Halogen Bonding DOI: 10.1002/ange.201108404 A Halogen-Bonding Catenane for Anion Recognition and Sensing** Antonio Caballero, Fabiola Zapata, Nicholas G. White, PauloJ. Costa, Vítor FØlix, and Paul D. Beer* The field of anion supramolecular chemistry has expanded enormously during the past few decades, inspired in large part by the realization of the many fundamental roles negatively charged species play in a range of chemical, biological, medical and environmental processes. [1] Through the incor- poration of complementary electrostatic, hydrogen bonding, Lewis acid–base [2] and anion–p non-covalent interactions [3] into acyclic and macrocyclic molecular framework design, numerous efficient synthetic anion receptors and sensors have been developed. However, the challenge to mimic the degree of selectivity exhibited by biological anion-binding proteins still remains. Halogen bonding (XB) is the attractive non- covalent interaction between an electron-deficient, positively polarized halogen atom, commonly bromine or iodine, and a Lewis base. [4] Anions have been exploited extensively as XB acceptors in the solid state crystal engineering [5] of conduct- ing, magnetic and liquid-crystalline materials. [6] Given XB)s complementary analogy to hydrogen bonding in terms of stringent directionality and bond strength, [7] it is surprising that solution phase applications of XB [8] in molecular recognition processes such as protein–ligand [9] complexes, anion receptor chemistry, [10] and catalysis [11] are only now beginning to emerge. By using anion templation, we have constructed three- dimensional interpenetrated and interlocked molecular host systems that exhibit a high degree of selectivity towards the templating anion through primarily electrostatics and con- vergent hydrogen bonding. [12] In an effort to influence significantly the steric, electronic and lipophilic character- istics, and hence anion recognition properties, of the inter- locked binding pocket, we recently reported the first XB- bonding rotaxane containing an iodotriazolium axle which selectively recognizes iodide through the combination of halogen and hydrogen bonding interactions from respectively the axle and macrocyclic components of the interlocked host system. [13] Herein we describe the anion-templated synthesis of the first XB catenane which selectively recognizes chloride and bromide anions solely by halogen bonding, through cooperative action of two bromine halogen bond donor atoms. Furthermore we demonstrate the XB catenane)s ability to optically sense anions using fluorescence spectros- copy. The target acyclic precursor bromine-functionalized imi- dazolium compound 6 + was designed to incorporate comple- mentary supramolecular interactions to facilitate the assem- bly of an orthogonal 2 :1 host-to-guest stoichiometric complex around a halide anion template using XB (Figure 1). The acyclic precursor molecule contains a bromine halogen bond-donating imidazolium group covalently linked through naphthalene and hydroquinone motifs to vinyl functional groups, such that highly directional, linear coop- erative XB halide interactions would favor formation of the orthogonal assembly. In addition, p–p stacking interactions between the intercalated electron deficient bromoimidazo- lium motif and electron-rich hydroquinones are designed to further stabilize the XB-associated assembly prior to ring closing metathesis (RCM) double cyclization catenane syn- thesis. Importantly, the naphthalene spacer unit of precursor 6 + not only serves as a rigid linker, but also has the potential to act as a fluorescent reporter group for catenane host anion sensing. Precursor compound 6 + ·Br À was prepared in a stepwise procedure shown in Scheme 1. The reaction of 4-(2-(allylox- y)ethoxy)phenol (1) [14] and 2,7-bis(bromomethyl)naphtha- lene (2) in the presence of base produced the bromomethyl- naphthalene derivative 3 in 31 % yield. Alkylation of bromoimidazole 4 [15] with 3 under basic conditions gave 5 in near quantitative yield. The coupling of 5 and 3 afforded the desired bis-vinyl appended precursor 6 + ·Br À in a yield of 60 % Figure 1. Schematic representation of halogen bonding anion tem- plated assembly of an orthogonal 2:1 stoichometric complex. [*] Dr. A. Caballero, Dr. F. Zapata, N. G. White, Prof. P. D. Beer Chemistry Research Laboratory, Department of Chemistry University of Oxford, Mansfield Road, Oxford, OX1 3TA (UK) E-mail: paul.beer@chem.ox.ac.uk Dr. P.J. Costa, Prof. V. FØlix Departamento de Química, CICECO and Secżo Autónoma de CiÞncias da Safflde, Universidade de Aveiro, 3810-193 Aveiro (Portugal) [**] A.C. thanks the European Union for a Marie Curie Postdoctoral Fellowship. F.Z. thanks the Ministry of Education of Spain for a postdoctoral contract (Programa Nacional de Movilidad y Recursos Humanos del Plan Nacional I + D + I 2008-2011). N.G.W. thanks the Clarendon Fund and Trinity College for a studentship. P.J.C. thanks FCT for the postdoctoral grant SFRH/BPD/27082/2006. We express our gratitude to Diamond Light Source for the award of beamtime on I19 (MT1858). Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201108404. . Angewandte Zuschriften 1912 # 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. 2012, 124, 1912 –1916