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-pi binding of substituted benzenes is presented. The anion-pi binding energies (E-bind) 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-pi E-bind values with the sum of the Hammett constants sigma(p) (Sigma sigma(p)) or sigma(m) (Sigma sigma(m)), 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 Ebind values, the correlation with the Sigma sigma(p) and Sigma sigma(m), values of aromatics with electron-withdrawing groups had r(2) values of 0.89 and 0.87 respectively. For the Br--substituted benzene E-bind values, the correlation with the Sigma sigma(p) and Sigma sigma(m) values of aromatics with electron-withdrawing groups had r(2) 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-bind values and the Hammett constants had r(2) = 0.81 for Sigma sigma(p) and r(2) = 0.84 for Sigma sigma(m). For the Br--substituted benzene complexes, the respective r(2) values were 0.71 for Sigma sigma(p) and 0.79 for Sigma sigma(m). The deterioration in correlation upon consideration of substituted benzenes with electron-donating substituents is due to the anion it 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-pi binding energies, and this discrepancy is further highlighted by the SAPT2+ calculated electrostatic component energies (E-ele). The E-ele values for the Cl--substituted benzene anion-pi complexes are all more binding than the E-ele 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-pi binding strength of substituted benzene is developed using the results of the SAPT2+ calculations.