Hexafluoroantimony(v) salts of the cationic Ti(iv) fluoride non metallocene complexes [TiF3(MeCN)3]+ and [TiF2L]2+ (L = 15-Crown-5 and 18-Crown-6). Preparation, characterization and thermodynamic stability

Abstract

The cationic titanium fluoride containing complexes [fac-TiF₃(MeCN)₃][SbF₆]·MeCN (1), [trans-TiF₂(15-Crown-5)][SbF₆]₂ (2) and [trans-TiF₂(18-Crown-6)][SbF₆]₂ (3), were prepared by the reaction of TiF₄, the molecular ligand and SbF₅ in MeCN. Complexes 1–3 were characterized by X-ray single crystal analysis, elemental analysis, IR, NMR and mass spectroscopy. Titanium tetrafluoride reacts with the SbF₅ in SO₂ with the formation of fac-[TiF₃(SO₂)₃]⁺, detected by ¹⁹F NMR. Application of the volume-based approach to thermodynamics (VBT) offers a means, for the first time, of exploring the energetics surrounding these materials and in the thermodynamic section a discussion of this new approach is provided. It emerges that the basis of the thermodynamic driving force of formation of [TiF₃L₃][SbF₆](s) salts, that enforces the unfavourable [ΔH° = + 237 (±20) kJ mol⁻¹] fluoride ion transfer from the Lewis acid TiF₄(s) to SbF₅(l) to give the hypothetical [TiF₃]⁺[SbF₆]⁻ (s), is the higher Ti–L (L = ligand) bond energy in the cationic complexes [TiF₃L₃]⁺ as compared to that in the molecular adducts TiF₄L₂(s) and SbF₅L(s) so giving rise to larger enthalpies of complexation of [TiF₃]⁺(g) by 3L(g) compared to those for complexation of TiF₄(g) by 2L(g) and SbF₅(g) by 1L(g). Formation of the trans-[TiF₂(15-Crown-5)]²⁺ and trans-[TiF₂(18-Crown-6)]²⁺ is accounted for the stabilization of [TiF₂]²⁺ cation by the five donor acceptor Ti–O contacts and the accompanying positive charge delocalization. Cationic titanium(IV) complexes fac-[TiF₃(MeCN)_3−nL_n]⁺ (n = 0–3) and cis-[TiF₃(18-Crown-6)]⁺, trans-[TiF₂(Crown)]²⁺ (Crown = 15-Crown-5 and 18-Crown-6) were obtained in MeCN solution by the reaction of fac-[TiF₃(MeCN)₃]⁺ and L = Et₂O, THF, H₂O or crown ethers. Complexes fac-[TiF₃(MeCN)_{3 − n}L_n][SbF₆] L = Et₂O, THF, H₂O, crown ethers are unstable in MeCN solution and slowly decompose giving molecular complexes cis-TiF₄L₂, cis-TiF₄ (Crown), SbF₅L, titanium oxofluoride and alkoxide complexes. The structure of the fac-[TiF₃(MeCN)₃]⁺ is similar to the fac-[TiCl₃(MeCN)₃]⁺ and the complexes trans-[TiF₂L]²⁺ L = 15-Crown-5, 18-Crown-6 have very similar geometries to that of trans-[TiCl₂(15-Crown-5)]⁺ showing that the essential features of coordination are the same for the cationic titanium chloride and fluoride complexes with MeCN and 15-Crown-5, 18-Crown-6.