Thermodynamics of Anion−π Interactions in Aqueous Solution

Thermodynamic parameters (Δ<i>G</i>°, Δ<i>H</i>°, <i>T</i>Δ<i>S</i>°), obtained by means of potentiometric and isothermal titration calorimetry (ITC) methods, for the binding equilibria involving anions of high negative charge, like SO₄^2–, SeO₄^2–, S₂O₃^2– and Co­(CN)₆^3–, 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)₆^3– 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