THERMODYNAMICS OF MACROCYCLE COMPLEXATION CHEMISTRY - INTERACTIONS OFMETAL-IONS WITH DOUBLE-ARMED N-PIVOT LARIAT ETHERS IN METHANOL AND METHANOL-WATER SOLUTIONS AT 25.0-DEGREES-C

The log K, DELTAH, and TDELTAS values for interactions of Na+, K+, Cu2 +, and Ca2+ With a series of N,N'-dipivot lariat ethers have been dete rmined from calorimetric titration data valid in 9:1 (v/v) CH3OH/H2O a nd absolute methanol at 25.0-degrees-C. Complex stability is increased greatly by introducing pendant arms containing oxygen or nitrogen ato ms to the parent macrocycles. The enthalpy and entropy changes support the idea that the side arms interact with the cations. Formation of c omplexes of Ca2+ is both enthalpy and entropy stabilized. All other in teractions are enthalpy driven, and the entropy changes are unfavorabl e. Effects of pendant-arm length and substituents on the complexation of the cations are discussed on the basis of the thermodynamic data. T he large negative DELTAH values for complexation of Cu2+ with lariat e thers indicate a strong interaction between Cu2+ and the nitrogen atom s of the ligands. Large entropic losses for some of the cation-ligand interactions indicate that the lariat ethers are flexible and readily change their conformations to accommodate the cations.