Magnetic ionic liquids (MILs) comprise a subcategory
of ionic liquids
(ILs) and contain a paramagnetic metal center allowing them to be
readily manipulated by an external magnetic field. While MILs are
popularly employed as solvents in catalysis, separations, and organic
synthesis, most low viscosity combinations possess a hydrophilic character
that limits their use in aqueous matrices. To date, no study has reported
the synthesis and characterization of hydrophobic MILs with viscosities
similar to those of hydrophilic MILs and organic solvents while simultaneously
exhibiting enhanced magnetic and thermal properties. In this study,
diglycolic acid esters are employed as ligands to chelate with paramagnetic
metals to produce cations that are paired with metal chelates composed
of hexafluoroacetylacetonate ligands to form MILs incorporating multiple
metal centers in the cation and anion. Viscosity values below 31.6
cP were obtained for these solvents, the lowest ever reported for
hydrophobic MILs. Solubilities in nonpolar solvents such as benzene
were observed to be as high as 50% (w/v) MIL-to-solvent ratio while
being insoluble in water at concentrations as low as 0.01% (w/v).
Effective paramagnetic moment values for these solvents ranged from
5.33 to 15.56 Bohr magnetons (μB), with mixed metal
MILs containing multiple lanthanides in the anion generally offering
higher magnetic susceptibilities. MILs composed of ligands containing
octyl substituents were found to possess thermal stabilities up to
190 °C. The synthetic strategies explored in this study exploit
the highly tunable nature of the employed cation and anion pairs to
design versatile ultra-low viscosity magnetoactive solvents that possess
tremendous potential and applicability in liquid–liquid separation
systems, catalysis, and microfluidics where the mechanical movement
of the solvent can be easily facilitated using electromagnets.
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