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−π
interactions, which displays a three-dimensional (3D) magnetic ordering
below the Néel temperature, TN =
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 Mössbauer 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 (P21 symmetry)
at room temperature with a = 6.745(3) Å, b = 14.364(3) Å, c = 6.759(3) Å,
and β = 90.80(2)°. 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···Fe distances
larger than 6.7 Å. Magnetization measurements show the presence
of 3D antiferromagnetic ordering below TN 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 imidazolium with the bromines of a [FeBr4]− metal complex confirms the existence
of the anion−π 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···Br–Fe interplane
interaction being defined as the main exchange pathway.