New superconducting charge-transfer salts (BEDT-TTF)4[A·M(C2O4)3]·C6H5NO2 (A = H3O or NH4, M = Cr or Fe, BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene)

Abstract

The syntheses, crystal structures, and physical properties of two new crystalline charge-transfer salts of BEDT-TTF, bis(ethylenedithio)tetrathiafulvalene, containing tris(oxalato)metallate(III) anions of 3d elements are reported. Electrochemical oxidation of BEDT-TTF in the presence of (NH 4 )The crystal structure of [1] has been solved at 120 K in the monoclinic space group C2/c, and that of [2] in the same space group at 298 and 120 K. For [1], a~10.273 A ˚, b~19.949 A ˚, c~35.030 A ˚, b~92.97u, V~7169.6(2) A ˚3, Z~8. For [2], at 298 K: a~10.304 A ˚, b~20.091 A ˚, c~35.251 A ˚, b~92.70u, V~7289.3(2) A ˚3, Z~8, and at 120 K a~10.283 A ˚, b~19.917 A ˚, c~34.939 A ˚, b~93.30u, V~7144.4(1) A ˚3, Z~8. The crystal structures of both compounds consist of alternating layers of BEDT-TTF cations and layers containing [M(C 2 O 4 ) 3 ] 32 , H 3 O z or NH 4 z , and PhNO 2 . The BEDT-TTF molecules are arranged in the b'' packing motif and the tris(oxalato)metallate(III) ions form the well-known honeycomb motif found in many molecular based magnets. SQUID magnetometry, Raman spectroscopy and electron paramagnetic resonance (EPR) measurements were performed on crystals of [1]. SQUID magnetometry, single-crystal four-probe conductivity measurements, Raman spectroscopy, EPR and polarised infrared reflectance were performed on crystals of [2]. Both compounds have metal to superconducting transitions with T c ~6.2 K for [1] and for [2], T c ~5.8 K.

Recent Advances on Anilato-Based Molecular Materials with Magnetic and/or Conducting Properties

“…The introduction of chirality in these materials represents one of the most recent advances [144] in material science and one of the milestones is represented by the first observation of the electrical magneto-chiral anisotropy (eMChA) effect in a bulk crystalline chiral conductor [145], as a synergy between chirality and conductivity [146][147][148]. However, the combination of chirality with electroactivity in chiral TTF-based materials afforded several other recent important results, particularly the modulation of the structural disorder in the solid state , [130][131][132][133][134][135][136][137][138] and hence a difference in conductivity between the enantiopure and racemic forms [149][150][151] and the induction of different packing patterns and crystalline space groups in mixed valence salts of dimethylethylenedithio-TTF (DM-EDT-TTF), showing semiconducting (enantiopure forms) or metallic (racemic form) behaviour [152]. Although the first example of an enantiopure TTF derivative, namely the tetramethyl-bis(ethylenedithio)-tetrathiafulvalene (TM-BEDT-TTF), was described almost 30 years ago as the (S,S,S,S) enantiomer [153,154], the number of TM-BEDT-TTF based conducting radical cation salts is still rather limited.…”

“…The development of these π-d systems as multifunctional materials represents one of the main targets in current materials science for their potential applications in molecular electronics [78,[127][128][129][130]. Important milestones in the field of magnetic molecular conductors have been achieved using as molecular building blocks the bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) organic donor [123,[131][132][133] or its selenium derivatives, and charge-compensating anions ranging from simple mononuclear complexes [MX 4 ] n− (M = Fe III , Cu II ; X = Cl, Br) [134][135][136] In these systems the shape of the anion and the arrangement of intermolecular contacts, especially H-bonding, between the anionic and cationic layers influence the packing motif of the BEDT-TTF radical cations, and therefore the physical properties of the obtained charge-transfer salt [141]. Typically, the structure of these materials is formed by segregated stacks of the organic donors and the inorganic counterions which add the second functionality to the conducting material.…”

The Highly Conducting Spin-Crossover Compound Combining Fe(III) Cation Complex with TCNQ in a Fractional Reduction State. Synthesis, Structure, Electric and Magnetic Properties

“…The trend is associated with the use of the octahedral cation complexes of Fe(II), Fe(III) and Co(II), showing reversible spin-crossover (SCO) between high-spin (HS) and low-spin (LS) states of the metal ion, in combination with the radical anion conducting subsystems [16,17]. The latter could be represented by the systems based on [M(dmit) 2 ] δ− complexes (M = Ni, Pd, Pt; dmit = 4,5-dithiolato-1,3-dithiole-2-thione; 0 < δ < 1) [18] and/or 7,7,8,8,-tetracyanoquinodimethane ((TCNQ) δ− , 0 < δ < 1) [19][20][21][22][23]. The availability of fractional oxidation ([M(dmit) 2 ] δ− ) or reduction states ((TCNQ) δ− ) is a necessary condition for the emergence of high conductivity in these systems.…”

“…In such materials, conductivity is associated with mobile electrons in organic layers, whereas magnetism usually originates from localized spins of transition metal ions in insulating counterion layers. In particular, salts of BEDT-TTF and its selenium-substituted derivative bis(ethylenedithio)tetraselenafulvalene (BETS) have been shown to combine (super)conducting and paramagnetic [7][8][9][10] and even antiferromagnetic [11,12] and ferromagnetic properties [13]. Moreover, interaction between localized spins in insulating magnetic layers and itinerant spins in conducting organic layers was found to lead to new fascinating phenomena such as field-induced superconductivity observed on λ-(BETS) 2 FeCl 4 [14] and κ-(BETS) 2 FeBr 4 [15].…”

Mn‐Containing Paramagnetic Conductors with Bis(ethylenedithio)tetrathiafulvalene (BEDT‐TTF)

“…[59] One of the advantages of these series of compounds is the possibility to tune the electrical properties by simply changing the guest solvent molecule (G) located in the centre of the hexagonal cavities formed by the anionic lattice. This guest molecule may interact with the ET molecules, promoting the ordering of the ethylene groups of the ET molecules and, thus, stabilizing the superconductor state [60] as in the case of G = PhCN and PhNO 2 [6,7,36,49,61] whose radical salts are superconductors and present the highest T c 's in these series: (T c = 6.0, 8.5, 5.8, 6.2 and 7.5 K for G/M = PhCN/Cr and PhCN/Fe, PhNO 2 /Cr, PhNO 2 /Fe and PhNO 2 /Ga, respectively, Table 1). For G = pyridine [35,49], dichloromethane [48] or dimethylformamide [41], the disorder remains down to very low temperatures and the salts are not superconductors or present very low T c .…”