Thermodynamic Characterization of Halide−π Interactions in Solution Using “Two-Wall” Aryl Extended Calix[4]pyrroles as Model System

Abstract

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.

Anion−π Interactions with Fluoroarenes

“…150 In acetonitrile the enthalpy term remains almost constant for all investigated receptor−anion complexes. The differences in the Gibbs free energy were exclusively assigned to changes of the entropy due to the strong solvation of the anions in the polar solvent.…”

“…69 Chemical Reviews ophenyl 53e, and dinitrophenyl 53f) ( Figure 64). 61,150 The anions are captured by NH groups in the center of the pyrrole in close proximity to the electron-deficient arenes. This is shown by the crystal structure of the pentafluorophenyl derivative 53e.…”

Substituent Effects in CH Hydrogen Bond Interactions: Linear Free Energy Relationships and Influence of Anions

“…16,17 It has also been recognized that an anion can interact with the face of an arene via anion–π or weak–σ interactions, leading to differentiation in both substituent and anion effects. 18–20 Computational studies indicate this type of dual anion and substituent dependence is important in CH HBs as well, although this has not been reduced to practice experimentally.…”

Anion-π Interactions in Supramolecular Chemistry and Catalysis

“…However, there are still few examples of attractive anion-π interactions in solution. Ballester's group has reported several examples of neutral molecular receptors that bind anions in solution as a combination of anion-π interactions and hydrogen bonding [97,98]. The strength of the anion-π interaction is indirectly detected as a modulation of the stronger hydrogen bonding interaction.…”