Rare-Earth Metal Complexes Derived From Linked Bis(?-ketoiminato)Ligands:Synthesis,Structure And Their Catalytic Properties For Rac-lactide Polymerization

In this paper,using bridged bis(?-ketoiminato)Ligands O[C6H4-2-N=C(CH3)CH2C=O(C6H5)]2(L1H2),O[C6H4-2-N = C(CH3)CH2C=O(CF3)]2(L2H2)and[CH2OC6H4-2-N=C(CH3)CH2C=O(C6H5)]2(L3H2)as ancillary ligands,11 rare-earth metal complexes were synthesized and all of them were characterized by X-ray single crystal diffraction,elemental analysis and IR.as well as 1H and 13C NMR for the yttrium complexes.Furthermore,the catalytic behavior of some of these rare-earth metal complexes for the ring-opening polymerization of rac-lactide was also studied.The main results are summarized as follows:1.Proton exchange reactions of bridged bis(?-ketoiminato)Ligand L1H2 with Ln(OAr)3(THF)(ArO = 2,6-But-4-MeC6H2O)afforded ?-ketoiminate rare-earth metal complexes L1LnOAr(THF)[Ln = Y(1),Sm(2),Nd(3)].2.Proton exchange reactions of bridged bis(?-ketoiminato)Ligand L2H2 with Y(OAr)3(THF)afforded ?-ketoiminate yttrium complex L2YOAr(THF)(4).3.Proton exchange reactions of bridged bis(?-ketoiminato)Ligand L3H2 with Ln(OAr)3(THF)afforded ?-ketoiminate rare-earth metal complexes L3LnOAr[Ln = Yb(5),Y(6)].However,the same reaction with Sm(OAr)3(THF)gave an unexpected homoleptic complex L23Sm2(7)probably due to the larger ionic radius of the latter.4.Treatment of Y[N(SiMe3)]3 with 1 equiv of L1H2 in THF and n-hexane,and subsequent protolysis reaction with 1 equiv of alcohols(EtOH,CH3OCH2CH2OH,p-cresol and guaiacol)afforded bridged bis(?-ketoiminato)yttrium complexes[L1YOR]2[R = OEt(8),OCH2CH2OCH3(9),OC6H4-4-Me(10),OC6H4-2-OMe(11)].They could also be synthesized by reactions of L1Na2 and YCl3;and then further reacted with LiOR.5.These bis(?-ketoiminato)rare-earth metal complexes could initiate the polymerization of rac-lactide at room temperature.With the increases of the ionic radii of the central metals,the observed activity increases,and the increasing order in stereoselectivity follows the decreasing order of the ionic radii.Complexes 8-11 have an unsolvated dimeric structure,and show lower catalytic activity than complexes 1-3 and 5.6.The highly heterotactic PLA(Pr up to 0.92)was obtained using complex 6 as the initiator at a polymerization temperature of 0 ?.