Naked (C₅Me₅)₂M Cations (M = Sc, Ti, and V) and Their Fluoroarene Complexes

The ionic metallocene complexes [Cp*₂M][BPh₄] (Cp* = C₅Me₅) of the trivalent 3d metals Sc, Ti, and V were synthesized and structurally characterized. For M = Sc, the anion interacts weakly with the metal center through one of the phenyl groups, but for M = Ti and V, the cations are naked. They each contain one strongly distorted Cp* ligand, with one (V) or two (Ti) agostic C−H···M interactions involving the Cp*Me groups. For Sc and Ti, these Lewis acidic species react with fluorobenzene and 1,2-difluorobenzene to yield [Cp*₂M(κF-FC₆H₅)_n][BPh₄] (M = Sc, n = 2; M = Ti, n = 1) and [Cp*₂M(κ²F-1,2-F₂C₆H₄)][BPh₄], the first examples of κF-fluorobenzene and κ²F-1,2-difluorobenzene adducts of transition metals. With the perfluorinated anion [B(C₆F₅)₄]^-, both Sc and Ti form [Cp*₂M(κ²F-C₆F₅)B(C₆F₅)₃] contact ion pairs. The nature of the metal−fluoroarene interaction was studied by density functional theory (DFT) calculations and by comparison with the corresponding tetrahydrofuran (THF) adducts and was found to be predominantly electrostatic for all metals studied.