SOLVENT EFFECTS ON COMPLEXATION OF CROWN-ETHERS WITH LICLO4, NACLO4 AND KCLO4 IN METHANOL AND ACETONITRILE

The thermodynamic formation constants K-f for complexation of Li+, Na and K+ with the crown ethers 12C4 and 15C5 have been determined in me thanol and acetonitrile at 25 degrees C using precision conductivity d ata. The method permits evaluation of very small K-f values (e.g., K-f = 6.98 mol(-1)-dm(3) for LiClO4 + 12C4 in methanol) as well as fairly large values (e.g., K-f = 2.73x10(4) mol(-1)-dm(3) for NaClO4 + 15C5 in acetonitrile). The determination of K-f values from conductivity da ta takes into consideration the often neglected ion pair formation of both the uncomplexed and the complexed cations. Our results for K-f ar e generally consistent with previously reported values based on potent iometry, calorimetry and polarography, but there are significant diffe rences in several cases which we attribute to neglect of ion associati on both for the uncomplexed or ''free'' cation K-a and the macrocyclic complexed cation K-a2. Our results are also consistent with the well known concepts relating the magnitude of K-f to both the cavity diamet er and ion-solvent interactions. Limiting molar conductivities Lambda( 2)(0) for the complex salt (M-crown ether) (ClO4) in both solvents wer e generally found to be smaller or very close to the corresponding qua ntity Lambda(1)(0) for the binary MClO(4)-solvent system. However, in methanol, single ion limiting molar conductivities for the cationic co mplexes lambda(2)(0) exhibit anomalous behavior which is attributed to solvation differences between ''free'' cations and complexed cations.