Capping the calix: how toluene completes cesium(i) coordination with calix[4]pyrrole.

The role of solvent in molecular recognition systems is under-researched and often ignored, especially when the solvent is considered "non-interacting". This study concerns the role of toluene solvent in cesium(i) recognition by calix[4]pyrrole. We show that π-donor interactions bind toluene molecules onto the open face of the cation-receptor complex, thus "capping the calix." By characterizing this unusual aromatically-saturated complex, we show how "non-interacting" aromatic solvents can directly coordinate receptor-bound cations and thus influence recognition.