Physicochemical And Spectral Properties Of Chelate-based Ionic Liquids

In this paper,we have a better understanding of the structure-property relations of the chelate-based ionic liquids(ILs)by exploring the physicochemical and spectral properties of these ILs.The chelate-based ILs possess the dual properties of metal complex and ILs which then have great application prospect.However,systematic studies on the physicochemical properties of the chelate-based ILs are so rare that this paper focuses on the study of the physicochemical and spectral properties of chelate-based ILs,and thus we have laid a good foundation for the further understanding and rational design of chelate-based ILs.In this work,a series of chelate-based ILs,[Cnmim][Cu(F6-acac)3](n = 6,8,10,12,14),were synthesized and purified.The density,viscosity,conductivity and surface tension of the ILs were measured at T = 298.15-323.15 K.The effects of temperature and chain length on these properties were investigated.The density and conductivity of the ILs decreased with the increase of chain length.However,the viscosity and surface tension first decreased and then increased with the increase of chain length,showing a typical V-shape.The melting point of these ILs was determined which decreased with the increase of chain length.The ILs possess a polar head and a non-polar tail,hence,there is microphase separation within the ILs.The microphase separation theory and intermolecular forces can explain the changes of these physicochemical properties.According to Glasser's theory,the standard molar entropy and crystal energy of the ILs were estimated.The molar enthalpy of vaporization was predicted using Kabo's method.These ILs have poor ionicity judging from the Walden rule.These properties were compared with the common ILs,so the similarities and differences between the chelate-based ILs and the common ILs were obtained.In addition,some similarities and differences between the chelate-based ILs and the complexes(including chelates)were also obtained from the comparison with the common complexes.The infrared(IR)spectra of the pure ILs were determined.We found that the red shift of stretching vibration of C-H bonds of the alkyl chain in the cation occurred with the increase of chain length.However,the frequencies of C-H bonds of n-alkane(n =6-14)were found to be almost constant.The electron paramagnetic resonance(EPR)spectra of the pure ILs were measured and the g-values and A-values of Cu2+ in the ILs were nearly unchanged with the increase of chain length.DFT calculations on EPR parameters of(Cu(F6-acac)2)and([Cu(F6-acac)3]-)were carried out.With closer EPR parameters to the experimental data,the six coordination structure of the anion was further proved.The results of IR and EPR spectra further verified the existence of microphase separation in the ILs.The properties of the chelated-based ILs are tunable by the choice of metals,ligands and substituents of cation or anion.For the ILs with long carbon chain,when the polar region(the backbone structure)is fixed,we can adjust the properties by changing the non-polar region(the alkyl chain)to make them suitable for specific applications,and thus the ILs can be designed according to the definite requirements.On the basis of our study,we have summarized the similarities and differences between the chelate-based ILs and the complexes,and between the chelate-based ILs and the common ILs,thus we have a comprehensive understanding of the structure and nature of chelate-based ILs.