Anion recognition through hydrogen bonding by adamantane-dipyrromethane receptors Marija Ales ˇkovic ´ a , Nikola Basaric ´ a , Kata Mlinaric ´-Majerski a, * , Kres ˇimir Molc ˇanov b , Biserka Kojic ´ -Prodic ´ b , Manoj K. Kesharwani c , Bishwajit Ganguly c, * a Department of Organic Chemistry and Biochemistry, RuCer Bos ˇkovic ´ Institute, Bijenic ˇka cesta 54, 10000 Zagreb, Croatia b Department of Physical Chemistry, RuCer Bos ˇkovic ´ Institute, Bijenic ˇka cesta 54, 10000 Zagreb, Croatia c Central Salt & Marine Chemicals Research Institute (CSIR), Bhavnagar-364 002, Gujarat, India article info Article history: Received 17 September 2009 Received in revised form 16 November 2009 Accepted 4 January 2010 Available online 11 January 2010 abstract Adamantane-dipyrromethane (AdD) receptors [di(pyrrole-2-yl)methyladamantane (1), 2,2-di(pyrrole-2- yl)adamantane (2), 1,3-bis[di(pyrrole-2-yl)methyl]adamantane (3), 2,2,6,6-tetra(pyrrole-2-yl)ada- mantane (4)] form complexes with F  , Cl  , Br  , AcO  , NO 3  , HSO 4  , and H 2 PO 4  . The association constants of the complexes were determined by 1 H NMR titrations, whereas the geometries of complexes 1$F  (2:1), 2$F  (2:1), 2$Cl  (2:1), 2$AcO  (2:1), and 4$F  (1:1) were determined by X-ray structural analysis. The most stable complexes are of 2:1 stoichiometry with F  and AcO  . The stability constants are in accordance with the anion basicity and the ability of AdD receptors to place the hydrogen bonding donor groups in a tetrahedral fashion around anions. The binding energies of the complexes between receptors 1–4 and F  anion are calculated using quantum chemical methods. The calculated results show that the solvent polarity is important for the complexation of fluoride ion with AdD receptors 1–4. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction In recent years, anion complexation has attracted attention and become an intensively studied area of supramolecular chemistry. 1 Considerable interest has been focused to produce a variety of new selective anion receptors 2 for different applications such as: fluo- rescent and chromogenic anion sensors, 3 ion selective electrodes, 4 phase-transfer catalysis, 5 or templates in supramolecular synthe- sis. 6 In biological systems, anions have important roles in signalling, and transport events and are becoming a subject of interest in medicine. These demands initiated the tremendous growth in the field of supramolecular chemistry of anions and there is a con- tinuing challenge to design new selective receptors for detection, transport or extraction of anionic species. One of the important interactions between receptor and anion is related to hydrogen bonding. Pyrrole derivatives can act as anion complexing agents due to their acidic NH protons, which can bind anions forming hydrogen bonds. Polypyrrolic macrocycles 7 (calixpyrroles, porphyrins) were the first examples of these receptors. Substitution of the pyrrole moiety with the additional hydrogen bond donating groups, such as amide 8 or guanidinium, 9 lead to the development of better hosts, potent in complexing an- ions with higher selectivity and affinity. On the other hand, dipyr- rolylquinoxalines are simple receptors bearing only two pyrrole units, which bind inorganic anions with appreciably high binding constants. 10 These findings motivated us to investigate whether the dipyrromethane moiety with two NH sites, can function as an anionic receptor. Dipyrromethanes are simple molecules that can be easily synthesized and purified, 11 and therefore, could be readily available for the anion binding essays. The representative examples of the anionic receptors containing rigid spacers are the urea and the thiourea derivatives of norbor- nene. 12 Recently we prepared a series of substituted dipyrro- methane derivatives bearing rigid adamantane subunits. 13 Incorporation of the rigid bulky adamantane unit was expected to decrease rotational mobility of the pyrroles and thus increase the stability of the complexes with anions. Furthermore, the lip- ophilicity of the adamantane could be a great advantage in the applicability of the dipyrromethanes as anion transfer agents in the extraction studies. We also reported that adamantane-dipyrro- methanes (AdD) receptors bind the following anions: F  , Cl  , Br  , HSO 4  and H 2 PO 4  . 14 In this paper we report further investigation of the anion binding ability of the AdD receptors including two additional oxo-anions with planar Y-shaped geometry, acetate and nitrate. * Corresponding authors. Tel.: þ385 1 4680 196; fax: þ385 1 4680 195. E-mail addresses: majerski@irb.hr (K. Mlinaric ´-Majerski), ganguly@csmcri.org (B. Ganguly). Contents lists available at ScienceDirect Tetrahedron journal homepage: www.elsevier.com/locate/tet 0040-4020/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2010.01.018 Tetrahedron 66 (2010) 1689–1698