Mono(cyclopentadienyl)titanium(II) Complexes with Hydride, Alkyl, and Tetrahydroborate Ligands: Synthesis, Crystal Structures, and Ethylene Dimerization and Trimerization Catalysis

The mono(cyclopentadienyl)titanium(II) complex CpTiCl(dmpe)2 has been prepared by treating (CpTiCl2)x with n-butyllithium in the presence of 1,2-bis(dimethylphosphino)ethane (dmpe). Subsequent treatment of this titanium(II) species with methyllithium or n-butyllithium affords the alkyl and hydride analogues CpTiMe(dmpe)2 and CpTiH(dmpe)2, respectively. The X-ray crystal structures of CpTiX(dmpe)2 (X = Cl, Me, H) have been determined, and all possess unusually long metal−ligand distances owing to steric crowding in the first coordination sphere of these formally eight-coordinate complexes. Analogous treatment of [Cp*Ti(BH4)Cl]2 with n-butyllithium in the presence of a chelating phosphine affords the mono(pentamethylcyclopentadienyl)titanium(II) complexes Cp*Ti(BH4)(η2-PP), where PP is dmpe or (tert-butyl)tris(dimethylphosphinomethyl)silane (trimpsi). All of these titanium(II) complexes are catalysts for the oligomerization of alkenes. For example, ethylene is dimerized to 1-butene; subsequent coupling reactions of 1-butene with ethylene gives the ethylene trimers 2-ethyl-1-butene and 3-methyl-1-pentene. The product distribution most strongly supports a coupling mechanism that proceeds via metallacyclopentane intermediates. As judged from the similarity of the product distributions, we propose that the catalytically active species in the Alphabutol process for the dimerization of ethylene to 1-butene are titanium(II) centers.