Redetermination of tetramethyl tetrathiafulvalene-2,3,6,7-tetracarboxylate

An improved crystal structure of the title compound, C14H12O8S4, is reported. The structure, previously solved using the heavy-atom method (R = 7.1%), has now been solved using direct methods. Due to the improved quality of the data set an R value of 2.06% could be achieved. In the crystal, C—H⋯S and C—H⋯O contacts link the molecules.

An improved crystal structure of the title compound, C 14 H 12 O 8 S 4 , is reported. The structure, previously solved using the heavy-atom method (R = 7.1%), has now been solved using direct methods. Due to the improved quality of the data set an R value of 2.06% could be achieved. In the crystal, C-HÁ Á ÁS and C-HÁ Á ÁO contacts link the molecules.
Data collection: APEX2 (Bruker, 2007); cell refinement: SAINT (Bruker, 2007); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: SHELXTL (Sheldrick, 2008). The present structure of the title compound (TTF) has been refined to an R-value of 2.06% which is clearly better than 7.1% of the previous study reported in 1976 by Belsky and Voet. In particular, this enabled us to refine the positions of hydrogen atoms of the methyl groups with improved accuracy making it possible to find potential hydrogen bonds. In conformity with previous findings, the TTF scaffold is planar and the methoxycarbonyl functions are slightly twisted out of the ring plane [interplanar angles 25.60 (1) and 42.77 (2)°] (Fig. 1).

Refinement
H atoms were positioned geometrically and allowed to ride on their parent atoms, with C-H = 0.96 Å, and U iso =1.5 U eq (parent atom).

Figure 1
Perspective view of the title compound showing thermal ellipsoids at the 50% probability level.

Special details
Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'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 > 2σ(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.