Neat and rapid preparation of hydrophobic magnetic ionic liquids composed of transition metal chelates featuring in situ formation capabilities in aqueous matrices

Abstract

Magnetic ionic liquids (MILs) form a subclass of ionic liquids (ILs) that possess paramagnetic properties and can respond to an external magnetic field, facilitating ease of manipulation in immiscible solvents. Despite being popular as solvents in catalysis, organic synthesis, and separations, MILs are obtained through complex and labor-intensive synthetic/purification routes that increase cost, are time-consuming, and require expertise in organic synthesis. To date, no study has successfully developed a straightforward and rapid procedure for MILs that also eliminates purification. In this study, transition metal-containing bis[(trifluoromethyl)sulfonyl]imide ([NTf₂⁻]) salts and N-alkylimidazoles, diglycolamides, and O-donor ligands were used to produce hydrophobic MILs that could maintain their chemical integrity under water for over 6 months. A neat heat/stir method was employed to form MILs for long-term storage or later use while an in situ method was used for select combinations to generate the same MIL under 30 s in the bulk presence of water. Viscosities as low as 198.3 cP at 22.8 °C were obtained that were comparable to previous classes of paramagnetic solvents. In contrast to MILs comprised of O-donor ligands, those formed with alkylimidazoles and diglycolamides were found to be soluble in non-polar solvents such as hexane at concentrations of up to 50% (w/v) MIL-to-solvent ratio while being insoluble in water even at 0.01% (w/v). Effective magnetic moment values for MILs ranged from 2.78 to 5.16 Bohr magnetons (μ_B) and were observed to be dependent on the metal center in the [NTf₂⁻] salts. The solvents possessed excellent thermal stabilities with diglycolamide-based MILs exhibiting structural resilience up to 345 °C. The synthetic design of these MILs has successfully achieved the primary objective of facilitating easy access to magnetoactive solvents by reducing their preparation to a simple step consisting of mixing two readily available reagents to expedite production in high-throughput laboratories where these compounds can be incorporated in automated separations and analytical testing systems where their magnetic properties can be conveniently exploited.