Understanding the driving force for the molecular recognition of S6-corona[3]arene[3]pyridazine toward organic ammonium cations

Abstract

The molecular recognition of S₆-corona[3]arene[3]pyridazine toward various N-alkyl ammonium cations was systematically studied by means of ITC titration, NMR spectroscopy, mass spectrometry and X-ray crystallography. As a powerful and selective macrocyclic host molecule, S₆-corona[3]arene[3]pyridazine was able to form dominantly 1 : 1 complexes with cations in a mixture of CH₃CN and 1,2-dichloroethane (v : v = 1 : 1) giving association constants in the range of (1.08 ± 0.01) × 10³ M⁻¹ to (1.48 ± 0.11) × 10⁵ M⁻¹. In all cases, the favorable host–guest complexation processes were driven by the combination of beneficial enthalpy and entropy effects. While the enthalpy effect was attributable to the multiple non-covalent bond attractions such as lpe/π, π/π and nonconventional hydrogen bonds between host and guest, the entropy increase was most likely due to the desolvation of the guests.