Nature of Electrophiles and Electron Acceptors. Comparison of Their Molecular Complexes with Aromatic Donors

The novel charge-transfer (CT) absorption bands observed with various arenes and mercuric trifluoroacetate are assigned to 1:1 electron donor-acceptor (EDA) complexes. Spectral data are also reported for the aromatic complexes of the halogen electrophiles Cl₂ and Br₂. Electron-poor organic compounds such as maleic anhydride, tetracyanoethylene, chloranil, tetracyanobenzene, and tetracyanopyrazine interact as acceptors with the same series of arenes to afford persistent CT absorption bands which can be shown to arise from 1:1 EDA complexes. An empirical parameter, K_s, is introduced to provide comparative measures of the strengths of EDA complexes from the readily available spectral parameters. Analysis indicates that electrophiles cannot be distinguished from π acceptors solely on the basis of values of K_s for their EDA complexes with aromatic donors. Furthermore, the CT transition energies h_vCT of electrophile and π-acceptor complexes show the same types of linear variations with the ionization potentials (I_D) of the aromatic donors, in accord with the predictions of the Mulliken theory. The variations in the slopes of the correlations, i.e., ƌ(hv_CT)/ƌ(I_D) are related to the changes in the mean separation r_DA, which are supported by intermolecular comparisons of the spectral data in Figure 8, as well as the intramolecular comparisons of the multiple CT bands in Figure 9. The structures of the EDA complexes with electrophiles are compared with those of other arene complexes.