Molecular Selective Binding of Pyridinium Guest Ions by Water‐Soluble Calix[4]arenes

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Abstract
The complex stability constants (KS) and thermodynamic parameters (ΔG°, ΔH°, and TΔS°) for 1:1 intermolecular complexation of water‐soluble calix[4]arene tetrasulfonate (CAS) and thiacalix[4]arene tetrasulfonate (TCAS) with pyridinium guest ions 1–10 have been determined by means of titration calorimetry in an acidic buffer solution (pH = 2.0) at 298.15 K, and their binding modes have been investigated by NMR spectroscopy. Possessing a smaller cavity and a higher π electron density than TCAS, CAS affords the stronger binding abilities for all guests examined, giving an exciting molecular selectivity of up to 354 for the 2,6‐dimethylpyridine/2,6‐pyridinedicarboxylic acid pair. The high binding affinities, attributed to the favorable enthalpic gains, are comprehensively discussed from the viewpoint of π–π stacking, electronic effect and size‐fit relationship between host and guest. The thermodynamic parameters that have been obtained, together with the NMR results, jointly demonstrate that the position, number, and type of substituent groups introduced onto the guest molecule are the key factors controlling the structural‐energetics correlation for the molecular selective binding of water‐soluble calixarenes. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

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