Interactions of Neodymium(III) with Small-Molecule Ligands and Coronene Evaluated with DLPNO-CCSD(T).

Abstract

Determination of the interactions between critical metal ions and single-molecule ligands is key to improving sustainable reclamation processes for critical metals such as electrodeposition of these metals from liquid media. Neodymium, Nd, is one such critical metal that faces rising demand due to its application in permanent strong magnets used in wind turbines and in electric motors of electric vehicles. Previous experimental work identified that trace quantities of water facilitated Nd electrodeposition from ionic liquid (IL) media. Here, interactions of Nd³⁺ with TFSI^- (bis(trifluoromethylsulfonyl)imide), a common anion in ILs, H₂O (water), C₂H₆ (ethane, an approximation for long-chained phosphonium and ammonium cations in ILs) and coronene (an approximation for a graphitic electrode surface) are obtained using high-level DLPNO-CCSD(T) calculations. A range of binding orientations/conformations for each small molecule ligand are investigated. The nonadditivity effect is explored to identify the scaling of the interaction energy when one TFSI^- is added to complexes of Nd(TFSI)_x (where x = 1-3) and when one water molecule is added to a Nd(TFSI)₃ complex. The most stable binding configuration is further analyzed using a relativistic Hamiltonian and also, using the local energy decomposition method. The LED method revealed that the interaction is mainly driven by electrostatics and polarization, and that correlation contributes insignificantly to the total interaction energy. This work provides high-quality benchmark data that cannot be obtained from experiment alone, to enable the design of high-quality force fields for simulation studies of the phenomena related to the electrodeposition processes.