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

The cationic titanium fluoride containing complexes [fac-TiF3(MeCN)(3)][SbF6] center dot 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 F-19 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 [Delta H degrees =+ 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-n)L-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. uoride complexes with MeCN and 15-Crown-5, 18-Crown-6.