The Series of Molecular Conductors and Superconductors ET₄[AFe(C₂O₄)₃]·PhX (ET = bis(ethylenedithio)tetrathiafulvalene; (C₂O₄)^2– = oxalate; A⁺ = H₃O⁺, K⁺; X = F, Cl, Br, and I): Influence of the Halobenzene Guest Molecules on the Crystal Structure and Superconducting Properties

An extensive series of radical salts formed by the organic donor bis­(ethylenedithio)­tetrathiafulvalene (ET), the paramagnetic tris­(oxalato)­ferrate­(III) anion [Fe­(C₂O₄)₃]^3–, and halobenzene guest molecules has been synthesized and characterized. The change of the halogen atom in this series has allowed the study of the effect of the size and charge polarization on the crystal structures and physical properties while keeping the geometry of the guest molecule. The general formula of the salts is ET₄[A^IFe­(C₂O₄)₃]·G with A/G = H₃O⁺/PhF (<b>1</b>); H₃O⁺/PhCl (<b>2</b>); H₃O⁺/PhBr (<b>3</b>), and K⁺/PhI (<b>4</b>), (crystal data at room temperature: (<b>1</b>) monoclinic, space group <i>C</i>2/<i>c</i> with <i>a</i> = 10.3123(2) Å, <i>b</i> = 20.0205(3) Å, <i>c</i> = 35.2732(4) Å, β = 92.511(2)°, <i>V</i> = 7275.4(2) ų, <i>Z</i> = 4; (<b>2</b>) monoclinic, space group <i>C</i>2/<i>c</i> with <i>a</i> = 10.2899(4) Å, <i>b</i> = 20.026(10) Å, <i>c</i> = 35.411(10) Å, β = 92.974°, <i>V</i> = 7287(4) ų, <i>Z</i> = 4; (<b>3</b>) monoclinic, space group <i>C</i>2/<i>c</i> with <i>a</i> = 10.2875(3) Å, <i>b</i> = 20.0546(15) Å, <i>c</i> = 35.513(2) Å, β = 93.238(5)°, <i>V</i> = 7315.0(7) ų, <i>Z</i> = 4; (<b>4</b>) monoclinic, space group <i>C</i>2/<i>c</i> with <i>a</i> = 10.2260(2) Å, <i>b</i> = 19.9234(2) Å, <i>c</i> = 35.9064(6) Å, β = 93.3664(6)°, <i>V</i> = 7302.83(18) ų, <i>Z</i> = 4). The crystal structures at 120 K evidence that compounds <b>1</b>–<b>3</b> undergo a structural transition to a lower symmetry phase when the temperature is lowered (crystal data at 120 K: (<b>1</b>) triclinic, space group <i>P</i>1̅ with <i>a</i> = 10.2595(3) Å, <i>b</i> = 11.1403(3) Å, <i>c</i> = 34.9516(9) Å, α = 89.149(2)°, β = 86.762(2)°, γ = 62.578(3)°, <i>V</i> = 3539.96(19) ų, <i>Z</i> = 2; (<b>2</b>) triclinic, space group <i>P</i>1̅ with <i>a</i> = 10.25276(14) Å, <i>b</i> = 11.15081(13) Å, <i>c</i> = 35.1363(5) Å, α = 89.0829(10)°, β = 86.5203(11)°, γ = 62.6678(13)°, <i>V</i> = 3561.65(8) ų, <i>Z</i> = 2; (<b>3</b>) triclinic, space group <i>P</i>1̅ with <i>a</i> = 10.25554(17) Å, <i>b</i> = 11.16966(18) Å, <i>c</i> = 35.1997(5) Å, α = 62.7251(16)°, β = 86.3083(12)°, γ = 62.7251(16)°, <i>V</i> = 3575.99(10) ų, <i>Z</i> = 2; (<b>4</b>) monoclinic, space group <i>C</i>2/<i>c</i> with <i>a</i> = 10.1637(3) Å, <i>b</i> = 19.7251(6) Å, <i>c</i> = 35.6405(11) Å, β = 93.895(3)°, <i>V</i> = 7128.7(4) ų, <i>Z</i> = 4). A detailed crystallographic study shows a change in the symmetry of the crystal for compound <b>3</b> at about 200 K. This structural transition arises from the partial ordering of some ethylene groups in the ET molecules and involves a slight movement of the halobenzene guest molecules (which occupy hexagonal cavities in the anionic layers) toward one of the adjacent organic layers, giving rise to two nonequivalent organic layers at 120 K (compared to only one at room temperature). The structural transition at about 200 K is also observed in the electrical properties of <b>1</b>–<b>3</b> and in the magnetic properties of <b>1</b>. The direct current (dc) conductivity shows metallic behavior in salts <b>1</b>–<b>3</b> with superconducting transitions at about 4.0 and 1.0 K in salts <b>3</b> and <b>1</b>, respectively. Salt <b>4</b> shows a semiconductor behavior in the temperature range 300–50 K with an activation energy of 64 meV. The magnetic measurements confirm the presence of high spin <i>S</i> = 5/2 [Fe­(C₂O₄)₃]^3– isolated monomers together with a Pauli paramagnetism, typical of metals, in compounds <b>1</b>–<b>3</b>. The magnetic properties can be very well reproduced in the whole temperature range with a simple model of isolated <i>S</i> = 5/2 ions with a zero field splitting plus a temperature independent paramagnetism (Nα) with the following parameters: <i>g</i> = 1.965, |<i>D</i>| = 0.31 cm^–1, and Nα = 1.5 × 10^–3 emu mol^–1 for <b>1</b>, <i>g</i> = 2.024, |<i>D</i>| = 0.65 cm^–1, and Nα = 1.4 × 10^–3 emu mol^–1 for <b>2</b>, and <i>g</i> = 2.001, |<i>D</i>| = 0.52 cm^–1, and Nα = 1.5 × 10^–3 emu mol^–1 for <b>3</b>

