Pressure Effects on Emim[FeCl₄], a Magnetic Ionic Liquid with Three-Dimensional Magnetic Ordering

We report a combined study using magnetization and Raman spectroscopy on the magnetic ionic liquid 1-ethyl-3-methyl­imidazolium tetrachloro­ferrate, Emim­[FeCl₄]. This material shows a long-range antiferromagnetic ordering below the Néel temperature <i>T</i>_N ≈ 3.8 K. The effects of pressure on the magnetic properties have been studied using a miniature piston–cylinder CuBe pressure cell. This three-dimensional ordering is strongly influenced when hydrostatic pressure is applied. It is observed that low applied pressure is enough to modify the magnetic interactions, inducing a transition from antiferromagnetic to ferrimagnetic ordering. Raman spectroscopy measurements reveal important information about the existence of isolated [FeCl₄]⁻ anions and the absence of dimeric [Fe₂Cl₇]⁻ units in the liquid and solid states. These features seem to suggest that the superexchange pathways responsible for the appearance of magnetic ordering are mediated through Fe-Cl–Cl-Fe. Furthermore, the liquid–solid phase transition exhibits a magnetic hysteresis near room temperature, which can be tuned by weak pressures

Series of 2D Heterometallic Coordination Polymers Based on Ruthenium(III) Oxalate Building Units: Synthesis, Structure, and Catalytic and Magnetic Properties

A series of 2D ruthenium-based coordination polymers with hcb-hexagonal topology, {[K­(18-crown-6)]₃[M^II₃(H₂O)₄{Ru­(ox)₃}₃]}<i>_n</i> (M^II = Mn (<b>1</b>), Fe (<b>2</b>), Co (<b>3</b>), Cu (<b>4</b>), Zn (<b>5</b>)), has been synthesized through self-assembly reaction. All compounds are isostructural frameworks that crystallize in the monoclinic space group <i>C</i>2/<i>c</i>. The crystal packing consists of a 2D honeycomb-like anionic mixed-metal framework intercalated by [K­(18-crown-6)]⁺ cationic template. Dehydration processes take place in the range 40–200 °C exhibiting two phase transitions. However, the spontaneous rehydration occurs at room temperature. Both hydrated and dehydrated compounds were tested as Lewis acids heterogeneous catalysts in the acetalyzation of benzaldehyde achieving high yields with the possibility to be recovered and reused. All the investigated materials do not show any long-range magnetic ordering down to 2 K. However, the Fe-based compound <b>2</b> presents a magnetic irreversibility in the ZFC-FC magnetization data below 5 K, which suggest a spin-glass-like behavior, characterized also by short-range ferromagnetic correlations. The coercive field increases as the temperature is lowered below 5 K, reaching a value of 1 kOe at 2 K. Alternating current measurements obtained at different frequencies confirm the freezing process that shows weak frequency dependence, being characteristic of a system exhibiting competing magnetic interactions

Selective Carbon Dioxide Hydrogenation Driven by Ferromagnetic RuFe Nanoparticles in Ionic Liquids

CO₂ is selectively hydrogenated to HCO₂H or hydrocarbons (HCs) by RuFe nanoparticles (NPs) in ionic liquids (ILs) under mild reaction conditions. The generation of HCO₂H occurs in ILs containing basic anions, whereas heavy HCs (up to C₂₁ at 150 °C) are formed in the presence of ILs containing nonbasic anions. Remarkably, high values of TONs (400) and a TOF value of 23.52 h^–1 for formic acid with a molar ratio of 2.03 per BMI·OAc IL were obtained. Moreover, these NPs exhibited outstanding abilities in the formation of long-chain HCs with efficient catalytic activity (12% conversion) in a BMI·NTf₂ hydrophobic IL. The IL forms a cage around the NPs that controls the diffusion/residence time of the substrates, intermediates, and products. The distinct CO₂ hydrogenation pathways (HCO₂H or FT via RWGS) catalyzed by the RuFe alloy are directly related to the basicity and hydrophobicity of the IL ion pair (mainly imposed by the anion) and the composition of the metal alloy. The presence of Fe in the RuFe alloy provides enhanced catalytic performance via a metal dilution effect for the formation of HCO₂H and via a synergistic effect for the generation of heavy HCs

3D Magnetically Ordered Open Supramolecular Architectures Based on Ferrimagnetic Cu/Adenine/Hydroxide Heptameric Wheels

