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)

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 (NH4)(3)[Fe(C2O4)(3)]. 3H(2)O or (NH4)(3)[Cr(C2O4)(3)]. 3H(2)O in C6H5NO2, yields crystals of beta"-(BEDT-TTF)(4)[A . Fe(C2O4)(3)].C6H5NO2 [1] or beta"-(BEDT-TTF)(4)[A . Cr(C2O4)(3)].C6H5NO2 [2] (A=H3O+ or NH4+). 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 Angstrom, b = 19.949 Angstrom, c = 35.030 Angstrom, beta = 92.97 degrees, V = 7169.6(2) Angstrom (3), Z = 8. For [2], at 298 K: a = 10.304 Angstrom, b = 20.091 Angstrom, c = 35.251 Angstrom, beta = 92.70 degrees, V = 7289.3(2) Angstrom (3). Z = 8, and at 120 K a = 10.283 Angstrom, b = 19.917 Angstrom, c = 34.939 Angstrom, beta = 93.30 degrees, V = 7144.4(1) Angstrom (3), Z = 8. The crystal structures of both. compounds consist of alternating layers of BEDT-TTF cations and layers containing [M(C2O4)(3)](3-), H3O+ or NH4+, and PhNO2. The BEDT-TTF molecules are arranged in the beta" 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.