Rare-earth alkyl complexes supported by formamidinate ligands: synthesis, structure, and catalytic activity for isoprene polymerization

Abstract

A series of rare-earth metal monoalkyl complexes supported by N,N′-di(2,6-dialkylphenyl)formamidinate ligand (L)₂RECH₂SiMe₃·thf [L¹ = HC(N-2,6-Me₂C₆H₃)₂, RE = Y (1), L² = HC(N-2,6-ⁱPr₂C₆H₃)₂, RE = Y (2), Er (3), Dy (4), Sm (5), and Nd (6)] were synthesized by alkyl elimination reaction or by salt metathesis reaction in good yields. All complexes were characterized by elemental analyses, FT-IR spectroscopy and single crystal X-ray diffraction. In combination with [Ph₃C][B(C₆F₅)₄] and alkylaluminium, these complexes displayed a good activity towards isoprene polymerization to give polyisoprenes with high molecular weight (M_n > 10⁴) and narrow molecular distribution (PDI < 2.0). The influence of alkylaluminium, central metal, temperature, sequence of addition of alkylaluminium and [Ph₃C][B(C₆F₅)₄] on the polymerization of isoprene was studied. It was interesting to find that addition of the cocatalysts sequence has a great influence on the regioselectivity of the polymerization. High 1,4-regioselectivity polymerizations of isoprene (as high as 98%) were observed when the catalysts were added in the order [RE]/[alkylaluminum]/[borate].