Gas-phase stability of sandwich complexes of crown ethers with metal cations - as studied by collision-induced dissociation tandem mass spectrometry

Rationale: The gas-phase stabilities of the sandwich complexes formed by 12-crown-4 or 15-crown-5 with metal cations (Na+, K+, Rb+, Tl+, Ag+, Ca2+, Sr2+, Ba2+, Pb2+, Hg2+, Cd2+) are compared with the stability of the sandwich complexes formed by benzo-12-crown-4 or benzo-15-crown-5 with the cations. It is interesting to check if the possible cation-pi interactions increase the gas-phase stabilities of the sandwich complexes of benzo-crown ethers with metal cations. Methods: The sandwich complexes were generated in the gas phase by electrospray ionization (ESI) and then they were subjected to collision-induced dissociation tandem mass spectrometry (CID-MS/MS) experiments. On the basis of the obtained product ion spectra, the respective breakdown plots of the ion abundances ratios against collision energy expressed in the terms of center-of-mass were made. Results: The gas-phase stabilities of [(B12C4)(2) + Tl](+), [(B15C5)(2) + SrNO3](+), [(B15C5)(2) + PbNO3](+) and [(B15C5)(2) + SrCl](+) were higher than those of [(12C4)(2) + Tl](+), [(15C5)(2) + SrNO3](+), [(15C5)(2) + PbNO3](+) and [(15C5)(2) + SrCl](+), respectively. For the other sandwich complexes the stabilities of the complexes with benzo-crown ethers were not higher than those of the complexes with simple crown ethers. Conclusions: It is reasonable to assume that the cation- interaction increases the stability of the sandwich complex of B12C4 with Tl+ and the stabilities of sandwich complexes of B15C5 with Sr2+ and Pb2+.