The Series of Molecular Conductors and Superconductors ET₄[AFe(C₂O₄)₃]·PhX (ET = bis(ethylenedithio)tetrathiafulvalene; (C₂O₄)^2– = oxalate; A⁺ = H₃O⁺, K⁺; X = F, Cl, Br, and I): Influence of the Halobenzene Guest Molecules on the Crystal Structure and Superconducting Properties

An extensive series of radical salts formed by the organic donor bis­(ethylenedithio)­tetrathiafulvalene (ET), the paramagnetic tris­(oxalato)­ferrate­(III) anion [Fe­(C₂O₄)₃]^3–, and halobenzene guest molecules has been synthesized and characterized. The change of the halogen atom in this series has allowed the study of the effect of the size and charge polarization on the crystal structures and physical properties while keeping the geometry of the guest molecule. The general formula of the salts is ET₄[A^IFe­(C₂O₄)₃]·G with A/G = H₃O⁺/PhF (<b>1</b>); H₃O⁺/PhCl (<b>2</b>); H₃O⁺/PhBr (<b>3</b>), and K⁺/PhI (<b>4</b>), (crystal data at room temperature: (<b>1</b>) monoclinic, space group <i>C</i>2/<i>c</i> with <i>a</i> = 10.3123(2) Å, <i>b</i> = 20.0205(3) Å, <i>c</i> = 35.2732(4) Å, β = 92.511(2)°, <i>V</i> = 7275.4(2) ų, <i>Z</i> = 4; (<b>2</b>) monoclinic, space group <i>C</i>2/<i>c</i> with <i>a</i> = 10.2899(4) Å, <i>b</i> = 20.026(10) Å, <i>c</i> = 35.411(10) Å, β = 92.974°, <i>V</i> = 7287(4) ų, <i>Z</i> = 4; (<b>3</b>) monoclinic, space group <i>C</i>2/<i>c</i> with <i>a</i> = 10.2875(3) Å, <i>b</i> = 20.0546(15) Å, <i>c</i> = 35.513(2) Å, β = 93.238(5)°, <i>V</i> = 7315.0(7) ų, <i>Z</i> = 4; (<b>4</b>) monoclinic, space group <i>C</i>2/<i>c</i> with <i>a</i> = 10.2260(2) Å, <i>b</i> = 19.9234(2) Å, <i>c</i> = 35.9064(6) Å, β = 93.3664(6)°, <i>V</i> = 7302.83(18) ų, <i>Z</i> = 4). The crystal structures at 120 K evidence that compounds <b>1</b>–<b>3</b> undergo a structural transition to a lower symmetry phase when the temperature is lowered (crystal data at 120 K: (<b>1</b>) triclinic, space group <i>P</i>1̅ with <i>a</i> = 10.2595(3) Å, <i>b</i> = 11.1403(3) Å, <i>c</i> = 34.9516(9) Å, α = 89.149(2)°, β = 86.762(2)°, γ = 62.578(3)°, <i>V</i> = 3539.96(19) ų, <i>Z</i> = 2; (<b>2</b>) triclinic, space group <i>P</i>1̅ with <i>a</i> = 10.25276(14) Å, <i>b</i> = 11.15081(13) Å, <i>c</i> = 35.1363(5) Å, α = 89.0829(10)°, β = 86.5203(11)°, γ = 62.6678(13)°, <i>V</i> = 3561.65(8) ų, <i>Z</i> = 2; (<b>3</b>) triclinic, space group <i>P</i>1̅ with <i>a</i> = 10.25554(17) Å, <i>b</i> = 11.16966(18) Å, <i>c</i> = 35.1997(5) Å, α = 62.7251(16)°, β = 86.3083(12)°, γ = 62.7251(16)°, <i>V</i> = 3575.99(10) ų, <i>Z</i> = 2; (<b>4</b>) monoclinic, space group <i>C</i>2/<i>c</i> with <i>a</i> = 10.1637(3) Å, <i>b</i> = 19.7251(6) Å, <i>c</i> = 35.6405(11) Å, β = 93.895(3)°, <i>V</i> = 7128.7(4) ų, <i>Z</i> = 4). A detailed crystallographic study shows a change in the symmetry of the crystal for compound <b>3</b> at about 200 K. This structural transition arises from the partial ordering of some ethylene groups in the ET molecules and involves a slight movement of the halobenzene guest molecules (which occupy hexagonal cavities in