Single-Component Polymerization Catalysts for Ethylene and Styrene: Synthesis, Characterization, and Reactivity of Alkyl and Hydrido Yttrium Complexes Containing a Linked Amido−Cyclopentadienyl Ligand

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Abstract

Yttrium alkyl complexes Y(η5:η1-C5Me4SiMe2NCMe2R)(CH2SiMe3)(THF) (R = Me, Et) and Y(η5:η1-C9H6SiMe2NCMe3)(CH2SiMe3)(THF) can be prepared in high yields by a σ-bond metathesis reaction between Y(CH2SiMe3)3(THF)2 and amino-functionalized cyclopentadienes or indene. The structure of Y(η5:η1-C5Me4SiMe2NCMe2Et)(CH2SiMe3)(THF) was shown by single-crystal X-ray diffraction to be that of a three-legged piano stool. Reaction of Y(CH2SiMe3)3(THF)2 with the tridentate linked amido−cyclopentadienyl ligands (C5Me4H)SiMe2NHR (R = CH2CH2OMe, CH2CH2NMe2, CH2CH2CH2OMe, CMe2CH2OMe), which contain an additional donor site, results in the cleavage of the silicon−cyclopentadienyl bond and the formation of the tetramethylcyclopentadienyl complexes Y(η5-C5Me4H){N(SiMe2CH2SiMe3)(R)}(CH2SiMe3)(THF)x (x = 0, 1). The dimeric hydrido complexes [Y(η5:η1-C5Me4SiMe2NCMe2R)(THF)(μ-H)]2 are prepared in good yields by hydrogenolysis of the corresponding alkyl complexes. Variable-temperature 1H, 13C, 29Si, and 89Y NMR spectroscopic data show that the hydrido complexes retain their dimeric structure in solution on the NMR time scale but that they undergo fluxional processes which include THF dissociation and cis−trans isomerization. The presence of monomeric species is inferred from 1H NMR spectroscopic detection of the crossover product [Y(η5:η1-C5Me4SiMe2NCMe3)(THF)(μ-H)2Y(η5:η1-C5Me4SiMe2NCMe2Et)(THF)], which forms within minutes at 50 °C when [Y(η5:η1-C5Me4SiMe2NCMe2R)(THF)(μ-H)]2 with R = Me and Et are mixed in C6D6. Ethylene is polymerized with moderate activity by the hydrido complexes, whereas styrene derivatives and 1-hexene are cleanly converted into the monoinsertion products. 1,5-Hexadiene reacts with the hydrido complexes to give the monomeric cyclopentylmethyl complex Y(η5:η1-C5Me4SiMe2NCMe3){CH2CH(CH2)4}(THF). The 1-phenylethyl complexes Y(η5:η1-C5Me4SiMe2NCMe3){CH(CH3)(C6H3-2-R-4-R‘)}(THF) (R = R‘ = H; R = H, R‘ = OMe; R = R‘ = Me), selectively formed by secondary insertion of the corresponding styrene derivatives, show fluxional η3-coordination in solution. A monomeric structure with a weak η3-interaction was confirmed by single-crystal X-ray diffraction of Y(η5:η1-C5Me4SiMe2NCMe2Et){CH(CH3)(C6H4-4-tBu)}(THF). The insertion product of 1-hexene is dimeric in solution, but partially loses THF under vacuum. This product initiates the polymerization of styrene to give atactic polystyrenes with narrow molecular weight distributions (Mw/Mn 1.10−1.23) and microstructures enriched in syndiotacticity (rr ≈ 70%).

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