Calix[4]pyrrole as a Chloride Anion Receptor: Solvent and Countercation Effects

The interaction of calixpyrrole with several chloride salts has been studied in the solid state by
X-ray crystallography as well as in solution by isothermal titration calorimetry (ITC) and 1H NMR spectroscopic
titrations. The titration results in dimethylsulfoxide, acetonitrile, nitromethane, 1,2-dichloroethane, and
dichloromethane, carried out using various chloride salts, specifically tetraethylammonium (TEA), tetrapropylammonium (TPA), tetrabutylammonium (TBA), tetraethylphosphonium (TEP), tetrabutylphosphonium
(TBP), and tetraphenylphosphonium (TPhP), showed no dependence on method of measurement. The
resulting affinity constants (Ka), on the other hand, were found to be highly dependent on the choice of
solvent with Ka's ranging from 102−105 M-1 being recorded in the test solvents used for this study. In
dichloromethane, a strong dependence on the countercation was also seen, with the Ka's for the interaction
with chloride ranging from 102−104 M-1. In the case of TPA, TBA, and TBP, the ITC data could not be fit
to a 1:1 binding profile.