Log K, ΔH and ΔS values for the 1 : 1 reactions at 25°C in methanol solution of Na+, K+ and Cs+ with 2-methoxy-1,3-xylyl-15-crown-4 (I), 2-methoxy-1,3-xylyl-18-crown-5 (II), 2-hydroxy-1,3-xylyl-15-crown-4 (III), and 2-hydroxy-1,3-xylyl-18-crown-5 (IV) were determined by a calorimetric titration procedure. Log K values for the systems with Na+ and K+ were also determined by potentiometry with ion-selective electrodes. For a given cation the stability constants for the complexes with the 18-membered cycles II and IV are higher than those for the complexes with the 15-membered cycles I and III. For the ligand having the same ring size the methoxy-crown complex is always more stable than the hydroxy-crown one. The macrocyclic effect shown by the two 18-membered cycles is the result of favourable entropic factors. © 1987.
Complexation of Some Macrocyclic Polyethers Containing Convergent Methoxyaryl or Phenolic Groups with Alkali Cations in Methanol.
ZANONATO, PIER LUIGI;DI BERNARDO, PLINIO;
1987
Abstract
Log K, ΔH and ΔS values for the 1 : 1 reactions at 25°C in methanol solution of Na+, K+ and Cs+ with 2-methoxy-1,3-xylyl-15-crown-4 (I), 2-methoxy-1,3-xylyl-18-crown-5 (II), 2-hydroxy-1,3-xylyl-15-crown-4 (III), and 2-hydroxy-1,3-xylyl-18-crown-5 (IV) were determined by a calorimetric titration procedure. Log K values for the systems with Na+ and K+ were also determined by potentiometry with ion-selective electrodes. For a given cation the stability constants for the complexes with the 18-membered cycles II and IV are higher than those for the complexes with the 15-membered cycles I and III. For the ligand having the same ring size the methoxy-crown complex is always more stable than the hydroxy-crown one. The macrocyclic effect shown by the two 18-membered cycles is the result of favourable entropic factors. © 1987.
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