Rare-Earth-Metal Allyl Complexes Supported by the [2-(N,N-Dimethylamino)ethyl]tetramethylcyclopentadienyl Ligand: Structural Characterization, Reactivity, and Isoprene Polymerization

摘要

Rare-earth-metal half-sandwich allyl complexes bearing an amino-functionalized cyclopentadienyl ligand (Cp-NMe2 = 1-[2-(N,N-dimethylamino)ethyl]-2,3,4,5-tetramethylcyclopentadienyl) were synthesized in a two-step salt-metathesis reaction. Treatment of LnCl(3)(THF)x with LiCpNMe2, followed by an in situ reaction with the Grignard reagent C3H5MgCl, generated the bis(allyl) half-sandwich complexes Cp(NMe2)Ln(eta(3)-C3H5)(2) only for the smaller rare-earth metals (Ln = Y, Ho, Lu) in good yields (82-88%). In case of the larger neodymium, the dimeric mono(allyl) chlorido half-sandwich complex [(CpNd)-Nd-NMe2(eta(3)-C3H5)(mu-Cl)](2) was obtained in 68% yield. All complexes show moderate to high activity in isoprene polymerization upon cationization with organoborates [Ph3C][B(C6F5)(4)] and [PhNMe2H][B(C6F5)(4)], the yttrium, holmium, and neodymium metal centers yielding mainly 3,4-microstructures (maximum 79%). Addition of 10 equiv of AlMe3 to the catalyst systems Cp(NMe2)Ln(eta(3)-C3H5)(2) (Ln = Y, Ho)/[PhNMe2H][B(C6F5)(4)] and [(CpNd)-Nd-NMe2(eta(3)-C3H5)(mu-Cl)](2)/[PhNMe2H][B(C6F5)(4)] switched the polyisoprene stereoregularity from 3,4-specific to trans-1,4-selective (maximum 85%). The use of AliBu3 instead led to polymers with mainly cis-1,4-microstructure for the monomeric yttrium and holmium complexes (maximum 74%). Treatment of the bis(allyl) complexes with Et2AlCl (as cocatalyst) did not provide active species for isoprene polymerization but led to [allyl] ? [Cl] exchange and isolation of the hexameric rare-earth-metal clusters [{(Cp-NMe2AlEt3)(2)(Cp-NMe2)Ln(3)(mu(2)-Cl)(3)(mu(3)-Cl)(2)}(mu(2)-Cl)](2) (Ln = Y, Ho). The complexes Cp(NMe2)Ln(eta(3)-C3H5)(2) (Ln = Y, Ho, Lu), [(CpNd)-Nd-NMe2(eta(3)-C3H5)(mu-Cl)](2), and [{(Cp-NMe2AlEt3)(2)(Cp-NMe2)Ln(3)(mu(2)-Cl)(3)(mu(3)-Cl)(2)}(mu(2)-Cl)](2) (Ln = Y, Ho) were analyzed by X-ray crystallography.