the anionic layers) toward one of the adjacent organic layers, giving rise to two nonequivalent organic layers at 120 K (compared to only one at room temperature). The structural transition at about 200 K is also observed in the electrical properties of <b>1</b>–<b>3</b> and in the magnetic properties of <b>1</b>. The direct current (dc) conductivity shows metallic behavior in salts <b>1</b>–<b>3</b> with superconducting transitions at about 4.0 and 1.0 K in salts <b>3</b> and <b>1</b>, respectively. Salt <b>4</b> shows a semiconductor behavior in the temperature range 300–50 K with an activation energy of 64 meV. The magnetic measurements confirm the presence of high spin <i>S</i> = 5/2 [Fe­(C₂O₄)₃]^3– isolated monomers together with a Pauli paramagnetism, typical of metals, in compounds <b>1</b>–<b>3</b>. The magnetic properties can be very well reproduced in the whole temperature range with a simple model of isolated <i>S</i> = 5/2 ions with a zero field splitting plus a temperature independent paramagnetism (Nα) with the following parameters: <i>g</i> = 1.965, |<i>D</i>| = 0.31 cm^–1, and Nα = 1.5 × 10^–3 emu mol^–1 for <b>1</b>, <i>g</i> = 2.024, |<i>D</i>| = 0.65 cm^–1, and Nα = 1.4 × 10^–3 emu mol^–1 for <b>2</b>, and <i>g</i> = 2.001, |<i>D</i>| = 0.52 cm^–1, and Nα = 1.5 × 10^–3 emu mol^–1 for <b>3</b>

Cobalt Clusters with Cubane-Type Topologies Based on Trivacant Polyoxometalate Ligands

Four novel cobalt-substituted polyoxometalates having cobalt cores exhibiting cubane or dicubane topologies have been synthesized and characterized by IR, elemental analysis, electrochemistry, UV–vis spectroscopy, X-ray single-crystal analysis, and magnetic studies. The tetracobalt­(II)-substituted polyoxometalate [Co₄(OH)₃­(H₂O)₆­(PW₉O₃₄)]^4– (<b>1</b>) consists of a trilacunary [B-α-PW₉O₃₄]^9– unit which accommodates a cubane-like {Co^II₄O₄} core. In the heptacobalt­(II,III)-containing polyoxometalates [Co₇(OH)₆­(H₂O)₆­(PW₉O₃₄)₂]^9– (<b>2</b>), [Co₇(OH)₆­(H₂O)₄­(PW₉O₃₄)₂]_<i>n</i>^9<i>n</i>– (<b>3</b>), and [Co₇(OH)₆­(H₂O)₆­(P₂W₁₅O₅₆)₂]^15– (<b>4</b>), dicubane-like {Co^II₆Co^IIIO₈} cores are encapsulated between two heptadentate [B-α-PW₉O₃₄]^9– (in <b>2</b> and <b>3</b>) or [α-P₂W₁₅O₅₆]^15– (in <b>4</b>) ligands. While <b>1</b>, <b>2</b>, and <b>4</b> are discrete polyoxometalates, <b>3</b> exhibits a polymeric, chain-like structure that results from the condensation of polyoxoanions of type <b>2</b>. The magnetic properties of these complexes have been fitted according to an anisotropic exchange model in the low-temperature regime and discussed on the basis of ferromagnetic interactions between Co²⁺ ions with angles Co–L–Co (L = O, OH) close to orthogonality and weakly antiferromagnetic interactions between Co²⁺ ions connected through central diamagnetic Co³⁺ ion. Moreover, we will show the interest of the unique spin structures provided by these cubane and dicubane cobalt topologies in molecular spintronics (molecular spins addressed though an electric field) and quantum computing (spin qu-gates)

