Homogeneous Ethylene-Polymerization Catalysts Based on Alkyl Cations of the Rare-Earth Metals:Are Dicationic Mono(alkyl)Complexes the Active Species?

摘要

Cationic alkyl complexes of Group 4 metals,for example,alkyl zirconocenium ions,currently play a pivotal role as single-site catalysts in homogeneous olefin-polymerization reactions.The first cationic alkyl rare earth metal complex [La(eta~5-C_5Me_5){CH(SiMe_3)_2}(thf)_x]~+[BPh_4]~-(x=o or 3) was reported in 1992,and since then,there has only been one report with evidence that such a complex,a scandium dimethyl cation,polymerizes ethylene.A number of cationic alkyl complexes of the rare-earth metals[Ln(L)R]~+supported by monoanionic ancillary noncyclopentadienyl ligands L,such as the amido-functionalized triazacyclononane,beta-diketiminate,or benzamidinate were surmised to also catalyze ethylene polymerization.We reported that the thermally sensitive tris(trimethylsilylmethyl))complexes [Ln(CH_2SiMe_3)_3(thf)_2](Ln=Lu,Y) can be transformed into robust monocationic bis(alkyl)complexes[Ln(CH_2SiMe_3)_2(thf)_x]~+ by the reaction with BPh_3.Herein we report that the tris(alkyl)complexes[Ln(CH_2SiMe_3)_2]without any ancillary ligand L can act as precursors for highly active catalysts for homogeneous ethylene polymerization.There is evidence that the active species are the dications[Ln(CH_2SiMe_3)(solv)_z]~2+,which are formed by the protonolysis of [Ln(CH_2SiMe_3)_3(thf)_2]via the dialkyl monocations [Ln(CH_2SiMe_3)_2(solv)_y]~+.