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
journal contributionposted on 02.12.2002, 00:00 by Peter Voth, Stefan Arndt, Thomas P. Spaniol, Jun Okuda, Lily J. Ackerman, Malcolm L. H. Green
The dimeric rare-earth hydrides [Ln(η5:η1-C5Me4SiMe2NCMe3)(THF)(μ-H)]2 (Ln = Y, Yb) react with excess α-olefin H2CCHR (R = Et, nPr, nBu) in a 1,2-insertion to give the series of THF-free dimeric n-alkyl complexes [Ln(η5:η1-C5Me4SiMe2NCMe3)(μ-CH2CH2R)]2 as isolable crystals. Single-crystal X-ray diffraction studies on the five derivatives [Y(η5:η1-C5Me4SiMe2NCMe2R‘)(μ-CH2CH2R)]2 (R‘ = Me, R = Et, nBu; R‘ = Et, R = Et, nPr) and [Yb(η5:η1-C5Me4SiMe2NCMe3)(μ-CH2CH2nBu)]2 revealed that the centrosymmetric dimeric complexes consist of two trans-arranged [Ln(η5:η1-C5Me4SiMe2NCMe2R‘)] fragments connected by two μ-alkyl ligands. Most strikingly, there is an agostic interaction of the n-alkyl groups' β-CH2 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-CH2 group and a dynamic β-agostic interaction. Addition of >10 equiv of THF per yttrium to a solution of [Y(η5:η1-C5Me4SiMe2NCMe3)(μ-CH2CH2Et)]2 results in the formation of the highly reactive, nonisolable, monomeric THF adduct [Y(η5:η1-C5Me4SiMe2NCMe3)(CH2CH2Et)(THF)]. Reaction of 1,2-dimethoxyethane (DME) with [Y(η5:η1-C5Me4SiMe2NCMe3)(μ-CH2CH2Et)]2 forms the crystalline compound [Y(η5:η1-C5Me4SiMe2NCMe3)(CH2CH2Et)(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.