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

• Published in: Organometallics 2015-01, Vol.34 (1), p.32-41
• Main Authors: Jende, Lars N, Hollfelder, Christoph O, Maichle-Mössmer, Cäcilia, Anwander, Reiner
• Format: Article
• Language: English
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om500725g

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)Ln3(μ2-Cl)3(μ3-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)Ln3(μ2-Cl)3(μ3-Cl)2}(μ2-Cl)]2 (Ln = Y, Ho) were analyzed by X-ray crystallography.