Magnetochemistry, Volume 3, Issue 1 (March 2017) – 15 articles

We describe the synthesis of a new tetratopic phenoxyl radical ligand based on the previously reported ditopic “baqp” radical. The tetratopic ligand is prepared in five steps from 4-tert-butylphenol. Coordination of the ligand with Cu(OAc)₂ produced a tetranuclear complex with an S = ½ ground state featuring a coordinated hexa-anion radical with very strong intramolecular antiferromagnetic exchange coupling. Full article

Structural, transport, and magnetic properties of new organic conductors composed of (BEDT-TTF) ${}_2$ TaF ${}_6$ , where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene, have been investigated. Two $\mathsf{\delta }$ -type polymorphs, monoclinic and orthorhombic phases are obtained by the electrocrystallization. Both phases show a semiconductor-insulator phase transition at 276 K and 300 K for the monoclinic and orthorhombic phases, respectively; the ground state of both salts is a nonmagnetic insulating state. The low-temperature X-ray diffraction measurements show two-fold superlattice reflections in the intercolumnar direction. The low-temperature crystal structures show a clear charge ordered state, which is demonstrated by the molecular shape and intramolecular bond lengths. The observed checkerboard charge ordered state is in agreement with the charge ordering in a dimer Mott insulator. If we distinguish between the monoclinic and orthorhombic phases, the transition temperature of the $\mathsf{\delta }$ -type (BEDT-TTF) ${}_{2}M$ F ${}_6$ conductors ( $M=$ P, As, Sb, and Ta) increases continuously with increasing the anion volume. Full article

We consider structural instabilities exhibited by the one-dimensional (1D) (arene)₂X family of organic conductors in relation with their electronic and magnetic properties. With a charge transfer of one electron to each anion X, these salts exhibit a quarter-filled (hole) conduction band located on donor stacks. Compounds built with donors such as fluorenthene, perylene derivatives and anions X such as PF₆ or AsF₆ exhibit a high temperature (T_P ~ 170 K) conventional Peierls transition that is preceded by a sizeable regime of 1D 2k_F charge density wave fluctuations (k_F is the Fermi wave vector of the 1D electron gas located on Per stack). Surprisingly, and probably because of the presence of a multi-sheet warped Fermi surface, the critical temperature of the Peierls transition is considerably reduced in the perylene series α-(Per)₂[M(mnt)₂] where X is the dithiolate molecule with M = Au, Cu, Co and Fe. Special attention will be devoted to physical properties of α-(Per)₂[M(mnt)₂] salts with M = Pt, Pd and Ni which incorporate segregated S = 1/2 1D antiferromagnetic (AF) dithiolate stacks coexisting with 1D metallic Per stacks. We analyze conjointly the structural and magnetic properties of these salts in relation with the 1D spin-Peierls (SP) instability located on the dithiolate stacks. We show that the SP instability of Pd and Ni derivatives occurs in the classical (adiabatic) limit while the SP instability of the Pt derivative occurs in the quantum (anti-adiabatic) limit. Furthermore, we show that in Pd and Ni derivatives 1st neighbor direct and frustrated 2nd neighbor indirect (through a fine tuning with the mediated 2k_F RKKY coupling interaction on Per stacks) AF interactions add their contribution to the SP instability to stabilize a singlet-triplet gap. Our analysis of the data show unambiguously that magnetic α-(Per)₂[M(mnt)₂] salts exhibit the physics expected for a two chain Kondo lattice. Full article

Molecular spins have shown interesting quantum features, which make them potential candidates for the implementation of quantum information processing. New challenges related to possible applications in the broader class of quantum technologies are currently under discussion. Here, we revisit some key features trying to learn something from experiences in related fields. Full article

