Synthesis and characterization of trinuclear and mononuclear rare-earth metal aryloxides supported by Salpn ligand and their application for the polymerization of rac-lactide

Four rare-earth metal complexes supported by Salpn ligand were synthesized, and their catalytic behaviors for the polymerization of rac-lactide (rac-LA) were explored. The amine elimination reactions of Salpn ligand LH2 (LH2 = (CH2)(3)[N = CH(C6H4-2-OH)](2)) with RE[N(SiMe3)(2)](3) (RE = Y, Yb, Sm, Nd) in a 1:1 M ratio, and then with 1 equiv. of phenol ArOH (ArO = 2,6-Bu-2(1)-4-MeC6H2O) in THF gave the trinuclear rare-earth metal aryloxo complexes [LRE(OAr)(HMPA)](3) (RE = Y (1), Yb (2), Sm (3)), and the mononuclear neodymium complex LNd(OAr)(HMPA)(2) (4), respectively, in good yields in the presence of HMPA (HMPA = hexamethylphosphoric triamide). Complexes 1 and 2 can also be prepared by the phenol elimination reactions of (ArO)(3)RE(THF) with LH2 in a 1:1 M ratio or salt metathesis reaction of LLi2(THF)(x) with anhydrous RECl3, then with NaOAr in THF. Complexes 1-4 were well characterized by elemental analyses and IR, as well as NMR spectroscopy in the case of complex 1. The definitive molecular structures of complexes 1-4 were determined by single-crystal X-ray analysis. The catalytic behaviors of these complexes for rac-LA polymerization were investigated. It was found that complexes 1-4 can initiate efficiently this polymerization and gave hetero-rich PLA. Kinetics study demonstrated that rac-LA polymerization initiated by complexes 1-3 has a first order relationship between polymerization rate and monomer concentration. A comparative kinetics study in THF and CHCl3 revealed that polymerization media have significant influence on the activity order of complexes 1-3 for rac-LA polymerization. The observed activity-increasing order of these complexes in THF is in agreement with the increasing order of their ionic radii, whereas it is in reverse order in CHCl3. A mechanism study demonstrated that the polymerization proceeded via a coordination insertion mechanism, and the aryloxo group in these complexes acted as the initiating group. (C) 2017 Elsevier Ltd. All rights reserved.