Thermodynamics of Anion−π Interactions in Aqueous Solution

Thermodynamic parameters (ΔG°,
ΔH°, TΔS°),
obtained by means of potentiometric and isothermal titration calorimetry
(ITC) methods, for the binding equilibria involving anions of high
negative charge, like SO42–, SeO42–, S2O32– and Co­(CN)63–, and nitroso-amino-pyrimidine
receptors in water suggested that anion−π interactions
furnish a stabilization of about −10 kJ/mol to the free energy
of association. These anion−π interactions are almost
athermic and favored by large entropic contributions which are likely
due to the reduced hydrophobic pyrimidine surface exposed to water
after anion aggregation, and the consequent reduced disruptive effect
on the dynamic water structure. The crystal structure of the {H4L­[Co­(CN)6]}·2H2O complex showed
strong anion−π interactions between Co­(CN)63– and the protonated H4L3+ receptor. The CN···centroid distance (2.786(3) Å),
occurring with a cyanide N atom located almost above the centroid
of the pyrimidine ring, is the shortest distance till now reported
for the interaction between CN– ions and heteroaromatic
rings.