On the stability of [Pb(Proline)]2+ complexes. Reconciling theory with experiment

Salt-bridge and canonical charge solvated complexes of neutral proline (Pro) and Pb(II), [Pb(Pro)]2+, are discussed for the first time. Although thermochemically stable with respect to their corresponding deprotonated forms [Pb(Pro-H)]+, the dicationic complexes are not observed experimentally. Indeed, for the deprotonated complexes a disagreement between IRMPD results and theoretical calculations was reported. We perform an exhaustive DFT assessment to correctly predict the experimental findings, and to rationalize why [Pb(Pro)]2+ complexes are not observed. The deprotonation is likely to occur through a highly exergonic proton transfer between [Pb(Pro)]2+ and a water molecule resulting in the observed [Pb(Pro-H)]+ singly charged ion.