Abstract
A 4-nitro-L-phenylalanine scaffold was used to construct effective ion pair receptors capable of binding anions in an enhanced manner with the assistance of alkali metal cations. A benzocrown ether was linked to a receptor platform via the amide function so as to support the squaramide function in anion binding and to allow all three NHs to act simultaneously. The binding properties of the receptors were determined using UV-vis, 1H NMR, 2D NMR, and DOSY spectroscopy in MeCN and in the solid state by X-ray measurements. Ion pair receptor 2 was found to interact with the most strongly with salts, and the removal of its key structural elements was shown to hinder the receptor action. The amide proton was recognized to switch from having involvement in an intramolecular hydrogen bond to interacting with anions upon complexation. Apart from carboxylates, which promote deprotonation, and other monovalent salts creating 1:1 complexes with the receptor, more complex equilibria were established upon the complexation of 2 with sulfates. Receptor 2 was shown to be capable of the extraction of ion pairs from the aqueous to organic phase and of the cation-enhanced transport chloride and sulfate anions across a bulk chloroform membrane. These features may open the door for its use in regulating ion concertation under interfacial conditions and acting as a potential drug to treat channelopathies.
Highlights
There has been significant progress in the design of molecular anion receptors characterized by high selectivity and efficiency in recent years
Ion pair receptors 1 and 2 as well as monotopic anion receptor 3 were synthesized in four steps, starting from commercially available Boc-4-nitro-L-phenylalanine, as outlined in Briefly, the DCC-promoted amide bond formation allowed benzocrown ethers or phenyl units to be introduced to the amino acid scaffold
Reference receptor 4 was synthesized from cinnamic acid through the heterogenous hydrogenation of the double bond followed by the nitration of the phenyl group, which resulted in a mixture of orto- and para- isomers of 3-(nitrophenyl) propionic acid with a ratio of 25:75
Summary
There has been significant progress in the design of molecular anion receptors characterized by high selectivity and efficiency in recent years. The use of anion binding domains tethered to appropriate molecular platforms has allowed a number of molecular receptors to be obtained, with applications in medicine, environmental remediation, and many industrial processes [1,2,3,4]. Choosing an appropriate molecular platform is crucial in the design of molecular receptors, and it has repeatedly been shown that amino acids are convenient candidates for such a purpose. The presence of both the amino and carboxylate groups in their structure allows the binding domains to be introduced in a relatively simple way [28].
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