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A Series of Amino Acid Functionalized Tripodal Hexaamide Anion Receptors: Ion‐Pair‐Assisted Capped‐Cleft Formation by a Pentafluorophenyl‐Functionalized Amide
Authors:Purnandhu Bose  Dr Iyyamperumal Ravikumar  Bidyut Akhuli  Prof Pradyut Ghosh
Institution:1. Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Kolkata 700 032 (India);2. Current address: Department of Chemistry and Biochemistry, The University of Texas at Austin, 1 University Station, A5300, Austin, TX, 78712‐0165 (USA)
Abstract:A new series of tris(2‐aminoethyl)amine (tren)‐based L ‐alanine amino acid backboned tripodal hexaamide receptors (L1–L5) with various attached moieties based on electron‐withdrawing fluoro groups and lipophilicity have been synthesized and characterized. Detailed binding studies of L1–L5 with different anions, such as halides (F?, Cl?, Br?, and I?) and oxyanions (AcO?, BzO? (Bz=benzoyl), NO3?, H2PO4?, and HSO4?), have been carried out by isothermal titration calorimetric (ITC) experiments in acetonitrile/dimethylsulfoxide (99.5:0.5 v/v) at 298 K. ITC titration experiments have clearly shown that receptors L1–L4 invariably form 1:1 complexes with Cl?, AcO?, BzO?, and HSO4?, whereas L5 forms a 1:1 complex only with AcO?. In the case of Br?, I?, and NO3?, no appreciable heat change is observed owing to weak interactions between these anions and receptors; this is further confirmed by 1H NMR spectroscopy. The ITC binding studies of F? and H2PO4? do not fit well for a 1:1 binding model. Furthermore, ITC binding studies also revealed slightly higher selectivity of this series of receptors towards AcO? over Cl?, BzO?, and HSO4?. Solid‐state structural evidence for the recognition of Cl? by this new category of receptor was confirmed by single‐crystal X‐ray structural analysis of the complex of tetrabutylammonium chloride (TBACl) and L1. Single‐crystal X‐ray diffraction clearly showed that the pentafluorophenyl‐functionalized amide receptor (L1) encapsulated Cl? in its cavity by hydrogen bonds from amides, and the cavity of L1 was capped with a TBA cation through hydrogen bonding and ion‐pair interactions to form a capped‐cleft orientation. To understand the role of the cationic counterpart in solution‐state Cl? binding processes with this series of receptors (L1–L4), a detailed Cl? binding study was carried out with three different tetraalkylammonium (Me4N+, Et4N+, and Bu4N+) salts of Cl?. The binding affinities of these receptors with different tetralkylammonium salts of Cl? gave binding constants with the TBA cation in the following order: butyl>ethyl>methyl. This study further supports the role of the TBA countercation in ion‐pair recognition by this series of receptors.
Keywords:host–  guest systems  ion pairs  receptors  thermodynamics  X‐ray diffraction
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