Topotactic desolvation and condensation reactions of 3D Zn3TiF7(H2O)2(taz)3·S (S = 3H2O or C2H5OH)

Abstract

Crystals of two new 3D hybrid compounds, trans-Zn₃TiF₇(H₂O)₂(taz)₃·3H₂O and cis-Zn₃TiF₇(H₂O)₂(taz)₃·C₂H₅OH, have been obtained by solvothermal synthesis ((taz)⁻ = 1,2,4-triazolate C₂H₂N₃ ligand). Their structures, determined from X-ray single crystal diffraction data in Cm and Pnma space groups, respectively, are based on Zn₃N₉(H₂O)₂F₃ trimers linked by TiF₆ octahedra that build trans- or cis-chains _∞[Zn₃TiN₉(H₂O)₂F₇]. Water or ethanol in the structure cavities is released below 110 °C to give trans- or cis-Zn₃TiF₇(H₂O)₂(taz)₃ and, on further heating, these intermediate phases dehydrate and lead to anhydrous trans- or cis-Zn₃TiF₇(taz)₃. At 110 °C, the loss of ethanol concerns only ≈1/3 of the weight of cis-Zn₃TiF₇(H₂O)₂(taz)₃·C₂H₅OH while the remaining part subsists up to 180 °C. This behaviour is attributed to a core–shell type configuration. Rehydration in humid air occurs at room temperature for the anhydrous cis-phase. All desolvated, dehydrated or rehydrated phase structures have been determined by X-ray powder diffraction and ab initio Rietveld refinements. All transformations from solvated or hydrated to anhydrous phases are realised without any symmetry change and the trans or cis connection of the TiF₆ octahedra is maintained together with the overall features of the 3D networks. The final loss of water molecules induces a condensation reaction that implies the connection of the trimers by fluorine atoms; the structures of the anhydrous phases Zn₃TiF₇(taz)₃ are then described by Zn₃N₉F₄ trimers. ¹H and ¹⁹F MAS NMR studies, coupled with DFT calculations of NMR parameters, confirm the water loss and support the strutural models while evidencing both the positional disorders, more likely of the organic parts, and the F motions within TiF₆ octahedra.