Binding of Acetylcholine and Tetramethylammonium to a Cyclophane Receptor: Anion?s Contribution to the Cation-pi Interaction

The interaction of the lipophilic cyclophane 1 with several acetylcholine (ACh) and tetramethylammonium (TMA) salts has been investigated in deuterochloroform to ascertain the influence of the counterion on the cation-pi interaction. Reliable association constants have been measured for 17 salts of commonly used anions; corresponding binding free energies -Delta G° ranged from over 8 kJ mol-1 down to the limit of detection. The dramatic dependence of the binding energy on the anion showed that the latter takes part in the process with a passive and adverse contribution, which inhibits cation binding even to complete suppression in unfavorable cases. Thermodynamic parameters for the association of 1 with TMA picrate demonstrate that binding is enthalpic in origin, showing a substantial enthalpy gain (Delta H° = -16.7 kJ mol-1) and an adverse entropic contribution (Delta S° = -27.9 J mol-1 K-1). A correlation has been found between the “goodness” of anions as binding partners and the solubility of their salts. Conversion of the anion into a more charge-dispersed species, e.g. conversion of chloride into dialkyltrichlorostannate, improves cation binding substantially, indicating that charge dispersion is a main factor determining the influence of the anion on the cation-pi interaction. DFT computational studies show that the variation of the binding free energy of TMA with the counterion is closely accounted for by the electrostatic potential (EP) of the ion-pair: guest binding appears to respond to the cation’s charge density exposed to the receptor, which is determined by the anion’s charge density through a polarization mechanism. A value of –Delta G° = 38.6 kJ mol-1 has been extrapolated for the free energy of binding of TMA to 1 in chloroform but in the absence of a counterion. The transmission of electrostatic effects from the ion-pair to the cation-pi interaction demonstrate that host-guest association is governed by coulombic attraction, as long as factors (steric, entropic, solvation, etc.) other than pure electrostatics are not prevalent.