The present work provides two new examples of supramolecular metal–organic frameworks consisting of three-dimensional extended noncovalent assemblies of wheel-shaped heptanuclear [Cu₇(μ-H₂O)₆(μ₃-OH)₆­(μ-adeninato-κ<i>N</i>3:κ<i>N</i>9)₆]²⁺ entities. The heptanuclear entity consists of a central [Cu­(OH)₆]^4– core connected to six additional copper­(II) metal centers in a radial and planar arrangement through the hydroxides. It generates a wheel-shaped entity in which water molecules and μ–κ<i>N</i>3:κ<i>N</i>9 adeninato ligands bridge the peripheral copper atoms. The magnetic characterization indicates the central copper­(II) center is anti-ferromagnetically coupled to external copper­(II) centers, which are ferromagnetically coupled among them leading to an <i>S</i> = 5/2 ground state. The packing of these entities is sustained by π–π stacking interactions between the adenine nucleobases and by hydrogen bonds established among the hydroxide ligands, sulfate anions, and adenine nucleobases. The sum of both types of supramolecular interactions creates a rigid synthon that in combination with the rigidity of the heptameric entity generates an open supramolecular structure (40–50% of available space) in which additional sulfate and triethylammonium ions are located altogether with solvent molecules. These compounds represent an interesting example of materials combining both porosity and magnetic relevant features

3D Magnetically Ordered Open Supramolecular Architectures Based on Ferrimagnetic Cu/Adenine/Hydroxide Heptameric Wheels

The present work provides two new examples of supramolecular metal–organic frameworks consisting of three-dimensional extended noncovalent assemblies of wheel-shaped heptanuclear [Cu₇(μ-H₂O)₆(μ₃-OH)₆­(μ-adeninato-κ<i>N</i>3:κ<i>N</i>9)₆]²⁺ entities. The heptanuclear entity consists of a central [Cu­(OH)₆]^4– core connected to six additional copper­(II) metal centers in a radial and planar arrangement through the hydroxides. It generates a wheel-shaped entity in which water molecules and μ–κ<i>N</i>3:κ<i>N</i>9 adeninato ligands bridge the peripheral copper atoms. The magnetic characterization indicates the central copper­(II) center is anti-ferromagnetically coupled to external copper­(II) centers, which are ferromagnetically coupled among them leading to an <i>S</i> = 5/2 ground state. The packing of these entities is sustained by π–π stacking interactions between the adenine nucleobases and by hydrogen bonds established among the hydroxide ligands, sulfate anions, and adenine nucleobases. The sum of both types of supramolecular interactions creates a rigid synthon that in combination with the rigidity of the heptameric entity generates an open supramolecular structure (40–50% of available space) in which additional sulfate and triethylammonium ions are located altogether with solvent molecules. These compounds represent an interesting example of materials combining both porosity and magnetic relevant features

Magnetic Structure, Single-Crystal to Single-Crystal Transition, and Thermal Expansion Study of the (Edimim)[FeCl₄] Halometalate Compound

This contribution addresses standing questions about the nature and consequences of the ion self-assembly and magnetic structures, as well as the molecular motion of the crystalline structure as a function of the temperature, in halometalate materials based on imidazolium cation. We present the magnetic structure and magnetostructural correlations of 1-ethyl-2,3-dimethylimidazolium tetrachloridoferrate, (Edimim)­[FeCl₄], resolved by neutron diffraction studies. Single-crystal, synchrotron powder X-ray diffraction and powder neutron diffraction techniques have been combined to follow the temperature evolution on its crystallographic structure from 2 K close to its melting point (340 K). In this sense, slightly above room temperature (307 K) (Edimim)­[FeCl₄] presents a single-crystal to single-crystal transition (SCSC), from phase <b>I</b> (space group <i>P</i>2₁/<i>n</i>) to phase <b>II</b> (<i>P</i>2₁/<i>m</i>), accompanied by a notable increase in the disorder of the imidazolium cation, as well as in the metal complex anion. The temperature evolution and solid-phase transitions of the presented compound were followed in detail by synchrotron X-ray powder diffraction (SXPD), which confirms the occurrence of another phase transition at 330 K, phase <b>III</b> (<i>P</i>2₁/<i>m</i>), the crystal structure of which was elucidated from the SXPD pattern. Moreover, this material presents an anisotropic thermal expansion with a switch from axial positive to negative thermal expansion coefficients as the temperature is raised above the first phase transition, which has been correlated with the molecular motion of the imidazolium-based molecules, producing not only a shortening of the counterion···counterion distances but also the occurrence of different quasi-isoenergetic crystal structures as a function of the temperature

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, <i>T</i>_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 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 (<i>P</i>2₁ symmetry) at room temperature with <i>a</i> = 6.745(3) Å, <i>b</i> = 14.364(3) Å, <i>c</i> = 6.759(3) Å, and β = 90.80(2)°. Its framework, projected along the <i>b</i> 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 <i>T</i>_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 <i>b</i> direction. The magnetic unit cell is the same as the chemical one, and the magnetic moments are aligned along the <i>c</i> 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