Weak Antiferromagnetic Coupling Research Articles

Structural Trends in a 3 d‐4 f System: Ln(NO3)3−Cu(2,2‐bipyridine)2(NO3)2 (Ln=La−Nd, Sm−Yb)

AbstractA series of thirteen complexes which may be formally considered as assembled from Cu(bpy)2(NO3)2 and Ln(NO3)3 (Ln=La−Yb excluding Pm, bpy=2,2’‐bipyridine) have been characterized. The crystals may be grouped into four classes: A, [Cu2(bpy)4(μ‐NO3)][Ln(NO3)6] (Ln=La−Nd); B, [Cu(bpy)2(NO3)]2[Ln(NO3)4(OH2)2](NO3) (Ln=Sm−Dy); C, [Cu(bpy)2(NO3)]2[Ln(NO3)5] (Ln=Ho−Tm); D, [Cu(bpy)2(NO3)]2 [Ln(NO3)3(OH2)3](NO3)2⋅2H2O (Ln=Yb). The coordination numbers of the nitrato or aquonitrato lanthanide anions range from 9 to 12. The dinuclear cation in Class A having a linear monoatomic nitrate bridge is new and is seen to produce weak antiferromagnetic coupling in these compounds. The other classes contain distortion isomers of [Cu(bpy)2NO3]+. The structural trends are analysed in terms of lanthanide contraction and non‐covalent interactions in the crystals. EPR and electronic spectral characteristics are discussed for the four classes of compounds.

Structural, Magnetic, and Optical Studies of the Polymorphic 9'-Anthracenyl Dithiadiazolyl Radical.

The fluorescent 9'-anthracenyl-functionalized dithiadiazolyl radical (3) exhibits four structurally determined crystalline phases, all of which are monomeric in the solid state. Polymorph 3α (monoclinic P21/ c, Z' = 2) is isolated when the radical is condensed onto a cold substrate (enthalpically favored polymorph), whereas 3β (orthorhombic P21 21 21, Z' = 3) is collected on a warm substrate (entropically favored polymorph). The α and β polymorphs exhibit chemically distinct structures with 3α exhibiting face-to-face π-π interactions between anthracenyl groups, while 3β exhibits edge-to-face π-π interactions. 3α undergoes an irreversible conversion to 3β on warming to 120 °C (393 K). The β-phase undergoes a series of reversible solid-state transformations on cooling; below 300 K a phase transition occurs to form 3γ (monoclinic P21/ c, Z' = 1), and on further cooling below 165 K, a further transition is observed to 3δ (monoclinic P21/ n, Z' = 2). Both 3β → 3γ and 3γ → 3δ transitions are reversible (single-crystal X-ray diffraction), and the 3γ → 3δ process exhibits thermal hysteresis with a clear feature observed by heat capacity measurements. Heating 3β above 160 °C generates a fifth polymorph (3ε) which is distinct from 3α-3δ based on powder X-ray diffraction data. The magnetic behavior of both 3α and the 3β/3γ/3δ system reflect an S = 1/2 paramagnet with weak antiferromagnetic coupling. The reversible 3δ ↔ 3γ phase transition exhibits thermal hysteresis of 20 K. Below 50 K, the value of χm T for 3δ approaches 0 emu·K·mol-1 consistent with formation of a gapped state with an S = 0 ground-state configuration. In solution, both paramagnetic 3 and diamagnetic [3][GaCl4] exhibit similar absorption and emission profiles reflecting similar absorption and emission mechanisms for paramagnetic and diamagnetic forms. Both emit in the deep-blue region of the visible spectrum (λem ∼ 440 nm) upon excitation at 255 nm with quantum yields of 4% (3) and 30% ([3][GaCl4]) affording a switching ratio [ΦF(3+)/ΦF(3)] of 7.5 in quantum efficiency with oxidation state. Solid-state films of both 3 and [3][GaCl4] exhibit emission bands at a longer wavelength (490 nm) attributed to excimer emission.

Rational Improvement of Single-Molecule Magnets by Enforcing Ferromagnetic Interactions.

