Synthesis and structure characterization of homoleptic lanthanide complexes stabilized by Schiff-base ligands and their application in the polymerization of ε-caprolactone

Abstract

A series of homoleptic lanthanide complexes [(L1–6)3Ln] (Ln = La, L11, L23, L35, L47, L59, L611; Ln = Gd, L12, L24, L36, L48, L510, L612) supported by Schiff-base ligands were synthesized by the amine elimination reactions of monophenol ligands [2,4-tBu2-6-CH=N-C6H4-p-CH3-PhOH (HL1), 2,4-tBu2-6-CH=N-C6H4-o-CH3-PhOH (HL2), 2,4-tBu2-6-CH=N-C6H4-o-C2H5-PhOH (HL3), 2,4-tBu2-6-CH=N-C6H4-p-OCH3-PhOH (HL4), 2,4-tBu2-6-CH=N-C6H4-p-Br-PhOH (HL5), 2,4-tBu2-6-CH=N-Cy-PhOH (HL6)] with Ln[N(SiMe3)2]3 (Ln = La, Gd) in 3: 1 M ration in THF. Complexes 1, 3, 5, 7, 9 and 11 were characterized by NMR spectra and all twelve complexes were characterized using elemental analysis and infrared spectra. Complexes 4, 5 and 6 have been determined by single-crystal X-ray diffraction, which reveals that they have possessed isostructural unsolvated mononuclear structures and the metal center has exhibited a distorted octahedral geometry, in which the O(1), O(2), O(3) atoms have occupied three positions and N(1), N(2), N(3) atoms have occupied the other three positions. The catalytic properties of complexes 1–12 for the ring-opening polymerization of ε-caprolactone were studied and found that all complexes were efficient initiators for this ring-opening polymerization reaction that afforded polycaprolactone with high molecular weights and moderate molecular-weight distributions (1.23–2.46).