Counterintuitive affinity of [2.2]paracyclophane to cations

Geometries and binding energies of complexes of cations with benzene, [2.2]paracyclophane and a [2.2]paraheterocyclophane are computed and compared using ab initio calculations. [2.2]Paracyclophane is not used for building cation receptors because of its small cavity. Here, we demonstrate that its binding capability toward cations using one aromatic ring is superior to benzene in ∼10 kcal/mol. This unexpected difference is explained by the reduction, upon complexation, of the repulsive interaction of the π-systems, which is due to the close proximity of the two benzene rings. Experimental results derived from the analysis of X-ray structures retrieved from the CSD support this explanation.