1-Ethyl-2,3-dimethylimidazolium paramagnetic ionic liquids with 3D magnetic ordering in its solid state: synthesis, structure and magneto-structural correlations

Abstract

Two novel paramagnetic ionic liquids, comprised of a 1-ethyl-2,3-dimethylimidazolium (Edimim) cation and a tetrahaloferrate(III) (FeX₄) (X = Cl and Br) anion were synthetized and characterized by thermal, structural, Raman spectroscopy and magnetic studies. The crystal structures, determined by synchrotron X-ray powder diffraction and single crystal X-ray diffraction at 100 K for Edimim[FeCl₄] and Edimim[FeBr₄] respectively, are characterized by layers of cations (in non-planar configuration) and anions stacked upon one another in a three-dimensional (3D) manner with several non-covalent interactions: halide–halide, hydrogen bond and anion–π. Magnetization measurements show the presence of three-dimensional antiferromagnetic ordering below the Néel temperature (T_N) with the existence of a noticeable magneto-crystalline anisotropy in the bromide compound. The corresponding magneto-structural correlations evidence that the 3D magnetic ordering mainly takes place via Fe–X⋯X–Fe (X = Cl and Br) interactions, displaying a higher superexchange magnetic interaction between the planes. Comparison with the Emim[FeX₄] (X = Cl and Br) phases (Emim: 1-ethyl-3-methylimidazolium) reveals that the methylation at the C(2) position onto the imidazolium cation ring causes an increase of the melting point and a decrease of the T_N. In contrast, the comparative study with Dimim[FeX₄] (X = Cl and Br) compounds (Dimim: 1,3-dimethylimidazolium) shows a lower T_N in the chloride compound, Edimim[FeCl₄], whereas it is higher for the bromide, Edimim[FeBr₄]. This fact is attributed to the spin delocalization of iron atoms in [FeBr₄]⁻ and discards the hypothesis that a bigger imidazolium ion size causes a weaker magnetic coupling in paramagnetic ionic liquids based on tetrahaloferrate anions and imidazolium cations with 3D magnetic ordering in its solid state.