The anisotropy barrier of polynuclear single-molecule magnets is expected to be higher with less tunneling the better stabilized the spin ground state is so that less MS mixing in the ground state and with excited spin states occur. We have realized this experimentally in two structurally related heptanuclear SMMs: the triplesalen-based [MnIII 6 CrIII ]3+ and the triplesalalen-based *[MnIII 6 CrIII ]3+ . The ligand system triplesalen was developed to enforce ferromagnetic interactions by the spin-polarization mechanism. However, we found weak antiferromagnetic couplings, that we assigned to an inefficient spin-polarization by a heteroradialene formation. To prevent this heteroradialene formation, the triplesalalen ligand H6 talalen was designed. Here, we present the building block [(talalen )MnIII 3 ]3+ and its application for the assembly of [{(talalen )MnIII 3 }2 {CrIII (CN)6 }]3+ (=*[MnIII 6 CrIII ]3+ ). Both the trinuclear and heptanuclear complexes are SMMs. The comparison to the related triplesalen complex [(feld )MnIII 3 ]3+ proves the absence of heteroradialene character and the enforcement of ferromagnetic MnIII -MnIII interactions in the (talalen )6- complexes. This results in an increase of the barrier for spin reversal Ueff from 25 K in the triplesalen-based [MnIII 6 CrIII ]3+ SMMs to 37 K in the triplesalalen-based *[MnIII 6 CrIII ]3+ SMM proving the success of our concept. Based on this study, the next step in the rational improvement of our SMMs is discussed.

Copper(II) complex with L-arginine – Crystal structure, DFT calculations, spectroscopic, thermal and magnetic properties

Abstract Complex of formula [Cu(L−Arg)(phen)Cl]Cl·2H2O (1) was synthesized in crystalline form (triclinic crystal system, P1 (No. 1) space group with a = 9.376(1) A, b = 10.415(1) A, c = 12.209(2) A and Z = 2). The relations between the crystal structure, DFT calculations, spectroscopic properties (FT−IR, FT−Raman, NIR−Vis−UV, EPR) as well as the parameters of magnetic data were investigated. The square pyramidal geometry around copper (II) cations is very slightly distorted toward bipyramid geometry. The DFT-calculated interaction energies (ΔE) for the L−Arg zwitterion and phen ligands are −135.31 and −54.17 kcal mol−1, respectively. The g tensor diagonal components obey the relationship g|| > g⊥ (g⊥ = 2.063, g|| = 2.23) which suggested the pure dx2-y2 ground state orbital. Analysis of the crystal structure shows two possible magnetic exchange pathways: the neighboring [Cu( l −Arg)(phen)Cl]+ cations are linked in dimeric units by the N−H⋯Cl type hydrogen bonds, and almost parallel phenanthroline molecules provide π–π stacking interaction between the dimers. Analysis of the magnetic data, using an alternating ferro− and antiferromagnetic chain model for S = 1/2 spins, reveals the existence of weak antiferromagnetic coupling within the dimers (JAF = −0.23 cm−1) and weak ferromagnetic interaction between dimers (JF = 0.11 cm−1).

A series of two-dimensional lanthanide coordination polymers: synthesis, structures, magnetism and selective luminescence detection for heavy metal ions and toxic solvents.

