Predicting the Strength of Anion-π Interactions of Substituted Benzenes: the Development of Anion-π Binding Substituent Constants.

A computational study aimed at accurately predicting the strength of the anion-π binding of substituted benzenes is presented. The anion-π binding energies (E) of 37 substituted benzenes and the parent benzene, with chloride or bromide were investigated at the MP2(full)/6-311++G** level of theory. In addition, energy decomposition analysis was performed on 27 selected chloride-arene complexes via symmetry adapted perturbation theory (SAPT), using the SAPT2+ approach. Initial efforts aimed to correlate the anion-π E values with the sum of the Hammett constants σ (Σσ) or σ (Σσ), as done by others. This proved a decent approach for predicting the binding strength of aromatics with electron-withdrawing substituents. For the Cl-substituted benzene E values, the correlation with the Σσ and Σσ values of aromatics with electron-withdrawing groups had r values of 0.89 and 0.87 respectively. For the Br-substituted benzene E values, the correlation with the Σσ and Σσ values of aromatics with electron-withdrawing groups had r values of 0.90 and 0.87. However, adding aromatics with electron-donating substituents to the investigation caused the correlation to deteriorate. For the Cl-substituted benzene complexes the correlation between E values and the Hammett constants had r = 0.81 for Σσ and r = 0.84 for Σσ. For the Br-substituted benzene complexes, the respective r values were 0.71 for Σσ and 0.79 for Σσ. The deterioration in correlation upon consideration of substituted benzenes with electron-donating substituents is due to the anion-π binding energies becoming more attractive regardless of what type of substituent is added to the aromatic. A similar trend has been reported for parallel face-to-face substituted benzene-benzene binding. This is certainly counter to what electrostatic arguments would predict for trends in anion-π binding energies, and this discrepancy is further highlighted by the SAPT2+ calculated electrostatic component energies (E). The E values for the Cl-substituted benzene anion-π complexes are all more binding than the E value for the Cl-benzene complex, with the exception of chloride-1,3,5-trimethylbenzene. Again, this is a similar trend to what has been reported for parallel face-to-face substituted benzene-benzene binding. A discussion on this surprising result is presented. In addition, an improved approach to predicting the relative anion-π binding strength of substituted benzene is developed using the results of the SAPT2+ calculations.