Self-assembled metallosupramolecular cages are discrete 3-dimensional assemblies with well-defined internal cavities that have myriad applications including catalysis and molecular sensing. Unambiguous characterisation of coordination cages is challenging due to their dynamic behaviour, such as isomer exchange and structure interconversion. While the population can be selectively tuned through ligand design, guest binding or crystallisation, the efficient separation of all isomers is necessary to investigate their individual properties. Here we demonstrate the utility of ion mobility mass spectrometry to characterise individual coordination cages within dynamic mixtures.
Coordination cages were prepared by self-assembly of metal salts and bis bidentate ligands. The resulting mixtures of cages were infused into a Waters Cyclic ion mobility mass spectrometer via electrospray ionisation. Intact coordination cage ions were mass-selected and separated over a variable number of passes around the cyclic ion mobility device prior to mass analysis.
Tetrahedral cages were formed as a mixture of stereoisomers, arising from different ligand arrangements around each metal vertex. The observed isomer distribution varied depending on the metal cation, ligand, and the templating counter ion. Stereoisomers were partially resolved after 30 passes of the cyclic ion mobility cell. Complete resolution required the selective ejection of one isomer population from the cyclic device and re-injection of the second population for further separation. By careful comparison to calculated collision cross sections, the identity of each peak could be assigned. Post-mobility collision-induced dissociation enabled the relative stability of each isomer within the mixture to be probed. The method was successfully applied to paramagnetic complexes comprising Co(II) or Ni(II), which are particularly problematic for NMR characterisation. Moreover, individual isomers of bimetallic cages could be resolved by the same approach.
Ion mobility-based approaches should encourage chemists to embrace coordination cages containing a broad range of metal ions and ligands, significantly expanding the scope of accessible supramolecular architectures.