1,1′:4′,1′′-Terphenyl-2′,5′-dicarboxylic acid dimethyl sulfoxide-d 6 disolvate

The asymmetric unit of the title solvate, C20H14O4·2C2D6OS, contains half of the substituted terephthalic acid molecule and one solvent molecule. The centroid of the central benzene ring in the acid molecule is coincident with a crystallographic inversion center. Neither the carboxyl nor the phenyl substituents are coplanar with the central aromatic ring, showing dihedral angles of 53.18 (11) and 47.83 (11)°, respectively. The dimethyl sulfoxide solvent molecules are hydrogen bonded to the carboxylic acid groups.

The asymmetric unit of the title solvate, C 20 H 14 O 4 Á2C 2 D 6 OS, contains half of the substituted terephthalic acid molecule and one solvent molecule. The centroid of the central benzene ring in the acid molecule is coincident with a crystallographic inversion center. Neither the carboxyl nor the phenyl substituents are coplanar with the central aromatic ring, showing dihedral angles of 53.18 (11) and 47.83 (11) , respectively. The dimethyl sulfoxide solvent molecules are hydrogen bonded to the carboxylic acid groups.
The compound has a central terephtalic acid core, substituted at positions 2 and 5 with phenyl groups. The molecule has been previoulsy described by Tanaka et al. (Tanaka et al., 2009)  The packing shows a deuterated dimethyl sulfoxide solvent molecule hydrogen bonded with each carboxylic acid group, with O1···O3 ii distance of 2.581 (2) Å (ii: x + 1, y, z). There are two molecules of solvent for a single diacid molecule, each one defining one of the aforementioned hydrogen bond with each carboxylic acid group. A closely related pattern occurs for the ethanol solvate of the title molecule and the dimethylformamide solvate of the p-fluoro derivative (Tanaka et al., 2009): two molecules of solvent, ethanol or dimethylformamide, are bonded by hydrogen bonds to both carboxylic acid groups.

Experimental
The compound was prepared by a method described in the literature (Deuschel, 1951;Ebel & Deuschel, 1956), slighty modified by using d 6 -DMSO for crystallization instead of C 6 H 5 CN, giving the DMSO-clathrate. The title compound was prepared in a 93% yield, mp. = 280°C (dec).

Refinement
The hydrogen atoms positions were calculated after each cycle of refinement with SHELXL (Bruker,1999) using a riding model, with C-H distances in the range of 0.93 to 0.96 Å. U iso (H) values were set equal to 1.5U eq of the parent carbon atom for methyl groups, and 1.2U eq for the others. The carboxylic acid hydrogen atom was located in the difference Fourier map, and its coordinates were subsequentely kept fixed (by adding 10 to the coordinates in SHELXL), while U iso (H) was left free to refine.

Computing details
Data collection: SMART-NT (Bruker, 2001); cell refinement: SAINT-NT (Bruker, 1999); data reduction: SAINT-NT (Bruker, 1999); program(s) used to solve structure: SHELXTL-NT (Sheldrick, 2008); program(s) used to refine structure: (Sheldrick, 2008); molecular graphics: SHELXTL-NT (Sheldrick, 2008); software used to prepare material for publication: SHELXTL-NT (Sheldrick, 2008). 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq C1 0.6023 (