A series of two-dimensional (2D) coordination polymers (CPs), namely poly[[bis(μ-acetato)diaqua(μ6-biphenyl-3,3',5,5'-tetracarboxylato)bis(N,N-dimethylacetamide)digadolinium(III)] N,N-dimethylacetamide monosolvate], {[Gd2(C16H6O8)(C2H3O2)2(C4H9NO)2(H2O)2]·C4H9NO}n (CP1), poly[[bis(μ-acetato)diaqua(μ6-biphenyl-3,3',5,5'-tetracarboxylato)bis(N,N-dimethylacetamide)didysprosium(III)] N,N-dimethylacetamide monosolvate], {[Dy2(C16H6O8)(C2H3O2)2(C4H9NO)2(H2O)2]·C4H9NO}n (CP2), poly[bis(μ-acetato)diaqua(μ6-biphenyl-3,3',5,5'-tetracarboxylato)bis(N,N-dimethylacetamide)dineodymium(III)], [Nd2(C16H6O8)(C2H3O2)2(C4H9NO)2(H2O)2]n (CP3), poly[bis(μ-acetato)diaqua(μ6-biphenyl-3,3',5,5'-tetracarboxylato)bis(N,N-dimethylacetamide)disamarium(III)], [Sm2(C16H6O8)(C2H3O2)2(C4H9NO)2(H2O)2]n (CP4), has been synthesized from rigid biphenyl-3,3',5,5'-tetracarboxylic acid under solvothermal conditions. Their structures have been determined by single-crystal X-ray diffraction analyses, elemental analyses, IR spectra, powder X-ray diffraction and thermogravimetric analyses, and CP1-CP4 crystallize in the monoclinic space group P21/n. CP1-CP4 are isomorphous and feature similar 2D double layers, which are further extended via interlayer hydrogen-bonding interactions into a three-dimensional (3D) supramolecular structure. Hydrogen-bonding interactions between N,N-dimethylacetamide molecules and carboxylate O atoms strengthen the packing of the layers. The organic ligands interconnect with metal ions to generate 2D layered structures with a (4,4)-connected net having {44.62} topology. CP1 has been investigated for its magnetic properties and magnetic susceptibility measurements were carried out in the range 2.0-300 K. The results of the magnetic measurements show weak antiferromagnetic coupling between the GdIII ions in CP1. Moreover, the strong luminescence of CP2 and CP4 can be selectively quenched by the Fe3+ ion and toxic solvents (e.g. acetone).

Metal-Bonded Redox-Active Triarylamines and Their Interactions: Synthesis, Structure, and Redox Properties of Paddle-Wheel Copper Complexes.

Four new triphenylamine ligands with different substituents in the para position and their corresponding copper(II) complexes are reported. This study includes their structural, spectroscopic, magnetic, and electrochemical properties. The complexes possess a dinuclear copper(II) paddle‐wheel core, a building unit that is also common in metal‐organic frameworks. Electrochemical measurements demonstrate that the triphenylamine ligands and the corresponding complexes are susceptible to oxidation, resulting in the formation of stable radical cations. The square‐wave voltammograms observed for the complexes are similar to those of the ligands, except for a slight shift in potential. Square‐wave voltammetry data show that, in the complexes, these oxidations can be described as individual one‐electron processes centered on the coordinated ligands. Spectroelectrochemistry reveals that, during the oxidation of the complexes, no difference can be detected for the spectra of successively oxidized species. For the absorption bands of the oxidized species of the ligands and complexes, only a slight shift is observed. ESR spectra for the chemically oxidized complexes indicate ligand‐centered radicals. The copper ions of the paddle‐wheel core are strongly antiferromagnetic coupled. DFT calculations for the fully oxidized complexes indicate a very weak ferromagnetic coupling between the copper ions and the ligand radicals, whereas a very weak antiferromagnetic coupling is found among the ligand radicals.

A family of lanthanide complexes with a bis-tridentate nitronyl nitroxide radical: syntheses, structures and magnetic properties.

Eleven new lanthanide complexes based on a bis-tridentate nitronyl nitroxide radical NIT-Pm2Py (2-(4,6-di(pyridin-2-yl)pyrimidin-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxy-3-oxide), namely (NIT-Pm2Py)Ln(hfac)3 (Ln = Gd (1Gd), Tb (2Tb), Dy (3Dy), Ho (4Ho), Er (5Er), Yb (6Yb)), [(NIT-Pm2Py)Ln2(hfac)6]·xH2O (Ln = Gd (7Gd), Tb (8Tb), Ho (10Ho), x = 0.5 for 7Gd and 1 for 8Tb and 10Ho) and (NIT-Pm2Py)Ln2(hfac)6 (Ln = Dy (9Dy), Er (11Er)) were prepared and characterized. These complexes can be selectively prepared by controlling the reaction ratio of Ln(hfac)3·2H2O to the radical ligand NIT-Pm2Py. Single crystal X-ray crystallographic analyses confirmed that 1Gd-6Yb are isostructural 2p-4f LnIII-radical complexes, in which the NIT-Pm2Py radical acts as a terminal tridentate ligand chelating to one LnIII ion. On the other hand, 7Gd-11Er are isostructural 4f-2p-4f LnIII-radical-LnIII complexes with the NIT-Pm2Py acting as a bridging ligand between two LnIII ions. 7Gd-11Er represent a rare family of complexes showing the NIT bridged 4f-2p-4f three-spin motif. Alternating-current (ac) magnetic susceptibility investigations revealed that complex 6Yb exhibits field-induced frequency dependence, suggesting a possible field-induced single-molecule magnet behavior. Ab initio calculations were performed on all these complexes. The fitting of the magnetic susceptibilities of these complexes indicates weak antiferromagnetic coupling between the LnIII and NIT radical.