We have prepared two dmit-based salts with a stable organic radical-substituted ammonium cation, (PO-CONH-C₂H₄N(CH₃)₃)[Ni(dmit)₂]₂·CH₃CN and (PO-CONH-C₂H₄N(CH₃)₃)[Pd(dmit)₂]₂ where PO is 2,2,5,5-Tetramethyl-3-pyrrolin-1-oxyl and dmit is 2-Thioxo-1,3-dithiol-4,5-dithiolate. The salts are not isostructural but have similar structural features in the anion and cation packing arrangements. The acceptor layers of both salts consist of tetramers, which gather to form 2D conducting layers. Magnetic susceptibility measurements indicate that the Ni salt is a Mott insulator and the Pd salt is a band insulator, which has been confirmed by band structure calculations. The cationic layers of both salts have a previously unreported polar structure, in which the cation dipoles order as ➚➘➚➘ along the acceptors stacking direction to provide dipole moments. The dipole moments of nearest neighbor cation layers are inverted in both salts, indicating no net dipole moments for the whole crystals. The magnetic network of the [Ni(dmit)₂] layer of the Ni salt is two-dimensional so that the magnetic susceptibility would be expected to obey the 1D or 2D Heisenberg model that has a broad maximum around T ≈ θ. However, the magnetic susceptibility after subtraction of the contribution from the PO radical has no broad maximum. Instead, it shows Curie–Weiss behavior with C = 0.378 emu·K/mol and θ = −35.8 K. The magnetic susceptibility of the Pd salt obeys a Curie–Weiss model with C = 0.329 emu·K·mol⁻¹ and θ = −0.88 K. Full article

The πd molecular conductor λ‐(BETS)2FeCl4, where BETS is bis(ethylenedithio) tetraselenafulvalene, has attracted considerable interest for the discovery of its field induced superconducting state. A mystery of this system is its antiferromagnetic insulating ground state. The point still under strong debate is whether the d spins in Fe3+ are ordered or not. Here, we review experimental and theoretical studies on the antiferromagnetic insulating phase in λ‐(BETS)2FeCl4 and mention our perspective based on our ESR measurements for λ‐(BETS)2FexGa1−xCl4. Our ESR results indicate that the π‐d interaction in the system is very strong and there is no sign of paramagnetic Fe spins in the antiferromagnetic ground state. Full article

Three systems [Fe(III)(sal₂-trien)](TCNQ)_n·X (n = 1, 2, X = MeOH, CH₃CN, H₂O) showing spin-crossover transition, conductivity and ferromagnetic coupling were synthesized and studied by X-ray diffraction, Montgomery method for resistivity, SQUID magnetometry and X-band EPR. Spin-spin interactions between local magnetic moments of Fe(III) ions and electron spins of organic TCNQ network were discovered and discussed within the framework of intermolecular superexchange coupling. Full article

To develop novel magnetic conductors exhibiting conducting/magnetic bifunctionalities and peculiar responses to applied magnetic fields, we synthesized new EDT-TTF (ethylenedithiotetrathiafulvalene) donor containing a 2,2,5,5-tetramethylpyrrolin-1-yloxyl radical through a π-conjugated vinylene spacer 1 and examined its electronic and crystal structures, and physical properties. We also prepared its cation radical salts by an electrochemical oxidation method and successfully cleared the crystal structures and magnetic properties of the cation radical salts, 1·FeCl4 and 1·GaCl4. These salts have strongly dimerized one-dimensional arrays of the fully oxidized donor molecules, giving rise to the formation of spin-singlet state of the π cation radical spins in the dimer. On the other hand, the FeCl4- anion locates on the side of the dimers with very short S-Cl contacts and mediates very strong π-d interaction between the donor and anion moieties, resulting in the antiferromagnetic behavior of the Weiss temperature of θ = -3.9 K through its d-π-d interaction. Full article

