engleski

Complexation of alkali cations by lower rim calix[4]arene tetra(O-[N-acetyl-R-phenylglycine methyl ester]) derivative

Calixarenes with carbonyl-containing substituents on phenolic groups are effective receptors for alkali and alkaline-earth cations. In calixarene derivatives that possess substituents with both, a hydrogen-bond acceptor (carbonyl group) and a hydrogen-bond donor, circular intramolecular hydrogen bonds are formed which have a strong influence on their ionophoric activity.1, 2 Complexation of alkali metal cations with 5, 11, 17, 23-tetra-tert-butyl-26, 28, 25, 27-tetrakis(O-methyl-D-a-phenylglycinecarbonylmethoxy)(L) was studied by means of spectrophotometric, conductometric and potentiometric titrations. The solvent effect on the binding ability of L was examined by using two solvents with different affinities for hydrogen bonding, viz. methanol and acetonitrile. The stability constants of metal complexes were determined in both solvents. The investigated calix[4]arene amino acid derivative was shown to be an efficient binder for the smaller Li+ and Na+ cations in acetonitrile, whereas the bigger Rb+ and Cs+ did not fit in the calix[4]arene hydrophilic cavity. The stability of the complexes in methanol was significantly lower which could be explained by different solvation of the cations and by competition between the cations and methanol molecules (via hydrogen bonds) for amide carbonyl oxygens. The influence of cation solvation on the complex stability was most pronounced in the case of Li+ (stability constant of LiL+ in acetonitrile was quite high, >106 mol-1 dm3, whereas the one in methanol was low, i.e. <10 mol-1 dm3).

Calix[4]arene ; Alkali cations ; Complexation ; Spectrophotometry ; Conductometry ; Potentiometry

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