ABSTRACT
An experimental gas-phase study of the intensities and fragmentation patterns of [Mn.(H(2)O)(n)](2+) and [Mn.(ROH)(n)](2+) complexes shows the combinations [Mn.(H(2)O)(4)](2+) and [Mn.(ROH)(4)](2+) to be stable. Evidence in complexes involving the alcohols methanol, ethanol, 1-propanol, and 2-propanol favors preferential fragmentation to [Mn.(ROH)(4)](2+), whereas the fragmentation data for water is less clear. Supporting density functional calculations show that both [Mn.(H(2)O)(4)](2+) and [Mn.(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 [Mn.(MeOH)(4)](2+) unit shows a preference for [Mn.(MeOH)(4)(MeOH)(1,2)](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 Mn(2+) have on solvent geometry. It is concluded that the experimental observations are best reproduced when the central Mn(2+) ion is in the high-spin (6)S ground state. The results are also considered in terms of the biochemical activity of Mn(2+) where the ion is capable of isomorphous substitution with Zn(2+), which itself exhibits a preference for tetrahedral coordination.
ABSTRACT
Experiments have been performed in the gas phase to investigate the stability of complexes of the general form [Pb(ROH)(N)](2+). With water as a solvent, there is no evidence of [Pb(H(2)O)(N)](2+); instead [PbOH(H(2)O)(N-1)](+) is observed, where lead is considered to be held formally in a +2 oxidation state by the formation of a hydroxide core. As the polarizability of the solvating ligands is increased through the use of straight chain alcohols, ROH, solvation of Pb(2+) is observed without proton transfer when R >or= CH(3)CH(2)CH(2)-. The relative stabilities of [Pb(ROH)(4)](2+) complexes with respect to proton transfer are further investigated through the application of density functional theory to examples where R = H, methyl, ethyl, and 1-propyl. Of three trial structures examined for [Pb(ROH)(4)](2+) complexes, in all cases those with the lowest energy comprised of three solvent molecules were directly bound to the central cation, with the fourth molecule held in a secondary shell by hydrogen bonds. The implications of this arrangement as a favorable starting structure for proton transfer are discussed. Conditions for the stability of particular Pb(II)/ligand combinations are also discussed in terms of the hard-soft acid-base principle. Charge population densities calculated for the central lead cation and oxygen donor atoms across the ROH range are used to support the proposal that proton transfer occurs when a ligand is hard. Stability of the [Pb(ROH)(4)](2+) unit is commensurate with a decrease in the ionic character of the bond between Pb(2+) and a ligand; this in turn reflects a softening of the ligand as the alkyl chain increases in length. From the calculations, the most favorable protonated product is, in all cases, (ROH)(2)H(+). The trends observed with lead are compared with Cu(II), which is capable of forming stable gas-phase complexes with water and all of the alcohols considered here.
