Crystal structure of the co-crystal 2-[(2-carboxyphenyl)disulfanyl]benzoic acid – 3-chlorobenzoic acid (2/1), C35H25ClO10S4

Abstract C35H25ClO10S4, triclinic, P1̄ (no. 2), a = 7.9798(2) Å, b = 10.2392(3) Å, c = 20.6956(4) Å, α = 103.632(2)°, β = 96.082(2)°, γ = 96.053(2)°, V = 1619.13(7) Å3, Z = 2, Rgt(F) = 0.0493, wRref(F2) = 0.1413, T = 100(2) K.


CCDC no.: 1890703
The molecular structure is shown in the figure. Tables 1 and  2, respectively, contain details on crystal characteristics and measurement conditions, and a list of the atoms including atomic coordinates and displacement parameters. CrysAlis PRO [1], SHELX [2,3], WinGX/ORTEP [4]

Source of material
The title co-crystal was prepared through solvent drop grinding of 2-mercaptobenzoic acid (2-MBA; Merck) and 3-chlorobenzoic acid (ClBA; Merck) in a 1:1 molar ratio (0.001 mol, 0.154 g for 2-MBA and 0.157 g for ClBA). The mixture was ground for 15 min in the presence of few drops of methanol; the procedure was repeated three times. Colourless block crystals were obtained by carefully layering benzene (1.5 mL) on a dichloromethane (1.

Experimental details
The C-bound H atoms were geometrically placed (C-H = 0.95 Å) and refined as riding with U iso (H) = 1.2Ueq(C

Comment
Owing to the presence of a central disulphide bond, 2-[(2carboxyphenyl)disulfanyl]benzoic acid (2,2′-dithiobenzoic acid; DTBA), is a flexible dicarboxylic acid. This conformational flexibility is emphasized in a recent report of a tri-polymorphic inclusion compound formed between DTBA and trimethylamine [5]. Indeed, most studies of DTBA are motivated by co-crystal formation and it is of interest that sometimes DTBA found in co-crystals is a product of cocrystallisation experiments starting with 2-mercaptobenzoic acid (2-MBA) [6,7] whereby 2-MBA is oxidised to DTBA. As a continuation of studies in this area [6,8,9], the title co-crystal was isolated from the attempted 1:1 co-crystallisation experiments between 2-MBA and a second carboxylic acid, namely 3-chlorobenzoic acid (3-ClBA). X-ray diffraction showed the asymmetric unit in title cocrystal comprises two independent molecules of DTBA and one of 3-ClBA, indicating oxidation of 2-MBA had occurred during co-crystal formation. Confirmation that the molecule of 3-ClBA exists as a carboxylic acid is readily seen in the pattern of C1-O1, O2 bond lengths, i.e. 1.243(4) Å and 1.288(4) Å. A small twist from planarity is noted in the molecule. The r.m.s. deviation from the least-squares plane through the ringcarbon and chloride atoms being 0.0036 Å, and the dihedral angle between this plane and that through the carboxylic acid residue is 7.55(16)°. The crystal structure of 3-ClBA was originally reported in 1975 with a low temperature (105 K) redetermination of this P21/c polymorph [11] subsequently complimented by the report of a C2/c polymorph [12].
The conformations of the two DTBA molecules comprising the asymmetric unit resemble each other as seen in the respective r.m.s. bond and angle fits of 0.0054 Å and 0.491°f or the inverted-S1 and S3-molecules [13]; each lacks crystallographically imposed symmetry. The patterns in the C-O(hydroxy), O(carbonyl) bond lengths are consistent with the presence of carboxylic acid residues. The C 6 /CO 2 dihedral angles are 3. . This effectively co-planar relationship coupled with the observation each carbonyl-O atom is orientated towards the adjacent disulphide-S atom is indicative of attractive, hypervalent S ← O interactions [14]. The shortest such interaction in the present co-crystal structure is 2.675(2) Å for S4· · · O9 and the longest, 2.737(2) Å, for O3· · · O7. The twists in the molecules is best indicated by the C14-S1-S2-C15 and C28-S3-S4-C29 torsion angles of −91.06(15)°and 87.15(15)°, respectively, and the dihedral angles between the two phenyl rings in the S1-and S3-molecules, i.e. 79.54(11)°a nd 80.76(11)°, respectively. The crystal structure of the parent DTBA molecule has been determined and shows the asymmetric unit to comprise one and a half molecules, with the latter disposed about a 2-fold axis of symmetry [15]. The comparable C-S-S-C torsion and C 6 /CO 2 dihedral angles are 86.12 (14)°and 74.66 (8)°for the molecule in the general position, and 87.90(13)°and 77.55(8)°for the symmetric molecule.
Co-crystals of 3-ClBA with other carboxylic acids have not been reported and the sole example of a carboxylic acid co-crystal of DTBA is its 1:1 co-crystal with 2-MBA [7]. In the structure of this co-crystal, two DTBA molecules associate via hydroxy-O-H· · · O(carbonyl) hydrogen bonds leaving each of the outer carboxylic acid groups free to form eight-membered synthons with a 2-MBA molecule to result in the formation of a four-molecule aggregate.