The Structure and Thermodynamics of Calix[n]arene Complexes with Dipyridines and Phenanthroline in Aqueous Solution Studied by Microcalorimetry and NMR Spectroscopy

The complex stability constants (KS) and thermodynamic parameters (ΔH° and TΔS°) for 1:1 intermolecular complexation of three water-soluble calixarenes, that is, p-sulfonato calix[4]arene (C4AS), p-sulfonato thiacalix[4]arene (TCAS), and p-sulfonato calix[5]arene (C5AS), with dipyridines (4-DPD and 2-DPD) and 1,10-phenanthroline (Phen) have been determined by means of titration microcalorimetry in an acidic buffer solution (pH = 2.0) at 298.15 K, and their binding modes have been investigated by 1H NMR and 2D ROESY NMR spectroscopy. The results obtained indicate that 4-DPD, 2-DPD, and Phen are included in the cavity of C5AS with the different patterns, this is, accumbent for 4-DPD, acclivitous for 2-DPD and Phen, while Phen is included upright in the cavity of C4AS. The KS values decrease with increasing cavity size of host molecules but enhance with extending conjugation degree of guest molecules, and thus C4AS exhibits an exceptionally high Phen/4-DPD selectivity of 22.5. Thermodynamically, the complexation of DPDs/Phen with the water-soluble calixarenes is obviously enthalpy-driven, but the molecular selectivity is mainly governed by the entropy term.