Construction of a General Library for the Rational Design of Nanomagnets and Spin Qubits Based on Mononuclear f‑Block Complexes. The Polyoxometalate Case

This paper belongs to a series of contributions aiming at establishing a general library that helps in the description of the crystal field (CF) effect of any ligand on the splitting of the J ground states of mononuclear f-element complexes. Here, the effective parameters associated with the oxo ligands (effective charges and metal–ligand distances) are extracted from the study of the magnetic properties of the first two families of single-ion magnets based on lanthanoid polyoxometalates (POMs), formulated as [Ln­(W₅O₁₈)₂]^9– and [Ln­(β₂-SiW₁₁O₃₉)₂]^13– (Ln = Tb, Dy, Ho, Er, Tm, Yb). This effective CF approach provides a good description of the lowest-lying magnetic levels and the associated wave functions of the studied systems, which is fully consistent with the observed magnetic behavior. In order to demonstrate the predictive character of this model, we have extended our model in a first step to calculate the properties of the POM complexes of the early 4f-block metals. In doing so, [Nd­(W₅O₁₈)₂]^9– has been identified as a suitable candidate to exhibit SMM behavior. Magnetic experiments have confirmed such a prediction, demonstrating the usefulness of this strategy for the directed synthesis of new nanomagnets. Thus, with an effective barrier of 51.4 cm^–1 under an applied dc field of 1000 Oe, this is the second example of a Nd³⁺-based single-ion magnet

Synthesis and Physical Properties of K₄[Fe(C₅O₅)₂(H₂O)₂](HC₅O₅)₂·4H₂O (C₅O₅^2– = Croconate): A Rare Example of Ferromagnetic Coupling via H-bonds

The reaction of the croconate dianion (C₅O₅)^2– with a Fe­(III) salt has led, unexpectedly, to the formation of the first example of a discrete Fe­(II)–croconate complex without additional coligands, K₄[Fe­(C₅O₅)₂(H₂O)₂]­(HC₅O₅)₂·4H₂O (<b>1</b>). <b>1</b> crystallizes in the monoclinic <i>P</i>2₁/<i>c</i> space group and presents discrete octahedral Fe­(II) complexes coordinated by two chelating C₅O₅^2– anions in the equatorial plane and two trans axial water molecules. The structure can be viewed as formed by alternating layers of <i>trans</i>-diaquabis­(croconato)­ferrate­(II) complexes and layers containing the monoprotonated croconate anions, HC₅O₅^–, and noncoordinated water molecules. Both kinds of layers are directly connected through a hydrogen bond between an oxygen atom of the coordinated dianion and the protonated oxygen atom of the noncoordinated croconate monoanion. A H-bond network is also formed between the coordinated water molecule and one oxygen atom of the coordinated croconate. This H-bond can be classified as strong–moderate being the O···O bond distance (2.771(2) Å) typical of moderate H-bonds and the O–H···O bond angle (174(3)°) typical of strong ones. This H-bond interaction leads to a quadratic regular layer where each [Fe­(C₅O₅)₂(H₂O)₂]^2– anion is connected to its four neighbors in the plane through four equivalent H-bonds. From the magnetic point of view, these connections lead to an <i>S</i> = 2 quadratic layer. The magnetic properties of <b>1</b> have been reproduced with a 2D square lattice model for <i>S</i> = 2 ions with <i>g</i> = 2.027(2) and <i>J</i> = 4.59(3) cm^–1. This model reproduces quite satisfactorily its magnetic properties but only above the maximum. A better fit is obtained by considering an additional antiferromagnetic weak interlayer coupling constant (<i>j</i>) through a molecular field approximation with <i>g</i> = 2.071(7), <i>J</i> = 2.94(7) cm^–1, and <i>j</i> = −0.045(2) cm^–1 (the Hamiltonian is written as <i>H</i> = –<i>JS_iS_j</i>). Although this second model might still be improved since there is also an extra contribution due to the presence of ZFS in the Fe­(II) ions, it confirms the presence of weak ferromagnetic Fe–Fe interactions through H-bonds in compound <b>1</b> which represents one of the rare examples of ferromagnetic coupling via H-bonds