Bis(ethylenedithio)tetrathiafulvalenium–tetrachloridocobaltate(II) (3/2)

The structure of the electrochemically crystallized title compound, (C10H8S8)3[CoCl4]2, consists of two types of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) radical cation stacks separated by sheets of tetrahedral [CoCl4]2− anions. One of the BEDT-TTF molecules is generated by inversion. There are short S⋯S contacts between the stacks in the a direction and short C—H⋯Cl contacts between the radical cations and the anions.

The structure of the electrochemically crystallized title compound, (C 10 H 8 S 8 ) 3 [CoCl 4 ] 2 , consists of two types of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) radical cation stacks separated by sheets of tetrahedral [CoCl 4 ] 2À anions. One of the BEDT-TTF molecules is generated by inversion. There are short SÁ Á ÁS contacts between the stacks in the a direction and short C-HÁ Á ÁCl contacts between the radical cations and the anions.

Comment
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 [CoCl 4 ] 2anion 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 [CoCl 4 ] 2anion ( Fig. 1). The [CoCl 4 ] 2anion 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 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.  (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 [CoCl 4 ] 2anions 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-ET 3 (CoCl 4 )(TCE) (Mori et al., 2002).
In the crystal α-ET 3 (CoCl 4 )(TCE), the oxidation on ET molecules is lower than that in the title compound, where only one [CoCl 4 ] 2anion 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 (CoCl 4 ) 1.34 (GaCl 4 ) 0.66 , α-ET 3 (CoCl 4 ) 0.38 (CoCl 4 ) 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 ET 3 (CoCl 4 )(TCE) belongs to α. But the 'anion cavity' formed by the ET cations as above two packing motif are nearly same.

Experimental
BEDT-TTF was synthesized following the method of Varma et al. (1987) and recrystallized from CHCl 3 . K 2 CoCl 4 .6H 2 O was synthesized by metathesis of CoCl 2 .6H 2 O and KCl in a molar ratio of 1:2 in water and recrystallized before use. All solvent were distilled before use.
Electrochemical crystal growth was carried out in conventional H-shaped cells with Pt electrodes in a constant temperature of 295 (2) K at a current of 0.2 µA. Each cell contained 7 mg of BEDT-TTF and 99.3 mg of K 2 CoCl 4 .6H 2 O together with 40 mg of 18-crown-6 in 35 mL of TCE(1,1,2-trichloroethane) under an inert atmosphere (N 2 ). Black needles of (I) were obtained after three months.

Refinement
The electrocrystallization process is a 'self-assembly' process which is difficult to control. The tendency to grow as bundles of fine needles has hampered our efforts to get a better crystal. The low quality of the crystal was the reason for low bond precision, but the obtained structure information of the title compound is meaningful.
The H atoms were geometrically placed (C-H = 0.97Å) and refined as riding with U iso (H)= 1.2 U eq (C). Fig. 1. Molecular structure of (I) with displacement ellipsoids for the non-hydrogen atoms at the 30% probability level. The unlabelled atoms of the ETB molecule are generated by the symmetry operation (1-x, 1-y, 1-x).

Bis(ethylenedithio)tetrathiafulvalenium-tetrachloridocobaltate(II) (3/2)
Crystal data (C 10   Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.