Rare earth metal bis(amide) complexes bearing amidinate ancillary ligands: Synthesis, characterization, and performance as catalyst precursors for cis-1,4 selective polymerization of isoprene

Abstract

A family of rare earth metal bis(amide) complexes bearing monoanionic amidinate [RC(N-2,6-Me₂C₆H₃)₂]⁻ (R = cyclohexyl (Cy), phenyl (Ph)) as ancillary ligands were synthesized and characterized. One-pot salt metathesis reaction of anhydrous LnCl₃ with one equivalent of amidinate lithium [RC(N-2,6-Me₂C₆H₃)₂]Li, following the introduction of two equivalents of NaN(SiMe₃)₂ in THF at room temperature afforded the neutral and unsolvated mono(amidinate) rare earth metal bis(amide) complexes [RC(N-2,6-Me₂C₆H₃)₂]Y[N(SiMe₃)₂]₂ (R = Cy (1); R = Ph (2)), and the “ate” mono(amidinate) rare earth metal bis(amide) complex [CyC(N-2,6-Me₂C₆H₃)₂]Lu[N(SiMe₃)₂]₂(μ-Cl)Li(THF)₃ (3) in 61–72% isolated yields. These complexes were characterized by elemental analysis, NMR spectroscopy, FT-IR spectroscopy, and X-ray single crystal diffraction. Single crystal structural determination revealed that the central metal in complexes 1 and 2 adopts a distorted tetrahedral geometry, and in complex 3 forms a distorted trigonal bipyramidal geometry. In the presence of AlMe₃, and in combination with one equimolar amount of [Ph₃C][B(C₆F₅)₄], complexes 1 and 2 showed high activity towards isoprene polymerization to give high molecular weight polyisoprene (M_n > 10⁴) with good cis-1,4 selectivity (>90%).