Facilitated ion-transfer of alkaline-earth metal cations by naphtho-15-crown-5 across the water|1,2-dichloroethane interface: voltammetric and electrospray ionization mass spectrometric studies

摘要

The facilitated ion-transfer processes of Ca{sup}(2+), Sr{sup}(2+), and Ba{sup}(2+) cations from an aqueous phase to a 1,2-dichloroethane (DCE) phase by naphtho- 15-crown-5 (N15C5) were studied by cyclic voltammetry. When the metal ion concentrations in the aqueous phase were much higher than the N15C5 concentrations in the organic phase clear voltammograms were observed: the peak currents were controlled by the diffusion of N15C5 in the DCE phase. The diffusion coefficient of N15C5 in the DCE phase was evaluated to be 5.3 × 10{sup}(-6) cm{sup}2 s{sup}(-1). The analyses of these voltammograms revealed that the transfer of Ca{sup}(2+) cation was facilitated due to the formation of 1:2 complex of cation to ligand ratio, Ca(N15C5)2{sup}(2+) and Sr{sup}(2+) and Ba{sup}(2+) cations were facilitated due to the formation of 1:1 complexes, Sr(N15C5){sup}(2+) and Ba(N15C5){sup}(2+), respectively. The stoichiometries of the complexes were also studied by electrospray ionization mass spectrometry (ESI-MS). Both 1:1 and 1:2 complexes were observed at low concentration of N15C5 for the Ca{sup}(2+) -N15C5 system. At high concentration of N15C5, only 1:2 complex was observed. In the cases of Sr{sup}(2+) -N15C5 and Ba{sup}(2+) -N15C5 systems, no signal was observed at low concentration of N15C5 whereas, at high concentration of N15C5, both 1:1 and 1:2 complexes were observed for the Sr{sup}(2+) -N15C5 system and only 1:1 complex was observed for the Ba{sup}(2+) -N15C5 system. Comparing the intensities in the ESI-MS spectra, the stabilities of different complexes in the gas phase were discussed.