What causes the weakest host to act as the strongest one? A theoretical study on the host–guest chemistry of five azacryptands and fluoride anions

Abstract

In this work we have attempted to computationally analyze the important parameters affecting the selectivity in host-guest systems in order to show that the solvent effect in some host-guest systems is even more important than the hole-size fitting and/or host-guest interaction energy. For this purpose, the fluoride anion selectivities of the five most studied azacryptands with different affinities, moieties, cavity sizes and degrees of preorganization, in their fully protonated forms, were studied at PBE/TZVP and B3P86/TZVP levels of theory. The factors affecting the selectivity such as hole size fitting, steric effects, electronic properties, preorganization of hosts, as well as solvent effects were investigated. The results showed that among the five studied azacryptands, one of them, which ranks fifth for selectivity according to its weak interaction energy with fluoride anions, ranks fourth due to its good preorganization. However, a surprising improvement occurs when its selectivity for fluoride anions ranks first due to the existence of less solvent hindrance. The results of the calculations were further confirmed by a good correlation between the calculated and experimental formation constants.

Where and How Does an Organic Molecule Having a C–X Bond Release X– Anion Like an Inorganic Compound? A Theoretical Study

“…Because the chemical reactions in the solution may show unexpected properties in comparison with the gas phase because of the solvation effect, − the study of reactions in the presence of different solvents is very important. Thus, we also studied the processes of hydride and fluoride release from the compounds studied in this work through reactions – in the solution.…”

Inorganic anion recognition in aqueous solution by coupling nearby highly hydrophilic and hydrophobic moieties in a macrocyclic receptor

“…Typical examples can be phosphates and sulphates, which are able to form stable adducts in aqueous solution with appropriately tailored receptors, including polyammonium-based ones. [28][29][30][31][32][33][34][35][36][37] Cl − , Br − and I − are the anions most difficult to target with polyammonium receptors, due to their lower charge density and tendency to behave as H-bonding acceptors. Nevertheless, examples of macropolycyclic polyammonium cations able to form stable complexes with these elusive anions have been reported.…”

“…In fact, polyamines normally occur as polycharged cations in water solution, even at neutral pH values, establishing strong charge–charge and hydrogen bonding interactions with the anionic species, which are a necessary pre-requisite for complex coordination in a solvating protic medium. Several examples of inorganic anion binding through encapsulation within cavities or clefts of protonated polyamine receptors in aqueous solution have been recently reported; 2,23–45 however, studies on metal-free fluorescent receptors of polyammonium type in pure water are still less common 38–45 and most of them are related to fluoride or phosphate anion recognition. Specific binding of fluoride is generally obtained by exploiting its high negative charge density and the consequent ability to form strong hydrogen bonding interactions with appropriate H-bonding donor groups, much higher than that found for other halides.…”