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

Abstract

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>