Open AccessIonic Liquid-Based Low-Temperature Synthesis of Crystalline Ti(OH)OF·0.66H2O: Elucidating the Molecular Reaction Steps by NMR Spectroscopy and Theoretical Studies
Author(s) -
Melanie Sieland,
Manuel Schenker,
Lars Esser,
Barbara Kirchner,
Bernd M. Smarsly
Publication year - 2022
Publication title -
acs omega
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.779
H-Index - 40
ISSN - 2470-1343
DOI - 10.1021/acsomega.1c06534
Subject(s) - ionic liquid , chemistry , hydrolysis , ionic bonding , nuclear magnetic resonance spectroscopy , fluoride , solvent , density functional theory , tungsten , molecular dynamics , crystal structure , crystallography , computational chemistry , ion , inorganic chemistry , stereochemistry , organic chemistry , catalysis
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·0.66H 2 O solid, using NMR analyses ( 1 H, 13 C, 19 F, and 11 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 x mim BF 4 ) 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 4 - -containing IL, TiCl 4 , H 2 O) 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 4 - is a crucial step since the latter provides fluoride anions, which are incorporated into the crystal structure. Contrary to expectations, BF 4 - does not hydrolyze in water at room temperature but interacts with TiCl 4 , possibly forming a TiCl 4 complex with one or two coordinated BF 4 - units. This interaction also prevents the heavy hydrolysis reaction of TiCl 4 with H 2 O but-on the other side-spurs the hydrolysis of BF 4 - 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 BF 4 - . 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 H 2 O are key requirements for this synthesis.