Anions tetrahedral receptors

The selectivity advantage of the ditopic design of 1 in relation to its monotopic parent compound 3 was investigated using the dimensional probes 4-8 each containing two anionic functions at a fixed distance. Analysis of the UV-absoption band shifts experienced by these solvato-chromic probes on inclusion into the molecular cavity of the larger tetrahedral receptor site 3 yielded the 1 1 association constants in water. 

Development of ditopic receptors 58 for both an anion and a cation is a logical extension [19b]. It should be stressed that the recognition by cyclodextrins, calixarenes, hemicarcerands, and many other systems does not involve specific recognition sites, whereas tetrahedral recognition by 53 and 54, the adenine 56 selective binding, and multiple recognition like the one shown in formula 59 [20] require the presence of specific sites in receptor capable of directional interactions. 

Tetrahedral tetraazonium compounds 5a and 5b, developed by Schmidt-chen, bind carboxylates, such as formiate, acetate, and benzoate, among a variety of other anions [8]. Although these receptors do not display hydrogen bond interactions, they are endowed with well localized polycationic centers, which are independent on the acidity of the medium. A ditopic receptor build up of both a 5a and a 5b subunits bridged by a p-xylylene spacer showed selectivity towards the largest members of a series of dianionic probes [9],... 

Anslyn and coworkers [56,57] synthesized receptors containing Cu(II) binding sites with appended ammonium 69 and guanidinium 70 groups. These receptors showed high affinities for tetrahedral anions such as HPO " and... 

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... 

Fig. 24 Consecutive equilibria for protonation of the spherical cryptand 13 and inclusion of CP in the tetraprotonated receptor 13H44+. In the [I3H4—Cl]3+ complex, the anion is tetrahedrally coordinated by four highly polarised N-H fragments, as shown in the crystal structure. Unavailability of coordinates at the Cambridge Crystallographic Database prevented any redrawing of such a structure...

The reversible imine chemistry has been employed by Sessler and Katayev for the development of anion receptors for tetrahedral anions like S042- and HP042 [28, 29]. Thus, the dipyrrole 9 functionalized with aldehyde groups, and 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>... 

Hosseini have devoted a lot of effort to aza crown complexes with ATP 48, showing them to be true yet weak ATPase mimics [24], Schmidtchen reported tetrahedral zwitterions 49 to discriminate between spherical anions [25]. Reetz was able to show that ditopic receptor 50, which includes both Lewis-acidic and basic centers will complex a whole ion pair [26]. Incorporation of transition metals into anion receptors such as 51 renders neutral species positively charged and susceptible to spectroscopic and electrochemical manipulations [27]. 

A stereospecific receptor site for tetrahedral oxometallate anions has been realized in the form of octahedral transition metal cation complexes [10]. For example, the Cu (II) tris (ethylenediamine) SAMMS (Cu-EDA SAMMS) have demonstrated excellent affinity for anions such as chromate and arsenate, and have significant potential for the sequestration of pertechnetate. With the global concerns over arsenic in drinking water, these materials can truly save lives. 

Iron(II) has been used as a supramolecular template for the formation of a tris-imidazoUum receptor from Ugand 117 [79]. NMR studies and X-ray crystal structure determination were used to demonstrate the encapsulation of bromide in the cavity of the receptor, with the anion coordinated by six C-H fragments (Fig. 2). Spectrophotometric titrations in acetonitrile solution revealed that this receptor binds halides with selectivity for chloride > bromide > iodide, but has no affinity for dihydrogenphosphate or hydrogensulfate. Presumably the restricted size of the receptor cavity excludes the binding of these larger tetrahedral anions. The Hnear anions azide, cyanate and thiocyanate also produced a response in the UV/vis spectrum, and azide was found to bind preferentially to 117 in comparison to the non-symmetrical linear anions. 

By arranging two amide groups in a parallel arrangement in space, Cameron and Loeb developed a set of receptors that are selective for Y-shaped carboxylate anions over tetrahedral or pseudotetrahedral OxO-anions. Calix[4]arenes 15.16,and 17 were synthesized by reaction of the upper-rim calix[4]arene h/.s-amine with mono-, di-. or trichloroacetyl chloride. Receptor 16 proved to have the highest affinity for anionic guests. For example, receptor... 

Receptors 21 and 22. reported by Choi and Hamilton, show a particularly high affinity for iodide and 0x0-anions such as nitrate."" The receptor features a central hole of around 5 A diameter lined with three amide NH groups and six aryl CH groups. The receptors are skewed in a way that allows the three NH groups to form a complementary hydrogen-bonding site suitable for pseudo-tetrahedral anions (e.g., p-tosylate.) Proton NMR spectroscopic titration studies were carried out in the... 

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