Determination of alkali metal binding selectivities of caged crown ligands by electrospray ionization quadrupole ion trap mass spectrometry

The alkali metal (Li+, Na+, K+. Rb+, and Cs+) binding selectivities of caged crown ether compounds are evaluated by way of electrospray ionization mass spectrometry and compared to the binding selectivities of three reference crown ethers, 15-crown-5, 1,7-diaza-15-crown-5, and 18-crown-6. The relative binding selectivities are estimated from the mass spectral intensities of the metal complexes and compared to calculated equilibrium distributions when possible. In general, the selectivities of the caged crown ethers parallel the selectivities for the noncaged reference crowns. However, as the cage moiety imparts a greater degree of rigidity in the polyether ring, the selectivities are shifted slightly toward larger cations. Further understanding of the metal binding preferences is obtained from molecular modeling calculations. (C) 2001 Elsevier Science B.V.