Anion-π bonding, a counter-intuitive attraction of anions to electron-deficient π-systems, has been a subject of significant interest during the last decade. A number of works have demonstrated its considerable potential for molecular recognition, anion transport, and catalysis. p-Benzoquinone is one of the most important acceptors in chemical and biochemical systems. Yet, experimental studies of the characteristics of its anion-π complexes are very rare. In the current work, we present the structural, spectral, and thermodynamic properties of the complexes of a series of p-benzoquinone derivatives, (e.g., dichloro, tetrachloro- and dichlorodicyano-p-benzoquinone) with halide anions. UV-Vis spectral measurements showed that mixing solutions of halides with p-benzoquinone derivatives resulted in the appearance of new absorption bands in the UV-Vis range. The energies of these bands showed Mulliken correlations with the difference between the redox-potentials of the benzoquinone acceptor and halide. Dependencies of the intensity of these bands on concentrations of reactants and temperature indicated that they are related to the formation of 1:1 complex. Treatments of the spectral data produced the values of equilibria constants which, in general, increased parallel with the strength of the acceptor. For example, in acetonitrile, complexes of chloride anion with 2,5-dichloro-p-benzoquinone have an equilibrium constant (k) of about 0.1 M-1 for complexes, while the k = 70 M-1 for the same complex with dichlorodicyano-p-benzoquinone (Ered = - 0.20 V and + 0.52 V vs SCE, respectively). X-ray structural analysis demonstrated that the contraction of the intermolecular separations follows the same trend. Specifically, the distances between halide anions and tetarfluoro-p-benzoquinone are only slightly less than their van der Waals separations, while in comparison, Cl…C distances in the complex of chloride with dichlorodicyano-p-benzoquinone are 18 % less than the sum of the van der Waals radii of carbon and chlorine atoms.

Spectra, structures, and thermodynamics of anion-π complexes of p-benzoquinones with halide anions

2019 Spring National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.