Two novel paramagnetic conductors have been prepared with the organic donor bis(ethylenedithio)tetrathiafulvalene (BEDT‐TTF = ET) and paramagnetic Mn‐containing metallic complexes: κ′‐ET4[KMnIII(C2O4)3]∙PhCN (1) and ET[MnIICl4]∙H2O (2). Compound 1 represents the first Mn‐containing ET salt of the large Day’s series of oxalato‐based molecular conductors and superconductors formulated as (ET)4[AM(C2O4)3]∙G (A+ = H3O+, NH4+, K+, ...; MIII = Fe, Cr, Al, Co, ...; G = PhCN, PhNO2, PhF, PhCl, PhBr, ...). It crystallizes in the orthorhombic pseudo‐κ phase where dimers of ET molecules are surrounded by six isolated ET molecules in the cationic layers. The anionic layers contain the well‐known hexagonal honey‐comb lattice with Mn(III) and H3O+ ions connected by C2O42- anions. Compound 2 is one of the very few examples of ET salts containing ET2+. It also presents alternating cationic‐anionic layers although the ET molecules lie parallel to the layers instead of the typical almost perpendicular orientation. Both salts are semiconductors with room temperature conductivities of ca. 2 x 10-5 and 8 x 10−5 S/cm and activation energies of 180 and 210 meV, respectively. The magnetic properties are dominated by the paramagnetic contributions of the high spin Mn(III) (S = 2) and Mn(II) (S = 5/2) ions. Full article

We have scaled up the production process of magnetic refrigerants near room temperature. The Mn_2−yFe_y−xRu_xP_1−zSi_z compounds with 0.03 ≤ x ≤ 0.16, y ≈ 0.75, and z ≈ 0.55 were synthesized and their magnetocaloric properties were examined. By changing the compositions and the annealing temperature, the Curie temperature was tuned between 275 and 315 K with 2~3 K steps. All the compounds underwent a first-order magnetic transition accompanied by thermal hysteresis of less than 2 K. The compounds showed excellent magnetocaloric properties: the magnetic entropy change was more than 10 J/K·kg and the refrigerant capacity was about 115 J/kg in a field change of 1.5 T. The detailed instructions to synthesize high-performance (MnFe)₂PSi materials are given. Full article

The 1:1 reactions between hydrated lanthanide(III) nitrates and triethanolamine (teaH₃) in MeOH, in the absence of external bases, have provided access to the dinuclear complexes [Ln₂(NO₃)₄(teaH₂)₂] (Ln = Pr, 1; Ln = Gd, 2; Ln = Tb, 3; Ln = Dy, 4; Ln = Ho, 5) containing the singly deprotonated form of the ligand. Use of excess of the ligand in the same solvent gives mononuclear complexes containing the neutral ligand and the representative compound [Pr(NO₃)(teaH₃)₂](NO₃)₂ (6) was characterized. The structures of the isomorphous complexes 1∙2MeOH, 2∙2MeOH and 4∙2MeOH were solved by single-crystal X-ray crystallography; the other two dinuclear complexes are proposed to be isostructural with 1, 2 and 4 based on elemental analyses, IR spectra and powder XRD patterns. The IR spectra of 1–6 are discussed in terms of structural features of the complexes. The two Ln^III atoms in centrosymmetric 1∙2MeOH, 2∙2MeOH and 4∙2MeOH are doubly bridged by the deprotonated oxygen atoms of the two η¹:η¹:η¹:η²:μ₂ teaH₂⁻ ligands. The teaH₂⁻ nitrogen atom and six terminal oxygen atoms (two from the neutral hydroxyl groups of teaH₂⁻ and four from two slightly anisobidentate chelating nitrato groups) complete 9-coordination at each 4f-metal center. The coordination geometries of the metal ions are spherical-relaxed capped cubic (1∙2MeOH), Johnson tricapped trigonal prismatic (2∙2MeOH) and spherical capped square antiprismatic (4·2MeOH). O–H∙∙∙O H bonds create chains parallel to the a axis. The cation of 6 has crystallographic two fold symmetry and the rotation axis passes through the Pr^III atom, the nitrogen atom of the coordinated nitrato group and the non-coordinated oxygen atom of the nitrato ligand. The metal ion is bound to the two η¹:η¹:η¹:η¹ teaH₃ ligands and to one bidentate chelating nitrato group. The 10-coordinate Pr^III atom has a sphenocoronal coordination geometry. Several H bonds are responsible for the formation of a 3D architecture in the crystal structure of 6. Complexes 1–6 are new members of a small family of homometallic Ln^III complexes containing various forms of triethanolamine as ligands. Dc magnetic susceptibility studies in the 2–300 K range reveal the presence of a weak to moderate intramolecular antiferromagnetic exchange interaction (J = −0.30(2) cm⁻¹ based on the spin Hamiltonian $\widehat{H}=-J({\widehat{S}}_{\mathrm{Gd}1}\cdot {\widehat{S}}_{\mathrm{Gd}1\prime })$ ) for 2 and probably weak antiferromagnetic exchange interactions within the molecules of 3–5. The antiferromagnetic Gd^III∙∙∙Gd^III interaction in 2 is discussed in terms of known magnetostructural correlations for complexes possessing the {Gd₂(μ₂-OR)₂}⁴⁺ core. Ac magnetic susceptibility measurements in zero dc field for 3–5 do not show frequency dependent out-of-phase signals; this experimental fact is discussed and rationalized for complex 4 in terms of the magnetic anisotropy axis for each Dy^III center and the oblate electron density of the metal ion. Full article