Coordination polymers of zinc(II) and manganese(II) made by complexation of calix[4]arene functionalized with carboxylates afford alveolar materials

25,27-bis(methoxycarboxylic acid)-26,28-dihydroxycalix[4]arene (calixH2), so call here calix[4]arene dimethoxycarboxylic acid, was investigated for its ability to build metal–organic frameworks. When reacted with zinc or manganese chloride in DMF and deprotonated with trimethylamine it gives Zn (1) and Mn (2) coordination compounds. These crystallize in the triclinic P-1 space group and are isostructural as shown by X-ray diffraction on single crystals. The crystal structures evidence the coordination of calix[4]arenedimethoxycarboxylic acid through the carboxylate groups. This makes a 1D coordination polymer running along the c axis direction in which [MII2(calix)2(H2O)3(DMF)2] (M = Zn or Mn) neutral units are connected. Last step refinements of the crystal structure reveal a lot of residual electron density which was ascribed to uncoordinated disordered water and DMF molecules. This was treated with the SQUEEZE routine of the program PLATON from which the solvent accessible voids per unit cell volume was calculated to be 24.5% for compound 1 (Zn) and 24.9% for compound 2 (Mn). Complementary differential thermal analyses (DTA) combined with thermogravimety analysis (TGA) and mass spectrometry (MS) are in agreement with large amount of incorporated and non-coordinated water and DMF molecules. Adsorption isotherms of N2 measured for compound 1 do not show any significant porosity. The temperature dependence of the magnetic susceptibility (χ) of compound 2 evidences antiferromagnetic interactions and was best fitted with a weak antiferromagnetic exchange coupling between the manganese(II) ions J = − 1.5 cm−1 and small antiferromagnetic inter-dinuclear interactions zJ = − 1.2 cm−1 (g = 2.02).

Antiferromagnetic Interfacial Coupling and Giant Magnetic Hysteresis in La0.5Ca0.5MnO3–SrRuO3 Superlattices

Superlattices are of great importance due to their potential as new materials genome to synthesize new functional materials. Thus, tuning of the ground state of superlattices is crucial to further control their physical properties. In this study, superlattices (SLs) consisting of alternating layers of SrRuO3 (SRO) (5 nm) and La0.5Ca0.5MnO3 (LCMO) (5 nm) are epitaxially grown on SrTiO3 (STO) and LaAlO3 (LAO) substrates with 10-unit-cell periods. A variation in the substrate-induced-strain for this choice of SLs triggers observation of remarkable properties, such as magnetic anisotropy and large magnetic hysteresis. The strain states experienced by LCMO and SRO in these SLs result in strong ferromagnetic interlayer coupling and weak antiferromagnetic interlayer coupling at low temperatures in SLs of LCMO–SRO/STO and a strong antiferromagnetic interlayer coupling in SLs of LCMO–SRO/LAO. Besides, a large magnetic hysteresis resulting from the predominant magnetic anisotropy of SRO together with the strength of magnetic coupling is observed in SLs of LCMO–SRO/LAO along the out-of-plane direction of the LAO substrate. These four different magnetic behaviors along four different directions of substrate orientations are interpreted in terms of preferential orbital occupation and competing magnetic exchange coupling together with magnetic anisotropy. This study demonstrates the subtleties in controlling the strength of magnetic coupling at the interface and stands as a model system to realize fascinating magnetic phenomena in layer-by-layer hetero-epitaxial oxide films.

