Job/Unit: I20184 /KAP1 Date: 02-05-12 16:54:45 Pages: 6 FULL PAPER DOI: 10.1002/ejic.201200184 Weak Intermolecular Anion–π Interactions in Pentafluorobenzyl-Substituted Ammonium Betaines Michael Giese, [a] Markus Albrecht,* [a] Katharina Wiemer, [a] Grzegorz Kubik, [a] Arto Valkonen, [b] and Kari Rissanen [b] Keywords: Anion–π interactions / Betaines / X-ray structures / Supramolecular chemistry / Noncovalent interactions A series of ammonium–carboxylate and ammonium–sulfon- ate betaines was synthesized and studied by single-crystal X-ray diffraction analysis to investigate the weak intermo- lecular interactions as well as the intramolecular interactions in the solid state. None of the expected intramolecular anion– π interactions could be observed, probably because of the Introduction Supramolecular chemistry is commonly defined as the chemistry of the noncovalent bond. [1] Noncovalent interac- tions can vary dramatically in their strength from strong electrostatic to weak dispersive attraction. They can even be repulsive. Because of their biological and chemical rele- vance, noncovalent interactions involving aromatic systems, such as π–π stacking or cation–π interactions, play a crucial role. [2] Recently, theoretical as well as crystallographic stud- ies showed that the interaction between anions and elec- tron-deficient arenes is attractive. [3] This led to intense stud- ies in the field of anion recognition aimed at using anion– π interactions for the development of superior anion recep- tors. [4] While the existence of anion–π interactions in solid phase is broadly accepted, the relevance of anion–π interac- tions in solution [4] or in the gas phase [5] remains an open question. We started our work on anion–π interactions in 2008. [6] First results showed that the position of an anion above the electron-deficient pentafluorophenyl unit in phosphonium and ammonium salts is flexible and can be controlled by directing substituents. [7] Moreover, we were able to prove that anion–π interactions depend on the fluorination degree of the phenyl group. When the number of fluorine atoms at the arene is successively reduced, its electron density is enhanced. Thereby, the attractive anion–π interaction turns [a] Institut für Organische Chemie, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany E-mail: markus.albrecht@oc.rwth-aachen.de [b] Department of Chemistry, Nanoscience Center, University of Jyväskylä, P. O. Box 35, 40014 University of Jyväskylä, Finland E-mail: kari.t.rissanen@jyu.fi Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejic.201200184. Eur. J. Inorg. Chem. 0000, 0–0 © 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1 steric demands and the reduced nucleophilicity of the an- ionic part of the betaines. Nevertheless, a weak intermo- lecular anion–π interaction between the anionic part of the betaine and the pentafluorophenyl unit is present in the structure of 5a. into a repulsive force. [8] Additionally, the effect of the anion geometry on the interaction of pentafluorophenyl ammo- nium cations with halides, nitrate, tetrafluoroborate, and hexafluorophosphate were investigated. However, no de- pendence of the anion–π interaction on the geometry of the studied anions could be found in the solid state. [9] Attempts to synthesize and crystallize an ammonium hydroxide salt within this series, by substituting the para fluorine atom with a hydroxy group, led to the corresponding phenolate betaine 1 (Figure 1). [9] Figure 1. Crystal structure of tetrafluorophenolate betaine 1. The cocrystallized methanol molecules were omitted for clarity. C (black), H (white), F (green), N (blue), O (red). [9] This observation raises the following question: Could be- taine structures manifest intramolecular anion–π interac- tions in the solid state, or are other interactions, such as electrostatics, C–H···O hydrogen bonds, π–π stacking, or C– H···π interactions, overruling the weaker anion–π interac- tions? Results and Discussion To study intramolecular anion–π interactions of penta- fluorobenzyl ammonium betaines in the solid state, a series