Solvation rules: aromatic interactions outcompete cation- interactions in synthetic host-guest complexes in water

Chemical double mutant cycles were used to measure the interaction of a N-methyl pyridinium cation with a pi-box in a calix[4]pyrrole receptor. Although the cation-pi interaction is attractive (-11 kJ mol-1), it is 7 kJ mol-1 less favourable than the corresponding aromatic interaction with the isosteric but uncharged tolyl group. Chemical double mutant cycles were used to measure the interaction of a pi-box in a calix[4]pyrrole receptor with a cationic N-methyl pyridinium group and with neutral isosteric analogues in water.