Ionic Liquid-Based Low-Temperature Synthesis of Crystalline Ti(OH)OF·0.66H₂O: Elucidating the Molecular Reaction Steps by NMR Spectroscopy and Theoretical Studies

Abstract

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 Cl4–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 Cl4–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.

1-Butyl-3-methylimidazolium tetrafluoroborate as suitable solvent for BF3: the case of alkyne hydration. Chemistry vs electrochemistry

“…However, the 19 F NMR spectrum showed no detectable peak around −122 ppm, reported in the literature for BMIm-F [ 110 ]. The only difference between IL 19 F NMR spectra before and after electrolysis is a peak at ‒148.7 ppm (referred to BF 4 − at −150.6 ppm), possibly due to BF 3 OH − or B 2 F 7 − [ 111 – 112 ] (see Supporting Information File 1 , Figure S1a vs f and b, c). This last hypothesis is corroborated by the 19 F NMR analysis of BMIm-BF 4 after anodic oxidation in a divided cell, which shows a peak at −147.3 ppm (besides the peak at −150.6 due to BF 4 − ) (see Supporting Information File 1 , Figure S1e), which is replaced by a peak at −144.0 ppm (referred to −150.6 ppm for BF 4 − ) when the electrolysis is carried out in an undivided cell (see Supporting Information File 1 , Figure S1d).…”