Reaction Of CO₂and Epoxide Catalysed By Amine Bridged Poly（Phenolate） Lanthanide Complexes

A series of lanthanide complexes supported by amine-bridged poly(phenolato) ligands were synthesized and their catalytic property for the reaction of CO2and epoxides was explored. The ligands we used include ethylenediamine-bridged tetra(phenolate)(N, N, N’, N’-tetra-(3,5-di-tert-butyl-2-hydroxybenzyl)-ethylene-1,2-diamine), abbreviated as L1H4and N,N,N’-tris-(3,5-di-tert-butyl-2-hydroxybenzyl)-benzene-1,2-diamine, abbreviated as L2H3.1. Synthesis of lanthanide complexes supported by amine-bridged poly(phenolato) ligandsThe synthesis of the following complexes was repeated the previous work of our laboratory, and the complexes have been well characterizaed previously.(1) Reactions of VH4with LnCp3(THF)[Ln=Yb, Y, Sm, Nd] in a1:1molar ratio in THF gave the corresponding lanthanide complexes HL]Ln(THF)[Ln=Yb (1), Y (2), Sm (3), Nd (4)] in good isolated yields. The structure in solution of complex2was further investigated by variable-temperature1H NMR spectrum.(2) The reaction mixture of complexes1-3was added to a THF solution of ZnEt2in a1:1molar ratio under-15℃, respectively. The obtained mixture was stirred for12h, and then acetic acid was added dropwise to the above mixture. After workup, the heterobimetallic lanthanide-zinc complexes L1Ln(THF)Zn(OOCCH3)[Ln=Yb (5), Y (6), Sm (7)] were obtained in good yields.(3) Reactions of NdCp3(THF) with L2H3in a1:1molar ratio in THF gave the corresponding lanthanide complex L2Nd(DME)(8). Reaction of benzyl alcohol with ZnEt2in THF in a2:1molar ratio under-15℃gave zinc akoxide [Zn(OCH2Ph)2]n (9). The solution of complex9generated in situ was added to the THF solution of complex8, and the obtained reaction mixture was stirred for10h at room temperature. After workup, the lanthanide-zinc heterometallic complex10was isolated. 2. Coupling reaction of CO2with epoxides to produce cyclic carbonates catalyzed by lanthanide complexes1-4The coupling reaction of CO2with epoxides to produce cyclic carbonates catalyzed by complexes1-4in the presence of quaternary ammonium salt was explored. Through the optimization of reaction tempareture, catalysts, molar ratio of catalyst to substrate, quaternary ammonium salts, as well as the loading of quaternary ammonium salts, the optimum reaction conditions were0.2mol%of4,0.8mol%of NBu4O, and85℃. A series of monosubstituted terminal epoxides bearing an alkyl, aryl, halogen, alkene, hydroxl, ether, morpholine or ester substituents were commendably converted to the corresponding cyclic carbonates in good to excellent yields (60to97%) under ideally atmospheric pressure of CO2. Furthermore, the bulky and/or internal epoxides, which were generally considered as substrates of low reactivity were also tested, and all of them were transformed into the corresponding cyclic carbonates in good toexcellent yields (66to97%) at elevated pressure (10bar CO2) after prolonged reaction time.3. Copolymerization of CO2with cyclohexene oxide catalyzed by the heterometallic lanthanide-zinc complexes5-7, and10(1) The catalytic property of complex5-7for the copolymerization of CO2with cyclohexene oxide was explored. It was found that all of the reaction temperature, solvent, catalyst as well the loading of the solvent had obvious influence on the catalytic activity and the content of polycarbonate of the obtained polymers. To our suprised, it was found that the presence of a small amount of water was beneficial for the improvement of the catalytic activity and the selectivity in this catalytic system.(2) Complex10was found to be an excellent catalyst for the copolymerization of CO2with cyclohexene oxide to produce polycarbonate through under mild polymerization conditions. A good TOF of400h-1could be obtained at25℃under0.7Mpa of CO2. It should also be noted that the up to99%selectivity for polycarbonate were obtained in most cases. In addition, polymerization tempareture had significant influence on the molecular weight of the resulting copolymers. The determined molecular weight of the polymers increased as the decrease of the polymerization tempareture. Extremely high molecular weight copolymers (up to295.8kg/mol) could be obtained at25℃. The produced copolymer was further analyzed by MALDI-TOF mass spectrometry, indicating the active site in this system was the Zn-O(benzyloxy) bond. Furthermore, it was found that the first step of the copolymerization could be either the insertion of carbon dioxide or the ring-opening of cyclohexene oxide.