Ion-pair electrostatic attraction-enhanced donor–acceptor interactions between the prototypic 1,4-dialkoxybenzene-viologen binding mode in water

Abstract

Three electron-rich donor molecules D1–D3, which consist of one, two or four 1,4-bis(carboxylatomethoxy)benzene units, respectively, have been prepared. Donor–acceptor interactions between the di-, tetra- and octaanionic donors and electron-deficient bipyridinium derivatives A1–A3, which bear two methyl, benzyl, or 4-formylphenyl subunits, respectively, have been investigated using X-ray diffraction analysis (for the complex of D2 and A1) and ¹H NMR and absorption experiments. It was revealed that D1 and D2 formed 1 : 1 donor–acceptor complexes with the three acceptors, while D3 formed 1 : 2 complexes with them. All the complexes exhibited high water-solubility. The crystal structure of complex D1·A1 showed that the biphenyl unit of D1 and the bipyridinium unit of A1 stacked alternately in the new perfect face-to-face pattern. By using isothermal titration calorimetric titration experiments, association constants for the 1 : 1 and 1 : 2 complexes in water were determined to be as high as 1.3 × 10⁴ M⁻¹ and 2.6 × 10¹¹ M⁻², respectively. The results supported that the intermolecular ion-pair electrostatic attraction between the carboxylate anions and the bipyridinium dication remarkably enhanced the intermolecular donor–acceptor interaction in water.