ION-MOLECULE CLUSTERS INVOLVING DOUBLY CHARGED METAL-IONS (M2+)

Doubly charged metal ion-ligand L clusters, M2+ (L)n, where M2+ = Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, can be produced in the gas phase by electrospray of solutions of chloride, bromide or nitrate salts of M2+ in methanol-water. L may be added to the solution or to the gas phase. The ions produced by electrospray at atmospheric pressure are transferred to an interface chamber containing pure N2. When known amounts of ligand vapour are added, the equilibria M2+ (L)n-1 + L = M2+ (L)n can be determined by sampling the ions escaping from an orifice in the interface chamber with a quadrupole mass spectrometer. Equilibria for Ni2+ (H2O)n are determined. The clusters are with n almost-equal-to 10 and the binding energies are in the 15 kcal mol-1 range. Bonding of the ligands at n < 6 is much stronger. It can be studied with collision-induced dissociation (CID) in a triple quadrupole mass spectrometer. The binding energies in the inner shell, n = 0-6, are in the 60 kcal mol-1 range. At low n, charge reduction to M+ may occur. The water clusters undergo the charge reduction M2+ (H2O)n = MOH+ (H2O)n-2 + H3O+ Different ligands may accelerate (NH3, pyridine) or retard (DMF, DMSO) the onset of charge reduction. Charge reduction with DMF and DMSO occurs by simple charge transfer M2+ (DMSO)n = M+ (DMSO)n-1 + DMSO+ Polydentate cage-type ligands protect ions from charge reduction. Thus the only triply charged ion, M3+, so far observed was Co3+ sepulchrate where the ion is coordinated to six nitrogens.