Interaction of TiF 4 with the donor solvents SO 2, PhCN, and MeCN : Isolation and structural characterization of the first trimeric fluorine bridged donor–acceptor adduct {TiF 4(PhCN)} 3

Abstract

There has been speculations on the structures of TiF 4 polymeric complexes {TiF 4L} n (L = molecular donor) for several decades, however no structurally characterized examples have been reported. In this work the complex {TiF 4(PhCN)} 3 was isolated from a solution of TiF 4 in PhCN (donor number DN = 11.9 kcal mol −1) as well as from the mixtures PhCN/CH 2Cl 2 and PhCN/toluene and characterized by X-ray, IR, NMR, EI-MS. The structure of the complex {TiF 4(PhCN)} 3 can be regarded as formed by combining three face-TiF 3(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 {TiF 4(PhCN)} 3 represents the first example of a trimeric pseudo pentahalide MX 4L (M = Ti, Zr, X = halogen, D = ligand), a class of potentially interesting Lewis acids. The characterization of {TiF 4(PhCN)} 3 by 19F NMR revealed that in solution it dissociated to a mixture of [TiF 3(PhCN) 3] +, TiF 4(PhCN) 2 and oligomers including [Ti 4F 18] 2− and {TiF 4(PhCN)} n . The existence of oligomeric complexes containing face-{TiF 3(PhCN) 3− n ( μ-F) n } ( n = 1–3) fragments was established by one- and two-dimensional variable temperature 19F NMR. In contrast, TiF 4 has a low solubility in SO 2, because the donor strength of SO 2 (DN = 6.5 ± 2.2 kcal mol −1) is too weak to fully convert polymeric TiF 4 into soluble TiF 4–SO 2 donor–acceptor adducts. TiF 4 and MeCN (DN = 14.1 kcal mol −1) formed only the molecular complex TiF 4(MeCN) 2, characterized by preliminary X-ray structure, IR and EI-MS. Thus mononuclear donor–acceptor complexes TiF 4L 2 can only be isolated from MeCN and stronger basic solvents.