Crystal structures and raman spectra of imidazolium poly[perfluorotitanate(IV)] salts containing the [TiF_6]~(2-), ([Ti_2F_9]~-)_∞, and [Ti _2F_(11)]~(3-) and the New [Ti_4F _(20)]~(4-) and [Ti_5F_(23)]~(3-) anions

摘要

Reactions between imidazole (Im, C_3H_4N_2) and TiF_4 in anhydrous hydrogen fluoride (aHF) in different molar ratios have yielded [ImH]_2[TiF_6]·2HF, [ImH] _3[Ti_2F_(11)], [ImH]_4[Ti _4F_(20)], [ImH]_3[Ti_5F_(23)], and [ImH][Ti_2F_9] upon crystallization. All five structures were characterized by low-temperature single-crystal X-ray diffraction. The single-crystal Raman spectra of [ImH]_4[Ti_4F_(20)], [ImH]_3[Ti_5F_(23)], and [ImH][Ti_2F _9] were also recorded and assigned. In the crystal structure of [ImH]_2[TiF_6]·2HF, two HF molecules are coordinated to each [TiF_6]~(2-) anion by means of strong F-H···F hydrogen bonds. The [Ti_2F _(11)]~(3-) anion of [ImH]_3[Ti_2F _(11)] results from association of two TiF_6 octahedra through a common fluorine vertex. Three crystallographically independent [Ti _2F_(11)]~(3-) anions, which have distinct geometries and orientations, are hydrogen-bonded to the [ImH]~+ cations. The [ImH]_4[Ti_4F_(20)] salt crystallized in two crystal modifications at low (α-phase, 200 K) and ambient (β-phase, 298 K) temperatures. The tetrameric [Ti_4F_(20)]~(4-) anion of [ImH]_4[Ti_4F_(20)] consists of rings of four TiF_6 octahedra, which each share two cis-fluorine vertices, whereas the pentameric [Ti_5F_(23)]~(3-) anion of [ImH] _3[Ti_5F_(23)] results from association of five TiF_6 units, where four of the TiF_6 octahedra share two cis-vertices, forming a tetrameric ring as in [Ti_4F _(20)]~(4-), and the fifth TiF_6 unit shares three fluorine vertices with three TiF_6 units of the tetrameric ring. The [ImH][Ti_2F_9] salt also crystallizes in two crystal modifications at low (α-phase, 200 K) and high (β-phase, 298 K) temperatures and contains polymeric ([Ti_2F_9] ~-)_∞ anions, which appear as two parallel infinite zigzag chains comprised of TiF_6 units, where each TiF_6 unit of one chain is connected to a TiF_6 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 [Ti_4F_(20)]~(4-) and [Ti_5F_(23)]~(3-) anions, which aided in the assignment of the experimental vibrational frequencies of the anion series.