A Solvent-Stable Zinc(II)–Gadolinium(III) Metal–Organic Framework Assembled with Furan-2,5-Dicarboxylic Acid: Synthesis, Crystal Structure and Magnetic Property

A novel three-dimensional 3d–4f metal–organic framework (MOF), {[(CH3)2NH2]2[Zn2Gd2(FDA)6(DMF)2]·2DMF}n (Zn–Gd) (H2FDA = furan-2,5-dicarboxylic acid) assembled with furan-2,5-dicarboxylic acid has been isolated and characterized by infrared spectra, elemental analysis, thermogravimetric analysis, powder X-ray diffraction and single-crystal X-ray diffraction. Structure analysis indicates this MOF is an interesting three-dimensional 8-connected bcu body centered cubic topological net. Moreover, Zn–Gd MOF shows a tolerance towards moisture and the corresponding solvents, demonstrating the excellent chemical stability. The magnetic property of Zn–Gd MOF suggests a weak antiferromagnetic coupling interaction between adjacent Gd(III) ions in Zn–Gd MOF.

Computational study of the structure and magnetic properties of the weakly-coupled tetranuclear square-planar complex of Cu(II) with a tetraporphyrin sheet

Owing to the fact that weakly-coupled Cu(II) complexes are potentially applicable in the fields of spintronics and single-molecule magnets we have studied one particularly conspicuous case – a tetranuclear square-planar complex of Cu(II) ions with a π-extended tetraporphyrin ligand (Tetrahedron, 2008, 64, 11433), the structural criteria of which, with very long distances between the paramagnetic centers, can sensibly define the exchange coupling constants and magnetic behaviour. We use the “broken symmetry” density functional theory formalism for various spin states (open-shell singlet, triplet and quintet states). The calculated value of the magnetic exchange integral corresponds to a very weak antiferromagnetic coupling between the neighboring Cu(II) ions (Jcalc = –1.48 cm−1(calculated) Jexp = −1.16 cm−1 (experiment)). The opposite Cu(II) ions are coupled ferromagnetically with an extremely small exchange constant (jcalc = 0.09 cm−1 (calculated) that corresponds to the open-shell singlet ground state of the following spin configuration↑↓↑↓. The exchange mechanism between the [Cu(n) + 4N] localized singly-occupied “magnetic” orbitals of the dxy(Cu) + 4sp2(N) nature indicates a diffuse non-zero overlap between them on the Carbon atoms of the organic ligand. The numerical analysis of atomic spin densities and corresponding decomposition coefficients for the singly-occupied “magnetic” orbitals confirm the proposed exchange mechanism. This superexchange pathway is mediated by the in-plane sp2-hybrid orbitals of the Carbon atoms that couple four [Cu + 4N] fragments by the σ-bond system. For more detailed characterization of the target system a topological analysis of electron density distributions in the high-spin quintet states has been carried out using the Bader’s method “quantum theory of atoms in molecules”.

A hydrostable and bromine-functionalized manganese-organic framework with luminescence sensing of Hg2+ and antiferromagnetic properties

A hydrostable and bromine-functionalized MOF with a sxd topology constructed from tetranuclear clusters, {[Mn2(Bript)2(4,4'-bpy).5(DMF)]·(H2O)}n (1) (H2Bript = 4-Bromoisophthalic acid, 4,4'-bpy = 4,4'-bipyridine, DMF = dimethylformamide) was synthesized and structurally characterized. TGA and PXRD results show high thermal and water stability for the framework. 1 displays a 3D framework with the channel of 4.6 × 4.6 Å2 along the c axis. The framework exhibits a reversible and highly selective luminescent sensing function with respect to Hg2+ cation. Magnetic measurements reveal that the presence of weak antiferromagnetic couplings between the four Mn(II) ions in 1.

