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

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journal contribution
posted on 12.01.2015, 00:00 by Lars N. Jende, Christoph O. Hollfelder, Cäcilia Maichle-Mössmer, Reiner Anwander
Rare-earth-metal half-sandwich allyl complexes bearing an amino-functionalized cyclopentadienyl ligand (CpNMe2 = 1-[2-(N,N-dimethylamino)­ethyl]-2,3,4,5-tetramethylcyclopentadienyl) were synthesized in a two-step salt-metathesis reaction. Treatment of LnCl3(THF)x with LiCpNMe2, followed by an in situ reaction with the Grignard reagent C3H5MgCl, generated the bis­(allyl) half-sandwich complexes CpNMe2Ln­(η3-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 [CpNMe2Nd­(η3-C3H5)­(μ-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 CpNMe2Ln­(η3-C3H5)2 (Ln = Y, Ho)/[PhNMe2H]­[B­(C6F5)4] and [CpNMe2Nd­(η3-C3H5)­(μ-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 [{(CpNMe2AlEt3)2(CpNMe2)­Ln32-Cl)33-Cl)2}­(μ2-Cl)]2 (Ln = Y, Ho). The complexes CpNMe2Ln­(η3-C3H5)2 (Ln = Y, Ho, Lu), [CpNMe2Nd­(η3-C3H5)­(μ-Cl)]2, and [{(CpNMe2AlEt3)2(CpNMe2)­Ln32-Cl)33-Cl)2}­(μ2-Cl)]2 (Ln = Y, Ho) were analyzed by X-ray crystallography.