Origins of Selectivity in Molecular and Supramolecular Entities: Solvent and Electrostatic Control of the Translational Isomerism in [2]Catenanes

The fundamental basis for the stabilization of molecular complexes of various dioxyarenes and/or dithiaarenes and a tetracationic cyclophane was established by empirical force field and ab initio quantum mechanical calculations. The more stable translational isomers of the related [2]catenanes do not necessarily correspond to the more stable complexes involving the individual components. The origin of this anomaly was investigated using the AMBER* force field. Each [2]catenane is composed of cyclobis(paraquat-p-phenylene)-the tetracationic cyclophane-and one constitutionally unsymmetrical macrocyclic polyether, incorporating 1,4-dioxybenzene and a dioxyarene or a dithiaarene unit as its two π-electron rich recognition sites. The calculated and experimental isomer ratios at equilibrium for these [2]catenanes are in good agreement. In two instances, the calculated ratios invert as a result of changing the solvation model from H₂ to CHCl₃. There is a correlation between the experimental and theoretical observations for the equilibrated isomer ratios and the dielectric constant of the solvent. Both the solvation energies of the translational isomers and the energy differences associated with the corresponding complexes govern the nature of the translational isomerism. The relative stabilities of isomers are controlled by the electrostatic potential at the surface of the π-electron rich aromatic units, rather than by charge- transfer interactions.