Synthesis and characterization of new divalent lanthanide complexes supported by amine bis(phenolate) ligands and their applications in the ring opening polymerization of cyclic esters

Abstract

Treatment of an array of bis(phenol)s H₂L^x {L^x = [(⁻OC₆H₂(2,4-R)(6-CH₂))₂ NCH₂CH₂X], where X = CH₂NMe₂, NMe₂, NEt₂, OMe and R = Bu^t, Pe^t where Pe^t = C(CH₃)₂Et} with [Ln(N(SiMe₃)₂)₂(THF)₂] (Ln = Yb, Sm) in a 1:1 molar ratio in hexanes afforded a multitude of new divalent lanthanide bis(phenolate) complexes: [L^2cYb]₂ (X = NEt₂, R = Bu^t) 1, [L^2bYb]₂ (X = CH₂NMe₂, R = Bu^t) 2, [L^3aYb]₂ (X = NMe₂, R = Pe^t) 3, [L^3bYb]₂ (X = OMe, R = Pe^t) 4, [L^2cSm] (X = NEt₂, R = Bu^t) 5, [L^3cYb] (X = NEt₂, R = Pe^t) 6, [L^3cSm] (X = NEt₂, R = Pe^t) 7. X-Ray crystallographic analyses of compounds 1 and 3 reveal dimeric, centrosymmetric structures with 5-coordinate ytterbium centers arising from bridging and terminal phenolate groups. A selection of divalent compounds (1, 3, 4, 5, 6 and 7) were tested as catalyst precursors in the polymerization of ε-caprolactone and/or L-lactide which resulted in high molecular weight polymers with PDIs of 1.11–2.81 and 1.13–1.56 for ε-caprolactone and L-lactide respectively. All polymerization studies were performed in either toluene or THF and at room temperature (ε-caprolactone) or 70 °C (L-lactide) at a variety of catalyst:monomer ratios (1:100–1:300). Kinetics analyses of the polymerization of L-lactide by compounds 1 and 5 indicated pseudo-first order response with respect to L-lactide.