Anion Binding Studies of Fluorinated Expanded Calixpyrroles

The anion binding properties of fluorinated calix[n]pyrroles (n = 4−6) in aprotic solvents (acetonitrile
and DMSO) and modified reaction conditions allowing for the synthesis and isolation of the hitherto
missing dodecafluorocalix[6]pyrrole from the condensation of 3,4-difluoro-1H-pyrrole and acetone
are described. In acetonitrile solution containing 2% water, the association constants for the 1:1
binding interaction between octafluorocalix[4]pyrrole and chloride anion obtained with isothermal
titration calorimetry (ITC) and 1H NMR titration methods were found to match reasonably well.
As compared to its nonfluorinated congener, octafluorocalix[4]pyrrole was found to display enhanced
binding affinities for several representative anions in pure acetonitrile as judged from ITC analyses.
Similar analyses of the fluorinated calix[n]pyrroles revealed an increase in the relative affinity for
bromide over chloride with increasing macrocycle size, as manifest in a decrease in the binding
ratio Ka(Cl)/Ka(Br). Anion binding studies in the solid state, involving single-crystal X-ray diffraction
analyses of the chloride and acetate anion complexes of octafluorocalix[4]pyrrole and decafluorocalix[5]pyrrole, respectively, confirmed the expected hydrogen bond interactions between the pyrrolic
NH protons and the bound anions.