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Crystal Structures and Raman Spectra of Imidazolium Poly[perfluorotitanate(IV)] Salts Containing the [TiF6]2–, ([Ti2F9]), and [Ti2F11]3– and the New [Ti4F20]4– and [Ti5F23]3– Anions

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posted on 22.02.2016 by Igor M. Shlyapnikov, Hélène P. A. Mercier, Evgeny A. Goreshnik, Gary J. Schrobilgen, Zoran Mazej
Reactions between imidazole (Im, C3H4N2) and TiF4 in anhydrous hydrogen fluoride (aHF) in different molar ratios have yielded [ImH]2[TiF6]·2HF, [ImH]3[Ti2F11], [ImH]4[Ti4F20], [ImH]3[Ti5F23], and [ImH]­[Ti2F9] upon crystallization. All five structures were characterized by low-temperature single-crystal X-ray diffraction. The single-crystal Raman spectra of [ImH]4[Ti4F20], [ImH]3[Ti5F23], and [ImH]­[Ti2F9] were also recorded and assigned. In the crystal structure of [ImH]2[TiF6]·2HF, two HF molecules are coordinated to each [TiF6]2– anion by means of strong F–H···F hydrogen bonds. The [Ti2F11]3– anion of [ImH]3[Ti2F11] results from association of two TiF6 octahedra through a common fluorine vertex. Three crystallographically independent [Ti2F11]3– anions, which have distinct geometries and orientations, are hydrogen-bonded to the [ImH]+ cations. The [ImH]4[Ti4F20] salt crystallized in two crystal modifications at low (α-phase, 200 K) and ambient (β-phase, 298 K) temperatures. The tetrameric [Ti4F20]4– anion of [ImH]4[Ti4F20] consists of rings of four TiF6 octahedra, which each share two cis-fluorine vertices, whereas the pentameric [Ti5F23]3– anion of [ImH]3[Ti5F23] results from association of five TiF6 units, where four of the TiF6 octahedra share two cis-vertices, forming a tetrameric ring as in [Ti4F20]4–, and the fifth TiF6 unit shares three fluorine vertices with three TiF6 units of the tetrameric ring. The [ImH]­[Ti2F9] salt also crystallizes in two crystal modifications at low (α-phase, 200 K) and high (β-phase, 298 K) temperatures and contains polymeric ([Ti2F9]) anions, which appear as two parallel infinite zigzag chains comprised of TiF6 units, where each TiF6 unit of one chain is connected to a TiF6 unit of the second chain through a shared fluorine vertex. Quantum-chemical calculations at the B3LYP/SDDALL level of theory were used to arrive at the gas-phase geometries and vibrational frequencies of the [Ti4F20]4– and [Ti5F23]3– anions, which aided in the assignment of the experimental vibrational frequencies of the anion series.