Anion-π Interaction with aromatic and non-aromatic compounds

During last twenty years, the studies of anion-π interaction, a counter-intuitive attraction of negatively charged species to the π-systems, were focused predominantly on the complexes of electron deficient aromatic molecules. There is very little data about the interaction of different anions with other π-systems. To compare anion-π interaction with aromatic and non-aromatic π-molecules, we characterized complexes of various anions with p-benzoquinone (BQ) derivatives and olefinic π-acceptor, tetracyanoethylene (TCNE). Co-crystallization of p-benzoquinones with a series of salt of oxo- and fluoro-anions, A-, such as PF6 -, ClO4 -, with bulky alkylammonium or cryptand-encapsulated cations produced structures consisting of the alternating BQ/A- stacks separated by the counter-ions. The close interatomic contacts between oxygen or fluorine substituents and BQ rings indicated that this selfassembly is a result of the anion-π bonding. Computational analyses confirmed that anion-π bonding of π-acceptors with the oxo-and fluoro-anions are characterized by the interaction energies analogous to that in the corresponding complexes of halides or pseudohalides. Yet, in contrast to the latter, they did not show substantial donor/acceptor charge transfer and intense charge-transfer absorption bands. These differences suggest that although charge-transfer interaction plays a critical role in the formation of the complexes of the π-acceptors with (pseudo-)halides, it is negligible in the complexes with the weakly coordinating fluoro- and oxoanions. The strong interaction in the latter is related to the multicenter interactions between oxygen or fluorine substituents in the anions with the π-acceptor moieties. Cocrystallization of TCNE with tetraalkylammonium salts of (pseudo-)halides or fluoro- or oxoanions also ii produced solid-state associations of anions arranged over the π-bond with one or several interatomic distances shorter than van der Waals separations. The addition of (pseudo)halide anions to the solution of TCNE in dichloromethane or acetonitrile resulted in the appearance of new absorption bands indicating either the formation of 1:1 complex or, with the strongest electron donor, iodide, TCNE anion radical. Energies of the absorption bands were following the same mulliken correlations, and their formation constants were comparable to that reported earlier for the complexes of these anions with aromatic or pbenzoquinone π-acceptors. All these suggested the common charge-transfer nature of these associations. On the other hand, the complex formation with fluoro and oxoanions in solution of TCNE modulated its absorption, but no new bands appeared in visible or near-UV range. This points out a critical role of the nature of anions in these bindings and the general equivalence of the interactions in the anion-π complexes with different π-acceptors.