Rare-Earth Metal Complexes Supported by 1,3-Functionalized Indolyl-Based Ligands for Efficient Hydrosilylation of Alkenes

Two different 1,3-functionalized indolyl-based proligands 1-(2-C₄H₇O)CH₂-3-(2-^tBuC₆H₅N═CH)C₈H₅N (HL¹) and 1-Me₂NCH₂CH₂-3-(2-ⁱPrC₆H₅N═CH)C₈H₅N (HL²) were designed, prepared in high yields, and successfully applied to rare-earth metal chemistry showing different reactivities and different bondings with the central metals. The reactions of HL¹ with RE(CH₂SiMe₃)₃(THF)₂ provided two types of rare-earth metal complexes: the pincer type mononuclear complexes κ³-(L¹)RE(CH₂SiMe₃)₂ [L¹ = 1-(2-C₄H₇O)CH₂-3-(2-^tBuC₆H₅N═CH)C₈H₄N, RE = Lu(1), Yb(2)], and the dinuclear rare-earth metal alkyl (per alkyl/per metal) complexes having the ligand in novel coordination modes {(η¹:(μ-η²:η¹):η¹-1-(2-C₄H₇O)CH₂-3-[2-^tBuC₆H₅NCH-(CH₂SiMe₃)]C₈H₄N)RECH₂SiMe₃}₂ [RE = Er(3), Y(4), Dy(5), and Gd(6)]. Meanwhile, the reactions of HL² with RE(CH₂SiMe₃)₃(THF)₂ led to the isolation and characterization of only the mononuclear rare-earth metal dialkyl complexes κ³-(L²)RE(CH₂SiMe₃)₂ [L² = 1-Me₂NCH₂CH₂-3-(2-ⁱPrC₆H₅N═CH)C₈H₄N, RE = Lu(7), Gd(8)] bearing the ligand in the pincer chelate form. The mononuclear complexes were formed through the sp² C-H activation of the 2-indolyl moiety, while the dinuclear complexes were produced unexpectedly through the tandem 2-indolyl sp² C-H activation and C═N insertion into the RE-CH₂SiMe₃ bond. These complexes were fully characterized by spectroscopic methods, elemental analyses, and single-crystal X-ray crystallography. The applications of the synthesized complexes as catalysts for the hydrosilylation of terminal alkenes with phenylsilane are described. Anti-Markovnikov addition products were produced by the hydrosilylation of aliphatic olefins, and Markovnikov addition products were isolated with aromatic olefins with high selectivity in the absence of cocatalysts. It is found that the dinuclear rare-earth alkyl complexes exhibited the best catalytic activity with the advantages of mild reaction conditions, short reaction time, low catalyst loading, and wide substrate applicability in comparison with the synthesized mononuclear complexes and the reported catalysts.

Controlled, one-pot synthesis of recyclable poly(1,3-diene)-polyester block copolymers, catalyzed by yttrium β-diketiminate complexes

The one-pot synthesis of well-defined block copolymers of olefins/1,3-dienes and polar monomers, such as cyclic esters and acrylates has long been the focus of intense research. Cationic alkyl rare earth metal catalysts, activated by organoborates, have shown to be promising for the polymerization of isoprene or styrene and ε-caprolactone. In this study, we synthesize a series of yttrium bis(alkyl) complexes supported by simple β-diketiminate ancillary ligands. Subtle changes have been made to the β-diketiminate ligand framework to elucidate the effect of ligand structure on the rate and selectivity of olefin/1,3-diene and cyclic ester polymerization, with small ligand changes having a large impact on the resulting polymerizations. Generation of the active cationic species was easily streamlined by identification of appropriate catalyst : organoborate ratios, allowing for high catalyst efficiencies. Notably, we demonstrate the first cationic rare earth metal alkyl-initiated polymerization of δ-valerolactone and ε-decalactone as well as introduced five new block copolymer morphologies. In addition, selective degradation of the ester block in poly(isoprene-b-caprolactone) enabled recovery of the polyisoprene block with identical spectroscopic and thermal properties. Significantly, recopolymerization of the recovered poly(1,3-diene) with fresh ε-caprolactone reproduced the desired diblocks with nearly identical thermal and physical properties to those of virgin copolymer, illustrating a plausible recycling scheme for these materials.

Ligand features that promote one-pot block copolymerization of 1,3-dienes and cyclic esters were realized with yttrium β-diketiminate complexes. Depolymerization and repolymerization of the polyester block introduced a plausible recycling strategy.