Synthesis Of Rare Earth And Zinc Complexes Stabilized By N-methylethylenediamine-bridged Tris?phenolate? And Their Catalytic Activity For Cycloaddition Of Epoxide And CO₂

With the increasing frequency of industrial activities,the content of CO2 in the atmosphere is gradually increasing.CO2 as an ideal C1 resource,and has many advantages such as rich,renewable and non-toxic,but its stable thermodynamic and dynamic properties greatly limit its application.Based on the synthesis of a series of N-methylethylenediamine-bridged tris(phenol)rare earth and zinc complexes,which showed activity in the coupling of terminal and internal epoxides with CO2.A series of N-methylethylenediamine-bridged tris(phenol)ligand precursors were synthesized as follows:[N-methyl-N,N',N'-tris(2-hydroxy-3,5-di-tert-butyl)ethane-l,2-diamine](L1H3);[N-methyl-N,N',N'-tris(2-hy droxy-3-methyl,5-tert-butylbenzyl)ethane-1,2-diamine](L2H3);[N-methyl-N,N',N'-tris(2-hydr-oxy-3,5-dimethylbenzyl)ethane-1,2-diamine](L3H3);[N-methyl-N,N',N'-tris(2-hydroxy-3,5-dichlorobenzyl)ethane-1,2-diamine](L4H3).1.Synthesis of rare earth(RE)metal complexes stabilized by N-methylethylene-diamine-bridged tris(phenol)ligands and their catalytic property for the cycloaddition of CO2 and epoxides(1)Reaction of RECp3(THF)with L1H3 in toluene in a 1:1 molar ratio,after workup,gave the rare earth metal complexes L1Y(THF)1(1)and L12RE2[RE=Sm(2),Nd(3)]in good isolated yields.Similar reaction of LaCp3(THF)with L2H3,L3H3 and L4H3 in THF in a 1:1 molar ratio gave the lanthanum metal complexes L1La(THF)(4),L2La(THF)2(5),L3La(THF)2(6)and L4La(THF)2(7),respectively.All the complexes were characterized by elemental analysis,and NMR for Y and La complexes.The single crystal structure of complexes 1-5 were determined by X-ray diffraction.(2)The catalytic activities of complexes 1-7 for the cycloaddition reaction of epoxides and CO2 were studied.The effects of reaction time,catalyst and co-catalyst,reaction temperature on the reaction were investigated using the reaction of 1,2-epoxyhexane and CO2 as a template reaction,and the optical reaction conditions were:room temperature,1 bar pressure of CO2,0.3 mol%of complex 4 as catalyst,0.6 mol%of TBAI as co-catalyst,and 18 h.In addition,under the optimal conditions,various mono-substituted epoxides can be converted to cyclic carbonate in 49-99%yields.For cyclohexene oxide,a di-substituted epoxide with large steric hindrance,the optimal reaction conditions should be 2 mol%of complex 4 as catalyst,4 mol%of TBAI as co-catalyst,1 bar CO2 pressure,60?,24 h.The cyclohexene oxide transferred to cyclic carbonate in 96%yield,with the ratio of cis and trans of 97:3.Under the optimal reaction conditions,a series of internal epoxides were transformed into cyclic carbonates in 70-99%yields.Complex 4 is a rare example that can catalyze the transformation of internal epoxide to cyclic carbonate under 1 bar CO2 pressure.(3)Kinetics of the cycloaddition of cyclohexene oxide and CO2 was conducted.It was found that the orders of cyclohexene oxide,CO2,complex 4 and TBAI were 1,0,1.26 and 0.4,respectively,when[TBAI]is more than 1.5[4].Whereas the order of TBAI is 1.33,when[TBAI]is less than 1.5[4].According to the Eyring equation,the thermodynamic parameters were obtained by plotting In(k/T)versus 1/T.The enthalpy,entropy,and Gibbs energy of activation were determined as follows:?H(?)?4.9 kcal/mol,?S(?)=-75 cal/(mol·K)and ?G(?)333=29.8 kcal/mol.2.Synthesis of zinc complexes stabilized by N-methylethylene-diamine bridged tris(phenol)ligands and their catalytic property for the cycloaddition of CO2 and epoxides(1)Under ice-salt bath(-15?)condition s,ZnEt2 reacted with the ligand precursor LH3 in toluene in a 1:1 molar ratio to give the zinc complex HL1Zn(8)and HL2Zn(9).Under the similar reaction conditions,ZnEt2 reacted with the ligand precursor LH3 in a molar ratio of 2:1 to afford the zinc complexes L1Zn2Et(10)and L2Zn2Et(11).The similar reaction of ZnEt2 with the ligand precursor L3H3 in a molar ratio of 3:2 gave the zinc complex L32Zn3(12).All the complexes were characterized by elemental analysis,1H and 13C NMR,and X-ray single crystal structure determination.In addition,complex 11 was characterized by COSY,HSQC and HMBC.(2)The catalytic activities of complexes 8-12 for the cycloaddition reaction of epoxides and CO2 were studied.The effects of reaction time,catalyst and co-catalyst,reaction temperature on the reaction were investigated using the reaction of 1,2-epoxyhexane and CO2 as a template reaction,and the optical reaction conditions were:room temperature,1 bar pressure of CO2,0.6 mol%of complex 11 as catalyst,1.2 mol%of TBAI as co-catalyst,and 18 h.In addition,under the optimal conditions,various mono-substituted epoxides can be converted to cyclic carbonate in 33-99%yields.For the mono-substituted epoxides with poor reaction activity,when the amount of co-catalyst was increased to 2 mol%and the reaction time was extended to 24 h,they can be converted into the corresponding cyclic carbonate in 48-96%yields.Under the same reaction conditions,kinetics study revealed that the apparent rate constant of complex 10 is 1.32 times to that of complex 8.Therefore,a catalytic mechanism based on a synergistic effect between double zinc metals is proposed.3.Synthesis of rare earth metal-zinc heteronuclear complexes stabilized by N-methylethylene-diamine-bridged tris(phenol)ligands(1)Under ice-salt bath(-15?)conditions,ZnEt2 was added slowly to benzyl alcohol in a 1:2 molar ratio in toluene to give the cluster[Zn(OCH2Ph)2]n in 3 hours.RECp3(THF)reacted with L1H3 in toluene in a 1:1 molar ratio in 12 hours,and then was added to a suspension of[Zn(OCH2Ph)2]n.After 3 days,the solvent was evaporated,and the solid was recrystallized from toluene and hexane to give heterobimetallic rare earth metal-zinc complexes(L1Y)2[Zn(OCH2Ph)2]2(13),(L1RE)2Zn(OCH2Ph)2(RE=Nd(14),La(15)).All complexes were characterized by elemental analysis,X-ray single crystal structure determination.Unfortunately,no resolvable NMR spectrum of these complexes was obtained.