Unexpected Binding Affinity of [2.2]Paracyclophane to Cations

Abstract

[eng] Motivation. The interaction of a cation with an aromatic ring, namely cation-pi interaction is a strong noncovalent force of great importance in many systems, including cation receptors and biomolecules. Cyclophanes and especially calixarenes are widely used cation receptors based on this interaction. [2.2]Paracyclophanes are not used for building cation receptors; however its binding capability toward cations is superior to benzene. Method. HF and B3LYP calculations have been used to carry out the geometry optimizations of [2.2]paracyclophane (1) complexes with lithium and sodium cation. Benzene complexes are also studied for comparison purposes. Comparative AIM and NICS analyses of the complexes have been performed. Results. Several cation-pi complexes have been optimized and compared. Complexes of 1 are considerably more stable (~10 kcal/mol) than benzene complexes. This unexpected difference is explained by the reduction of the repulsive interaction of the pi-systems in 1 due to the close proximity of the two benzene rings upon complexation. The AIM analysis is in agreement with this explanation. Conclusions. From the results presented here, derived from the higher binding affinity of 1 in comparison to benzene toward cations, the following conclusion arises: [2.2]paracyclophane is an excellent binding unit for the construction of cation receptors.