A series of reports from this laboratory have described computational studies defining the first general binding motif yet discovered for ion pairing in conventional electrochemical media (1-4). These studies have shown that tetraalkylammonium cations interact in identical fashion with a variety of organic and inorganic electron donor species. For example, the structures (see image) generated by interaction between tetrabutylammonium cation and nitrobenzene anion radical, dianion, and even neutral nitrobenzene ,are identical. Remarkably similar structures have been found without exception for more than one hundred other species formed by interaction between tetralkylammonium cations where the alkyl group ranges from ethyl through heptyl and electron donors of diverse structural type (polycyclic aromatic hydrocarbon dianions, substituted benzaldehydes, nitrosobenzenes, and even inorganic anions).

 

The uniformity of binding between over a broad range of cationic and anionic structures in these many species is remarkable. Surely electrostatic attractions are important. Yet, in light of the high specificity of structure exhibited by these ion pairs and related species, one may well inquire into the reasons why this is so. Is the explanation simply electrostatic attraction between cations and anions? We will show evidence instead that formation of these structures involves a complex set of phenomena including electrostatics (of course) but also induced charges on each component and electron transfer between the two.

References

1. Fry, A. J. Tetrahedron 2006, 62, 6558.
2. Fry, A. J. J. Org. Chem. 2013, 78, 2111.
3. Fry, A. J. J. Org. Chem. 2013, 78, 5476.
4. Fry, A. J. J. Org. Chem. 2015, 80, 3758.