Electrostatic molecular recognition in rigid frameworks

Abstract

Complexation of six organic dianionic hosts (1cc–2tt) with methylguanidinium, tetramethylammonium and ammonium salts in various protic solvents has been studied by NMR spectroscopy. Stability of the 1∶1 electrostatic complexes is shown to decrease with increasing distances between the carboxylate groups in the host compounds. Spectroscopic data and the results of molecular modelling support non-symmetrical structures of some of the complexes which contain one tight and one loose ion pair. The difference between the contributions of the tight and loose ion pair formation into the binding free energy can be predicted by the Fuoss theory of ion pairing on the basis of interionic distances calculated by molecular modelling.