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

Abstract

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 1:1M ratio, and then with 1 equiv. of phenol ArOH (ArO=2,6-But2-4-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)3RE(THF) with LH2 in a 1:1M 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.