The magnetostructural transitions, magnetocaloric effects, and magnetoresistance properties of Ni₄₅Mn₄₃CrSn₁₁ Heusler alloys were investigated using X-ray diffraction (XRD), field-dependent magnetization, and electrical resistivity measurements. A large inverse and direct magnetocaloric effect has been observed in Ni₄₅Mn₄₃CrSn₁₁ across the martensitic and Curie transition temperature, respectively. The values of the latent heat (L = 15.5 J/g) and corresponding magnetic (ΔS_M) and total (∆S_T) entropy changes (ΔS_M = 35 J/kg·K for ΔH = 5T and ∆S_T = 39.7 J/kg·K) have been evaluated using magnetic and differential scanning calorimetry (DSC) measurements, respectively. A substantial jump in resistivity was observed across the martensitic transformation. A large negative magnetoresistance (~67%) was obtained at the magnetostructural transition for a field change of 5 T. The roles of the magnetic and structural changes on the transition temperatures and the potential application of Ni₄₅Mn₄₃CrSn₁₁ Heusler alloys for refrigerator technology are discussed. Full article

The complex [Dy(L)(tta)₃] with L the chiral 3-(2-pyridyl)-4-aza[6]-helicene ligand (tta⁻ = 2-thenoyltrifluoroaacetonate) has been synthesized in its racemic form and structurally and magnetically characterized. [Dy(L)(tta)₃] behaves as a single molecule magnet in its crystalline phase with the opening of a hysteresis loop at 0.50 K. These magnetic properties were interpreted with ab initio calculations. Full article

The syntheses and characterization of four new hybrid coordination networks based on lanthanide ions (Ln = Nd, Sm) and 1,3-carboxymethylimidazolium (L) salt in the presence of oxalic acid (H₂ox) are reported. The influence of the synthesis parameters, such as the nature of the lanthanide ion (Nd³⁺ or Sm³⁺), the nature of the imidazolium source (chloride [H₂L][Cl] or zwitterionic [HL] form) and the presence or not of oxalic acid (H₂ox), is discussed. In the presence of oxalic acid, the samarium salt gives only one compound [Sm(L)(ox)(H₂O)]·H₂O, whatever the nature of the imidazolium ligand, while the neodymium salt leads to three different compounds, [Nd(L)(ox)(H₂O)]·H₂O, [Nd(L)(ox)_0.5(H₂O)₂][Cl] or [Nd₂(L)₂(ox)(NO₃)(H₂O)₃][NO₃], depending on the imidazolium ligand. In the absence of oxalic acid, gels are obtained, except for the reaction between the neodymium salt and [H₂L][Cl], which leads to [Nd(L)(ox)(H₂O)]·H₂O. All compounds crystallized and their structures were determined by single crystal diffraction. The description of these new phases was consistently supported by ancillary techniques, such as powder X-ray diffraction, thermal analyses and UV-visible-near infrared spectroscopy. The luminescent and magnetic properties of the three pure compounds [Sm(L)(ox)(H₂O)]·H₂O, [Nd(L)(ox)(H₂O)]·H₂O and [Nd₂(L)₂(ox)(NO₃)(H₂O)₃][NO₃] were also studied. Full article