Abstract
A series of ditopic ion pair receptors equipped with 4-nitrophenylurea and 1-aza-18-crown-6-ether linked by ortho-(1), meta-(2), and para-(3) substituted benzoic acid were readily synthesized in three steps from commercially available materials. The binding properties of these regioisomeric receptors were determined using UV-vis and 1H NMR spectroscopy in MeCN and in the solid state by single-crystal X-ray diffraction crystallography. The solution studies revealed that, apart from carboxylates, all the anions tested formed stronger complexes in the presence of sodium cations. Receptors 2 and 3 were found to interact with ion pairs with remarkably higher affinity than ortho-substituted 1. 1H NMR titration experiments showed that both urea NH protons interacted with anions with comparable strength in the case of receptors 2 and 3, but only one of the NHs was effective in anion binding in the case of receptor 1. X-ray analysis of the crystal structure of receptor 1 and 1·NaPF6 complex showed that binding was hampered due to the formation of an intramolecular hydrogen bond. Analysis of the crystal structures of 2·NaBr and 3·NaBr complexes revealed that proper mutual orientation of binding domains was responsible for the improved binding of the sodium salts.
Highlights
IntroductionThe design and synthesis of heteroditopic receptors capable of simultaneously binding both counterions of an ion pair has recently emerged as an important area of supramolecular chemistry [1,2,3].The proper embedding of distinct cation and anion recognition motifs in a single receptor entity might allow for the cooperative binding of ion pairs due to electrostatic attraction and/or allosteric effects [4,5,6,7,8,9].In contrast, homotopic receptors possess a recognition motif exclusively for a cation or anion, they must compete with the counterion for binding of the corresponding ion [10]
Receptors 1–3 were designed so as to reinforce the anion binding by linking the crown ether unit
We have presented the synthesis of three regioisomeric urea-based heteroditopic receptors 1–3 possessing a crown ether unit and studied their anion and ion pair recognition characteristics in solution and in the solid state
Summary
The design and synthesis of heteroditopic receptors capable of simultaneously binding both counterions of an ion pair has recently emerged as an important area of supramolecular chemistry [1,2,3].The proper embedding of distinct cation and anion recognition motifs in a single receptor entity might allow for the cooperative binding of ion pairs due to electrostatic attraction and/or allosteric effects [4,5,6,7,8,9].In contrast, homotopic receptors possess a recognition motif exclusively for a cation or anion, they must compete with the counterion for binding of the corresponding ion [10]. The binding of a whole ion pair by ditopic receptor improves the salt’s lipophilicity, facilitating its solubilization, extraction, and membrane transport [11,12,13,14,15,16,17,18,19]. For these reasons, such systems are extremely important in the context of biology, environmental protection, and medicine [20,21,22]. The latter feature can be used for Molecules 2019, 24, 2990; doi:10.3390/molecules24162990 www.mdpi.com/journal/molecules
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