An Unsymmetrical Tetrathiafulvalene with a Fused 1,2,5-Thiadiazole Ring and Methylthio Groups

The title compound, 5-(4,5-dimethylthio-1,3-dithiol-2-ylidene)-1,3-diaza-2,4,6-trithiapentalene (4,5-dimethylthio[1,2,5]thiadiazolotetrathiafulvalene, molecular formula C8H6N2S7) crystallizes in the P21/n space group with one molecule in the asymmetric unit. The molecular framework is planar within 0.19 Å. The molecules form a head-to-tail type of π-stacking dimer with an interplanar distance is 3.50(1) Å, where methylthio groups face away from each other. The molecules are also linked by weak intermolecular S···S heteroatom interactions [3.497(1) and 3.572(1) Å] to construct a unique one-dimensional molecular tape structure.


OPEN ACCESS
S···N and S···S heteroatom contacts may increase the dimensionality in solid states and suppress metalinsulator transitions [9,10]. In addition, such interactions may lead to the formation of unique molecular networks which have special functions such as inclusion properties [11]. We report here the molecular and crystal structure of an unsymmetrical TTF derivative, (1, Figure 1), which contains a fused 1,2,5-thiadiazole ring and two methylthio groups. We have found that the crystal structure of 1 differs dramatically from that of an unsymmetrical TTF derivative with a fused 1,2,5-thiadiazole ring and an ethylenedioxy group [6]. The molecules of 1 form a unique one-dimensional molecular tape structure linked by intermolecular S···S heteroatom interactions in the crystalline state.

Bond lengths (Å) Bond angles (°)
The packing diagram of 1 viewed along the a axis is shown in Figure 3. The molecules form a headto-tail type of π-stacking dimer and the dimer stacks along the a axis ( Figure 4). In the dimer, an interplanar distance is 3.50(1) Å and the S7-C8 methylthio groups face away from each other. On the other hand, the dimers are linked by an intermolecular C8-H8A···N1(-x, -y + 1, -z) [3.382(5) Å] hydrogen bond and an interdimer distance is 3.71(1) Å. Between the dimers, the S7-C8 methylthio groups are placed across from each other. The overlap modes of the dimer of 1 are shown in Figure 4. In the intradimer overlapping, long axes of the molecules are fully superposed, while less overlap of π-conjugated system is observed in the interdimer overlapping. This is due to steric hindrance of the S7-C8 methylthio group. The intradimer arrangement of the molecules corresponds to the overlap of HOMO and LUMO for 1. As illustrated in Figure 5, electron transfer from the large lobe of HOMO on the 1,3-dithiol-2-ylidene unit to the large lobe of LUMO on the 1,2,5-thiadiazole ring causes the intradimer overlapping in Figure 5.  The molecules 1 are also linked by weak intermolecular S···S heteroatom interactions [3.497(1) for S2···S7(x + 1/2, -y + 3/2, z + 1/2) and 3.572(1) Å for S5···S6(x + 1/2, -y + 3/2, z + 1/2)] in the crystalline state. The S···S interactions are 0.8-2.9 % shorter than the sum of the corresponding van der Waals radii and build up a unique one-dimensional molecular tape structure (Figure 7). The molecular tape is flat and takes a zigzag conformation. The crystal structure of 1 differs dramatically from that of 4,5-ethylenedioxy [1,2,5]thiadiazolotetrathiafulvalene, in where a T-shaped molecular arrangement connected with intermolecular S···N interactions was observed [6]. The conformations around the methylthio groups of 1 bring the formation of the molecular tape structure with the S···S interactions. No short intermolecular S···N interaction was found in the crystal structure of 1.

Conclusions
We have determined the crystal structure of the unsymmetrical tetrathiafulvalene derivative with a fused 1,2,5-thiadiazole ring and methylthio groups by X-ray diffraction. The molecular framework is planar within 0.19 Å. The molecules form a head-to-tail type of π-stacking dimer with an interplanar distance is 3.50(1) Å, where methylthio groups face away from each other. A unique one-dimensional molecular tape network can be built by the usage of intermolecular S···S heteroatom interactions [3.497(1) and 3.572(1) Å] as a connection tool.

X-Ray crystallography
The data of a single crystal with a size of 0.35 × 0.05 × 0.05 mm was collected at 295 K on a Rigaku Mercury CCD diffractometer. No absorption correction was applied. The crystal structure was solved by direct methods and refined by full-matrix least-squares on F 2 with SHELXS97 and SHELXL97 [16]. All non-hydrogen atoms were refined anisotropically. After anisotropic refinement of all non-hydrogen atoms, all hydrogen atoms were placed in geometrically calculated positions and refined using a riding model, with C-H = 0.96 Å and U iso (H) = 1.5U eq (C). The final least-squares cycle was based on 2,555 observed reflections [I > 2σ(I)] and 156 variable parameters, converged with R 1 = 0.055 and wR 2 = 0.125. Crystal data and refinement details are summarized in Table 2. All molecular and crystal graphics were drawn using PLATON [17] and RasMol [18]. CCDC 675501 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/data_request/cif, by e-mailing data_request@ccdc.cam.ac.uk or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; Fax: +44-1223-336033.

Theoretical calculations
The molecular orbital shapes of HOMO and LUMO of 1 were evaluated in single point 3-21G calculations using the crystallographic geometry of 1 with Gaussian 98 [19] and were visualized with PGV [20].