Thermodynamic Characterization of Halide-pi Interactions in Solution using "Two-wall" Aryl extended Calix[4]pyrroles as Model System

Abstract

[eng] Herein, we report our latest experimental investigations of halide−π interactions in solution. We base this research on the thermodynamic characterization of a series of 1:1 complexes formed between halides (Cl−, Br−, and I−) and several α,α-isomers of "two-wall" calix[4]pyrrole receptors bearing two six-membered aromatic rings in opposed meso positions. The installed aromatic systems feature a broad range of electron density as indicated by the calculated values for their electrostatic surface potentials at the center of the rings. We show that a correlation exists between the electronic nature of the aromatic walls and the thermodynamic stability of the X−⊂receptor complexes. We give evidence for the existence of both repulsive and attractive interactions between π systems and halide anions in solution (between 1 and −1 kcal/ mol). We dissect the measured free energies of binding for chloride and bromide with the receptor series into their enthalpic and entropic thermodynamic quantities. In acetonitrile solution, the binding enthalpy values remain almost constant throughout the receptor series, and the differences in free energies are provoked exclusively by changes in the entropic term of the binding processes. Most likely, this unexpected behavior is owed to strong solvation effects that make up important components of the measured magnitudes for the enthalpies and entropies of binding. The use of chloroform, a much less polar solvent, limits the impact of solvation effects revealing the expected existence of a parallel trend between free energies and enthalpies of binding. This result indicates that halide−π interactions in organic solvents are mainly driven by enthalpy. However, the typical paradigm of enthalpy−entropy compensation is still not observed in this less polar solvent.