Interactions of some crown ethers, cyclam and its tetrathia analogue with alkali, alkali earth and other metal ions: an electrospray mass spectrometric study

Abstract

Electrospray mass spectrometry (ESMS) has been used to investigate the interactions in solution between three crown ethers (12-crown-4, 18-crown-6, dibenzyl-18-crown-6), 1,4,8,11-tetraazacyclotetradecane (cyclam) and its tetrathia analogue (S-crown) and a variety of metal ions. These include the alkali and alkali earth metal ions, various divalent transition metal ions and main group metal ions such as Cd 2+ and Pb 2+. The previously proposed relationships between the cation radius and the radius of the crown ether cavity which determine the stoichiometry of the complexes formed are dominant in solution. Although the cavity size of DB-18-C-6 is similar to that of 18-C-6, it behaves as though the cavity is effectively smaller as a result of its less flexible structure. Cyclam gave no complexes containing alkali and alkali earth cations, but ionic species were observed with the transition metal ions. S-crown did form ionic complexes with the alkali metal ions and transition metal ions, but not the alkali earth cations.