Synthesis of Ti(OH)OF ⋅ 0.66 H2 O in Imidazolium-based Ionic Liquids

Ionic Liquid-Based Low-Temperature Synthesis of Crystalline Ti(OH)OF·0.66H2O: 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·0.66H₂O solid, using NMR analyses (¹H, ¹³C, ¹⁹F, and ¹¹B) 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 _x mim BF₄) 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 (BF₄ ^--containing IL, TiCl₄, H₂O) 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 BF₄ ^- is a crucial step since the latter provides fluoride anions, which are incorporated into the crystal structure. Contrary to expectations, BF₄ ^- does not hydrolyze in water at room temperature but interacts with TiCl₄, possibly forming a TiCl₄ complex with one or two coordinated BF₄ ^- units. This interaction also prevents the heavy hydrolysis reaction of TiCl₄ with H₂O but-on the other side-spurs the hydrolysis of BF₄ ^- already at room temperature, releasing fluoride and building F-containing Ti(OH) _x Cl_4-x F _y 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) _x Cl_4-x F _y complexes to form the desired solid product but not for the hydrolysis of BF₄ ^-. 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 H₂O are key requirements for this synthesis.