Anion−π Complexes of Halides with p‑Benzoquinones: Structures, Thermodynamics, and Criteria of Charge Transfer to Electron Transfer Transition

Interchange of complex formation and electron-transfer reactions between halide anions and p-benzoquinones were established via UV–vis spectral and X-ray structural measurements and computational analysis. Solution-phase interaction of the p-benzoquinone acceptors with Cl^–, Br^–, or I^– donors led to the formation of anion−π complexes showing strong absorption bands in the UV–vis range. Formation constants and calculated interaction energies of these complexes increased, and donor/acceptor separations decreased with increasing reduction potentials of p-benzoquinones. Mulliken correlation and NBO analysis indicated a charge-transfer nature of these anion−π associates. Most notably, the increase of the acceptor strength led to a transition between the formation of the persistent anion−π complexes and electron-transfer reactions. Thermodynamic analysis accounted for the experimental observations of anion radicals and trihalide anions in solutions of p-benzoquinones with iodide or (for the strongest acceptor) bromide donors. Kinetics of these processes indicated that anion−π complexes represent critical intermediates of the redox reactions. In contrast to Cl^–, Br^–, or I^– anions, interaction of p-benzoquinones with F^– anions led to the formation of σ-complexes, and the appearance of anion radicals in such systems was related to the follow-up reactions of these complexes.