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

Abstract

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.

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

“…The peaks at n s ¼ 110 and n as ¼ 135 cm À1 are attributed to the bending modes and the other two, n s ¼ 332 and n as ¼ 338 cm À1 , respectively are related to the stretching modes of the Fe-Cl bond. This agrees with ndings in Emim[FeX 4 ] (T N : 4.2 K for X ¼ Cl and 12.5 K for X ¼ Br) 38,51 and Dimim[FeX 4 ] (T N : 5.6 K for X ¼ Cl and 7.7 K for X ¼ Br) 40,41 paramagnetic ILs. 6 and Table 4) of Edimim [FeCl 4 ] are assigned to the Edimim + cation.…”

“…S2 †). 41,52 Furthermore, the conformational equilibria of the imidazolium cation has a gauche (non-planar) conformation with respect to NCC angle of the ethyl group, in good agreement with the conformation found in other ILs with 1-ethyl-3-methylimidazolium cation. The topological description of the [FeX 4 ] À units (X ¼ Cl and Br) of both compounds involves a tetrahedral geometry, with a mean X-Fe-X bond angle of 109.5(2) and 109(1) and mean Fe-X bond distances of 2.21(1) and 2.36(1)Å for chloride and bromide derivatives, respectively.…”

“…We observe four Raman-active modes in the low-frequency region (see le-inset of Fig. 41 In the low temperatures regime, c m increases and reaches a maximum at 2.9 K and 9 K for Edimim [FeCl 4 ] and Edimim [FeBr 4 ], respectively, (see the insets of Fig. The peaks at n s ¼ 110 and n as ¼ 135 cm À1 are attributed to the bending modes and the other two, n s ¼ 332 and n as ¼ 338 cm À1 , respectively are related to the stretching modes of the Fe-Cl bond.…”

“…These bands may be compared with the theoretical and experimental vibrational frequencies of Emim +39 and Dimim + cation; 40 which have been previously reported for other members of the imidazolium-based paramagnetic ILs. 41 Fig . This peak is also observed in the bromide compound at 303 cm À1 .…”

“…In addition, if these non-bonding contacts are strong, interesting collective electrical and magnetic phenomena can arise, such as ferroelectricity in the bis(imidazolium) pentachloroantimonate(III), (C 3 N 2 H 5 ) 2 SbCl 5 , or a threedimensional (3D) magnetic ordering in ILs based on tetrahaloferrate ions. 41 Interestingly, it has also been shown that changes in the alkyl chain length of imidazolium cation cause variations in other physicochemical properties, including viscosity, density, conductivity, and melting point. 39 The learning of magneto-structural correlations in ILs based on tetrahaloferrate anion and imidazolium cation need a comprehensive understanding through a systematic investigation of a wide variety of compounds by changing the cationic and anionic structures.…”