spectroscopy and Theoretical Studies.

We present an in-depth mechanistic study of the first steps of the solution-based synthesis of the peculiar hexagonal tungsten bronze-type Ti(OH)OF·0.66HO solid, using NMR analyses (H, C, F, and B) as well as modeling based on density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulation. The reaction uses an imidazolium-based ionic liquid (IL, e.g., C mim BF) as a solvent and reaction partner. It is puzzling, as the fluorine-rich crystalline solid is obtained in a "beaker chemistry" procedure, starting from simple compounds forming a stable solution (BF -containing IL, TiCl, HO) at room temperature, and a remarkably low reaction temperature (95 °C) is sufficient. Building on NMR experiments and modeling, we are able to provide a consistent explanation of the peculiar features of the synthesis: evidently, the hydrolysis of the IL anion BF is a crucial step since the latter provides fluoride anions, which are incorporated into the crystal structure. Contrary to expectations, BF does not hydrolyze in water at room temperature but interacts with TiCl, possibly forming a TiCl complex with one or two coordinated BF units. This interaction also prevents the heavy hydrolysis reaction of TiCl with HO but-on the other side-spurs the hydrolysis of BF already at room temperature, releasing fluoride and building F-containing Ti(OH) Cl F complexes. The possible complexes formed were analyzed using DFT calculations with suitable functionals and basis sets. We show in addition that these complexes are also formed using other titanium precursors. As a further major finding, the heating step (95 °C) is only needed for the condensation of the Ti(OH) Cl F complexes to form the desired solid product but not for the hydrolysis of BF . Our study provides ample justification to state a "special IL effect", as the liquid state, together with a stable solution, the ionic nature, and the resulting deactivation of HO are key requirements for this synthesis.