Enthalpies and entropies of complexation of silver(I) and diaza-18-crown ethers in polar solvents

The enthalpies of complexation of Ag+ with diaza-18-crown ethers containing various combinations of donor atoms have been determined in polar organic solvents using calorimetric and potentiometric techniques. Enthalpies of complexation are also reported for two silver ion cryptates. The results are combined with previously determined free energies of complexation to obtain the corresponding entropies. In dipolar aprotic solvents, the enthalpies of transfer of the complexes are very similar to those of the free ligands, so that changes in enthalpies of complexation are dominated by the enthalpies of solvation of the silver cation. The thermodynamics of ligation, i.e., of complex formation between the gaseous silver ion and the diaza-crown and cryptand ligands in solution, are calculated and, for the free energies, partitioned into contributions from each of the donor atoms (N, O, and S). Among the aprotic solvents, contributions of oxygen and sulphur atoms in the ligand bridges to the stabilities of diaza-crown and cryptand complexes are additive and independent of solvent. In methanol, hydrogen bonding between methanol molecules and the free ligands reduces the magnitudes of both the free energies and enthalpies of formation of the silver complexes.