Four 3D Mn(II) coordination polymers with diverse carboxylate-bridged magnetic chains: Syntheses, crystal structures, magnetic properties

Four new three-dimensional Mn(II) coordination polymers with two isomeric ligands, 5-(3-nitro-4-carboxylphenyl)isophthalic acid (H3L1) and 5-(2-nitro-4-carboxylphenyl)isophthalic acid (H3L2), have been prepared and structurally and magnetically characterized. They are of the formula [Mn(HL1)(CH3CN)] (1), [Mn(L2)]·H2O·H3O (2), [Mn3(L2)2(DMF)2]·H2O (3), [Mn4(HL2)4(H2O)5]·2H2O (4). Compounds 1–3 contain diverse alternating magnetic chain’s sequences in which the chains are linked by the organic spacers into 3D structures. In 1, the two double [(μ-Ocarboxylate)2 and (μ-syn-anti-COO)2] bridges with apparently different structural parameters alternate in the − ABAB − sequence. While in 2, the two sets of double μ-syn-anti-carboxylate bridges with slightly different structural parameters also alternate in the -ABAB- sequence. In 3, the double (μ-syn-anti-COO)2 bridges and (μ-ODMF)(μ-syn-anti-COO)(μ3-COO) triple bridges alternate in the -ABB- sequence. Compound 4 contains two sets of independent motifs: one is 1D chain based on three different double (μ-syn-anti-COO)2 bridges alternate in the -ABC- sequences; the other is 1D tape-like motifs with three different single μ-syn-anti-COO bridges. Two sets of motifs in 4 are alternately linked together by the organic backbones into a complicated 3D structure. Compounds 1, 2 and 4 exhibit weak antiferromagnetic couplings. A magnetostructural survey indicates that the antiferromagnetic coupling through the (μ-syn-anti-COO)2 bridges decreases as the Mn⋯Mn distances increases and the magnitude of antiferromagnetic coupling mainly depends on the MnOMn angle for the double (μ2-Ocarboxylate)2 bridges.

FeII Complexes with Triple N1,N2‐Triazole Bridge Schiff Base Ligand: Antiferromagnetic Dimer vs. Spin Conversion Trimer

Based on the p‐tolyl functionalized Schiff base 1,2,4‐triazole ligand 4‐(p‐tolylidene‐amino)‐4H‐1,2,4‐triazole (toltrz), a series of three triazole‐bridged FeII complexes have been prepared. The compounds represent a range of dinuclear and trinuclear species, with µ2‐N1,N2‐triazole bridges linking the FeII centers. Mössbauer spectroscopy and magnetic susceptibility measurements reveal a gradual single‐step spin crossover (SCO) behavior for the trimeric compound 3 on the central FeII site. Single‐crystal X‐ray diffraction at 120 K confirms that the central FeII site undergoes a HS to LS transition while the peripheral FeII remain in HS state (HS = high spin; LS = low spin). Compounds 1 and 2 are isostructures with different terminal ligand XCN (X = S, Se), these two compounds stay in high spin state over the whole temperature range but display weak antiferromagnetic exchange coupling.

Magnetic interaction in a 2D solid through hydrogen bonds and π-π stacking

The intercalation of pyridine and its 3-substituted halogen derivatives (L) between layers of nickel (T) tetracyanonickelate provides a beautiful example of magnetic interaction through π-π stacking. The intercalation process results in the formation of 3D solids with formula unit TL2[Ni(CN)4]. In the interlayer region, the intercalated molecules are found coordinated to the axial positions for the metal T (Ni). Neighboring molecules remain interacting through their dipole and quadrupole moments. For a coplanar configuration of their aromatic rings, the corresponding π-π clouds appear overlapped, which makes possible the appearance of a weak ferromagnetic interaction between T metal centers, to form an ordered system of magnetic chains separated about 10 Å. The halogen substituent in the pyridine molecule modulates the π-π clouds overlapping and, in consequence, the magnetic interaction through the chain. Such interaction coexists with a weak antiferromagnetic coupling between T atoms through the CN bridges in the two dimensional system, {T[Ni(CN)4]}∝, but at low temperature, the interaction through the π-π clouds overlapping dominates. This contribution includes the study of the magnetic properties of the layered T(H2O)2[Ni(CN)4]∙xH2O precursor solid where the magnetic interaction takes place through the hydrogen bonding network formed by the water molecules found in the interlayer region. The magnetic properties of both series of layered solids were evaluated from low temperature SQUID measurements complemented with structural and spectroscopic information. The interpretation of the magnetic and structural data is supported by computational calculations on the intermolecular interactions in the interlayer region.

