Solvent and Ionic Atmosphere Effects in Anion-π Interactions: Complexes of Halide Anions with p-Benzoquinones

The interplay between the solvent polarity and ionic atmosphere in anion-π association was evaluated via an experimental and a computational study of the BQ·X^- complexes between benzoquinones (BQ) and halide anions (X^-). The UV-Vis spectral measurements showed that these complexes are characterized by the strong absorption bands in the 300-450 nm range and their effective formation constants, K_eff, measured in dichloromethane in the absence (or at low concentrations) of the supporting electrolyte, Bu₄NPF₆, were higher than those in acetonitrile. The experimental data were consistent with the results of the computations, which showed that magnitudes of the interaction energy, ΔE, between BQ and X^- decreased considerably with the increase in the polarity of the media. The addition of auxiliary electrolytes (e.g., Bu₄NPF₆) led to a decrease in the concentration of the BQ·X^- complexes. These changes were related to the competing associations of the π-acceptors with halides and PF₆^- anions (since the interaction energies between BQ acceptors and common non-halide anions, e.g., PF₆^-, BF₄^-, and NO₃^-, were comparable to those in the BQ·X^- complexes) and to the increased ionic strength of the solutions. The variations in strength of anion-π interactions with the solvent polarity and ionic atmosphere were related to the higher effective ionic radii of the complexes. Due to the larger effects of the auxiliary electrolytes in dichloromethane, the formation constants for the BQ·X^- complexes measured at high ionic strength in this solvent were lower than those in more polar acetonitrile or propylene carbonate. Such a combination of the effects of the solvent and ionic atmosphere should be taken into account when comparing experimental data with the results of the calculations and in design of the systems for molecular recognition and catalysis.