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.单（环戊二烯基）钛(II)配合物CpTiCl(dmpe)₂已通过在1,2-双(二甲基膦基)乙烷（1,2-bis(dimethylphosphino)ethane，缩写dmpe）存在下，将(CpTiCl₂)ₓ与正丁基锂反应制得。随后用甲基锂或正丁基锂处理该钛(II)物种，可分别得到烷基和氢化物类配合物CpTiMe(dmpe)₂与CpTiH(dmpe)₂。已测定CpTiX(dmpe)₂（X=Cl、Me、H）的X射线晶体结构，所有配合物均展现出异常长的金属-配体键长，这归因于这些形式上为八配位的配合物的第一配位球内存在空间位阻效应。采用类似方法，在螯合膦配体存在下，用正丁基锂处理[Cp*Ti(BH₄)Cl]₂，可得到单（五甲基环戊二烯基）钛(II)配合物Cp*Ti(BH₄)(η²-PP)，其中PP为dmpe或(叔丁基)三(二甲基膦甲基)硅烷（(tert-butyl)tris(dimethylphosphinomethyl)silane，trimpsi）。上述所有钛(II)配合物均可作为烯烃低聚反应的催化剂。例如，乙烯可被二聚为1-丁烯；1-丁烯与乙烯的后续偶联反应则生成乙烯三聚产物2-乙基-1-丁烯与3-甲基-1-戊烯。产物分布特征最有力地支持了该反应通过金属环戊烷中间体进行的偶联反应机理。基于产物分布的相似性，我们提出，在将乙烯二聚为1-丁烯的Alphabutol工艺中，其催化活性物种为钛(II)中心。