Voltammetric and electrospray ionization mass spectrometric studies of the facilitated ion transfer of sodium and potassium cations by naphtho-15-crown-5 across the water|1,2-dichloroethane interface

The transfer Na+ and K+ cations, facilitated by naphtho-15-crown-5, N15C5, from an aqueous phase to a 1,2-dichloroethane (DCE) phase were studied by cyclic voltammetry under two following conditions, the metal ion concentration in the aqueous phase was much higher than the N15C5 concentration in the organic phase (N15C5 diffusion controlled process) and the N15C5 concentration was much higher than metal ion concentration in the aqueous phase (metal ion diffusion controlled process). In both conditions, diffusion controlled waves were observed and the diffusion coefficients of Na+ and K+ in the aqueous phase and that of N15C5 in the organic phase were evaluated to be 1.03 x 10(-5), 1.28 x 10(-5) and 5.3 x 10(-6) cm(2) s(-1), respectively. The stoichiometries of the complexes were determined to be 1:1 (cation to ligand) for Na+-N15C5 and 1:2 for K+-N15C5. Under the both conditions, the stability constants, beta(1), of the Na(N15C5)(+) complex, in the organic phase were evaluated to be 3.9 x 10(8) M-1 and 2.3 x 10(5) M-1 because two reference standard potentials were reported elsewhere. The beta(2) values of the K(N15C5)(2)(+) complex were 2.5 x 10(11) M-2 and 1.2 x 10(11) M-2 under the conditions of N15C5 and K+ diffusion controlled processes, respectively. The stoichiometries of the complexes were also studied by electrospray ionization mass spectrometry, The complexes of Na(N15C5)(+) and Na(N15C5): were observed and their stabilities were almost similar to each other in the gas phase. On the other hand, the K(N15C5)(+) complex was very unstable and the K(N15C5)(2)(+) complex was only observed in the gas phase. This result is very consistent with the electrochemical results. Comparing the intensities in the ESI-MS spectra, the K(N15C5): was more stable than Na(N15C5)(2)(+) in the gas phase. (C) 1998 Elsevier Science Ltd. All rights reserved.