Complexation of Li+, Na+ and K+ Ions by [222], [222D], [221], [221D] Cryptands in Acetonitrile at 25 °C: Conductometric Determination of the True Thermodynamic Formation Constants

A new method is presented for the analysis of precise conductance data to obtain the true thermodynamic formation constants of macrocyclic–cation complexes. The method, based on the Lee–Wheaton theory on mixed electrolytes, takes into consideration the ion pair formation of both the uncomplexed and complexed cations and avoids the use of the simple additivity assumption of the conductances of two electrolytic species present in salt/ligand/solvent systems. The method has been applied to determine the thermodynamic complexation constants of lithium, sodium and potassium ions with the cryptands [221], [222] and their decyl derivatives [221D], [222D] in acetonitrile. The results show that the presence of an alkyl chain in the molecular structure of the cryptands decreases the macrocyclic–cation complexation constant with respect to the values obtained for the parent compounds by almost an order of magnitude. Such a finding has been explained in terms of the asymmetric position in the space of the oxyethylenic bridges of the macrocyclic ligand promoted by the presence of the linked hydrocarbon chain. The above explanation has been confirmed by the anomalous behavior of both the ion-pair association constants of complexed salts and their limiting molar conductivity.