Journal article

Authors list: Sieland, M; Schenker, M; Esser, L; Kirchner, B; Smarsly, BM

Publication year: 2022

Pages: 5350-5365

Journal: ACS Omega

Volume number: 7

Issue number: 6

ISSN: 2470-1343

Open access status: Gold

DOI Link: https://doi.org/10.1021/acsomega.1c06534

Publisher: American Chemical Society

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 center dot 0.66H(2)O solid, using NMR analyses (H-1, C-13, F-19, and B-11) 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)min 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 degrees 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-xFy) , 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 degrees C) is only needed for the condensation of the Ti(OH)(x)Cl4-xFy 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.

Harvard Citation style: Sieland, M., Schenker, M., Esser, L., Kirchner, B. and Smarsly, B. (2022) Ionic Liquid-Based Low-Temperature Synthesis of Crystalline Ti(OH)OF·0.66H2O: Elucidating the Molecular Reaction Steps by NMR Spectroscopy and Theoretical Studies, ACS Omega, 7(6), pp. 5350-5365. https://doi.org/10.1021/acsomega.1c06534

APA Citation style: Sieland, M., Schenker, M., Esser, L., Kirchner, B., & Smarsly, B. (2022). Ionic Liquid-Based Low-Temperature Synthesis of Crystalline Ti(OH)OF·0.66H2O: Elucidating the Molecular Reaction Steps by NMR Spectroscopy and Theoretical Studies. ACS Omega. 7(6), 5350-5365. https://doi.org/10.1021/acsomega.1c06534