Di and trinuclear rare-earth metal complexes supported by 3-amido appended indolyl ligands: synthesis, characterization and catalytic activity towards isoprene 1,4-cis polymerization

Abstract

Different di and trinuclear rare-earth metal complexes supported by 3-amido appended indolyl ligands were synthesized and their catalytic activities towards isoprene polymerization were investigated. Treatment of [RE(CH₂SiMe₃)₃(thf)₂] with 1 equiv. of 3-(CyNCH)C₈H₅NH in toluene or in THF afforded dinuclear rare-earth metal alkyl complexes having indolyl ligands in different hapticities with central metals {[η²:η¹–μ–η¹-3-(CyNCH(CH₂SiMe₃))Ind]RE-(thf)(CH₂SiMe₃)}₂ (Cy = cyclohexyl, Ind = Indolyl, RE = Yb (1), Er (2), Y (3)) or {[η¹–μ–η¹-3-(CyNCH(CH₂SiMe₃))Ind]RE-(thf)₂(CH₂SiMe₃)}₂ (RE = Yb (4), Er (5), Y (6), Gd (7)), respectively. These two series of dinuclear complexes could be transferred to each other easily by only changing the solvents in the process. Reaction of [Er(CH₂SiMe₃)₃(thf)₂] with 1 equiv. of 3-t-butylaminomethylindole 3-(^tBuNHCH₂)C₈H₅NH in THF afforded the unexpected trinuclear erbium alkyl complex [η²:η¹–μ–η¹-3-(^tBuNCH₂)Ind]₄Er₃(thf)₅(CH₂SiMe₃) (8), which can also be prepared by reaction of 3 equiv. of [Er(CH₂SiMe₃)₃(thf)₂] with 4 equiv. of 3-(^tBuNHCH₂)C₈H₅NH in THF. Accordingly, complexes [η²:η¹–μ–η¹-3-(^tBuNCH₂)Ind]₄RE₃(thf)₅(CH₂SiMe₃) (RE = Y (9), Dy (10)) were prepared by reactions of 3 equiv. of [RE(CH₂SiMe₃)₃(thf)₂] with 4 equiv. of 3-(^tBuNHCH₂)C₈H₅NH in THF. Reactions of [RE(CH₂SiMe₃)₃(thf)₂] with 1 equiv. of 3-t-butylaminomethylindole 3-(^tBuNHCH₂)C₈H₅NH in THF, followed by treatment with 1 equiv. of [(2,6-ⁱPr₂C₆H₃)NCHNH(C₆H₃ⁱPr₂-2,6)] afforded, after workup, the dinuclear rare-earth metal complexes [η¹–μ–η¹:η¹-3-(^tBuNCH₂)Ind][η¹–μ–η¹:η³-3-(^tBuNCH₂)Ind]RE₂(thf)[(η³-2,6-ⁱPr₂C₆H₃)NCHN(C₆H₃ⁱPr₂-2,6)]₂(RE = Er (11), Y (12)) having the indolyl ligands bonded with the rare-earth metal in different ligations. All new complexes 1–12 were fully characterized by spectroscopic methods and elemental analyses, and their structures were determined by X-ray crystallographic analyses. It was found that, except for complexes 1, 4, 11 and 12, all complexes were highly efficient catalysts for selective isoprene polymerization (up to 99% 1,4-cis selectivity) with the cooperation of co-catalysts, and the trinuclear complexes displayed advantages over dinuclear complexes in terms of molecular weight of polymers.