The Nature of Cation–Anion Interactions in Magnetic Ionic Liquids as Revealed Using High-Pressure Fourier Transform Infrared (FT-IR) Spectroscopy

Abstract

Magnetic ionic liquids are a group of magneto-responsive compounds that typically possess high ionic conductivities and low vapor pressures. In spite of the general interest in these materials, a number of questions concerning the fundamental interactions among the ions remain unanswered. We used vibrational spectroscopy to gain insight into the nature of these interactions. Intramolecular vibrational modes of the ions are quite sensitive to their local potential energy environments, which are ultimately defined by cation–anion coordination schemes present among the ions. Ambient pressure Fourier transform infrared (FT-IR) spectroscopy indicates comparable interaction motifs for 1-ethyl-3-methylimidazolium tetrachloroferrate(III), [emim]FeCl4, and 1-ethyl-3-methylimidazolium tetrabromoferrate(III), [emim]FeBr4, magnetic ionic liquids. However, the vibrational modes of [emim]FeCl4 generally occur at slightly higher frequencies than those of [emim]FeBr4. These differences reflect different interaction strengths between the [emim]+ cations and [Formula: see text] or [Formula: see text] anions. This conclusion is supported by gas-phase ab initio calculations of single [emim]FeCl4 and [emim]FeBr4 ion pairs that show longer C–H···Br–Fe interaction lengths compared to C–H···Cl–Fe. Although the IR spectra of [emim]FeCl4 and [emim]FeBr4 are comparable at ambient pressure, a different series of spectroscopic changes transpire when pressure is applied to these compounds. This suggests [emim]+ cations experience different types of interaction with the anions under high-pressure conditions. The pressure-dependent FT-IR spectra highlights the critical role ligands attached to the tetrahalogenoferrate(III) anions play in modulating cation–anion interactions in magnetic ionic liquids.

Synthesis, Characterization, and Magnetic Properties of Lanthanide-Containing Paramagnetic Ionic Liquids: An Evan’s NMR Study

“…The data presented in Table and Figure provide a comprehensive overview of the magnetic properties of various ILs, including their molar magnetic susceptibility (χ m ), mass magnetic susceptibility (χ g ), magnetic susceptibility (χ), and effective magnetic moment (μ eff ). The effective magnetic moment of the iron paramagnetic ILs aligns with previous reports, suggesting the presence of five unpaired electrons . This is consistent with the electronic configuration of Fe 3+ (3d 5 ), which indeed has five unpaired electrons.…”

“…The effective magnetic moment of the iron paramagnetic ILs aligns with previous reports, suggesting the presence of five unpaired electrons. 24 This is consistent with the electronic configuration of Fe 3+ (3d 5 ), which indeed has five unpaired electrons.…”

Confinement Effects on the Magnetic Ionic Liquid 1-Ethyl-3-methylimidazolium Tetrachloroferrate(III)

“…All of these bands gradually shift to higher frequencies as the pressure is increased from 0.7 to 2.5 GPa. These spectroscopic changes indicate phase transformation occurs between 0.4 and 0.7 GPa [ 12 , 80 ].…”

Magnetic Ionic Liquids: Current Achievements and Future Perspectives with a Focus on Computational Approaches

“…However, the analysis of the crystal structure of [C 2 C 1 im][FeCl 4 ] revealed that the C2−H bond does not interact with the [FeCl 4 ] − anion. 68 Based on the infrared spectrum analysis, it appears that the key bands are associated with the C4−H and C5−H bonds, possibly due to the acidic Lewis character of the [FeCl 4 ] − anion. 15 The elongation of the imidazolium cation and the replacement of the halide atom in the anion structure have also been investigated.…”

“…Its imidazolium cation is generally considered unpopular due to the presence of the C2–H bond (see Figure ), which is the most acidic site in the imidazolium ring and prone to forming H-bonds with the anions. However, the analysis of the crystal structure of [C 2 C 1 im]­[FeCl 4 ] revealed that the C2–H bond does not interact with the [FeCl 4 ] − anion . Based on the infrared spectrum analysis, it appears that the key bands are associated with the C4–H and C5–H bonds, possibly due to the acidic Lewis character of the [FeCl 4 ] − anion …”