supplementary materials

There is a large variety of quasi-two-dimensional organic conductors based on the radical cation salts of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) which also is abbreviated to ET. They show a wealth of structural modifications, but the conducting properties of the radical salts are essentially determined by the packing patterns of their BEDT-TTF layers (Shibaeva & Yagubskii, 2004). So, according to packing patterns of the BEDT-TTF layers in the various crystal structure, it is customary classified as α, β, δ, χ and so on. (Mori, 1998; Mori, 1999; Mori et al., 1999)

In the title structure, (I), the unit cell contains three ET donors and two anions. Since one BEDT-TTF molecule (ETA) and [CoCl₄]^2- anion are on general positions, and the other BEDT-TTF molecule (ETB) is positioned on an inversion center, the crystallographically independent molecules are 1.5 ET molecules and one [CoCl₄]^2- anion (Fig. 1). The [CoCl₄]^2- anion shows a slightly distorted tetrahedral configuration (Table 1). The two independent ET molecules have different molecular configuration. In the ETA, the terminal ethylene C2 and C9 atoms extend from the molecular plane with deviations of -0.499 (16) Å and -0.579 (18) Å, but C1 and C10 atoms [with deviations of 0.169 (17) Å and 0.057 (19) Å] almost share the same plane with the rest of the molecule. ETA shows a boat conformation in which C2 and C9 are on the both sides of the molecular plane. Otherwise, ETB shows a chair conformation in which C11 and C12 deviate from the molecular plane by 0.658 (18) Å and 0.02 (2) Å respectively and the inversion center positioned on the middle of the C15—C15A bond. The dihedral angle between the molecular planes of ETA and ETB is 16.88 (16)° and their molecular planes make angles of 43.93 (10)° and 27.10 (13)° with [010] directions respectively.

ETA and ETB form stack A and stack B along the b axis in a face to face manner with different intra-stack packing modes. In the stack A, ETA molecules are dimerized and the intra-stack packing mode alternates between the ROB [ring over bond, ET molecules stack on top of each other with a relative displacement between stacking moleculaes along their long in-plane axes] and ROA [ring over atom, ET molecules stack on above or below with a large displacement betweentacking moleculaes along their short in-plane axes] modes, as defined by Williams et al. (1984). The mean interplannar distance in the dimersied ET molecules is 3.606 Å and between the dimerized is 3.082 Å. There are three kinds of intra-dimer S···S short distances within each dimmer shorter than the combined van der Walls radii of two sulfur atoms (3.60 Å), but there are no obvious inter-dimer S···S short contacts between the dimers. Stack B contains only ETB molecules overlapped with ROB mode, and there are no intrastack S···S short contacts. The mean interplannar distance in stack B is 4.392 Å.

Stack A and Stack B form layer A and layer B in the ab plane respectively and these layers are interleaved by the inorganic anions. In layer A, one ET molecule has four pairs of inter-stack S···S short contacts between the stacks in the a-direction [S1···S2 = 3.518 (6) Å, S1···S4 = 3.562 (6) Å, S7···S8 = 3.549 (6) Å] as shown in Fig. 2. Combined with intra-stack S···S short contacts within Stack A, it is suggested that the possibility of a quasi-two-dimensional interaction in the ab-plane. In contrast to the layer A, there are no S···S distance are shorter than the sum of van der Walls radii of two sulfur atoms in the layer B, which indicates the absence of any intermolecular interaction in the layer B.

Between layer A and layer B, there are layers of [CoCl₄]^2- anions, which engage in ionic donor-anion interactions. The anions sandwich the layers A and layers B along the c axis. The [CoCl₄]^2- anions are situated in the so called 'anion cavity', which is bonded by the terminal ethylene groups of the ET cations. Short C—H···Cl interactions (Table 2) probably stabilize the crystal structure.

Among the molecular conductors based on the ET, there are three major determinant of packing of a crystal: (1) the ET network packing motif; (2) the anion layer motif; and (3) the interatomic environment at the interface of the donor and anion layers. For the larger size of the ET and fractional charge on it, the coulomb interaction between the ET cations and anions is inferior to the π-π interaction. In the title crystal, the ET molecules form poly-ET cations by the π-π interaction and [CoCl₄]^2- anions fill the cavity formed by the ethylene groups of the surrounding ET molecules to balance charge. Hence, the 'anion cavity' is mainly determined by the ET packing motif where the 'anion cavity' is a trigonal channel along the [100] directions. If the size of anion is not enough to fill the anion cavity, the solvent molecules will be crystallized in the crystal to stabilize the structure, such as TCE(1,1,2-trichloroethane) molecules in the crystal α-ET₃(CoCl₄)(TCE)(Mori et al., 2002). In the crystal α-ET₃(CoCl₄)(TCE), the oxidation on ET molecules is lower than that in the title compound, where only one [CoCl₄]^2- anion is needed to balance charge, so the TCE solvent molecules are filled in the 'anion cavity' together. At the same time, if some of the anions are replaced by the other anion with same geometry, size and charge, the crystal structure will be an isomorphic structure, for example, β-(ET)₃(CoCl₄)_1.34(GaCl₄)_0.66, α-ET₃(CoCl₄)_0.38(CoCl₄)_0.62(TCE) (Mori et al., 2002). On the other hand, the oxidation on ET molecules and the interaction between the anions and the ethylene groups of the ET will significant affect the packing motif of the ET cations, so the ET packing motif of the title crystal belongs to β' and ET₃(CoCl₄)(TCE) belongs to α. But the 'anion cavity' formed by the ET cations as above two packing motif are nearly same.