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 þ , 222 cryptates, comparing two counterions X 2 (Chloride Cl 2 versus Picrate Pic 2) 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 2 or Pic 2 counterions, the cryptates and the X 2 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 2 than with Pic 2 counterions. In water, hydrophobic forces lead to still different anion dependent arrangements. The hydrophilic Cl 2 anions are diluted in water, without pairing with the cryptates which tend to "attract each other," in spite of their coulombic repulsions. The Pic 2 anions are more hydrophobic than Cl 2 and display p2stacking 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 Ba 2þ , 222 cryptates, comparing the Cl 2 to the Pic 2 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.

Interactions between Keggin Anions in Water: The Higher Their Charge, the Higher Their Condensation? A Simulation Study

“…Similar features are observed with big “spherical” ions without amphiphilic topology 70. See for instance simulation results on tetrahedral AsPh 4 + and BPh 4 – ions,71 cryptates,72 other macrocyclic receptors,73 and charged metallic complexes74 that concentrate at the water surface and “attract each other” in bulk water. Dicarbollides that aggregate in water also adsorb at aqueous interfaces with marked counterion effects 49,50,75.…”

Polyoxometalate Keggin Anions at Aqueous Interfaces with Organic Solvents, Ionic Liquids, and Graphite: a Molecular Dynamics Study

“…For instance, according to MD simulations on X − halides, “attraction” at the interface increases when the anion gets less hydrophilic, i.e., when it increases in size . Likewise big spherical polyvalent ions, tetrahedral AsPh 4 + and BPh 4 − ions, 222-cryptates, tetra-charged tetrahedral macrocycles, or even a −6-charged metallic complex, although lacking the amphiphilic topology, have been found to be “attracted” at aqueous interfaces and to condense in water. Recent experiments , and simulations on the “ellipsoidal” dicarbollide CCD − anions have evidenced their aggregation in water and their interfacial activity.…”

Do Keggin anions repulse each other in solution? The effect of solvent, counterions and ion representation investigated by free energy (PMF) simulations

“…For instance, dicarbollide anions [(B 9 C 2 H 8 Cl 3 ) 2 Co] À have been found by MD simulations and light scattering experiments to condense in water in the form of large aggregates whose size, shape and dynamics depend on the M n+ counterion [46]. Likewise, mono-or di-charged cryptates can display short contacts in water, with marked counterions effects [47,48]. These species can be viewed as hydrophobic (relatively small) macro-ions with reduced charges that tend to self-assemble in water.…”

“…Generally speaking, there is a deep relationship between ion aggregation in water and surface activity, even for big ''spherical'' ions [65]. For instance, according to MD simulations, spherical S n+ and S nÀ ions [66], tetrahedral AsPh 4 + and BPh 4 À ions [67], 222-cryptates [47], +4 charged tetrahedral macrocyclic receptors [68], or a À6 charged metallic complex [69] display surface activity, and also aggregate somewhat in water. Likewise, dicarbollide anions that aggregate in water also adsorb at aqueous interfaces, with marked counterion effects [46,70].…”