Novel Topology in Semiconducting Tetrathiafulvalene Lanthanide Metal‐Organic Frameworks

AbstractTetrathiafulvalene tetrabenzoate (TTFTB) and several lanthanide ions self‐assemble into metal‐organic frameworks (MOFs) that exhibit a novel topology, a (3,3,3,6,6)‐coordinated net, which features an unusual ligand coordination mode and stacking motif. The Yb and Lu MOFs are electrically conductive, with pellet conductivity values of 9(7) × 10−7 and 3(2) × 10−7 S/cm, respectively. The crystallographically‐determined bond lengths indicate partial oxidation of the ligand, with close S ⋅ ⋅ ⋅ S contacts between ligands providing likely charge transport pathways in the material. Magnetometry reveals temperature‐independent paramagnetism, consistent with the presence of ligand‐based radicals, as well as weak antiferromagnetic coupling between Yb3+ centers. These results illustrate the diversity of MOF structures and properties that are accessible with the TTFTB ligand owing to its electroactive nature, propensity for intermolecular interactions, and conformational flexibility.

Abrupt Spin Crossover Behavior in a Linear N1,N2-Triazole Bridged Trinuclear Fe(II) Complex

The synthesis, structures and magnetic properties of a new trinuclear spin crossover complex [FeII3(pyrtrz)6(TsO)6]·10H2O·2CH3OH (C2) and its analogue binuclear [FeII2(pyrtrz)5(SCN)4]·7H2O (C1), are reported here. These two compounds are synthesized based on the pyrrolyl functionalized Schiff base 1,2,4-triazole ligand 4-((1H-pyrrol-2-yl)methylene-amino)-4H-1,2,4-triazole (pyrtrz), which represent rare discrete multi-nuclear species, with µ2-N1,N2-triazole bridges linking the FeII centers. DC magnetic susceptibility measurements revealed an abrupt single-step spin crossover (SCO) behavior for compound 2 on the central FeII site and single-crystal X-ray diffraction (173 K) showed that this compound crystallizes in the monoclinic space group (P21/c), and multiple intramolecular interactions were found responsible for the abrupt transition. Compound 1 is a binuclear complex with thiocyanate as terminal ligands. This compound stays in high spin state over the whole temperature range and displays weak antiferromagnetic exchange coupling.

Crystalline functionalized endohedral C60 metallofullerides

Endohedral metallofullerenes have been extensively studied since the first experimental observation of La@C60 in a laser-vaporized supersonic beam in 1985. However, most of these studies have focused on metallofullerenes larger than C60 such as (metal)@C82, and there are no reported purified C60-based monomeric metallofullerenes, except for [Li@C60]+(SbCl6)− salt. Pure (metal)@C60 compounds have not been obtained because of their extremely high chemical reactivity. One route to their stabilization is through chemical functionalization. Here we report the isolation, structural determination and electromagnetic properties of functionalized crystalline C60-based metallofullerenes Gd@C60(CF3)5 and La@C60(CF3)5. Synchrotron X-ray single-crystal diffraction reveals that La and Gd atoms are indeed encapsulated in the Ih-C60 fullerene. The HOMO-LUMO gaps of Gd@C60 and La@C60 are significantly widened by an order of magnitude with addition of CF3 groups. Magnetic measurements show the presence of a weak antiferromagnetic coupling in Gd@C60(CF3)3 crystals at low temperatures.

A Multifunctional Lanthanide Carbonate Cluster Based Metal-Organic Framework Exhibits High Proton Transport and Magnetic Entropy Change.

