Low-Valent Titanium Species Stabilized with Aluminum/Boron Hydride Fragments

Low-valent titanium species were prepared by reaction of [TiCp*X₃ ] (Cp*=η⁵ -C₅ Me₅ ; X=Cl, Br, Me) with LiEH₄ (E=Al, B) or BH₃ (thf), and their structures elucidated by experimental and theoretical methods. The treatment of trihalides [TiCp*X₃ ] with LiAlH₄ in ethereal solvents (L) leads to the hydride-bridged heterometallic complexes [{TiCp*(μ-H)}₂ {(μ-H)₂ AlX(L)}₂ ] (L=thf, X=Cl, Br; L=OEt₂ , X=Cl). Density functional theory (DFT) calculations for those compounds reveal an open-shell singlet ground state with a Ti-Ti bond and can be described as titanium(II) species. The theoretical analyses also show strong interactions between the Ti-Ti bond and the empty s orbitals of the Al atom of the AlH₂ XL fragments, which behave as σ-accepting (Z-type) ligands. Analogous reactions of [TiCp*X₃ ] with LiBH₄ (2 and 3 equiv.) in tetrahydrofuran at room temperature and at 85 °C lead to the titanium(III) compounds [{TiCp*(BH₄ )(μ-X)}₂ ] (X=Cl, Br) and [{TiCp*(BH₄ )(μ-BH₄ )}₂ ], respectively. The treatment of [TiCp*Me₃ ] with 4 and 5 equiv. of BH₃ (thf) produces the diamagnetic [{TiCp*(BH₃ Me)}₂ (μ-B₂ H₆ )] and paramagnetic [{TiCp*(μ-B₂ H₆ )}₂ ] complexes, respectively.