An archetype of hydrogen bonding observed in cationic dimers of carboxy-functionalized ionic liquids by means of NMR solid state spectroscopy – reminiscent of salt bridges, peptides and DNA†

Ion pair formation is a fundamental concept in chemistry. The association between ions of opposite charge is widely used in synthesis and catalysis. In contrast, there is little evidence for the formation of cationic or anionic dimers in solution. We report the strength and distribution of doubly hydrogen bonded cationic dimers (c⁺c⁺) in carboxy-functionalized ionic liquids [HOOC-(CH₂)_n-py][NTf₂] with n = 2, 4, 5, 6, 7, and 9, probed by NMR solid-state spectroscopy. The two OH⋯OC H-bonds of the cationic dimers resemble the archetype H-bond motif known for formic acid. Herein, we clarify how the propensity for the formation of (c⁺c⁺) H-bonds depends on the alkyl chain length between the pyridinium ring and the carboxy group of the cations. For n = 9, the H-bond population is primarily dominated by cationic dimers (c⁺c⁺). Obviously, cooperative H-bond attraction is not only able to compensate for the repulsive Coulomb forces but also to ignore the H-bond accepting capabilities of the counter anion completely. In this regard, we provide the first evidence for quasi-isolated cationic dimers in solution that are stabilized by strong and directional (c⁺c⁺) H-bonds being as strong as the (mm) H-bonds between molecular mimics of the IL cations.