Computational study of the interaction between the [Pb(H2O)3]2+ cation and ligands containing oxygen, nitrogen and sulfur donor atoms

Abstract

The Pb(II) cation is a toxic species for both the human body and the environment. The design of compounds that can remove it from the medium is therefore of continuous interest. B3LYP/6-311+G(d,p) calculations were performed to study the affinity of 14 monodentate model neutral ligands to the Pb(II) cation. Compounds containing oxygen, nitrogen and sulfur donor atoms, with a series of different functional groups, were selected as ligands to complex the cation. A hemidirected complex, with coordination number 4, was simulated with four water molecules surrounding the cation ([Pb(H2O)4]2+). In this complex one water molecule was exchanged by a ligand to evaluate the efficiency of the interaction between the ligand and the Pb(II) cation. The strength of the interaction was analyzed in terms of geometric, energetic and electronic parameters inherent to the isolated ligands and to the metal ligand complex. The enthalpy and the free energy for substitution of one water molecule in the [Pb(H2O)4]2+ complex by one ligand have negative values showing that the exchange of one water molecule by a ligand is a spontaneous and exothermic process. The phosphine oxide shows the strongest interaction with the metal cation, followed by functional groups with a double bonded interacting atom and then by groups with a single bonded interacting atom. Considering the atom type, functional groups containing oxygen atom have the strongest interaction with the metal center, followed by groups with nitrogen and sulfur atoms, respectively. Energy decomposition analysis shows that both the covalent and the electrostatic components of the interaction are important to determine the strength of the complex. For the covalent component the polarization term is the largest, followed by the exchange component. The dispersion term for all complexes is almost constant with small variation and magnitude. The polarization term was found as the most relevant to modulate the affinity of each ligand for the metal center. The electronic charges on the whole ligand after complexation, the ligand HOMO energy and the softness of the ligand are all correlated with the polarization component of the interaction, thus dictating its magnitude. The phosphine oxide has the highest affinity for the Pb(II) cation, being the molecular block of choice for building a chelating agent for this cation.