GAS-PHASE COMPLEXATION OF MONOPOSITIVE ALKALINE-EARTH METAL-IONS WITH POLYETHERS - COMPARISON WITH ALKALI-METAL ION AND ALUMINUM ION COMPLEXATIONS

The gas-phase reactions of the monopositive alkaline earth metal ions (Ca+ and Mg+) with a variety of crown ethers and their acyclic analogues have been studied by laser desorption/quadrupole ion trap mass spectrometry. The initial monopositive Mg+ and Ca+ ions were generated directly from laser desorption of a metal foil or salt and then reacted with a variety of neutral polyethers. Collisional activated dissociation (CAD) techniques were employed to evaluate the structures of these metal/polyether adducts. The reactions of these initially monopositive ions (M(+)) are of special interest because the reactions with crown ethers or glymes involve incorporation of ''OH'', corresponding to formation of (L + (M(2+)OH(-)))(+) ions, in which the metal ions attain their favored oxidation states. The alkaline earth metal ions apparently react with H2O, which may be loosely bound to a polyether ligand, by donation of the lone s electron to the oxygen atom in conjunction with elimination of H-.. The polyether ligand then solvates the (M(2+)OH(-)) species by formation of multiple electrostatic bonds between the ether oxygen atoms and the metal center. The reactions of the glycols with the metal ions are somewhat different in that the resulting products may be assigned as (L + (M(2+)OH(-)) - H2O)(+), from unstable (L + (M(2+)OH(-)))(+) complexes that spontaneously dehydrate, or as (L + M(2+) - H-.)(+), in which the initial Mg+ ion reacts directly with a hydroxyl terminus of the glycol molecule. Ligand-exchange methods used to measure the relative order of (Mg2+OH-) binding energies for the polyethers indicate that the relative binding energy increases with the size of the polyether. A comparison of the results with those for reactions of alkali metal ions and aluminum ions with polyethers in the gas phase reveals further insight into the nature of the binding interactions.