Cation–Cation Pairing by N-C-H···O Hydrogen Bonds

The pairing of ions of opposite charge is a fundamental principle in chemistry, and is widely applied in synthesis and catalysis. In contrast, cation–cation association remains an elusive concept, lacking in supporting experimental evidence. While studying the structure and properties of 4-oxopiperidinium salts [OC₅H₈NH₂]X for a series of anions X^- of decreasing basicity, we observed a gradual self-association of the cations, concluding in the formation of an isolated dicationic pair. In 4-oxopiperidinium bis(trifluoromethylsulfonyl) amide, the cations are linked by N-H···O=C hydrogen bonds to form chains, flanked by hydrogen bonds to the anions. In the tetra(perfluoro-tert-butoxy)aluminate salt, the anions are fully separated from the cations, and the cations associate pairwise by N-C-H···O=C hydrogen bonds. The compounds represent the first genuine examples of self-association of simple organic cations based merely on hydrogen bonding as evidenced by X-ray structure analysis, and provide a paradigm for an extension of this class of compounds