Ion-molecule clusters involving doubly charged metal ions (M 2+)

Abstract

Doubly charged metal ion-ligand L clusters, M 2+ (L) n , where M 2+ = Mg 2+, Ca 2+, Sr 2+, Ba 2+, Mn 2+, Fe 2+, Co 2+, Ni 2+, Cu 2+, Zn 2+, can be produced in the gas phase by electrospray of solutions of chloride, bromide or nitrate salts of M 2+ 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 N 2. When known amounts of ligand vapour are added, the equilibria M 2+ (L) n−1 + L = M 2+ (L) n can be determined by sampling the ions escaping from an orifice in the interface chamber with a quadrupole mass spectrometry. Equilibria for Ni 2+ (H 2O) n are determined. The clusters are with n ≈ 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 ▪ Different ligands may accelerate (NH 3, pyridine) or retard (DMF, DMSO) the onset of charge reduction. Charge reduction with DMF and DMSO occurs by simple charge transfer ▪ Polydentate cage-type ligands protect ions from charge reduction. Thus the only triply charged ion, M 3+, so far observed was Co 3+ sepulchrate where the ion is coordinated to six nitrogens.