Crystal structure of tetramethyltetrathiafulvalenium (1S)-camphor-10-sulfonate dihydrate

In this salt, two types of TMTTF units are present as TMTTF. + radical cations which form one-dimensional stacks in which they are associated two by two, forming dimers with short S⋯S contacts. The S-camphSO3 anions also form stacks and are connected with each other via O—H⋯O hydrogen bonds. The columns of cations and anions are connected through C—H⋯O hydrogen bonds.


Chemical context
Chiral molecular conductors may display interesting properties such as the magneto-chiral anisotropy effect; the different strategies of access to these materials have been recently reviewed (Avarvari & Wallis, 2009;Pop et al., 2014). Among these possible strategies, a straightforward a priori approach consists of combining, through electrocrystallization experiments, chiral counter-anions, existing in enantiopure form, to TTF-based radical-cations; in this context, due to the commercial availability of the parent acid S-camphSO 3 H, the anion S-camphSO 3 À appears to be a ready choice and, in fact, it has already been used to obtain the salt (EDT-TTFI 2 ) 2 ÁS-camphSO 3 ÁH 2 O, where EDT-TTFI 2 is diiodoethylenedithiotetrathiafulvalene (Brezgunova et al., 2010). In addition, it is worth mentioning a more general review relating to conducting radical cation salts with organic anions, especially anions derived from carboxylic and sulfonic organic acids (Geiser & Schlueter, 2004). ISSN 2056-9890

Structural commentary
The title compound crystallizes with two independent TMTTF cations, two independent S-camphSO 3 anions and four water molecules ( Fig. 1) in the asymmetric unit. The geometries of the two types (A and B) of TMTTF units (Fig. 1), are rather similar despite the fact that A and B are crystallographically independent; in both case, the observed bond lengths (see e.g. Penicaud et al., 1990;Shibaeva, 1984) and especially the central C=C distance [1.392 (6) Å in A and 1.378 (6) Å in B] are in agreement with a complete oxidation of TMTTF which is thus present as TMTTF .+ radical-cations, in agreement also with the 1/1 cation/anion balance of this salt.

Packing of the donors
The cations form columns along the a axis in which the two types, A and B, of TMTTF units alternate (Fig. 2). The overall arrangement of the donors can be described as monodimensional since these stacks are isolated. Starting from one particular column, a set of equivalent columns may be deduced by translation along b, thus generating a cationic layer lying in the ab plane; however, there is no vicinity relation between two successive donors belonging to two different stacks of the same layer, except for proximity of the external methyl groups. When looking in the c-axis direction, successive layers are completely separated by slabs of anions; moreover, the orientation of the donors is different in two consecutive cationic layers since they adopt a herringbone arrangement.

Figure 2
Overall view, along the a axis, of the crystal packing.

Figure 3
Packing of the donors: SÁ Á ÁS contact distances within a stack, in the case of the two different inter-donor intervals.
planes being only 0.24 . Within a stack, two independent intermolecular intervals alternate with mean inter-plane distances of 3.40 and 3.71 Å . One can conclude in favour of the presence of dimers since the four intermolecular SÁ Á ÁS contacts corresponding to the smaller interval range from 3.472 (1) to 3.554 (2) Å ( Fig. 3) and thus are shorter than twice the van der Waals radius of sulfur (3.6-3.7 Å : Bondi, 1964;Pauling, 1960); within a dimer the A and B units adopt a bondover-ring (Williams et al., 1992) relative arrangement. On the other hand, all SÁ Á ÁS distances across the larger interval exceed the van der Waals distance, ranging from 4.026 (2) to 4.050 (2) Å .

Supramolecular features
The S-camphSO 3 anions stack along the a axis and are connected with each other via the water molecules with many O-HÁ Á ÁO hydrogen bonds ranging from 1.86 (3) Å to 2.15 (4) Å (Table 1). The oxygen from one sulfonate is linked to the oxygen of the neighbouring sulfonate through a bridging water molecule, while the oxygen of this latter is linked to the H atom of another water molecule, which is also connected to the oxygen of the ketone group, through O-HÁ Á ÁO interactions (Fig. 4). Thus, in Etter's classification (Etter, 1990), the O-HÁ Á ÁO hydrogen-bonding network can be described as being constituted of C 2 2 (6) chains bearing R 3 3 (11) lateral rings. On the other hand, the columns of cations and anions are connected through C-HÁ Á ÁO hydrogen bonds, forming a system expanding in all three directions ( Fig. 5 and Table 1); finally, the result is a three-dimensional network of O-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds.
Electrocrystallization of TMTTFÁS-camphSO 3 Á2H 2 O A conventional H-shaped cell was charged with 142 mg (0.3 mmol) of Bu 4 N + ÁS-camphSO 3 À dissolved in 20 ml of a 98/2 (v/v) tetrahydrofuran-water mixture, degassed with argon. TMTTF (7.8 mg, 0.03 mmol) was introduced in the anodic arm and was then electro-oxidized under galvanostatic conditions with stepwise increases of the applied current (Anzai et al., 1995): 0.5 mA for 3 days, then 1 mA for 4 days, 2 mA for 3 days and finally 5 mA for 8 days; afterwards, the black needles of TMTTFÁS-camphSO 3 Á2H 2 O, deposited at the platinum wire anode, were collected. The electrocrystallization was conducted at room temperature except during the 6 last days during which the cell was cooled to 283 K.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. 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 R-factors(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. The H atoms on the water molecules were added by Fourier difference map and then restrained with 13 DFIX commands between O and H and H and H on the 4 water molecules.