DISSOCIATION OF POLYETHER TRANSITION-METAL ION DIMER COMPLEXES IN A QUADRUPOLE ION-TRAP

The formation and dissociation of dimer complexes consisting of a tran sition metal ion and two polyether ligands is examined in a quadrupole ion trap mass spectrometer. Reactions of three transition metals (Ni, Cu, Co) with three crown ethers and four acyclic ethers (glymes) are studied. Singly charged species are created from ion-molecule reaction s between laser-desorbed monopositive metal ions and the neutral polye thers. Doubly charged complexes are generated from electrospray ioniza tion of solutions containing metal salts and polyethers. For the singl y charged complexes, the capability for dimer formation by the ethers is dependent on the number of available coordination sites on the liga nd and its ability to fully coordinate the metal ion. For example, 18- crown-6 never forms dimer complexes, but 12-crown-4 readily forms dime rs. For the more flexible acyclic ethers, the ligands that have four o r more oxygen atoms do not form dimer complexes because the acyclic li gands have sufficient flexibility to wrap around the metal ion and pre vent attachment of a second ligand. For the doubly charged complexes, dimers are observed for all of the crown ethers and glymes, thus showi ng no dependence on the flexibility or number of coordination sites of the polyether. The nonselectivity of dimer formation is attributed to the higher charge density of the doubly charged metal center, resulti ng in stronger coordination abilities. Collisionally activated dissoci ation is used to evaluate the structures of the metal-polyether dimer complexes. Radical fragmentation processes are observed for some of th e singly charged dimer complexes because these pathways allow the mono positive metal ion to attain a more favorable 2+ oxidation state. Thes e radical losses are observed for the dimer complexes but not for the monomer complexes because the dimer structures have two independent li gands, a feature that enhances the coordination geometry of the comple x and allows more flexibility for the rearrangements necessary for los s of radical species. Dissociation of the doubly charged complexes gen erated by electrospray ionization does not result in losses of radical neutrals because the metal ions already exist in favorable 2+ oxidati on states. (C) 1997 American Society for Mass Spectrometry.