Gas Phase Measurements of the Stabilization and Solvation of Metal Dications in Clusters of Ammonia and Methanol

An experimental study has been undertaken of the ability of small numbers of either ammonia or methanol molecules (XH) to form stable solvated complexes with each of nine metal dications, M2+. Complexes have been generated using a combination of the pick-up technique and electron impact ionization, and individual ions were monitored for evidence of metastability in the form of Coulomb fission or charge separation: [M(XH)(n)](2+) -> [M+X](XH)(n-m) + H2X+(XH)(m-2). Values have been assigned to a quantity n(sj) which is identified as the minimum number of molecules required to suppress the above reaction. These values were found to range from 3 for Sr2+ complexed with methanol to 19 for Sn2+ complexed with ammonia. Comparisons are made with results published previously for the same metal dications complexed with water (Chen, X.(j) Stace, A. J. Chem. Commun. 2012, 10292), and for the most part, it is found that ions solvated with either ammonia or methanol are less stable than their water counterparts. To account for differences in stability, several criteria have been examined, and of those, the most satisfactory correlation is between n, and M2+-XH bond strength; the stronger the bond, the larger n(s) has to be in order for a complex to be stable. However, for complexes where n(s) is large, such as those involving Zn2+, Cu2+, and especially Sn2+ and Pb2+, it is proposed that the geometry adopted by solvent molecules also has a significant influence on proton transfer. By comparing the ease with which proton transfer occurs for the three protic solvents, water, ammonia, and methanol, it is possible to comment on metal ion acidity in nonaqueous solutions, for which condensed phase data are nonexistent; the results suggest that most of the nine metals would be stronger Lewis acids in ammonia than in water.