Anomalous Melting Point of Multicharge Ionic Liquids: Structural, Electrostatic, and Orbital Properties of [Ln(NO)] (Ln = Ce, Pr) Anions.

Salts composed of multicharged cations/anions usually exhibit a large lattice energy and strong Coulomb force, which results in high melting points. However, an increasing number of highly charged ionic liquids exceed expectations based on conventional experience; even their melting points are much lower than those found for simple ionic liquids composed of monovalent ions. To further study this phenomenon, we studied a group of stable ionic liquids containing tricharged [Ce(NO)] and [Pr(NO)] anions. The structures for [Cmim][Ce(NO)] and [Cmim][Pr(NO)] were determined by single-crystal X-ray diffraction with triclinic and 1̅ space groups. The electrostatic potential density per unit ion surface and volume was proposed and calculated. Additionally, theoretical analysis based on Hirshfeld surface and charge decomposition was carried out to explore the intermolecular interaction and electronic structure of the lanthanide anions. The electrostatic and orbital properties were found to be more useful for understanding the melting points of highly charged salts compared with the sole use of lattice energy. The electrostatic potential density per unit ion surface and volume showed a linear relationship with the melting point of ionic liquids composed of monovalent to trivalent ions. These structure-melting point relationships will be beneficial for expounding new low-melting-point ionic liquids with a wide liquidus range.