Analysis of Fe2+ and Mn2+ ions in DES and water: A theoretical study using molecular dynamic simulations, QTAIM and NCI-RDG

Abstract

This work was essential to analyze the behavior of Fe2+ and Mn2+ ions in water and deep eutectic solvent (DES) based on choline chloride and ethylene glycol (1ChCl:2EG (choline chloride and ethylene glycol, DES)) to evaluate the electrodeposition of metallic ions. Due to their low toxicity, high solubility, biodegradability and thermal stability, DES have shown promise as alternatives to traditional solvents such as water in various applications, including biological studies. They also dissolve insoluble substances in other types of solvents, working from the purification of chemicals to the manufacture of semiconductor devices. Water, choline and ethylene glycol molecules were optimized by Density Functional Theory (DFT) through B3LYP/6–311 +G(d,p) model implemented with the Gaussian 09 W software. Molecular Dynamics (MD) simulations were performed by GROMACS software 2020.04 in a cubic box of dimensions 8 nm × 8 nm × 8 nm. Quantum Theory of Atoms in Molecules (QTAIM) and analysis of Non-covalent Interactions (NCI) used Multiwfn 3.8 software. The preliminary results of MD simulations showed that gradually adding water increased the interaction between the metal center and water. Fe2+ and Mn2+ ions were placed in water (OW) and 1ClCh:2EG, indicating that the interaction between the cations and the chloride anion in DES was strong, followed by the ethylene glycol molecules (O1, O2). In the presence of water, however, Fe-OW and Mn-OW interactions were more substantial in all systems, either with the ions alone or in a mixture. Electrostatic forces acting on the electrons in the last filled layer of the iron atom provided the formation of shorter and stronger bonds than the Mn2+ ions. When placed in a 1:1 equimolar mixture, they showed little difference from when they were isolated. Thus, the Mn-Cl interaction was more significant in the presence of a eutectic solvent than the Fe-Cl interaction. QTAIM analysis showed for the FeDES and MnDES systems that the Electron Localization Function (ELF) values increased for the Fe-(O1, O2) and Mn-(O1, O2) interactions but decreased for the Fe-Cl and Mn-Cl interactions in the metal mixture. It revealed strong hydrogen bonding interactions between the water molecules, Fe2+ and Mn2+, which may affect their chemical reactivity, as the Fe-Cl interaction in DES was stronger than Fe-Cl in any water. The Laplacians of the electronic densities were positive, and so were the ELF values, indicating a predominance of non-covalent interactions of the van der Waals forces type. Considering the two metal ions alone, in the presence of DES, the interaction strength followed Fe-Cl > Mn-Cl > Fe-(O1, O2) > Mn-(O1, O2). In the mixture, Fe-Mn followed the pattern Mn-Cl > Fe-Cl > Fe-(O1, O2) > Mn-(O1, O2). Thus, comparing the 3 systems, the interaction strength was Fe-Cl > Mn-Cl > Mn-Clmixture > Fe-Cl mixture e Fe-(O1, O2) mixture > Fe-(O1, O2) > Mn-(O1, O2)mixture > Mn-(O1, O2). The interaction strength for the systems with water was Fe-Clmixture > Fe-Cl > Mn-Cl > Mn-Clmixture. However, for the interaction with OW, the observed behaviors were: W300, Mn-OW > Fe-OWmixture > Mn-OWmixture > Fe-OWmixture; W600, Mn-OWmixture > Fe-OWmixture > Mn-OW > Fe-OW; W900, Mn-OW > Fe-OW > Mn-OWmixture > Fe-OWmixture; W1200, Mn-OW > Mn-OWmixture > Fe-OWmixture > Fe-OWmixture; W5580, Mn-OW > Fe-OW > Mn-OWmixture > Fe-OWmixture. The mixtures in the 1:1 molar ratio of the cations presented a little discrepant from their isolated ions, suggesting a new approach to observing the behavior of these systems and the coordination possibilities. One of the prospects for expanding the study will be temperature variation, modification of metal ions and molar proportions, and the effects of these changes.