Solution Thermodynamics of Ba(II)−Cryptand 222 in Various Solvents and Derived Coordination Data in the Solid State

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Enthalpies of complexation of barium and cryptand 222 in N,N-dimethylformamide, dimethyl sulfoxide, acetonitrile, and propylene carbonate at 298.15 K determined calorimetrically are reported. The stability constant for this system in acetonitrile at the same temperature was measured by competitive potentiometric titration using silver electrodes. Gibbs energies of complexation in a variety of solvents are analyzed in terms of corresponding data for the transfer of ligand and free and complexed cation. Standard enthalpies of solution of barium perchlorate in N,N-dimethylformamide, dimethyl sulfoxide, acetonitrile, and propylene carbonate and for barium−cryptand 222 perchlorate in water, methanol, N,N-dimethylformamide, and dimethyl sulfoxide measured calorimetrically at 298.15 K are reported. The data show that in these solvents the former electrolyte is more solvated than the latter. Single-ion values of transfer enthalpies for the free and complexed cations are calculated on the basis of the Ph4AsPh4B convention. These data and those previously reported for the transfer enthalpy of cryptand 222 among these solvents are used to explain the variation observed in the complexation enthalpies of Ba2+ and this ligand in the various solvents. Coordination enthalpies for the process in the solid state derived from six different solvents are for the first time reported. The agreement found between the values derived from independent measurements reflect the reliability of complexation and solution data reported in this paper. The relationship between complexation and solvation entropy of barium is discussed taking into account previous thermodynamic data on univalent cations and cryptand 222. It is concluded that the effective shielding effect of the ligand for univalent cations weakens considerably for barium and breaks completely for some of the lanthanide cations as assessed from thermochemical data on these systems.