A new compound in the BEDT-TTF family [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene] with a tetrathiocyanatocuprate(II) anion, (BEDT-TTF)4[Cu(NCS)4]

The new compound (BEDT-TTF)4[Cu(NCS)4] based on the organic donor BEDT-TTF [bis(ethylenedithio)tetrathiafulvalene] has been obtained during a galvanostatic electrocrystallization process with Cu(NCS). It exhibits a pseudo-κ arrangement, never observed up to now in the BEDT-TTF family with thiocyanatocuprate(II) anions.

A new phase combining BEDT-TTF and [Cu(NCS) 4 [Cu(NCS) 4 ] was obtained during a galvanostatic electrocrystallization process. As previously observed with BEDT-TTF-based compounds with oxalatometallate anions, the BEDT-TTF molecules in (BEDT-TTF) 4 [Cu(NCS) 4 ] exhibit the so-called pseudo-arrangement, with two BEDT-TTF molecules being positively charged and two electronically neutral. The bond lengths and angles in the two unique BEDT-TTF molecules differ slightly. The crystal structure consists of layers of BEDT-TTF molecules extending parallel to (001). The width of this layer corresponds to the length of the a axis [16.9036 (17) Å ]. The BEDT-TTF layers are separated by layers of centrosymmetric square-planar [Cu(NCS) 4 ] 2dianions.

Chemical context
For several years, we have been interested in synthesizing molecular (super)conductors as nanoparticles (Chtioui-Gay et al., 2016;Valade et al., 2016;de Caro, Jacob et al., 2013;de Caro, Souque et al., 2013;de Caro et al., 2014;Winter et al., 2015) in order to study the effects of size reduction on the properties of this kind of material. As there are numerous structuring agents such as ionic liquids based, for instance, on imidazolium cations (Fig. 1), and long alkyl chains in ammonium salts or neutral amines, it is possible to obtain nanoparticles of these materials, either by electrochemical oxidation or by chemical reaction. Recently, we have focused on BEDT-TTF-based compounds [BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene]. The BEDT-TTF family is one of the most studied in the field of molecular superconductors because it exhibits the largest number of superconductors with T c above 10 K (Ishiguro et al., 1998). During the planned electrosynthesis of (BEDT-TTF) 2 [Cu(NCS) 2 ] as nanoparticles from BEDT-TTF and Cu(SCN) in the presence of (EMIM)(SCN) (EMIM = 1-ethyl-3-methylimidazolium), a few crystals were formed as a minor product besides the desired powder as the main phase. A structure determination of these crystals revealed a new salt-like compound, based on the BEDT-TTF donor and the [Cu(NCS) 4 ] 2dianion, namely pseudo--(BEDT-TTF) 4 [Cu(NCS) 4 ].

Structural commentary
The asymmetric unit of the title salt contains two well-ordered BEDT-TTF molecules and one Cu(NCS) 2 entity with the Cu II cation lying on an inversion centre (Fig. 2). This results in the composition (BEDT-TTF) 4 [Cu(NCS) 4 ], and thus is different from the well-known -phase (BEDT-TTF) 2 [Cu(NCS) 2 ] (Hiramatsu et al., 2015;Schultz et al. 1991;Urayama et al., 1988) and also from (BEDT-TTF)[Cu 2 (NCS) 3 ] (Geiser et al., 1988). One of the two BEDT-TTF molecules (central bond C7-C8) forms a dimer that is related through an inversion centre, whereas the other BEDT-TTF molecules (central bond C17-C18) are farther away from each other. To our knowledge, this feature has not been observed within the (BEDT-TTF)[Cu(NCS) x ] family, but it has been found in BEDT-TTF compounds with tris-(oxalato)metallate anions, such as (BEDT-TTF) 4 [AM(C 2 O 4 ) 3 ]Ásolv. (A = K, NH 4 , H 3 O; M = Fe, Cr, Co, Ru; solv. = benzonitrile, 1,2-dichlorobenzene, bromobenzene) (Kurmoo et al., 1995;Martin et al., 2001;Prokhorova et al., 2011Prokhorova et al., , 2013. The latter compounds are representatives of the pseudo -phase where the two independent BEDT-TTF molecules show some slight structural differences. Similarly, the bond lengths within the central C 2 S 4 core in the BEDT-TTF molecules of the title salt deviate by up to 0.035 Å . The bond lengths in the TTF core are indicative of the degree of charge in this family of BEDT-TTF compounds. According to Guionneau et al. (1997), this allows the charge Q of the two BEDT-TTF molecules in the title salt to be calculated. Whereas each BEDT-TTF molecule in the dimer carries a charge of +1 (Q = 0.83), the other BEDT-TTF molecule is neutral (Q = 0.18). Not only do the bond lengths of the BEDT-TTF molecules in the title salt show some differences, but the overall shape of the molecules also differs. The BEDT-TTF molecule in the dimer deviates less from planarity [r.m.s. deviation of 0.0853 Å neglecting the outer ethylene bridges, with the largest deviation being 0.1579 (19) Å for S5] than the other BEDT-TTF molecule [r.m.s. deviation of 0.1431 Å ; highest deviation = 0.3273 (12) Å for S12]. Moreover, the outer ethylene groups tend to be more eclipsed in the molecule of the dimer whereas they tend to be more staggered in the other molecule (Fig. 3 (Wang et al., 2008;Chekhlov, 2009). It should be noted that one Cu-NCS fragment is more bent than the other one [angle Cu-N2-C2 of 151.6 (4) versus 175.2 (3) for Cu-N1-C1; Fig. 2].

Supramolecular features
The BEDT-TTF molecules in the dimer stack face-to-face. The interplanar distance within the dimer is 3.62 (3) Å , considering the least-squares planes of the molecule except for the terminal ethylene groups. In addition, there are two pairs of short SÁ Á ÁS contacts within the dimer [S5Á Á ÁS8 = 3.461 (1) and S6Á Á ÁS7 = 3.515 (1) (Table 1) and the two others are almost parallel to the dimer [angle of 7.09 (4) ] with short SÁ Á ÁH contacts (Table 1). This arrangement leads to layers of BEDT-TTF donors, extending parallel to (001). The layers have a width that corresponds to the length of the a axis and are separated from each other by layers of [Cu(NCS) 4 ] 2dianions (Fig. 5).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The crystal diffracted rather weakly (2 max = 47.42 ). The hydrogen atoms of the ethylene bridges were placed in idealized positions and were refined with C-H = 0.99 Å and with U iso (H) = 1.2U eq (C). Reflections (100) and (110) were obstructed by the beam stop and thus were excluded from the refinement. program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2018 (Sheldrick, 2015); molecular graphics: Mercury (Macrae et al., 2006); software used to prepare material for publication: WinGX (Farrugia, 2012).

Bis[bis(ethylenedithio)tetrathiafulvalenium] tetrathiocyanatocuprate(II) bis[bis(ethylenedithio)tetrathiafulvalene]
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.