ESI-MS detection of very weak pi-stacking interactions in the mixed-ligand sandwich complexes formed by substituted benzo-crown ethers and metal cations.

The stability of the mixed-ligand complex formed by amino-benzo-15-crown-5 and nitro-benzo-15-crown-5, with a metal cation, [NH(2)B15C5 + NO(2)B15C5 + K](+), was found to be enhanced by pi-stacking interactions. This conclusion was deduced by comparison of the abundance of the mixed-ligand complex with the abundances of homo-ligand complexes ([(NH(2)B15C5)(2) + K](+), [(NO(2)B15C5)(2) + K](+)), as well as with those of 1:1 complexes ([NH(2)B15C5 + K](+), [NO(2)B15C5 + K](+)). Some solvents and some metal cations with large radii were also found to prevent the existence of pi-stacking interactions. 2010 American Society for Mass Spectrometry. Published by Elsevier Inc. All rights reserved.