Synthesis And Characterization Of Aminophenolate Group ⅢB And ⅣB Metal Complexes And Their Application In The ROP Of Cyclic Esters

Polyester materials,such as polylactide（PLA）,poly（ε-caprolactone）（PCL）and their copolymers,not only have mechanical and physical properties similar to traditional petroleum-based polymers,but also have good biodegradability,biocompatibility and permeability.Moreover,they could be synthesized from a wide range of renewable sources with pollution-free.Therefore,PLA,PCL and other polyester materials are widely considered as “green environmentally-friendly materials” anticipated to replace the traditional petroleum-based polymers.They are expected to have a good prospect of development and application,and important commercial value in the agricultural production,biomedicine and food packaging fields.At present,the most efficient method employed to synthesize these polyester materials is the ring-opening polymerization（ROP）of the related cyclic esters initiated by using metal complexes as catalysts（or initiators）.Numerous studies show that the metal complexes used for the ROP of cyclic esters have not only high reactivity but also good controllability for the molecular weight and molecular weight distribution of the polyester materials.Furthermore,the selection of appropriate catalytic system based on metal complexes can also achieve stereoselective ROP of chiral cyclic ester monomers.Hence,on the basis of controlling the molecular weight distribution of the polyester materials,many efforts have been devoted to the design and synthesis of metal complexes with high activity and high stereoselectivity.In this dissertation,we described the design and synthesis of a family of chiral and achiral group IIIB and IVB metal complexes.The structures of these complexes have been fully characterized and analyzed in detail,and the reaction performance of these complexes toward the ROP of cyclic esters have been systematically investigated.The primary content in this dissertation was summarized as follows:Chapter 1.A brief review of the research progress of the mechanism for the ROP of cyclic esters,metal complexes applied to the ROP of cyclic esters,characterisation of the stereoselective ROP of lactides,stereocontrol mechanism of the ROP of lactides and the stereocontrol ROP of rac-lactide.Chapter 2.The group 4 metal complexes [L^1-23M3（OiPr）3]（1-6）（M = Ti,Zr,Hf）and [L13Hf3（OiPr）3（HOiPr）]（7）based on amine-bridged bis（phenolate）ligands L^1-2-H₃ were synthesized and fully characterized.X-ray diffraction studies revealed that complexes 1-2 and 4-6 were trinuclear species in which the six-coordinate metal centers were bonded to three oxygen atoms and one nitrogen atom of one ligand,one oxygen atom of the isopropoxy group,and one oxygen atom of the bridging carboxyl group of another.The coordination geometry around the metal ion center adopted a distorted octahedral geometry.Complex 7 was also trinuclear species in which the six-coordinate metal centers,but there was one 2-propanol ligand coordinated to one of the three metal centers,which was different with the observation in complexes 1-2 and 4-6.Under solvent-free conditions,except complex 4,other complexes were excellent single-site catalysts/initiators for the ROP of ε-caprolactone（ε-CL）,L-lactide（L-LA）and rac-lactide（rac-LA）to give the corresponding polyesters with good controlled manner.Furthermore,complex 7 turned out to be an efficient initiator for the copolymerization of ε-CL with lactides to produce tapered copolymers.Chapter 3.The neutral rare-earth complexes [M2（L）2（THF）4]（L = L1,M = La（8）,Nd（9）,Sm（10）,Gd（11）,Y（12）;L = L2,M = La（13）,Nd（14）,Sm（15）,Gd（16）,Y（17））were prepared by the reaction of amine-bridged bis（phenolate）ligands L^1-2-H₃ with one equiv.Ln[N（SiMe3）2]3（Ln = La,Nd,Sm,Gd,Y）in THF at room temperature.All of these complexes were characterized by single-crystal X-ray diffraction,IR spectroscopy,elemental analysis,and NMR spectroscopy in the case of yttrium and lanthanum complexes.X-ray diffraction determination indicated that complex 8 was dinuclear species in which the lanthanum centers were eight-coordinated with three oxygen atoms and one nitrogen atom of one ligand,two oxygen atoms of two THF molecules,and two oxygen atoms of carboxyl group of another ligand.The coordination geometry around lanthanum is a distorted square antiprism.Complexes 9-17 illustrated analogous dinuclear species and the metal centers were seven-coordinated with three oxygen atoms and one nitrogen atom of one amine-bridged bis（phenolate）ligand,two oxygen atoms of two THF molecules,as well as one oxygen atom of another ligand.The coordination geometry around rare-earth metal ions was described as a distorted octahedral.Further treatment of complex 8 with access benzyl alcohol provided dinuclear complex [La2（L1）2（BnOH）6]（18）,in which each lanthanum ion was eight-coordinated to three oxygen atoms and one nitrogen atoms of one ligand,three oxygen atoms of three Bn OH molecules,as well as one oxygen atom of bridging carboxyl group of the other ligand.In the presence of BnOH,complexes 8-17 efficiently catalyzed the ROP of L-lactide in a controlled manner and gave polymers with relatively narrow molecular weight distributions.In the absence of BnOH,complex 18 could efficiently catalyze the ROP of L-lactide and the resultant polymers had controlled molecular weights with narrow molecular weight distributions.In addition,according to the results of the kinetics studies of 8/Bn OH initiating system,crystal structures of 8 and 18 the possible reaction mechanism of the ROP of LA for this catalytic system had been proposed.Chapter 4.Six new monomeric rare-earth alkoxides（19-24）were successfully synthesized by one-pot reaction of chiral [NNOO]-type amine-bridged bis（phenolate）ligand(（S）-L^3-6-H₂),Ln[N（SiMe3）2]3（Ln = Y,Sm,Nd）and 2-propanol in a 1:1:1 molar ratio in THF at room temperature.These complexes had been well characterized with X-ray crystallography,elemental analyses,IR spectra and NMR spectrum in the case of yttrium complexes.Furthermore,complexes 19-24 were employed as efficient initiators/catalysts for the stereoselective ROP of rac-lactide in THF at 20 oC to provide heterotactically enriched polymers（Pr = 0.80-0.95）.All of these polymerizations proceeded in a controlled manner,giving polymers with good controlled molecular weights and very narrow molecular weight distributions（PDI = 1.03-1.12）.The natures of the ligand substituents had profound impact on the reactivity of rare-earth alkoxides but no significant impact on the stereoselectivity.Moreover,the reactivity and hereotacticity were strongly dependent on the ionic radii of rare-earth metals and solvent used in polymerization.Chapter 5.A family of zirconium and hafnium complexes supported by chiral salalen-type [ONNO] tetradentate ligands were successfully prepared.All of these complexes were well characterized by NMR spectroscopy,elemental analyses and single-crystal X-ray analysis in the case of complexes 25 and 27-29.The NMR and X-ray structural determination revealed that these complexes were analogous mononuclear species and had a similar structure in which the metal centers were six-coordinated to two oxygen atoms and two nitrogen atoms of one ligand and two oxygen atoms of two isopropoxy groups.The coordination geometry around the metal center adopted a distorted octahedron.All of these complexes were employed as efficient initiators for the ROP of L-and rac-LA both under melt conditions and in toluene solution at 80 oC to afford polymers with controlled molecular weight and narrow polydispersity.Furthermore,the homonuclear decoupled 1H NMR spectra indicated that complexes 25-29 showed good isoselectivity for the ROP of rac-LA both in solution（Pm = 0.74-0.80）and under melt（Pm = 0.63-0.72）conditions.