Thermodynamics of lithium-crown ether (12-crown-4 and 1-benzyl-1-aza-12-crown-4) interactions in acetonitrile and propylene carbonate. The anion effect on the coordination process

Titration microcalorimetry in nonaqueous media (acetonitrile and propylene carbonate) has been used for the determination of stability constants (log K_s) and enthalpies of complexation of lithium and crown ethers (12-crown-4 and 1-benzyl-1-aza-12-crown-4 at 298.15 K. To ensure that the data are referred exclusively to the complexation process, salts containing highly polarizable anions (hexafluoroasenate, tetrafluoroborate, trifluoromethanesulfonate, and perchlorate) are used as sources for lithium. From stability constants and standard enthalpies, standard Gibbs energies and standard entropies are calculated. In propylene carbonate, a correlation is found between the stability of the lithium crown complex and the increase in conductance of the complexed relative to the free cation. Eight new lithium coronand salts of 12-crown-4 and 1-benzyl-1-aza-12-crown-4 were isolated. Standard enthalpies of these salts and crown ethers in acetonitrile and propylene carbonate at 298.15 K measured calorimetrically are used to explain (i) the higher molar ionic conductivities, observed for lithium coronand relative to lithium electrolytes, and (ii) the effect of the solution properties of ligand, free, and complexed cation in the binding of these ligands with lithium in these solvents. Enthalpies of coordination first reported show the anion effect in the process involving reactants and product in their pure physical state. The strength of cation-anion interaction follows the sequence ClO₄^- > CF₃SO₃^- > AsF₆^- > BF₄^-.