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.