Lead-enhanced gas-phase stability of multiply charged EDTA anions: a combined experimental and theoretical study

Besides their fundamental importance, multiply charged anions (MCAs) are considered as promising molecular capacitors for which their intrinsic stabilities are of great significance. Herein, the gas-phase stabilities of ethylenediaminetetraacetic acid (EDTA) anions (i.e. EDTA-nH](n-), n = 1-4) and their Pb(II) complexes (i.e. EDTA + Pb-nH]((2-n)-), n = 3, 4) have been investigated using an approach that combines extractive electrospray ionization mass spectrometry (EESI-MS) measurements, Car-Parrinello molecular dynamics simulations and density functional theory/Tao-Perdew-Staroverov-Scuseria calculations. The EESI-MS data showed that the doubly charged EDTA anions in the form of EDTA-2H](2-) and EDTA + Pb-4H](2-) were much more abundantly observed than the singly charged species such as EDTA-H](-) and EDTA + Pb-3H](-), respectively. The calculation results indicated that EDTA-2H](2-) and EDTA + Pb-4H](2-) anions were thermodynamically more stable than the EDTA-H](-) and EDTA + Pb-3H](-) species in the gas phase, respectively. The EDTA + Pb-3H](-) anions preferred five-coordinated structure, whereas EDTA + Pb-4H](2-) anions formed either five-coordinated or six-coordinated structures. The calculations further revealed that significant electron clouds drifting from the ligand EDTA to the metal Pb(II) ions and the large distances between the carboxylic groups reduced the Coulomb repulsion among the excess electrons of these MCAs. Our data demonstrated that EESI-MS combined with theoretic calculations were able to provide a deep insight into the fundamental behavior of stability of MCAs in the gas phase and, thus, might be useful tools for studying MCAs for potential molecular capacitors.