Investigation of metal complex coordination structure using collision-induced dissociation and ion–molecule reactions in a quadrupole ion trap mass spectrometer

Abstract

A technique described previously using ion–molecule (I–M) reactions to determine metal complex coordination number has been combined with collision-induced dissociation (CID) in a quadrupole ion trap mass spectrometer. Nitrogen-containing tripodal ligands complexed with some first-row transition metals [i.e. Mn(II), Co(II), Fe(II), and Ni(II)] are generated by electrospray ionization, and the coordination number of these metal complex ions are determined in the gas phase using I–M reactions. Information about the coordination sphere structure (i.e. metal–ligand connectivity) is then gathered by dissociating the metal complexes, reacting their product ions, and comparing the coordination number of these product ions with their parent ions. Results suggest that a change in the coordination number upon dissociation allows identification of the functionality involved in coordination to the metal. Conversely, no change in the coordination number indicates either the lost functional group is not involved in the coordination or the ligand rearranges to fill a coordination site vacated by the dissociated group. Distinguishing between the two processes (i.e. when coordination number does not change) was very difficult in these studies, but results suggest that, in some cases, ligand rearrangement does not occur. Also, some product ions did not provide any useful coordination information upon reaction. In these cases the product ions either did not contain the metal or contained the metal in a reduced oxidation state. The preliminary results presented here are promising and suggest that with further investigation of the dynamics of metal complex I–M reactions, a combination of CID and I–M reaction may allow metal-complex coordination structure to be determined using mass spectrometry.