title: Cation-cation pairing by N-C-H⋅⋅⋅O hydrogen bonds.
abstract: 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 [OC5 H8 NH2 ]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.
authors.affiliation: Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr (Germany) http://www.kofo.mpg.de/poerschke.html.
authors.forename: Waltraud
authors.initials: W
authors.lastname: Gamrad
authors.forename: Angelika
authors.initials: A
authors.lastname: Dreier
authors.forename: Richard
authors.initials: R
authors.lastname: Goddard
authors.forename: Klaus-Richard
authors.initials: KR
authors.lastname: Pörschke
keywords: hydrogen bonds; piperidine derivatives; polycations; structure elucidation