Conducting and Magnetic Properties of 1-Ethyl-3-methylimidazolium (EMI) Salts Containing Paramagnetic Irons: Liquids [EMI][MIIICl4] (M = Fe and Fe0.5Ga0.5) and Solid [EMI]2[FeIICl4]

An EMI-based room-temperature (RT) ionic liquid containing d5 trivalent iron(III) ions [EMI][FeIIICl4] was fully investigated, where EMI is 1-ethyl-3-methylimidazolium. The viscosity of the salt is 14 cP at 30 °C, and its ionic conductivity is as high as 1.8 × 10−2 S cm−1 at 20 °C. The high conductivity and fluidity can be attributed to the reduced interionic Coulomb attractions owing to the nephelauxetic effect. Magnetic susceptibility shows the Curie–Weiss behavior arising from S = 5/2 high-spin electronic state on iron(III) ions in both liquid and solid states, leading to the conductive–paramagnetic bifunctional RT ionic liquid. The solidified salt passes through an antiferromagnetic transition at 4.2 K. Diamagnetic [EMI][GaIIICl4] and paramagnetic [EMI][FeIIICl4]0.5[GaIIICl4]0.5 are also RT ionic liquids and show similar conductivities (1.8–2.0 × 10−2 S cm−1 at 20 °C) and viscosities (12–13 cP at 30 °C). These results indicate that the influence of paramagnetic iron(III) ions upon the ionic conductivity and viscosity is negligible in the present system. A 2:1 EMI salt containing d6 divalent iron(II) ions [EMI]2[FeIICl4] melts at 86 °C. Its crystal structure determined by a synchrotron X-ray powder diffraction measurement is analogous to those of the reported [EMI]2[CoIICl4] and [EMI]2[NiIICl4] with expanded lattice.