Synthesis Of Rare Earth Metal Complexes Containing Imidazolium Cations And Their Application In The Carbon Dioxide Transformations

A series of rare earth metal complexes were synthesized and characterized bearing1,3-bis-?2,6-diisopropylphenyl?imidazolium chloride,1,3-bis-?2,6-di-tert-butyl?imidazolium chloride and amine-bridged poly?phenolato?ligands,and their catalytic activities for reactions of carbondioxidewithepoxideswereexplored.Ligandsusedinclude1,3-bis-?2,6-diisopropylphenyl?imidazoliumchloride,abbreviatedasL^1.HCl,1,3-bis-?2,6-di-tert-butyl?imidazoliumchloride,abbreviatedasL^2.HCland N,N,N'-tris-?3,5-di-tert-butyl-2-hydroxybenzyl?-1,2-phenylenediamine,abbreviatedas L³H₃.1.Synthesis of rare earth complexes containing imidazolium cations?1?Compound 1,3-bis-?2,6-diisopropylphenyl?imidazolium chloride?L¹×HCl?was added to RECl₃ in a 1:1 molar ratio in THF for 24 h,and rare earth complexes[HL¹][RECl₄?THF?_n]^-[RE=La?1?,n=1;Sm?2?,Yb?3?,Y?4?,n=2]were obtained in good yields of 81-86%,which were characterized by elemental analysis,¹H and ¹³C NMR spectroscopy?in the case of complexes 1 and 3?.Solid state structures of complexes 2 and4 were confirmed by X-ray diffraction analysis on single crystals.?2?Compound 1,3-bis-?2,6-di-tert-butyl?imidazolium chloride?L²×HCl?was added to YCl₃ in a 1:1 molar ratio in acetonitrile at 80 ^oC for 48 h,and rare earth complexes[HL²][YCl₄?CH₃CN?]^-?5?was obtained in good isolated yields.The obtained rare earth complex was characterized by elemental analysis and ¹H NMR,¹³C NMR spectroscopy.2.Reaction of carbon dioxide with epoxides to produce cyclic carbonates catalyzed by rare earth complexes?1-5?containing imidazolium cationsIt was found that complexes 1-5 could catalyze the cycloaddition reaction of carbon dioxide with epoxides to produce cyclic carbonates without co-catalyst under solvent free conditions.Through the screening of the reaction conditions such as catalysts,reaction time,reaction temperature and molar ratio of catalyst to substrate,optimal conditions were confirmed as follows:0.2 mol%complex 2,90?,12 h,1 bar CO₂,solvent free.A series of monosubstituted epoxides bearing chloromethyl,methyl,phenyl,alkenyl,morpholine,ether,halogen,hydroxyl or alkene substituent were converted to the corresponding cyclic carbonates in good to excellent yields?60 to 97%?under atmospheric pressure of CO₂.Furthermore,thebulky/internalepoxides?suchascyclohexeneoxide,3,4-epoxytetrahydrofuran,cyclopentene oxide?,which were generally considered as substrates of low reactivity were also tested,and all of them were transformed into the corresponding cyclic carbonates in the yields of 40-95%at elevated pressure to10 bar CO₂and after prolonged reaction time to 40 h.Compared with the catalytic system reported in the literature,such catalysts are relatively rare catalysts which are capable of catalyzing such reactions at atmospheric pressure without co-catalyst.In addition,complex 2 was reused for six successive cycles without any significant loss in its catalytic activity.3.Synthesis of rare earth complexes stabilized by amine-bridged poly?phenolato?ligands and reaction of epoxides with carbon dioxide to produce cyclic carbonates catalyzed by 6-8.?1?Reactions of L³H₃ with RECp₃?THF?in a 1:1 molar ratio in THF gave the corresponding rare earth complexes L³Yb?THF??6?,L³Sm?DME??7?,L³La?DME??8?in good isolated yields.The catalytic property of complexes 6–8 for the coupling reaction of carbon dioxide with epoxides to produce cyclic carbonates in the presence of quaternary ammonium salt was explored,under solvent-free conditions.Through the screening of a series of reaction conditions,optimal conditions were confirmed as follows:0.2 mol%of 6,0.8 mol%of NBu₄Br,and60?,1 bar CO₂.A series of monosubstituted terminal epoxides were commendably converted to the corresponding cyclic carbonates in good to excellent yields?60 to 98%?under atmospheric pressure of CO₂.Furthermore,the bulky 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 excellent yields?92 to 98%?after elevated temperature?85??,pressure?10 bar CO₂?and prolonged reaction time to 40h.Compared with the catalytic system reported in the literature,such catalysts are relatively rare catalysts which are capable of catalyzing such reactions at relatively low temperatures?60??.4.Synthesis of heterometallic rare earth-zinc complexes supported by nitrogen-bridged tri?phenolato?ligandsWe try to synthesize heterometallic rare earth-zinc complexes through the reaction of rare earth metal complexes supported by nitrogen-bridged tri?phenolato?ligands with benzyloxy zinc or phenoxy zinc.