Stable [Pb(ROH)N]2+ complexes in the gas phase: softening the base to match the Lewis acid

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(H2O)N]2+; instead [PbOH(H2O)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 Pb2+ is observed without proton transfer when R = CH3CH2-CH2-. 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 Pb2+ 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)2H+. 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.