Anion binding amide-based receptors

In early 1980s, chemists started to develop amide-based receptors for selective recognition of anions. In 1986, Pascal and co-workers reported a pioneering work about the synthesis and X-ray crystal structure of an amide-based cryptand 1 [12]. Although it was not certain that fluoride was contained within its cavity, and NMR studies suggested that 1 could bind to fluoride through H-bonding. 

As a seminal contribution to anion recognition, in 1997, Crabtree and co-workers reported the synthesis and anion-binding properties of isophthalamide-linked bidentate amide-based receptors 4a,b [21]. Crystal stmcture of 4a Br showed 1 1 complexation with Br (Fig. 5.2). In the stmcmre, the bromide was held with two H-bonds by the amide NH groups at N Br distances of 3.436 and 3.634 A, and N-H—Br angles of 116 and 172 . The isophthalamide spacer could be replaced with a 2,6-diamidopyridine unit to give 4b, which was selective for fluoride anion in CD2CI2 ( Ta = 2.4 x 10 M ) [22]. 

Jurczak and co-workers reported cyclic and acyclic amide-based receptors 9a-e, 10 and 11 [33, 34]. The crystal stmctures of 9a F and 9a-Cl indicated that the anion was coordinated by four H-bonds with N F distance ranging from 2.742 to 2.881 A and N C1 from 3.211 to 3.520 A, respectively (Figs. 5.6a and 5.7a). However, the fluoride ion was almost in the plane of the macrocycle (Fig. 5.6b), while the larger chloride hovered above the plane of the macrocycle (Fig. 5.7b). Solution-phase binding studies also proved that the smaller fluoride ion fit better to the macrocycle K = 830 M ) than chloride K = 65 M ). Base on these results, Kanbara et al. transformed amide 9a to thioamide 9f which was found to exhibit strong affinity toward anions [35]. Although acyclic receptors 10 and 11 have... 

Appropriately strapped calixarenes have also been shown to complex anions and cations simultaneously. Such a system is the ditopic, calixarene-based receptor 11 (Scheme 9.4) reported by Tumcharern and coworkers [51[. Receptor 11, in which the strap incorporates two different binding motifs (urea and amide), showed selectivity for the tetrahedral phenylphosphinate anion (PPhH02) over simple Y-shaped anions, such as acetate. After initial screening studies, quantitative analyses were carried out using NMR spectroscopic methods. Specifically, titrations were carried out in CD3CN using the TBA salts of three promising anions, namely acetate, phenylphosphinate, and diphenylphosphate. Standard analyses of the... 

The tren-based heteroditopic receptor 22, featuring a tripodal tetrahedral amide hydrogen-bond anion-recognition site in combination with benzo-15-crown-5 ether cation-binding moieties, has been found to cooperatively bind chloride, iodide, and perrhenate anions via co-bound crown ether-complexed sodium cations. It also can efficiently extract the radioactive sodium pertechnetate from simulated aqueous nuclear waste streams. The anion-binding affinity of 22 is considerably reduced in the absence of a co-bound cation <1999CC1253>... 

Another way of avoiding the problems of pH range and counterion competition is to use a neutral hydrogen-bonding receptor based on amide functionalities. Peptide groups from the protein backbone are, of course, well known to be involved in enzyme anion binding as discussed earlier. Amide involve-... 

Abstract This review article provides a broad overview to the area of anion coordination by synthetic organic receptors and includes examples of different functional groups used to bind anions. The first section examines neutral anion receptors containing amide-, sulfonamide-, urea- and thiourea-based receptors. Then aromatics such as pyrrole, car-bazole and indole are discussed before concluding the discussion of neutral systems with examples of hydroxy OH donors. A brief overview of charged systems is also provided. 

Prohens and co-workers have synthesized compounds 8a and 8b, simple squaramido-based receptors and investigated their ability to coordinate car-boxylate anion in competitive solvents [16]. The amide NH groups of the squaramide form a more open cleft (similar to ureas) than the isophthala-mides. Receptors 8a and 8b therefore adopt a more suitable geometry for the coordination of bidentate anions, such as carboxylate anions, through two approximately linear hydrogen bonds. Proton NMR titration experiments revealed association constants of 217 M and 1980 for the binding of acetate by 8a and 8b respectively in DMSO-de at 295 K. 

Another class of mixed-metal anion receptors has been investigated which possess redox reporter groups based on two different metal complexes. This enables the quahtative comparison of their comparative anion-sensing abih-ties. Macrocycles 35 and 36 combine the Ru (bpy)3 moiety with a bridging ferrocene or cobaltocenium imit [29]. Electrochemical experiments in acetonitrile solution revealed that the Ru VRu redox potential was insensitive to anion binding, whereas the ferrocene/ferrocenium (in 35) and cobal-tocene/cobaltocenium (in 36) redox couples were shifted cathodically (by 60 mV and 110 mV respectively with chloride). However, the first reduction of Ru°(bpy)3, a Hgand-centred process based on the amide substituted bipyridyl, was also found to imdergo an anion induced cathodic shift (40 mV and 90 mV with chloride for 35 and 36, respectively). 

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