A novel multifunctional, three-dimensional (3D) lanthanide carbonate cluster based metal-organic framework (MOF) with the general formula {[Gd2(CO3)(ox)2(H2O)2]·3H2O} n (1) has been synthesized via self-assembly of gadolinium (Gd) carbonate and oxalate under hydrothermal conditions. Single-crystal X-ray diffraction reveals that the compound 1 consists of the Gd carbonate cluster with oxalic acid ligands, which form a 3D framework structure with an ordered one-dimensional (1D) pore channel along the a-axis. The coordination water molecules of Gd3+ ions point to the interior of the pore and form a 1D hydrogen bond pathway with oxygen atoms in adjacent oxalic acid that is stable at high temperature (up to 150 °C). The compound 1 features multiple hydrogen-bonding walls and good thermal stabilities, and shows the highest proton conductivity of 1.98 × 10-3 S cm-1 at T = 150 °C and in room air without additional humidity. Magnetic investigations of compound 1 demonstrate that weak antiferromagnetic couplings between adjacent Gd3+ ions bring about large cryogenic magnetocaloric effects. Remarkably, the maximum entropy change (-Δ Sm) of compound 1 reaches 58.5 J kg-1 K-1 at 2 K for a moderate field change (Δ H = 7 T). Moreover, the isomorphous MOFs: {[Ln2(CO3)(ox)2(H2O)2]·3H2O} n (Ln3+ = Ce3+(2), Pr3+(3), Nd3+(4), Tb3+(5)) also are structurally and functionally characterized, and compounds 2-5 exhibit proton conductivity above 10-3 S cm-1 in room air and without additional humidity.

Enantiopure versus Racemic Mixture in Reversible, Two-Step, Single-Crystal-to-Single-Crystal Transformations of Copper(II) Complexes.

The reaction of chiral sodium complexes, 1∞ [Na(S-valmetH)]⋅H2 O (1-S) and 1∞ [Na(R-valmetH)]⋅H2 O (1-R), with copper(II) acetate affords chiral one-dimensional coordination polymers with the formulas 1∞ [Cu(S-valmet)(H2 O)]⋅H2 O (2-S) and 1∞ [Cu(R-valmet)(H2 O)]⋅H2 O (2-R) (R/S-valmetH2 are Schiff base proligands resulting from the condensation reactions between o-vanillin and R/S-methionine). The copper ions are connected by the carboxylato groups belonging to the amino-acid moieties, resulting in infinite chains showing syn-anti out-of-plane bridging mode. The circular dichroism spectra of 1-S, 1-R, 2-S, and 2-R confirm their enantiomeric nature. Compounds 2-S and 2-R undergo a two-step single-crystal-to-single-crystal transformation, with the elimination of the lattice and coordinated water molecules: 1∞ [Cu(S-valmet)(H2 O)]⋅H2 O (2-S)→1∞ [Cu(S-valmet)]⋅H2 O (3-S⋅H2 O)→1∞ [Cu(S-valmet)] (3-S) and 1∞ [Cu(R-valmet)(H2 O)]⋅H2 O (2-R)→1∞ [Cu(R-valmet)]⋅H2 O (3-R⋅H2 O)→1∞ [Cu(R-valmet)] (3-R), respectively. During these transformations, every pair of face-to-face chains present in 2-S (or 2-R) has been "zipped up" into a chiral double chain through the removal of the aqua ligands and their replacement by the carboxylato oxygen atoms from the neighboring chain. Consequently, each carboxylato group now bridges three copper ions. The conversion of the single chains, 2-S and 2-R, into the double chains, 3-S and 3-R, is accompanied by a change of the strength of the exchange interactions between the copper ions: weak antiferromagnetic couplings are observed in compound 2-S (J/kB =-1.23(5) K, H=-2J ΣSi Si+1 ) and relatively strong in compound 3-S (J/kB =-76.0(8) K). When the racemic mixture of the ligands, R,S-valmetH2 , is employed, in the same experimental conditions, a racemic mixture of mononuclear compounds, [Cu(R,S-valmet)(H2 O)2 ]⋅H2 O (4-RS), is obtained. Compound 4-RS also undergoes a SCSC transformation with the elimination of the lattice and one of the coordinated water molecules, resulting in a racemic mixture of chiral chains, 1∞ [Cu(R-valmet)(H2 O)]⋅1∞ [Cu(S-valmet)(H2 O)] (5-RS). In this compound, the coupling of the copper(II) ions within the chains is weak and ferromagnetic (J/kB =+0.10(2) K). These results prove that the chirality of the valmetH2 ligands (optically pure or racemic mixture) plays a key role in the self-assembly process of the copper(II) complexes.