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

The molecular recognition of S-6-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, S-6-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)x10(3) M-1 to (1.48 +/- 0.11)x10(5) 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/, / and nonconventional hydrogen bonds between host and guest, the entropy increase was most likely due to the desolvation of the guests.