Crystal Chemistry and Physical Properties of Superconducting and Semiconducting Charge Transfer Salts of the Type (BEDT-TTF)4[AIMIII(C2O4)3]·PhCN (AI= H3O, NH4, K; MIII= Cr, Fe, Co, Al; BEDT-TTF = Bis(ethylenedithio)tetrathiafulvalene)

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Abstract
Synthesis, structure determination by single-crystal X-ray diffraction, and physical properties are reported and compared for superconducting and semiconducting molecular charge-transfer salts with stoichiometry (BEDT-TTF)4[AIMIII(C2O4)3]·PhCN, where AI = H3O, NH4, K; MIII = Cr, Fe, Co, Al; BEDT-TTF = bis(ethylenedithio) tetrathiafulvalene. Attempts to substitute MIII with Ti, Ru, Rh, or Gd are also described. New compounds with M = Co and Al are prepared and detailed structural comparisons are made across the whole series. Compounds with A = H3O+ and M = Cr, Fe are monoclinic (space group C2/c), at 150, 120 K a = 10.240(1) Å, 10.232(12) Å; b = 19.965(1) Å, 20.04(3) Å; c = 34.905(1) Å, 34.97(2) Å; β = 93.69(1)°, 93.25(11)°, respectively, both with Z = 4. These salts are metallic at room temperature, becoming superconducting at 5.5(5) or 8.5(5) K, respectively. A polymorph with A = H3O+ and M = Cr is orthorhombic (Pbcn) with a = 10.371(2) Å, b = 19.518(3) Å, c = 35.646(3) Å, and Z = 4 at 150 K. When A = NH4+, M = Fe, Co, Al, the compounds are also orthorhombic (Pbcn), with a = 10.370(5) Å, 10.340(1) Å, 10.318(7) Å; b = 19.588(12) Å, 19.502(1) Å, 19.460(4) Å; c = 35.790(8) Å, 35.768(1) Å, 35.808(8) Å at 150 K, respectively, with Z = 4. All of the Pbcn phases are semiconducting with activation energies between 0.15 and 0.22 eV. For those compounds which are thought to contain H3O+, Raman spectroscopy or CC and CS bond lengths of the BEDT-TTF molecules confirm the presence of H3O+ rather than H2O. In the monoclinic compounds the BEDT-TTF molecules adopt a β‘ ‘ packing motif while in the orthorhombic phases (BEDT-TTF)2 dimers are surrounded by monomers. Raman spectra and bond length analysis for the latter confirm that each molecule of the dimer has a charge of +1 while the remaining donors are neutral. All of the compounds contain approximately hexagonal honeycomb layers of [AM(C2O4)3] and PhCN, with the solvent occupying a cavity bounded by [M(C2O4)3]3- and A. In the monoclinic series each layer contains one enantiomeric conformation of the chiral [M(C2O4)3]3- anions with alternate layers having opposite chirality, whereas in the orthorhombic series the enantiomers form chains within each layer. Analysis of the supramolecular organization at the interface between the cation and anion layers shows that this difference is responsible for the two different BEDT-TTF packing motifs, as a consequence of weak H-bonding interactions between the terminal ethylene groups in the donor and the [M(C2O4)3]3- oxygen atoms.

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