Unusual aggregation of bis(2-quinuclidinium-butyrate) hydrobromides Z. Dega-Szafran ⇑ , A. Katrusiak, M. Szafran Faculty of Chemistry, Adam Mickiewicz University, ul. Grunwaldzka 6, 60-780 Poznan ´, Poland article info Article history: Received 26 August 2010 Received in revised form 20 September 2010 Accepted 20 September 2010 Available online 25 September 2010 Keywords: Quinuclidinium-alkanoates inner salts X-ray diffraction DFT calculations FTIR spectroscopy OHO hydrogen bonds Electrostatic interactions abstract The molecular structure of di-[bis(2-quinuclidinium-butyrate) hydrobromide], [(QNBu) 2 HBr] 2 (1), has been characterized by single-crystal X-ray diffraction, infrared spectroscopy and DFT calculations. The crystals (1) are monoclinic, space group P2 1 /c with [(QNBu) 2 HBr] 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 OÁÁÁHÁÁÁO hydrogen bonds of 2.418(12) and 2.411(13) Å, respectively. The bromide anions interact electrostatically with the one pos- itively charged nitrogen atom of each cation. The presence of short OHO hydrogen bonds is confirmed by the broad absorption in the 1500–400 cm À1 region, with the center of gravity, m H , at ca. 900 cm À1 , in the solid-state FTIR spectrum. In the structure of [(QNBu) 2 HBr] 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–HÁÁÁO 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. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Understanding the structure and dynamics of hydrogen bonded species is crucial for prediction and modeling of biological systems and in material science [1]. The short and strong hydrogen bond has been the subject of renewed attention over the past several decades, because it may be viewed as a model of a purported tran- sition state or an intermediate in enzymatic processes involving proton transfer [2]. Very short OHO hydrogen bonds (2.39– 2.50 Å) have been found in several acid salts of carboxylic acids [3–5], 2:1 complexes of heterocyclic N-oxides [6–9], betaines [10–30] and amino acids [31–33]. Olovsson et al. [34] and Gilli et al. [35–38] divided the complexes with very short hydrogen bond into several groups: symmetric or centered, with the proton localized precisely at the center of the OÁÁÁO distance; statically disordered with the proton closer to one or the other oxygen atom in different domains in the crystal; dynamically disordered, where the proton jumps between two positions in the same hydrogen bonds; and asymmetric with the proton closer to one of the oxygen atoms. Recently, we have described the synthesis and structure of bis- (quinuclidinium acetate) hydrochloride, (QNB) 2 HCl [29] and bis(2- quinuclidinium-propionate) hydrobromide, (QNPr) 2 HBr [30]. This paper is a continuation of our structural studies of the hydrogen- bonded complexes of betaines and it is focused on di-[bis(2-qui- nuclidinium-butyrate) hydrobromide], [(QNBu) 2 HBr] 2 , (Scheme 1), its crystal and optimized molecular structures, and vibrational data. The structure of 2-(quinuclidinium)-butyric acid bromide hydrate has been recently described [39]. The ethyl group is at- tached to the chiral center in the N-CH(C 2 H 5 )COOH substituent; [a] D = À2.6 o in water. It is a homologue of quinuclidine betaine hydrochloride [40] and 2-(quinuclidinium)-propionate acid bro- mide hydrate [41]. Quinuclidine (1-azabicyclo[2.2.2]octane) is a globular molecule and it is a strong base (pK a = 11.15) [42]. 2. Experimental 2.1. Synthesis The equimolar amounts of 2-(quinuclidinium)-butyrate inner salt and 2-(quinuclidinium)-butyric acid bromide hydrate [39] were dissolved in methanol and mixed together. The crystals of di-[bis(quinuclidinium-butyrate) hydrobromide], [(QNBu) 2 HBr] 2 , were filtered off and recrystalized from a mixture of acetoni- trile–methanol (1:1), m.p. 207–208 °C, [a] D = 0.0(16)° in water (Scheme 1). 2.2. Measurements The crystal structure of [(QNBu) 2 HBr] 2 (1) was determined by X-ray diffraction, measured with a KUMA KM-4 CCD diffractometer 0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2010.09.021 ⇑ Corresponding author. Tel.: +48 61 8291216; fax: +48 61 8291505. E-mail address: degasz@amu.edu.pl (Z. Dega-Szafran). Journal of Molecular Structure 984 (2010) 157–163 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc