Metal ions in ionic liquids are laid under an unprecedented reaction field. In order to assess the reaction thermodynamics of metal ions in such a situation, Cu2+-chloro complex formation was examined with spectroscopic and calorimetric titrations in an ionic liquid, 1-buthyl-3-methylimidazolium trifluoromethanesulfonate (C4mimTfO). In addition, the effect of the structure of the solvated complexes on the complexation mechanism was investigated with the aid of DFT calculations. Chloro complexation successively proceeded and finally provided a [CuCl4]2– species, which is also the final product in conventional molecular solvents. Their stability constants were comparable to those in molecular solvents. Interestingly, in spite of the charged solvent in the ionic liquid, the entropy profile of the complexation resembled that in the conventional molecular liquids. This indicates that the entropy gain of the released solvent species from the complexes is the main driving force of the chloro complexation in the ionic liquid. In contrast, unlike the major molecular solvents, the total coordination number of Cu2+ is saturated to 4 in the ionic liquid, and the Cl– complexation tends to be accompanied by a 1:1 exchange of the solvent TfO– from the complex. In addition, this ligand exchange was almost athermal. This possibly indicates that the coordination number is dominated by the electrostatic hindrance among the ligands including the solvent ions in the primary coordination sphere.

Proton-donating and ionization properties of several protic ionic liquids (PILs) made from N-methylimidazole (Mim) and a series of acids (HA) have been assessed by means of potentiometric and calorimetric titrations. With regard to strong acids, bis(trifluoromethanesulfonyl) amide (Tf2NH) and trifluoromethanesulfonic acid (TfOH), it was elucidated that the two equimolar mixtures with Mim almost consist of ionic species, HMim+ and A–, and the proton transfer equilibrium corresponding to autoprotolysis in ordinary molecular liquids was established. The respective autoprotolysis constants were successfully evaluated, which indicate the proton-donating abilities of TfOH and Tf2NH in the respective PILs are similar. In the case of trifluoroacetic acid, the proton-donating ability of CF3COOH is much weaker than those of TfOH and Tf2NH, while ions are predominant species. On the other hand, with regard to formic acid and acetic acid, protons of these acids are suggested not to transfer to Mim sufficiently. From calorimetric titrations, about half of Mim is estimated to be proton-attached at most in the CH3COOH–Mim equimolar mixture. In such a mixture, hydrogen-bonding adducts formation has been suggested. The autoprotolysis constants of the present PILs show a good linear correlation with dissociation constants of the constituent acids in an aqueous phase.