Trimesic acid dimethyl sulfoxide solvate: space group revision

The structure of the title solvate, C9H6O6·C2H6OS, was determined 30 years ago [Herbstein, Kapon & Wasserman (1978 ▶). Acta Cryst. B34, 1613–1617], with data collected at room temperature, and refined in the space group P21. The present redetermination, based on high-resolution diffraction data, shows that the actual space group is more likely to be P21/m. The crystal structure contains layers of trimesic acid molecules lying on mirror planes. A mirror plane also passes through the S and O atoms of the solvent molecule. The molecules in each layer are interconnected through strong O—H⋯O hydrogen bonds, forming a two-dimensional supramolecular network within each layer. The donor groups are the hydroxyls of the trimesic acid molecules, while the acceptors are the carbonyl or the sulfoxide O atoms.

The structure of the title solvate, C 9 H 6 O 6 ÁC 2 H 6 OS, was determined 30 years ago [Herbstein, Kapon & Wasserman (1978). Acta Cryst. B34, [1613][1614][1615][1616][1617], with data collected at room temperature, and refined in the space group P2 1 . The present redetermination, based on high-resolution diffraction data, shows that the actual space group is more likely to be P2 1 /m. The crystal structure contains layers of trimesic acid molecules lying on mirror planes. A mirror plane also passes through the S and O atoms of the solvent molecule. The molecules in each layer are interconnected through strong O-HÁ Á ÁO hydrogen bonds, forming a two-dimensional supramolecular network within each layer. The donor groups are the hydroxyls of the trimesic acid molecules, while the acceptors are the carbonyl or the sulfoxide O atoms.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: FB2098).

Comment
The title compound was obtained during attempts to prepare coordination compounds with transition metals and benzene-1,3,5-tricarboxylic acid. The latter compound is also known as trimesic acid, TMA. It is a rigid, planar molecule that is soluble in a number of solvents. Its three exo-carboxylic acid groups are arranged symmetrically around the benzene ring, forming a flat, trigonal molecule, which can be used as a building block in the construction of organic crystals and multidimensional metalorganic frameworks (e.g. Almeida Paz & Klinowski, 2004).
The clathration ability of TMA allowed to prepare a number of solvate structures, including hydrates, and consequently determination of these structures. Among them, the dimethylsulfoxide (DMSO) solvate has been reported already 30 years ago (Herbstein et al., 1978). The data were collected at room temperature with Mo-Kα radiation. Laue symmetry as well as systematic extinctions are in agreement with the space group P2 1 /m or P2 1 . Herbstein et al. applied the Hamilton test, i.e. essentially based their choice on final R residuals. The space group P2 1 was eventually retained (R = 0.084) and P2 1 /m rejected (R = 0.092), despite the E statistics, which favoured a centrosymmetric space group. The authors, however, commented in their publication that "there is some doubt about the correctness of this decision".
We have now collected an accurate high-resolution diffraction pattern for this compound. Wilson statistics are not in agreement with the non-centrosymmetric space group, for instance 〈E 2 -1〉 = 1.002 for 4589 E values. Refinement in space group P2 1 converges to R 1 = 0.035 for 1772 F o >4σ(F o ). However, abnormally high correlation matrix elements are observed for methyl groups in DMSO, and methyl H atoms, if refined freely, exhibit unrealistic C-H bond lengths, ranging from 0.68 Å to 1.48 Å. Finally, refinement using non-merged data (286 measured Friedel pairs) gives an inconsistent Flack parameter, 0.23 (13).
All these symptoms indicate that the space group should be rather P2 1 /m. All the atoms with exception of the methyl group (C10) of the DMSO molecule lie in the mirror plane. The methyl group (C10) occupies a general position (Fig. 1).
Expected geometry for both moieties is observed.
The displacement parameters deserve a careful examination. The longest axes of the displacement parameters are perpendicular to the molecular planes. In the case of the TMA molecule, the U 3 /U 1 ratios of non-hydrogen atoms lie in the range 2.17-6.60. A similar thermal behaviour is observed for DMSO atoms lying in the m plane, S1 (U 3 /U 1 = 3.69) and O7 (U 3 /U 1 = 5.77). Such motions suggest another possibility that the crystal can contain statistically distributed non-centrosymmetric domains; i.e. the structure can be non-centrosymmetric on a shorter scale. Therefore, the space group P2 1 can not be totally ruled out, and the actual space group may also be dependent on the choice of the particular sample. Further work, like multi-temperatures data collections, would be desirable in order to determine the symmetry unambiguously.
On the other hand, the molecular motion within the molecular planes would be affected by stronger intermolecular interactions that take place within each molecular layer. The molecules are involved in a two-dimensional supramolecular network through the strong hydrogen bonds (Desiraju & Steiner, 1999; Tab. 1). All the hydroxyl groups of the TMA molecule supplementary materials sup-2 form O-H···O hydrogen bonds using carbonyl and sulfoxide O atoms as acceptors. As a result, the molecular layers are formed in the crystal structure (Fig. 2), parallel to (010). These layers correspond to the crystallographic m planes, and are thus separated by b/2 = 3.42 Å.

Experimental
Copper (0.1 g, 1.5 mmol), TMA (0.32 g, 1.5 mmol), and DMSO (3.3 g, 42.2 mmol) were placed in a flask and the mixture was heated at 338 K with magnetic stirring until total dissolution of the metal was observed (0.5-2 hours). The solution was filtered and allowed to stand at room temperature for 12 hours, after which the crystals of the title compound were formed.

Refinement
Hydroxyl H atoms were found in a difference map, and refined freely. Other H atoms were placed in idealized positions, with C-H bond lengths fixed to 0.93 (aromatic CH) or 0.96 Å (methyl CH 3 ) and refined using a riding model approximation, with U iso (H) = 1.5U eq (carrier C) for the methyl group and U iso (H) = 1.2U eq (carrier C) for the aryl groups. The methyl group is considered as a rigid group free to rotate about the S1-C10 bond. Fig. 1. The structure of the title compound, with displacement ellipsoids for non-H atoms at the 50% probability level. Symmetry code: (i) x, 1/2 -y, z.