Anion-π and Halide-Halide Nonbonding Interactions in a New Ionic Liquid Based on Imidazolium Cation with Three-Dimensional Magnetic Ordering in the Solid State

We present the first magnetic phase of an ionic liquid with anion-pi interactions, which displays a threedimensional (3D) magnetic ordering below the Neel temperature, T-N = 7.7 K. In this material, called Dimim[FeBr4], an exhaustive and systematic study involving structural and physical characterization (synchrotron X-ray, neutron powder diffraction, direct current and alternating current magnetic susceptibility, magnetization, heat capacity, Raman and Mossbauer measurements) as well as first-principles analysis (density functional theory (DFT) simulation) was performed. The crystal structure, solved by Patterson-function direct methods, reveals a monoclinic phase (P2(1) symmetry) at room temperature with a = 6.745(3) angstrom, b = 14.364(3) angstrom, c = 6.759(3) angstrom, and beta = 90.80(2)degrees. Its framework, projected along the b direction, is characterized by layers of cations [Dimim](+) and anions [FeBr4](-) that change the orientation from layer to layer, with Fe center dot center dot center dot Fe distances larger than 6.7 angstrom. Magnetization measurements show the presence of 3D antiferromagnetic ordering below T-N with the existence of a noticeable magneto crystalline anisotropy. From low-temperature neutron diffraction data, it can be observed that the existence of antiferromagnetic order is originated by the antiparallel ordering of ferromagnetic layers of [FeBr4](-) metal complex along the b direction. The magnetic unit cell is the same as the chemical one, and the magnetic moments are aligned along the c direction. The DFT calculations reflect the fact that the spin density of the iron ions spreads over the bromine atoms. In addition, the projected density of states (PDOS) of the irnidazolium with the bromines of a [FeBr4] metal complex confirms the existence of the anion-pi interaction. Magneto structural correlations give no evidence for direct iron-iron interactions, corroborating that the 3D magnetic ordering takes place via superexchange coupling, the Fe-Br center dot center dot center dot Br-Fe interplane interaction being defined as the main exchange pathway.