Crown ether receptors

We have recently demonstrated (Beer et al., 1995b,c) that it is possible electrochemically to detect simultaneously the presence of two different cations bound in the redox-active ferrocene bis-crown ether receptor [15] as shown in Fig. 10. 

Success in complexing laiger oiganic cations through encirclement or encapsulation by crown ether receptors is enhanced when benzo rings are lused on to the 3n-crown-n framework, as in 3-6. Three examples are worthy of brief consideration in the context of this review. 

Closely related to the synthetic work reported in the previous section is the incorporation (131) of a 2,5-anhydro-3,4Hdi-0-methyl-D-mannitol residue (Figure 15) into the 18-crown-6 derivative d-91. Other derivatives of D-mannitol that have been built into crown ether receptors include l,4 3,6-dianhydro-D-maiuiitol (132), l,3 4,6-di-0-methylene-D-marmitol (13 134), and 1,3 4,6-di-O-benzylidene-D-mannitol (134). Examples of chiral crown compounds containing these residues include dd-92, dd-93, d-94, and d-95. Although not derived from carbohydrates—but rather (135) from the terpene, (-t-)-pulegone—... 

Such systems can undergo ion recognition when made up of a ruthenium or rhenium polypyridyl lumophore and a crown ether receptor for metal ions, similar to the ICT examples given above. There are many examples in the hterature, but one will have to sufhce in this context. Compound (3.85) is interesting because it shows significant luminescence enhancement with Pb. ... 

Figure 16.25 Semiconductor nanoparticle-based fluorescent sensors (a) Forster resonant energy transfer (FRET) between two nanoparticles induced by analyte, (b) crown ether receptor for potassium ions, and (c) operation principle of maltose fluorescent sensor. (Adapted from Chen et at. [144] and Medintz et at. [146])...

The macrocyclic revolution in metal ion coordination chemistry [115] soon had repercussions on the design of fluorescent sensors for alkali cations. Before the advent of PET sensors, the first examinations of crown ether receptors with adjacent n-eleetron systems such as naphtho- and benzo crown ethers (50) [116] and (51) [117, 118] showed small but significant alkali cation induced fluore-... 

A novel multisite crown ether receptor based on a P3N3 heterocycle has been synthesized by Mitjaville et al. (Figure 8) [18]. This class of receptors will be of interest for multiple metal ion binding. 

Inhibition is observed when excess of KBF4 is added the hydrogen transfer becomes intermolecular and follows second-order kinetics with identical rate constants (kj = 1.2 X 10 /M/sec) for hydrogen transfers between l,l-(52) and (50) and l,l-(52) and (51). This results from the exclusion of the substrates from the crown ether-receptor sites by the more strongly complexed ion. When inhibition is caused by a... 

Bell and Liu prepared newly designed heterocyclic receptors via the Friedlander condensation and used them in the complexation of urea. For example, urea complex 376 (88JA3673) is at least 10 times more stable than the best crown ether receptor that also solubilizes solid urea in chloroform (Scheme 83) (90AGE931). 

Hosts 23, 24 and 28 are ditopic and contain two electron rich receptor sites they are therefore suitable hosts for bis-cations or two mono-cations. The development of synthetic receptors for anionic species has been rather slower than the development of synthetic receptors for cations and cryptand species of this type are rather rare, although some interesting examples have been reported. For ditopic hosts, electron deficient receptor sites can be incorporated to give the new cryptand species shown diagrammatically in Scheme 3 in which the ellipses represent electron rich (for example diaza-crown ether) receptor sites and the rectangles represent electron deficient receptor sites. This scheme also indicates possible guest species below each class of ditopic host and clearly the mixed system 29 is of particular interest because it can potentially complex both components of a guest salt. 

The electron deficient receptors, indicated by rectangles in the new classes of ditopic host 29 and 30, are conveniently based upon metalloporphyrins 31 because their coordination chemistry has been extensively investigated [30] and they are readily available synthetically with side chain functionality that permits their incorporation into macropolycyclic systems of types 29 and 30. Whereas a suitable choice of ring size for the aza-crown ether receptor ensures guest binding only in the inwards direction, as indicated by the arrows in Scheme 3, a metalloporphyrin can... 

V. CHIRAL CONJUGATED POLYMERS CONTAINING CROWN ETHER RECEPTORS [28b]... 

Kbnig prepared guanidinium-crown ether receptors (24) for the recognition of zwitterionic substrates. They investigated the effects of the spacer between the guanidinium and the crown ether units on the association of the guest. The 1,4-disubstituted benzyl spacer was optimal for binding peptides that contain carboxylates. 

The stimuli-responsive supramolecular dendronized polymers produced by Stoddart and coworkers used a so-called hook-and-eye approach, harnessing the interaction between ammonium ions and crown ether receptor units (Scheme 10). The assembly was constructed by the addition of Frdchet-type benzyl ether dendrons with benzyl ammonium at the focal point (first to third generation, only G2 shown, 44, Figure 13) to either polystyrene (45, M 85kDa)- or polyacetylene (46, M 13kDa)-based polymers containing DB-24-C8 pendent eye groups. 

Yamaguchi et al. conducted a more detailed study of the correlation between ion recognition and the LCSTs of PNIPAM-bearing crown ethers. They focused on the quantities of the complex of the crown ether receptor and ions in order to quantify the influence of the addition of ions. They used KCl, SrCl2 and BaClj as additives and PNIPAM-bearing benzo[18]-crown-6-ether 5. 

Another model was developed, by the group of J. M. Lehn (278) of Strasbourg. They prepared a complex between a pyridinium substrate and a chiral crown ether receptor molecule having four dihydronicotinamide derivatives as the side chain (Fig. 7.1). 

The discotic 2,3,6,7,10,1 l-hexa-(l,4,7-trioxaocetyltriphenylene) amphiphilic molecule [13] in Fig. 4 (cf. also Section n.E and this volume. Chapter 2) can form single polymeric columnar assemblies stabilized by supramolecular bonds perpendicular to the monomeric surface in nematic solutions containing up to 50% solvent. The tapered 12-ABG-B15C5 compound [14] in Fig. 5, complexed with 0.4 M triflate salt per mole of the crown ether receptor B15C5, self-assem-bles into an hexagonal columnar (( ), ) mesophase revealed by X-ray and optical microscopy in the undiluted system. The column cross-section is based on disks... 

Barboiu and co-workers reported heteroditopic ureido crown ether receptor 28. The coordination modes differed, depending on the anion, as illustrated by two complexes, one with NaCl and the other with NaNOa. In the NaNOa complex, the crown ether portion, containing the sodium ion, lies adjacent to a neighboring urea podand chain that binds the nitrate by two long H-bonds (N 0 = 3.063 and 3.073 A) (Fig. 5.20a). In the NaCl complex, the appended urea chains use four donor hydrogen atoms to hold the chloride ion, with N C1 distances ranging from 3.333 to 3.517 A (Fig. 5.20b) [66]. 

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