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

The dimeric rare-earth hydrides [Ln( η 5: η 1 -C 5 Me 4 SiMe 2 NCMe 3 )(THF)(μ-H)] 2 (Ln = Y, Yb) react with excess α-olefin H 2 C CHR (R = Et, n Pr, n Bu) in a 1,2-insertion to give the series of THF-free dimeric n -alkyl complexes [Ln( η 5: η 1 -C 5 Me 4 SiMe 2 NCMe 3 )( μ -CH 2 CH 2 R)] 2 as isolable crystals. Single-crystal X-ray diffraction studies on the five derivatives [Y( η 5: η 1 -C 5 Me 4 SiMe 2 NCMe 2 R‘)( μ -CH 2 CH 2 R)] 2 (R‘ = Me, R = Et, n Bu; R‘ = Et, R = Et, n Pr) and [Yb( η 5: η 1 -C 5 Me 4 SiMe 2 NCMe 3 )( μ -CH 2 CH 2 n Bu)] 2 revealed that the centrosymmetric dimeric complexes consist of two trans-arranged [Ln( η 5: η 1 -C 5 Me 4 SiMe 2 NCMe 2 R‘)] 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 5 Me 4 SiMe 2 NCMe 3 )( μ -CH 2 CH 2 Et)] 2 results in the formation of the highly reactive, nonisolable, monomeric THF adduct [Y( η 5: η 1 -C 5 Me 4 SiMe 2 NCMe 3 )(CH 2 CH 2 Et)(THF)]. Reaction of 1,2-dimethoxyethane (DME) with [Y( η 5: η 1 -C 5 Me 4 SiMe 2 NCMe 3 )( μ -CH 2 CH 2 Et)] 2 forms the crystalline compound [Y( η 5: η 1 -C 5 Me 4 SiMe 2 NCMe 3 )(CH 2 CH 2 Et)(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.