Diazacrown

Diazacrown compounds have been studied extensively by the groups of Sutherland and Stoddart . Particular emphasis has been placed on the inter-... 

Lehn s approach is slightly more complex than that illustrated above in that the diol is chloromethylated and then treated with cyanide. Hydrolysis then affords the diacid which may be carried through as shown. It should also be noted that once the bis-acyl halide is in hand, it may be treated directly with an open-chained amine to yield a lipophilic diazacrown, after reduction ... 

Calcium-selective electrodes have long been in use for the estimation of calcium concentrations - early applications included their use in complexometric titrations, especially of calcium in the presence of magnesium (42). Subsequently they have found use in a variety of systems, particularly for determining stability constants. Examples include determinations for ligands such as chloride, nitrate, acetate, and malonate (mal) (43), several diazacrown ethers (44,45), and methyl aldofuranosides (46). Other applications have included the estimation of Ca2+ levels in blood plasma (47) and in human hair (where the results compared satisfactorily with those from neutron activation analysis) (48). Ion-selective electrodes based on carboxylic polyether ionophores are mentioned in Section IV.B below. Though calcium-selective electrodes are convenient they are not particularly sensitive, and have slow response times. 

The easily available hexa-O-benzylsucrose (187) was used as a starting material for the preparation of macrocyclic receptors. Several derivatives of type 188 with various cavities and different number of nitrogen and oxygen atoms were prepared (Fig. 60).78 The azacrown macrocycles showed remarkable enantioselectivity towards a-phenylethylammonium cations. For example diazacrown derivative 188 (Y = O X = NBn n = 2) did... 

PET-19 (Figure 10.16) consists of a diazacrown ether with two pendant pyrene groups. As expected, cation binding results in a large change in the monomer/... 

SBFI has been designed for probing intracellular sodium ions and PBFI for potassium ions. In both compounds, the photophysical changes are likely to be due to the reduction of the electron-donating character of the nitrogen atoms of the diazacrown by the complexed cation. 

J. Bourson, J. Pouget, and B. Valeur, Ion-responsive fluorescent compounds. 4. Effect of cation binding on the photophysical properties of a coumarin linked to monoaza- and diazacrown ethers,... 

The title ligands are the monoanionic 6-10 and the dianionic 11 and 12. Doubly deprotonated diazacrowns have also featured in Ln " chemistry, as well as A,A -dianionic macrocycles. Exceptionally, an [N(H)aryl] ligand figured as a chelate, implicating the aryl 7i-electrons, as in 2. ° Structurally characterised complexes using these ligands included [Nd(6)3], Li[Ho(7)4] (the homoleptic compound was not accessible from H0CI3 I Li N-... 

Scheme 25 Liquid crystals 48 devoid of peripheral chains Diazacrowns...

Another significant deviation from known rules has been observed in the rather low affinity of K+ with the 1,10-diaza crown 18-C-6 with only AG=10 kJ/mol (in methanol). It has been shown that the free energy of binding AG in crown ether and ciyptand complexes usually is an additive function of number and electron donicity of the host donor atoms which are in contact with the metal ion.[17] Molecular mechanics calculations suggest the reduced affinity with the diazacrown to be due to the N-lone pairs in pseudoaxial position, pointing away from the metal ion (Figure 4). This has led to experiments with the N-methyl substituted crown here the N-alkyl substituents would clash which each other inside the macrocycle, therefore a pseudoequatorial lone pair orientation towards the cation is enforced, and the stability of the complex indeed returns to the normal scale with an increase to 29.5 kJ/mol.[ 18]... 

Akabori et al. employed an interesting tethering system by linking cinnamyl groups onto the amine of a diazacrown ether [22]. Since the... 

Alkali cations have also been involved in two interesting photophysical studies concerned with PET processes. In the first of these, sodium ion com-plexation to a diazacrown ether spacer between a PET donor-acceptor pair causes an acceleration of the electron transfer supporting a superexchange interaction [129]. The second case relies on sodium ion induced conformational changes in a calix[4]arene tetraester diagonally substituted with a PET donor-acceptor pair [130]. This results in a good fluorescent sensory action. 

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