Reports: GB4
47159-GB4 The Scope and Chemical Relevance of Anion-Pi Interactions Involving Aromatics: Computational and Solid-State Studies
Our proposal outlined three related areas of research on the interaction between anions and aromatic pi-electron density. Over the past year we have made significant progress on all three fronts, as summarized below.
Correlation Between Anion-Arene Binding Energies and the Polarizability of the Aromatics. We have calculated the anion-arene binding energies between halo-, methyl-, amino-, hydroxyl-, nitro-, and cyano-substituted aromatics and chloride and bromide anions and found an excellent correlation with the absolute value of the sum of the Hammett substituent constants sigma-m of the substituted aromatics. This is an exciting result, and it coincides with recent studies showing correlation between Hammett substituent constants and either anion-arene or arene-arene binding energies. Given that dispersion and polarizability are both known to be factors that contribute to the binding between anions and aromatic pi-electron density, this result suggests that Hammett sigma-m values describe the polarizability of substituted aromatics. Our ongoing efforts in this area involve calculating the polarizability values of the substituted aromatics, determining how broad the correlation is between anion-arene binding energies and the Hammett constants of substituted aromatics, determining if a correlation exists between anion-arene binding energies and the polarizability values of substituted aromatics, and determining why the Hammet values seem to be indicative of aromatic polarizability.
Anion Binding to Transition Metal-Complexed Aromatics. We have calculated the chloride and bromide binding of Ar-FeII-Cp complexes (Ar = substituted aromatic; Cp = cyclopentadienyl anion) where the anion binds to the pi-face of the substituted aromatic Ar and compared that to the anion binding to the FeII center of the complex. We determined the preference for anion-pi binding by subtracting the binding energy when the anion is complexed to the metal from the binding energy when the anion is complexed to the aromatic pi-cloud. This gave the very surprising result that the preference for anion-pi binding increased with an increase in electron donating groups on the aromatic. This work is nearing completion and will be disseminated in the near future.
Preparation of Solid-State Host for Anion-Pi Binding Studies. Our synthetic approach to the preparation of an anion-binding host involves a low-yielding macrocyclization step and we are currently working on optimizing this step.
