Cation–π and Lone Pair–π Interactions Combined in One: The First Experimental Evidence of (H₃O‐lp)⁺⋅⋅⋅π‐System Binding in a Crystal

In addition to hydrogen bonding and p–p stacking, two types of interactions involving aromatic systems have been recently recognized as strong noncovalent binding forces in supramolecular chemistry. These are the interactions of a p cloud with an electron lone pair (lp) contributed by a neutral or an anionic moiety (lp–p interactions) and cation–p interactions (where the cation is an alkali metal or an alkali-earth metal, but may be any onium ion); 7] both having great implications for small organic molecules and biological macromolecules. With the aromatic fragment acting as an acceptor of electron density in one case and its donor in the other, these interactions can nevertheless coexist and even enhance each other when present in the same system. 9] Cationic species with free electron lone pairs, such as the oxonium-type ions H3O + , H5O2 + , H7O3 + , etc. , 11] are of importance in various fields of physical chemistry, materials science, and biology; among them, the proton conductivity in the solid state 13] and proton-transfer processes in chemical and biochemical systems are particularly interesting. The main type of interactions an oxonium moiety is involved in are H bonds with different proton acceptors, p systems included. 15] Having a high proton-donating ability, H3O + is a very poor acceptor: although its faint energetic preference for O H···OH3 + binding was theoretically predicted, the corresponding non-directional contacts rarely occur in crystals. Such a behavior of an oxonium moiety was shown to be the result of a “compact” character of the lp of its oxygen atom. At the same time, recent investigations 19] on H3O and H5O2 -containing salts have shown that due to the formation of interionic H bonds the positive charge of these species is significantly reduced; this may appear sufficient to involve an oxonium cation into the lp–p binding. Although such interactions are well-known for neutral and negatively charged species, those involving cations as the lp donors have—to the best of our knowledge—not been observed or even proposed yet in the literature. Although DFT and MP2 calculations 20] carried out on the model oxonium–benzene complex (H3O···C6H6) + always lead to O H···p or C H···O interactions instead of the awaited (H3O lp) ···p binding, there is a system—bis(oxonium) 4-hydroxy-1,3-benzenedisulfonate (1)—where the latter might occur. (The Cambridge Structural Database, release 2010, contains several structures where the same type of contacts with aromatics might also be expected: their ref. codes SODFOG (benzene), TIJKAZ (pyrazine) and YUYRAL (triazine), just to name a few.) According to our X-ray diffraction (XRD) data for 1, the oxygen atom of one of the two oxonium cations [both with the typical flattened geometry: the sum of the HOH bond angles is 330.6(13) and 326.9(13)8] points to the aromatic moiety of the dianion; the smallest O···C distance being 3.0639(6) . With numerous proton-donor and -acceptor groups in the molecule (Figure 1), the crystal structure of 1 is