Synthesis And Characterization Of Lanthanide Complexes Bearing Bridged Schiff-base Ligands And Their Catalytic Behavior For The Ring-opening Polymerization Of Lactones

Using bridged Schiff-base ligands NH[CH2CH2N=CH(2-OH-3,5-But2C6H2)]2andNCH3[CH2CH2N=CH(2-OH-3,5-But2C6H2)]2as ancillary ligands,13lanthanidecomplexes were synthesized and11of them were characterized by X-ray single crystaldiffraction. Furthermore, the catalytic behavior of some of these lanthanide complexesfor the ring-opening polymerization of L-lactide and rac-lactide was also studied. Themain results are summarized as follows:1. Reactions of bridged Schiff-base ligandNH[CH2CH2N=CH(2-OH-3,5-But2C6H2)]2(L1H3) with Ln[N(SiMe3)2]3(Ln=La, Y)afforded homodinuclear lanthanide Schiff-base complexes [L1Ln(THF)]2[Ln=La(1),Y(2)]. Complexes1and2were characterized by element analysis, FT-IR, NMRspectroscopy and single crystal X-ray diffraction in case of complex2.2. Reactions of Ln[N(SiMe3)2]3(Ln=La, Nd, Sm, Y) with bridged Schiff-baseligands NH[CH2CH2N=CH(2-OH-3,5-But2C6H2)]2(L1H3), and then with2,6-Di-tert-butyl-4-methylphenol or different alcohols afforded the lanthanidearyloxides (L1H)La(OC6H2-2,6-But2-4-CH3)(THF)(3) or the lanthanide alkoxides suchas (L1H)La(OBut)(THF)(4),[(L1H)La(OPri)]2(5),[(L1H)La(OPh)]2(6),[(L1H)La(OEt)]2(7) and [(L1H)Ln(OCH2CH2OCH3)]2[Ln=La (8), Nd (9), Sm (10), Y(11)]. All complexes were characterized by element analysis, FT-IR and NMRspectroscopy in case of complexes3-8and11. Moreover, Complex3-9werecharacterized by single crystal X-ray diffraction. Structural determination revealed thatcomplexes3and4have monomeric structure, while complexes5-11have dimericstructure. 3. Reactions of Y[N(SiMe3)2]3with bridged Schiff-base ligandNCH3[CH2CH2N=CH(2-OH-3,5-But2C6H2)]2(L2H2), and then with paretert-butylphenol or alcohol afforded the lanthanide aryloxides L2Y(OC6H4-4-But)(12) or thelanthanide alkoxides [L2Y(OEt)]2(13). Both complexes12and13were characterizedby element analysis, FT-IR, NMR spectroscopy and single crystal X-ray diffraction.4. The salen-type lanthanide complexes3-11could initiate the polymerization ofL-lactide under mild conditions, and the solvent greatly effect on the catalytic activity.These complexes show higher acticity in toluene than that in THF. Whether in tolueneor THF, the observed activity increasing order is in agreement with the increasing orderof the ionic radii. The polymerization proceeds via a common “coordination-insertion”mechanism by the end group analysis of the oligomer.5. The schiff base complexes3-13could initiate the polymerization of rac-lactideunder mild conditions. The observed activity increasing order is in agreement with theincreasing order of the ionic radii, whereas the order for stereoselectivity is in reverseorder. The lanthanum complexes3-8show high catalytic activity, giving atactic polymer.While the yttrium complex11-13show relatively poor catalytic activity, but displayedthe better stereoselectivity (Pr up to0.78).