Ethers, Cryptates, and Sepulchrates

Crown Ethers, Cryptates, and Sepulchrates.— The results of an ultrasonic absorption study of the complexation of bivalent metal ions with 18-crown-6 and 15-crown-5 are consistent with a two-step mechanism, 

1 Metal Ions in Biological Systems , ed. H. Siegel, Vol. 7, Marcel Dekker, New York, 1978. Metal Ions in Biological Systems , ed. H. Siegel, Vol. 8, Marcel Dekker, New York, 1979. 3 The Porphyrins , ed. D. Dolphin, Vols. 1—7, Academic Press, New York, 1978. 

The complexing behaviour of crown ethers towards acetonitrile has been used to enhance the rate of hydrolysis of /ra/ts-[Co(en)2(N02)(MeCN)] +. Weak pre-association of the crown ether with metal-bound acetonitrile is followed by a rate-determining interchange. The overall second-order rate constant is 8.7 x 10 M s at 20 °C for 18-crown-6 and about half that value for 15-crown-5. 

Kinetic behaviour of the alkali-metal ions with cryptands (1) has been investigated by pH jump in methanol. 

Dissociation rates were obtained at high acidity where kn is fast and were corrected for ionic strength using a Debye-Huckel equation. The results show that the cryptate selectivity results mainly from kh and that the transition state for the reaction has little interaction between the metal and the cryptand to differentiate between metals. Rates kt increase with increasing cavity size. The thermodynamics of cryptand formation in water and methanol have also been used to calculate the free energy and enthalpy of transfer of the free ions between the two media. 

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