Thermodynamics of solvent extraction of metal picrates with crown ethers: enthalpy–entropy compensation. Part 2. Sandwiching 1 : 2 complexation

Abstract

Thermodynamics of sandwich complexation in solvent extractions were investigated for the first time. Quantitative solvent extractions over a wide range of ligand concentration in the water-dichloromethane system at 10–25 °C gave the overall and stepwise extraction equilibrium constants and the thermodynamic quantities for both 1 : 1 and 1 : 2 complexations of some alkali, alkaline earth, and/or heavy metal picrates with 15-crown-5 (1), benzo-15-crown-5 (2), and cis-cyclohexano-15-crown-5 (3). The overall sandwich complexation process was deemed as being composed of the first stoicheiometric and the subsequent sandwiching complexation equilibrium. The feasibility of the second, sandwiching complexation in the solvent extraction and in the homogeneous solutions is discussed. Global treatments of the thermodynamic parameters obtained in the solvent extraction and of those reported in the homogeneous phase led to a comparable linear relationship between the enthalpy and entropy changes of complexation. The large slope (α) of the ΔH°vs. TΔS° plot indicates the substantial conformational change of not only the second but also of the originally ligating crown ether molecule upon the second, sandwiching complexation.