Unusual aggregation of bis(2-quinuclidinium-butyrate) hydrobromides

The molecular structure of di-[bis(2-quinuclidinium-butyrate) hydrobromide], [(QNBu)2HBr]2 (1), has been characterized by single-crystal X-ray diffraction, infrared spectroscopy and DFT calculations. The crystals (1) are monoclinic, space group P21/c with [(QNBu)2HBr]2 symmetry-independent units. The complex 1 consists of two independent homoconjugated cations, in which two (S) QNBu semications, and (S) and (R) QNBu semications are joined by short, symmetrical O⋯H⋯O hydrogen bonds of 2.418(12) and 2.411(13) Å, respectively. The bromide anions interact electrostatically with the one positively charged nitrogen atom of each cation. The presence of short OHO hydrogen bonds is confirmed by the broad absorption in the 1500-400 cm−1 region, with the center of gravity, νH, at ca. 900 cm−1, in the solid-state FTIR spectrum. In the structure of [(QNBu)2HBr]2 (2) optimized at the B3LYP/6-31G(d,p) level of theory, the 2-quinuclidinium-butyrate units are non-equivalent. In each homoconjugated cation the 2-quinuclidinium-butyric acid interacts with the QNBu inner salt by the short, asymmetric O-H···O hydrogen bonds of 2.440 and 2.446 Å, respectively. Each bromide anion interacts electrostatically with the positively charged nitrogen atoms from both homoconjugated cations, which fold into a globular supramolecular aggregate.