Thermodynamic studies of cryptand 222 and cryptates in water and methanol

Abstract

Enthalpies of solution in water and in methanol are reported for a number of cryptate electrolytes ([M+222]+X–) and in conjunction with data on X– yield values for the enthalpy of transfer, ΔH°t, of [M+222] ions from water to methanol. Enthalpies of complexing of cations with cryptand 222 have been determined in water and methanol; when combined with known values of ΔH°t(M+) and the presently determined value of ΔH°t(222), they yield, via a thermodynamic cycle, values of ΔH°t for the ions [M+222] where M+= Li+, Na+, K+, Rb+, Cs+ and Ag+. The two methods of obtaining the ΔH°t values are in good agreement with each other.Solubility measurements on the perchlorates of [Na+222] and [K+222] yield values of ΔG°t([M+222]) from water to methanol. These values are also obtained from a thermodynamic cycle involving known values of ΔG°t(M+), the free energies of complex formation in water and methanol and the presently determined value of ΔG°t(222). The direct values and the cycle values are again in good agreement.ΔH°t, ΔG°t and ΔS°t values for transfer of the complexed ions [M+222] vary considerably with the central cation M+(M+= Li+, Na+, K+, Rb+, Cs+ and Ag+) and it is clear that the surrounding cryptand does not isolate the central ion from the environment. Also, various single-ion assumptions that require the constancy of ΔG°t([M+222]) and ΔH°t([M+222]) with M+ are not valid for the water to methanol transfer.Partition coefficients for the hypothetical extraction process M+(aq)+222(aq)→[M+222](methanol) have been obtained and it is shown that by comparison with the simple partition M+(aq)→ M+(methanol), greatly enhanced cation selectivities are observed. The largest selectivity enhancement occurs with the ions Ag+ and Li+, where the complex extraction equilibrium favours the extraction of Ag+ by a factor of 4 × 109 over the simple partition.Ion-pair partition coefficients have also been obtained for the species [Na+222]ClO–4 and [K+222]ClO–4; extraction of the latter is favoured by a factor of 1.6 × 102. This may be compared to a factor of 2.7 × 102 in favour of the potassium salt when extracted as the pair of ions ([M+222]+ ClO–4) and to a factor of 0.61 for extraction as the uncomplexed K+ and Na+ cations.