Interaction of TiF4 with the donor solvents SO2, PhCN, and MeCN: Isolation and structural characterization of the first trimeric fluorine bridged donor–acceptor adduct {TiF4(PhCN)}3

There has been speculations on the structures of TiF4 polymeric complexes {TiF4L}n (L = molecular donor) for several decades, however no structurally characterized examples have been reported. In this work the complex {TiF4(PhCN)}3 was isolated from a solution of TiF4 in PhCN (donor number DN = 11.9 kcal mol−1) as well as from the mixtures PhCN/CH2Cl2 and PhCN/toluene and characterized by X-ray, IR, NMR, EI-MS. The structure of the complex {TiF4(PhCN)}3 can be regarded as formed by combining three face-TiF3(PhCN)(μ-F) units, containing octahedrally coordinated titanium centers surrounded by three terminal fluorine atoms on the face of the octahedron and the bridging fluorine atoms in cis-positions with respect to each other. The structure of {TiF4(PhCN)}3 represents the first example of a trimeric pseudo pentahalide MX4L (M = Ti, Zr, X = halogen, D = ligand), a class of potentially interesting Lewis acids. The characterization of {TiF4(PhCN)}3 by 19F NMR revealed that in solution it dissociated to a mixture of [TiF3(PhCN)3]+, TiF4(PhCN)2 and oligomers including [Ti4F18]2− and {TiF4(PhCN)}n. The existence of oligomeric complexes containing face-{TiF3(PhCN)3−n(μ-F)n} (n = 1–3) fragments was established by one- and two-dimensional variable temperature 19F NMR. In contrast, TiF4 has a low solubility in SO2, because the donor strength of SO2 (DN = 6.5 ± 2.2 kcal mol−1) is too weak to fully convert polymeric TiF4 into soluble TiF4–SO2 donor–acceptor adducts. TiF4 and MeCN (DN = 14.1 kcal mol−1) formed only the molecular complex TiF4(MeCN)2, characterized by preliminary X-ray structure, IR and EI-MS. Thus mononuclear donor–acceptor complexes TiF4L2 can only be isolated from MeCN and stronger basic solvents.