18-crown hydrates

Calcium-binding proteins, 6, 564, 572, 596 intestinal, 6, 576 structure, 6, 573 Calcium carbonate calcium deposition as, 6, 597 Calcium complexes acetylacetone, 2, 372 amides, 2,164 amino acids, 3, 33 arsine oxides, 3, 9 biology, 6, 549 bipyridyl, 3, 13 crown ethers, 3, 39 dimethylphthalate, 3, 16 enzyme stabilization, 6, 549 hydrates, 3, 7 ionophores, 3, 66 malonic acid, 2, 444 peptides, 3, 33 phosphines, 3, 9 phthalocyanines, 2,863 porphyrins, 2, 820 proteins, 2, 770 pyridine oxide, 3,9 Schiff bases, 3, 29 urea, 3, 9... 

A possible explanation comes from X-ray analyses of the sulfonic acids [45]. All X-rayed crown ether crystals contained water and the sulfonic acid moiety was dissociated. Therefore in crystals of [45], macrocyclic ben-zenesulfonate anions and hydronium ions (sometimes hydrated) are present. The ions are bound to each other by hydrogen bonds. The size of the included water-hydronium ion cluster (varying by the number of solvating water molecules) depends on the ring size. In the 15-membered ring, HsO" was found, whereas in a 21-membered ring HsO and in the 27-membered ring were present. This means the sulfonic acid functions in [45] are... 

Bi-0 2.54(l)-2.68(2) A], In contrast, the nine-coordinate capped square antiprism geometry for bismuth in [Bi(N03)3(H20)3] (18-crown-6) does not involve the expected multidentate ether coordination to bismuth, but rather a hydrogen-bonded interaction of the crown ether with the hydrated bismuth center chelated by bidentate nitrate groups [Bi-0 2.38(2)—2.56(2) A] 32, implying that the hexado-... 

Some caution is required when comparing the association constants obtained from extraction experiments with those measured under anhydrous, homogeneous conditions. Iwachido et al. (1976, 1977) have shown that the extracted cation retains part of its aqueous solvation shell on complexation. In particular, the small univalent cations (Li+, Na+) and bivalent cations give high hydration numbers for their crown-ether complexes. Water molecules completing the co-ordination sphere of the cation have frequently been encountered in the solid state of crown-ether complexes (Bush and Truter, 1970, 1971). The effect of small amounts of water on the equilibria (1) has not been studied yet for crown ethers. However, it has been found that the presence... 

Abstract Starting with tetracyanodibenzo(l,4,7,10-tetrathia-(12-crown-4)) (1) and 4-nitro-l,2-dicyanobenzene (2), nitro-substituted dimeric phthalocyanine (3) was synthesized. In the second step, using hydrazine hydrate as a reductant, amine-substituted dimeric phthalocyanine (4) was synthesized from nitro-substituted dimeric phthalocyanine. Stractmres of all synthesized compounds were determined by elemental analyses, UV/vis, H-NMR and IR spectroscopy. 

With soft anions crown-ethers are more efficient than quaternary salts, the reverse being observed when less polarizable nucleophiles are used. This is explained by the different extent of complexation of crown-ethers which depends not only on the complexed cation, but also on the anionic counterpart. Swelling and hydration measurements of polymer-supported crown-ethers in toluene/aqueous KY showed that the content of water in the imbibed solvent increases with the loading. This leads to a progressive polarity increase within the polymers and to a better crown-ether complexing... 

The effect of the hydration radius of these cations is very important, and mobilities are sometimes very close or the same as for potassium and ammonium. For this reason, a complexing agent is added to the buffer. Several complexing agents such as a-hydroxyiso-butyric acid (HIBA), 18-crown-6, phthalic, malonic, tartaric, lactic, citric, oxalic, or glycolic acid may be used. 

It is probable that a primary reason for the lower stability of complexes formed between dicyclohexyl-18-crown-6 and cations larger than the optimum size (e.g. Cs+) is that these cations are too large to "fit into the ligand cavity. On the other hand, as cation size decreases from that affording maximum stability, the hydration energy of the cation becomes predominant and little or no reaction is found, as in the case of Ca2+. Very large cations such as di, tri, and tetramethylammo-nium and trimethyl sulfonium do not appear to form complexes wi h dicyclohexyl-18-crown-6 in aqueous solution (4). Also, tetramethyl-ammonium ion (radius = 3.47 A (30)) complexes less strongly (log K =... 

The dilactone 6 dihydrate isolated from hexane has a melting point of 65-66 °C. At 60° in vacuo the waters of hydration are lost, and the anhydrous compound melts at 83-84 °C. In the dihydrate the donor centers lie within 0,26 A of the mean plane. One water molecule coordinates directly with the host and lies 0.14(9) A below the plane, 0 -0(4) = 2.92 A (Fig. 33). The pyridino nitrogen, which usually functions as a donor atom, does not participate in the binding of the water. The second water molecule is hydrogen bonded to the first (0- 0 = 3.01 A), and does not interact significantly with the crown ether. 

The activity of polymer-supported crown ethers is a function of % RS as shown in Fig. 11 149). Rates for Br-I exchange reactions with catalysts 34, 35, and 41 decreased as % RS increased from 14-17% to 26-34%. Increased % RS increases the hydro-philitity of the catalysts, and the more hydrated active sites are less reactive. Less contribution of intraparticle diffusion to rate limitation was indicated by less particle size dependence of kohMi with the higher % RS catalysts149). 

The activity of polymer-supported crown ethers depends on solvent. As shown in Fig. 11, rates for Br-I exchange reactions with catalysts 34 and 41 increased with a change in solvent from toluene to chlorobenzene. Since the reaction with catalyst 34 is limited substantially by intrinsic reactivity (Fig. 10), the rate increase must be due to an increase in intrinsic reactivity. The reaction with catalyst 41 is limited by both intrinsic reactivity and intraparticle diffusion (Fig. 10), and the rate increase from toluene to chlorobenzene corresponds with increases in both parameters. Solvent effects on rates with polymer-supported phase transfer catalysts differ from those with soluble phase transfer catalysts60. With the soluble catalysts rates increase (for a limited number of reactions) with decreased polarity of solvent60), while with the polymeric catalysts rates increase with increased polarity of solvent74). Solvents swell polymer-supported catalysts and influence the microenvironment of active sites as well as intraparticle diffusion. The microenvironment, especially hydration... 

The transport velocity of Li+ is faster than that of Na+ and K+ due to the size of the cation. The data are consistent with a hopping transport mechanism of the cations accompanied by a non-specific co-transport of the anions. The transport rates for N03 > Cl- > C104 are related to the adjacent hydrate shell and not yet fully understood. Anyway, a path in the center of the supramolecular tubes, where the crown ethers assemble, must exist and allow for the co-transport of the anions. By forming the membranes in the pores of track-etched membranes, the transport rates could be improved by an order of magnitude due to the orientation of the channels perpendicular to the membrane surface. 

Albert, A., Mootz, D., Formation and crystal structures of the hydrates of 18-crown-6. Zeit. Naturforsh. Tell. B1997, 615-619. 

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