Conformational Analysis and Binding Affinity Determination for Host—Guest Complexation of Alkali Metal Ions with Bis-Crown Ethers by Electrospray Mass Spectrometry and Molecular Modeling

This study presents the application of electrospray ionization mass spectrometry (ESI/MS) to investigate the host–guest complexation phenomenon for an array of alkali metal ions with bis[(benzo-15–crown-5)-15–ylmethyl] pimelate (BBCP). The results have shown that potassium ions possess the best binding affinity with the BBCP, owing to formation of the very stable sandwich-type (1:1) complexes and the strong K⁺ cation-π electron interactions between the K⁺ ion and the benzene rings of BBCP. The results of the competition experiments indicate the selectivity for BBCP toward the alkali metal ions is K⁺ > Rb⁺ > Cs⁺ > Na⁺ > Li⁺. In addition, combining the results of ESI/MS and molecular mechanics conformational searches reveal that the formation of sandwich-type (1:1) complexes can be observed for larger metal ions including K⁺, Rb⁺ and Cs⁺. As for Na⁺ ions, both 1:1 and 1:2 complexes can be observed.