Dimeric n-Alkyl Complexes of Rare-Earth Metals Supported by a Linked Amido−Cyclopentadienyl Ligand: Evidence for β-Agostic Bonding in Bridging n-Alkyl Ligands and Its Role in Styrene Polymerization

Abstract

The dimeric rare-earth hydrides [Ln(η^5:η^1-C_5Me_4SiMe_2NCMe_3)(THF)(μ-H)]_2 (Ln = Y, Yb) react with excess α-olefin H_2C ═ CHR (R = Et, ^nPr, ^nBu) in a 1,2-insertion to give the series of THF-free dimeric n-alkyl complexes [Ln(η^5:η^1-C_5Me_4SiMe_2NCMe_3)(μ-CH_2CH_2R)]_2 as isolable crystals. Single-crystal X-ray diffraction studies on the five derivatives [Y(η^5:η^1-C_5Me_4SiMe_2NCMe_2R‘)(μ-CH_2CH_2R)]_2 (R‘ = Me, R = Et, ^nBu; R‘ = Et, R = Et, ^nPr) and [Yb(η^5:η^1-C_5Me_4SiMe_2NCMe_3)(μ-CH_2CH_2^nBu)]_2 revealed that the centrosymmetric dimeric complexes consist of two trans-arranged [Ln(η^5:η^1-C_5Me_4SiMe_2NCMe_2R‘)] fragments connected by two μ-alkyl ligands. Most strikingly, there is an agostic interaction of the n-alkyl groups' β-CH_2 hydrogen atoms with the formally 12-electron lanthanide metal center. Variable-temperature NMR spectroscopic data suggest a fluxional process that interconverts the diastereotopic protons of the α-CH_2 group and a dynamic β-agostic interaction. Addition of >10 equiv of THF per yttrium to a solution of [Y(η^5:η^1-C_5Me_4SiMe_2NCMe_3)(μ-CH_2CH_2Et)]_2 results in the formation of the highly reactive, nonisolable, monomeric THF adduct [Y(η^5:η^1-C_5Me_4SiMe_2NCMe_3)(CH_2CH_2Et)(THF)]. Reaction of 1,2-dimethoxyethane (DME) with [Y(η^5:η^1-C_5Me_4SiMe_2NCMe_3)(μ-CH_2CH_2Et)]_2 forms the crystalline compound [Y(η^5:η^1-C_5Me_4SiMe_2NCMe_3)(CH_2CH_2Et)(DME)] with a terminal n-butyl group that contains a slightly distorted α-carbon atom according to a crystallographic study. α-Olefins having two or more substituents on the γ-carbon do not react with the hydride complexes. The role of these n-alkyl complexes in the controlled polymerization of styrene is discussed.