FULL PAPER DOI: 10.1002/ejoc.200900814 Resorcinarene Podand with Amine-Functionalized Side Arms – Synthesis, Structure, and Binding Properties of a Neutral Anion Receptor Kirsi Salorinne, [a] Dominik P. Weimann, [b] Christoph A. Schalley, [b] and Maija Nissinen* [a] Keywords: Supramolecular chemistry / Host-guest systems / Anions / Receptors / Resorcinarene The synthesis and structure of a neutral resorcinarene host bearing four amine-functionalized side arms is described. The anion binding properties were investigated in solution by 1 H NMR spectroscopic titration and diffusion experiments and in the gas phase by mass spectrometric studies. It was observed that in solution 1:2 (host/guest) complexes were Introduction The development and synthesis of anion receptors has gained a considerable amount of attention over the last dec- ades. [1] The interest in anions arises from their importance in biological, environmental and chemical processes, which has induced a number of papers dealing with anion sensing, transport, and extraction as well as the use of anions in organic reactions. [2] The varying geometry and larger radii of anions compared to the most common cations greatly influences the binding, which again set the demands on the design of the receptor molecule in terms of host-guest com- plementarity and selectivity. [3] In addition, the solvent has a more pronounced role in anion binding in terms of sol- vation and desolvation effects, which are namely attributed to the protic, polar, or hydrogen-bonding nature of thesol- vent, and furthermore, the basicity of certain anions may vary drastically depending on the solvent used. [1,3] Calixarenes have been extensively utilized in the design of anion receptors. [1c] The calixarene core provides a preor- ganized three-dimensional structure, which is easily modi- fied by incorporation of anion binding ligands in the form of bridges or pendant side chains at the upper or lower rim of the calixarene framework. [1c] In neutral anion receptors, anion recognition is mainly based on hydrogen bonding in- teractions, which are generally provided in the binding site through amine, amide, urea, or thiourea moieties. [1c,4] Al- though hydrogen bonding interactions are not as strong as [a] Nanoscience Center, Department of Chemistry, University of Jyväskylä, P. O. Box 35, 40014 JYU, Finland Fax: +358-14-260-4756 E-mail: maija.nissinen@jyu.fi [b] Institut für Chemie und Biochemie, Freie Universität Berlin, Takustrasse 3, 14195 Berlin, Germany Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejoc.200900814. Eur. J. Org. Chem. 2009, 6151–6159 © 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 6151 formed between the resorcinarene host and the basic fluoride and acetate anions, whereas in the gas phase 1:1 complexes with other anions (Cl – , HCOO – , NO 3 – , and BF 4 – ) were de- tected additionally. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) electrostatic interactions, they are directional, which con- tributes to the recognition and selectivity of certain anion geometries. Our approach utilizes tetramethoxyresorcinarene [5] as the platform, which has a well-defined macrocyclic structure yet enough freedom to reorganize in order to best accom- modate the guest molecule. Thus, four amine-functionalized side arms were attached on the upper rim of the resorcinar- ene bowl to create a neutral tetrapodal structure, in which the resorcinarene core adopts a boat conformation. Two binding sites are formed between the neighboring out- stretched side arms at both ends of the resorcinarene cavity, the amine groups offering hydrogen bonding interactions with the anion (Scheme 1). The anion binding properties were investigated in solution and in the gas phase. In solu- tion, binding was observed only between the more basic F – Scheme 1. A schematic presentation of the suggested anion binding mode for the tetrapodal resorcinarene receptor.