Ionic liquids in green chemistry: catalytic reaction of cyclic ...

L.W. Wijayanti et al. Proceeding of The International Seminar on Chemistry 2008 (pp. 356-362)Jatinangor, 30-31 October 2008ambiguous spectroscopic and elemental analysis dataled us to carry out single crystal X-ray diffractionstudy, which revealed that the white powder was[Dmim] 2 ZnCl 2 Br 2 . The crystals of [Dmim] 2 ZnCl 2 Br 2were grown in a DMSO/toluene solvent mixture atroom temperature. An ORTEP of its molecularstructure is shown in Fig. 1, where selected bonddistances and angles are also provided.The crystal of [Dmim] 2 ZnCl 2 Br 2 contains twoindependent molecules in the asymmetric-crystal unit.Each zinc center is surrounded by four halide ligandsand possesses a distorted tetrahedral ligandenvironment. Unfortunately, the exact position ofbromide and chloride in tetrahedral zinc center couldnot be determined due to a disorder in the crystalsystem. Interestingly, two imidazolium cations werepaired with a tetrahalo zincate dianion ([ZnBr 2 Cl 2 ] 2- )in the solid state.In order to confirm that [Dmim] 2 ZnCl 2 Br 2 is areal-active species for the coupling reactionperformed in the presence of [Dmim]Br and ZnCl 2 ,the activity of [Dmim] 2 ZnCl 2 Br 2 was compared withthat of the catalytic system composed of [Dmim]Brand ZnCl 2 for the coupling reaction of CO 2 andethylene oxide at 100 °C and 500 psig with a molarratio of EO/Zn = 500.As can be seen in Table 1, [Dmim] 2 ZnCl 2 Br 2 wasfound to exhibit the similar activity to the catalyticsystem consisting of [Dmim]Br and ZnCl 2 . The activespecies, [Dmim] 2 ZnCl 2 Br 2 was obtained in high yieldover 90% either by reacting [Dmim]Br with ZnCl 2 orfrom the reaction of [Dmim]Cl and ZnBr 2 , suggestingthat bromide and chloride anions in [Dmim] 2 ZnCl 2 Br 2can be exchanged. Other imidazolium zinc complexeswere similarly prepared.The catalytic activity of [Dmim] 2 ZnCl 2 Br 2 wasevaluated for the coupling reactions of CO 2 andvarious alkylene oxide to produce correspondingalkylene carbonates, and the results are listed in Table2. Most of the alkylene oxides tested were readilyconverted to the corresponding alkylene oxides butthe reactivity of alkylene oxide was greatly affectedby the substituent at the carbon atom of the epoxide.358