Synthesis And Characterization Of Lanthanide Complexes Stabilized By Bridged Schiff-base Ligands And Their Performance In The Ring-opening Polymerization Of Lactide

Using bridged schiff-base ligands NCH3[CH2CH2N=CH(2-OH-3,5-But2C6H2)]2 (L1H2) and NCH2C6H5[CH2CH2N=CH(2-OH-3,5-But2C6H2)]2 (L2H2) as ancillary ligands, a series of lanthanide complexes were synthesized and characterized. What’s more, the catalytic behavior of some of these complexes was also researched. The main contents were listed below.1. Synthesis of lanthanide complexes:the reaction of the ligand L1H2 with Ln[N(SiMe3)2]2 and then with tBuOH produced the salen lanthanide tert-butoxy1 complexes L1Ln(O’Bu)(THF) [Ln=Sm (1), Ln=Nd (2), Ln=La (3)]. Similar reaction gave the alkoxyl complexes [L1Sm(μ-OEt)2 (4) and [L1Sm(μ-OCH2CH2OCH3)]2 (5) when EtOH and CH3OCH2CH2OH were used instead of tBuOH, respectively. However, when Yb[N(SMe3)]3 was used as the precursor, the similar reaction produced homoleptic ytterbium complex (L1)2Yb2(μ-L1) (6). As the procedure described above, the reaction of the ligand L2H2 with Y[N(SMe3)2]2 and then with p-tert-buty1 phenol produced the corresponding aryloxide complex L2Y(p-tBuC6H4O)(THF) (8); the proton exchange reacition of the schiff-base ligand L2H2 with Y(OAr)3 afforded the salen yttrium complex L2Y(2,6-tBu-4-MeC6H2O) (9). Among these complexes, the complexes 1-3,8 and 9 were monomeric structures, and complexes 4,5 and 6 were unsolvated dimeric structures, which were different from each other. All the complexes were characterized by IR analysis, elemental analysis,1H NMR and 13C NMR analysis for the yttrium and lanthanum complexes.2. The performance of complexes 1-5 and 7-9 in the polymerization of L-LA was investigated and the influence of the bridging group, the polymerization solvents and central metals was discussed. (1) The polymerizations display a higher activity in toluene than in tetrahydro furan; (2) The increasing order of observed activity is in agreement with the increasing sequence of the ionic radii; (3) The influence sequence of the initiating groups on the activity is tBuO<CH3OCH2CH2O<EtO; (4) The polymerization follows the rate equation of the form -d[M]/dt=kp[M][I]; (5) The end group analysis and MALDI-TOF mass spectrum of the oligomer indicate the polymerization proceed via a coordination-insertion mechanism.3. The lanthanide complexes 1-5 and 7-9 could efficiently initiate the polymerization of rac-LA under mild conditions, affording heterotactic-rich polylactides. (1) The polymerizations display a better heteroselectivity in tetrahydrofuran than in toluene; (2) The decreasing order in heteroselectivity is in agreement with the increasing sequence of the ionic radii; (3) The initiating groups have no effect on the stereoselectivity, but can affect the catalytic activity.