Solvent coordination in gas phase [Mn.(H2O)n]2+ and [Mn.(ROH)n]2+ complexes: Theory and Experiment

An experimental gas-phase study of the intensities and fragmentation patterns of [M(H2O)n]2+ and [M(ROH)n]2+ complexes shows the combinations [M(H2O)4]2+ and [M(ROH)4]2+ to be stable. Evidence in complexes involving the alcohols methanol, ethanol, 1-propanol, and 2-propanol favors preferential fragmentation to [M(ROH)4]2+, whereas the fragmentation data for water is less clear. Supporting density functional calculations show that both [M(H2O)4]2+ and [M(MeOH)4]2+ adopt stable tetrahedral configurations, similar to those proposed for biochemical systems where solvent availability and coordination is restricted. Calculated incremental binding energies show a gradual decline on going from one to six solvent molecules, with a step occurring between four and five molecules. The addition of further solvent molecules to the stable [M(MeOH)4]2+ unit shows a preference for [M(MeOH)4(MeOH)2]2+ structures, where the extra molecules occupy hydrogen-bonded sites in the form of a secondary solvation shell. Very similar behavior is seen on the part of water. As part of an analysis of the experimental data, the calculations have explored the influence different spins states of Mn2+ have on solvent geometry. It is concluded that the experimental observations are best reproduced when the central Mn2+ ion is in the high-spin 6S ground state. The results are also considered in terms of the biochemical activity of Mn2+ where the ion is capable of isomorphous substitution with Zn2+, which itself exhibits a preference for tetrahedral coordination.