Département de chimie moléculaire

The paper reports a mol. dynamics study on the solvation of M+ (Na+ to Cs+) alkali cations and of their LM+ complexes with a calix[4]arene host (L = 1,3-dimethoxy-calix[4]arene-crown-6 in the 1,3-alternate conformation) in the [BMI][PF6] and [BMI][Tf2N] room-temp. ionic liqs. "ILs" based on the BMI+ (1-butyl-3-methylimidazolium) cation. The comparison of the two liqs. and the dry vs. humid form of the former one (with a 1:1 ratio of H2O and BMI+PF6- species) reveals the importance of humidity, namely in [BMI][PF6]-dry as in the [BMI][Tf2N] liq., the first solvation shell of the "naked" M+ ions is composed of solvent anions only (four PF6- anions, and from four to five Tf2N- anions, resp., quasi-neutralized by a surrounding cage of BMI+ cations), while in the [BMI][PF6]-humid IL, it comprises from one to three solvent anions and about four H2O mols. In the LM+ complexes, the cation is shielded from solvent, but still somewhat interacts with a solvent anion in the dry ILs and with water in the humid IL. The authors also report tests on M+ interactions with solvent anions PF6- and Tf2N- in the gas phase, showing that the AMBER results are in satisfactory agreement with QM results obtained at different levels of theory. The question of ion recognition by L is then examd. by free energy perturbation studies in the three liqs., predicting a high Cs+/Na+ selectivity upon liq. extn. from an aq. phase, in agreement with exptl. results on a parent calixarene host. A similar Cs+/Na+ selectivity is predicted upon complexation in a homogeneous IL phase, mainly due to the desolvation energy of the free cations. Thus, despite their polar character, ionic liqs. qual. behave as classical weakly polar org. liqs. (e.g., chloroform) as far as liq.-liq. extn. is concerned but more like polar liqs. (water, alcs.) as far as complexation in a single phase is concerned. [on SciFinder(R)]