Supramolecular Interactions of Cryptates in Concentrated Solutions: The Effect of Solvent and Counterions Investigated by MD Simulations

Abstract We present a molecular dynamics study of concentrated solutions of K+⊂222 cryptates, comparing two counterions X− (Chloride Cl− versus Picrate Pic−) and three solvents (water, acetonitrile, and chloroform), with the main aim to investigate the distribution of the ions in the different solutions. The simulations reveal marked differences from one system to the other. In acetonitrile, with either Cl− or Pic− counterions, the cryptates and the X− anions are well diluted in the solvent box, without revealing specific ion pairing. In the less polar chloroform solutions, the complexes aggregate, and aggregation is more pronounced with Cl− than with Pic− counterions. In water, hydrophobic forces lead to still different anion dependent arrangements. The hydrophilic Cl− anions are diluted in water, without pairing with the cryptates which tend to “attract each other,” in spite of their coulombic repulsions. The Pic− anions are more hydrophobic than Cl− and display π−stacking attractions, forming negatively charged oligomers surrounded by cryptates that are therefore close to each other. The role of counterion is further demonstrated with aqueous solutions of the more charged Ba2+⊂222 cryptates, comparing the Cl− to the Pic− counterions. Beyond the case of the studied model solutions, the results have a bearing on the aggregation phenomena of other big ions (e.g. cation complexes, or bulky anions like polyoxometallates, or carborane derivatives) in solution.