Synthesis and characterization of the physicochemical and magnetic properties for perfluoroalkyl ester and Fe(III) carboxylate-based hydrophobic magnetic ionic liquids

Magnetic ionic liquids (MILs) are a new class of ionic liquids (ILs) that incorporate a paramagnetic component in their chemical structure. Although imidazolium-based MILs can be synthesized using inexpensive and relatively straightforward procedures, these compounds often are water soluble which limits their usefulness in aqueous applications. In this study, two classes of hydrophobic MILs, including perfluorobutyryl ester-based and Fe(III) carboxylate-based MILs, were synthesized and characterized. Functionalization of the cation with fluorinated substituents yielded MILs that were insoluble in aqueous solution at concentrations as low as 0.1% (w/v). In contrast to conventional MILs that rely on paramagnetic anions, Fe(III) carboxylate-based MILs were prepared featuring carboxylate ligands in the cationic moiety capable of chelating a paramagnetic Fe(III) center. The hydrophobic character of the Fe(III) carboxylate-based MILs was subsequently controlled by incorporating the bis[(trifluoromethyl)sulfonyl]imide ([NTf2−]) anion, resulting in MILs that were insoluble in aqueous solutions at 0.1% (w/v). This synthetic strategy has the potential to impart dual functionality to MILs by providing the flexibility to incorporate a task specific anion without sacrificing paramagnetic properties. The molar magnetic susceptibilities (χm) and effective magnetic moments (μeff) of the studied MILs were determined using superconducting quantum interference device (SQUID) magnetometry. Consistent with the Curie–Weiss law, a linear relationship between temperature and inverse magnetic susceptibility (χm−1) was observed for the hydrophobic MILs. The μeff values of the MILs examined in this study ranged from 3.56 to 8.06 Bohr magnetons (μB).