Cryptands binding properties

A number of bridged crown ethers have been prepared. Although the Simmons-Park in-out bicyclic amines (see Sect. 1.3.3) are the prototype, Lehn s cryptands (see Chap. 8) are probably better known. Intermediates between the cryptands (which Pedersen referred to as lanterns ) and the simple monoazacrowns are monoazacrowns bridged by a single hydrocarbon strand. Pedersen reports the synthesis of such a structure (see 7, below) which he referred to as a clam compound for the obvious reason . Although Pedersen appears not to have explored the binding properties of his clam in any detail, he did attempt to complex Na and Cs ions. A 0.0001 molar solution of the clam compound is prepared in ethanol. The metal ions Na and Cs are added to the clam-ethanol solutions as salts. Ultraviolet spectra of these solutions indicate that a small amount of the Na is complexed by the clam compound but none of the Cs . 

X 10 forNa+,2.8 X 10 for, and 1.3 x 10 for Li These constants were of a magnitude that persuaded the authors to liken the binding properties of these calixar-enes to cryptands and spherands. 

The selective cation binding properties ol crown ethers and cryptands have obvious commercial applications in the separation of metal ions and these have recently been reviewed (B-78MI52103.79MI52102, B-81MI52103). Many liquid-liquid extraction systems have been developed for alkali and alkaline earth metal separations. Since the hardness of the counterion is inversely proportional to the extraction coefficient, large, soft anions, such as picrate, are usually used. 

Hossain, M. A. Llinares, J. M. Powell, D. Bowman-James, K. Structure and binding properties of the tiny octaaza cryptand, Supramol. Chem. 2002, 2, 143-151. 

Recently, efforts have been initiated to examine intrinsic host-guest chemistry in the solvent-free environment of a mass spectrometer. Of present interest are preliminary reports on perfluorinated hosts, crown ethers and cryptands, which are physiologically compatible and may possess important biological properties, such as the ability to carry oxygen and transport ions through membranes. Specifically, the oxygen-binding properties of... 

The anion-binding carcerand 11 was described by the Amouri group [31]. This complex contains a tetrafluoroborate anion coordinated to two cobalt(II) ions. Each cobalt ion adopts a square-pyramidal geometry. Four benzimidazole arms of the bridging ligands fill the equatorial positions, and solvent molecules (acetonitrile) coordinate to the outside axial positions. Inside the complex the included tetrafluoroborate anions interacts with the cobalt ions whose inside axial positions are otherwise coordinatively unsaturated. No exchange of the anion was observed even at 60 °C. A detailed study of the anion-binding properties in the crystal state of similar metalla-macrotricyclic cryptands has been performed by Adarsh et al. [32],... 

By replacement of just one oxygen atom in 5 by a methylene group, affinity of the corresponding cryptand 15 to NH4+ decreases by a factor of ca. 100 (NH4+ c 15 log /Ca = 4.3). The macrobicycle 16 has almost completely lost the complexation ability and the selectivity of 5 (NH4+ c 16 log /Ca = 1.7). This dramatic effect results from the removal of one bridge of 5, i.e., from a decrease in cyclic order from the tricyclic to the bicyclic ring system, demonstrating the importance of the spherical macrotricyclic structure for the binding properties of 5. 

Polyammonium cryptands and tricyclic lactams probably represent the largest family of cryptand-type anion receptors known today and a detailed overview of the properties of these compounds lies outside the scope of this review. The interested reader is therefore referred to two recent reviews [11, 53]. Compound 35b in the following section is the only polyammonium cryptand besides 5 whose properties are described in this review in more detail to illustrate the effect of the incorporation of additional hydrogen bond acceptor sites on binding properties. 

Recently, Tobe and co-workers have designed cryptand-like macrocycles based on homobenzylic tripodal thiourea and compared their anion-binding properties to a series of acyclic tripod-type receptors [46]. 

As a new series of "open-chain cryptands", "multi-armed cyclams" were designed and employed in the cation transport experiments. Parent polyamine macrocycle (cyclam) is well known to show somewhat different cation binding properties from those of macrocyclic polyethers. It effectively bind ammoniiim cations via strong hydrogen bonding as well as transition metal cations via coordination interaction. Hence, new "multi-armed cyclams" are expected to exhibit unique cation transport abilities. Their cation transport results are summarized in Table 3. 

Bucher C, Zimmerman RS, Lynch V, Sessler JL (2001) First cryptand-Uke caUxpyrrole synthesis, X-ray structure, and anion binding properties of a bicyclic[3,3,3]nonapyrrole. J Am Chem Soc 123 9716-9717... 

J. P. Konopelski, F. Kotzyba-Hibert, J.-M. Lehn, J.-P. Desvergne, F. Fages, A. Castellan, and H. Bouas-Laurent, Synthesis, cation binding, and photophysical properties of macrobicyclic an-thraceno-cryptands,./. Chem. Soc., Chem. Commun. 433 (1985). 

Although some scattered examples of binding of alkali cations (AC) were known (see [2.13,2.14]) and earlier observations had suggested that polyethers interact with them [2.15], the coordination chemistry of alkali cations developed only in the last 30 years with the discovery of several types of more or less powerful and selective cyclic or acyclic ligands. Three main classes may be distinguished 1) natural macrocycles displaying antibiotic properties such as valinomycin or the enniatins [1.21-1.23] 2) synthetic macrocyclic polyethers, the crown ethers, and their numerous derivatives [1.24,1.25, 2.16, A.l, A.13, A.21], followed by the spherands [2.9, 2.10] 3) synthetic macropolycyclic ligands, the cryptands [1.26, 1.27, 2.17, A.l, A.13], followed by other types such as the cryptospherands [2.9, 2.10]. 

---