Synthesis of pincer rare-earth metal chlorides bearing indol-2-yl motifs with nonsymmetrical hard and soft appendant side arms and their catalytic applications

Abstract

Two types of pincer rare-earth metal chlorides bearing indolyl backbone and appendant by hard and soft side arms of OP-, NP- chelated groups were synthesized by the metathesis reactions between corresponding lithium compounds (Li-L¹ = [(1-(CH₃OCH₂CH₂)-3-(Ph₂PCH₂)C₈H₄N)]₂Li₂, Li-L² = [(1-(2-(CH₂)₄NCH₂CH₂)-3-(Ph₂PCH₂)C₈H₄N)]₂Li₂) and rare-earth chlorides RECl_3. NMR and X-ray diffraction analyses revealed that the chlorides (REL₂Cl₂Li(THF)_n, n = 2 for 2, n = 1 for 3) are heteronuclear bimetal complexes with two ligands chelating the RE³⁺ ions and Cl^- bridging RE³⁺ and Li⁺. Complex 2 adopts a homoleptic κ³_OCP ligation mode with a coordination number of 8. While complex 3 possesses an uneven κ³_NCP/κ²_CP coordination mode, thus generating a seven-coordinate metal center. The catalytic activity of these complexes towards isoprene polymerization reveals that ionic radii and substituents on the ligands greatly affect their catalytic performances and stereoselectivity of the resulting polymers. Amongst, the gadolinium complexes cooperating with cocatalysts display the best catalytic performance among the surveyed catalytic systems.