Synthesis and Characterization of Yttrium and Ytterbium Complexes Supported by Salen Ligands and Their Catalytic Properties for rac-Lactide Polymerization

A series of yttrium and ytterbium complexes supported by Salen ligands with different steric and electronic properties were synthesized, and their catalytic performances for the polymerization of rac-lactide (rac-LA) were explored. The phenol elimination reactions of (ArO)3Ln­(THF) (ArO = 2,6-But2-4-MeC6H2O) with a number of Salen ligands (CH3)2C­[CH2NCH­(C6H2-2-OH-3,5-R2)]2 [L1H2, R = H; L2H2, R = Cl; L3H2, R = But; L4H2, R = CMe2Ph] and CH2[CH2NCH­(C6H2-2-OH-3,5-Cl2)]2 (L5H2), in a 1:1 molar ratio gave yttrium and ytterbium Salen aryloxides L1Ln­(OAr)­(THF)2 (Ln = Y (1), Yb (2)), L2Ln­(OAr)­(THF)n (Ln = Y (3), n = 2; Ln = Yb (4), n = 1), L3Y­(OAr) (5), L4Y­(OAr)­(THF) (6), and L5Yb­(OAr)­(THF) (7), respectively. The amine elimination reactions of L3H2 with Y­[N­(SiMe3)2]3 in a 1:1 molar ratio and then with 1 equiv of phenol ArOH, and alcohols PhCH2OH and PriOH, produced complex 5 and the yttrium Salen alkoxides [L3Y­(μ-OCH2Ph)]2 (8) and [L3Y­(μ-OPri)]2 (9), respectively. X-ray structural determination showed that complexes 1–4 and 7 have a THF-solvated monomeric structure, and complex 5 has an unsolvated monomeric structure, whereas complex 8 has an unsolvated dimeric structure. All of these complexes can initiate the ring-opening polymerization of rac-LA at 30 °C in THF. It was found for the first time that the overall coordination environment around the metal center has an obvious influence on the stereoselectivity of these lanthanide Salen complexes. Five-coordinated complex 5 with bulky tert-butyl substituent groups on the phenyl rings displayed apparently lower stereoselectivity than seven-coordinated complex 1, although there is no substituent on the ortho-positions of the Salen ligand L1 in the latter. Complex 7 bearing Cl substituents on the Salen ligand showed the highest stereoselectivity among these lanthanide complexes for rac-LA polymerization, and heterotactic polylactides (Pr up to 0.88) can be obtained at 30 °C in THF.