Rare-earth metal alkyl complexes bearing an alkoxy N-heterocyclic carbene ligand: synthesis, characterization, catalysis for isoprene polymerization

Abstract

Treatment of imidazolium iodide [R-NHC-CH₂CH(ⁿBu)OH]I (1a (R = methyl), 1b (R = isopropyl)) and benzimidazole iodide [R-NHC(C₆H₄)-CH₂CH(ⁿBu)OH]I (2a (R = methyl), 2b (R = isopropyl)), respectively, with ((trimethylsilyl)methyl)lithium (LiCH₂SiMe₃) followed by rare-earth metal tris(alkyl)s (Ln(CH₂SiMe₃)₃(THF)₂) (Ln = Sc, Y, Lu) afforded the alkoxy-modified N-heterocyclic carbene ligated rare-earth metal bis(alkyl) complexes {[R-NHC-CH₂CH(ⁿBu)OLn(CH₂SiMe₃)₂]₂ (3a (R = methyl, Ln = Sc), 3b (R = methyl, Ln = Lu), 4a (R = isopropyl, Ln = Y), 4b (R = isopropyl, Ln = Lu)) and [(CH₃)₂CH-NHC(C₆H₄)-CH₂CH(ⁿBu)OY(CH₂SiMe₃)₂]₂ (5)} via double-deprotonation reactions. All complexes were characterized by NMR spectroscopy, elemental analysis and X-ray crystallography. They are dimers in which two rare-earth metal ions are bridged by two μ²-O atoms. Under the activation of an organoborate, complex 3a showed cis-1,4 enriched regioselectivity for isoprene polymerization (84.3%).