Spectrophotometric study of the complexation of iodine with macrocycles in chloroform solution

Abstract

The complex-formation reactions between iodine and aza-18-crown-6 (1,4,7,10,13-pentaoxa-16-azacyclooctadecane), 7,16-diaza-18-crown-6 (1,4,10,13-tetraoxa-7,16-diazacyclooctadecane and 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane (cryptand C222) have been studied spectrophotometrically in chloroform solution at various temperatures. In the cases of 7,16-diaza-18-crown-6 and cryptand C222 the resulting 1 : 2 (macrocycle to iodine) molecular complex is formulated as (macrocycle ⋯ I⁺)I₃^–, while with aza-18-crown-6 a 2 : 2 molecular complex of the type [(macrocycle)₂⋯ I⁺]I₃^– is formed. The spectrophotometric results, as well as conductance measurements, revealed that the gradual release of triiodide ion from its contact ion pair in the molecular complex into solution is the rate-determining step of the reaction. The formation constants of the resulting molecular complexes were evaluated from computer fitting of absorbance vs. mole ratio data and found to vary in the order cryptand C222 > 7,16-diaza-18-crown-6 > aza-18-crown-6. The enthalpy and entropy of complexation of iodine with aza- and 7,16-diaza-18-crown-6 were determined from the temperature dependence of the formation constants. In both cases, the complexation reactions are enthalpy stabilized, but entropy destabilized.