Synthesis and crystal structures of two (cyclopentadienyl)titanium(III) hydroborate complexes, [CpTiCl(BH4)]2 and Cp2Ti(B3H8)

Treatment of Cp^∗TiCl₃ and Cp₂TiCl₂ with NaB₃H₈ affords the titanium(III) hydroborate compounds [Cp^∗TiCl(BH₄)]₂ and Cp₂Ti(B₃H₈), respectively. The former compound arises by means of a new reaction, the metal-induced fragmentation of the B₃H₈ anion, and can also be made by treating Cp^∗TiCl₃ with LiBH₄. The latter compound has been previously described, but not characterized crystallographically. Both compounds have been studied by single crystal X-ray diffraction. Dimeric [Cp^∗TiCl(BH₄)]₂ has bridging chloride ligands and terminal Cp^∗ and BH₄ ligands. The Ti-Ti distance is 3.452(1) Å, which indicates that there is no metal-metal bonding interaction. The Ti-Cl distances are 2.440(2) Å and the Ti-Cl-Ti and Cl-Ti-Cl angles of 89.97(8) and 90.03(8)° so that the Ti₂Cl₂ unit is nearly a perfect square. The BH₄ groups are each tridentate, with a Ti-B distance of 2.220(9) Å and an average Ti-H distance of 1.98(5) Å. In Cp₂Ti(B₃H₈), the B₃H₈ ligand is bidentate, as is usually seen, and the Ti-B and Ti-H distances are 2.600(3) and 1.96(2) Å. The dihedral angle between the Ti-B(1)-B(2) plane and the B(1)-B(2)-B(3) plane is 123.4°. The Ti-B distances are 0.04 Å longer than those in niobium analog, Cp₂Nb(B₃H₈), despite the fact that the single bond metallic radius of Ti is 0.02 Å smaller than that of Nb. This lengthening of the bond is probably a consequence of the presence of one fewer skeletal bonding electron in Cp₂Ti(B₃H₈).