The interactions of chelate-based ionic liquids: from the perspective of vaporization enthalpy

The vaporization enthalpies of [C_nTPP][Cu(F₆-acac)₃] (n = 14, 16) and [C_nmim][Cu(F₆-acac)₃] (n = 8, 10), a new type of chelate-based ionic liquids (ChILs), were measured by thermogravimetric analysis (ΔglHom(T_av) = 73.2-83.7 kJ mol^-1), which decreased compared to their non-chelate counterparts (difference up to 34.1 kJ mol^-1). This can be explained by the fact that the increase in the polar region size by chelate ions mainly leads to the decrease in Coulomb forces based on the micro-biphasic separation in ionic liquids. Vaporization enthalpies at experimental temperatures were adjusted to a reference temperature by the Verevkin's method, and compared with the data calculated by the Kabo's method, which illustrated clear differences on ChILs. In addition, some physicochemical properties of [C_nTPP][Cu(F₆-acac)₃] (n = 14, 16) were measured including density, viscosity, conductivity, and surface tension.

Special Mixing Behavior of Chelate-based Ionic Liquid with Methanol

Compared to the general ionic liquids (ILs), a significant deviation of the binary mixtures of 1-decyl-3-methylimidazolium tri(hexafluoroacetylaceto)-copper(II) ([C₁₀ mim][Cu(hfacac)₃ ]) with methanol was found, indicating the way methanol interacts with ILs might be governed by the special structure of the chelating anion. IR results showed that the v (C2-H) of 1-decyl-3-methylimidazolium hexafluoroacetylacetonate ([C₁₀ mim][hfacac]) blue-shifted more significantly than that of [C₁₀ mim][Cu(hfacac)₃ ], meanwhile the v (C=O) red-shifted in [C₁₀ mim][Cu(hfacac)₃ ], which is contrast with that in [C₁₀ mim][hfacac]. Two-dimensional correlation analysis of the FTIR spectra indicated that the chelating cavity has little effect on the sequence of the ILs sites that interact with methanol. Combined with small angle X-ray scattering (SAXS) results, the picture of mixing processes in these two systems were proposed. Methanol interacts directly with the anion followed by the cation in [C₁₀ mim][hfacac], while methanol preferentially enters the chelating cavity and enhances the packing effect in the [C₁₀ mim][Cu(hfacac)₃ ] system.

Structural and electronic properties of CuII, CoII, and NiII-containing chelate-based ionic liquids

Potential applications of chelate-based ionic liquids depend on the structural and electronic properties of this class of liquid materials. Due to the large size of chelated metal anions, the high number of potential interaction sites could lead to complex intermolecular interactions, but metal-based anions have many degrees of freedom, and they have great potential in the structural modification of the physical properties of ionic liquids. To explore the influence of varying the metal center of anions and the length of carbon chains of cationic species, four single crystal structures of chelate-based ionic liquids, ([C₁₀mim][M(F₆-acac)₃], M = Cu, Co, and Ni) and [C₆mim][Cu(F₆-acac)₃] were obtained. Taking these as the initial configurations, theoretical efforts were made to understand the structural and electronic properties. The hydrogen bonding energies of the primary hydrogen bonding interaction, in the range of 17.7-20.9 kJ mol^-1, follow the order of [C₁₀mim][Ni(F₆-acac)₃] < [C₁₀mim][Co(F₆-acac)₃] <[C₁₀mim][Cu(F₆-acac)₃], and [C_nmim][Cu(F₆-acac)₃] (n≠ 10) < [C₁₀mim][Cu(F₆-acac)₃], while the experimental viscosities exhibit an opposite trend. Furthermore, by NBO analysis, the more negative charge on the oxygen atoms of anions shows the stronger hydrogen bonding of imidazolium with C²H. The reliability of the theoretical method was supported by the comparison between the simulated and experimental infrared and UV/vis spectra. This work is useful in increasing the understanding of the structure-property relationship of chelate-based ionic liquids and furthering the rational design of novel ionic liquids.