First crystal structure of a Pigment Red 52 compound: DMSO solvate hydrate of the monosodium salt

The crystal structure of a DMSO monosolvate monohydrate of a previously unknown monosodium salt of the industrial intermediate Pigment Red 52 (P.R.52) with the formula Na+[C18H12ClN2O6S]−·H2O·C2H6OS. The compound was obtained by in-house synthesis. The crystals have triclinic symmetry at 173 K. The crystal structure is built up by Na—O chains, which arrange the anions in polar/non-polar double layers.

Pigment Red 52, Na 2 [C 18 H 11 ClN 2 O 6 S], is an industrially produced hydrazonelaked pigment. It serves as an intermediate in the synthesis of the corresponding Ca 2+ and Mn 2+ salts, which are used commercially for printing inks and lacquers. Hitherto, no crystal structure of any salt of Pigment Red 52 is known. Now, single crystals have been obtained of a dimethyl sulfoxide solvate hydrate of the monosodium salt of Pigment Red 52, namely, monosodium 2-[2-(3-carboxy-2-oxo-1,2-dihydronaphthalen-1-ylidene)hydrazin-1-yl]-5-chloro-4-methylbenzenesulfonate dimethyl sulfoxide monosolvate monohydrate, Na + ÁC 18 H 12 ClN 2 O 6 S À ÁH 2 OÁC 2 H 6 OS, obtained from in-house synthesized Pigment Red 52. The crystal structure was determined by single-crystal X-ray diffraction at 173 K. In this monosodium salt, the SO 3 À group is deprotonated, whereas the COOH group is protonated. The residues form chains via ionic interactions and hydrogen bonds. The chains are arranged in polar/non-polar double layers.

Chemical context
Pigment Red 52 (P.R.52, Na 2 [C 18 H 11 N 2 ClO 6 S]), is produced industrially as an intermediate in the synthesis of Pigment Red 52:1 (Ca [C 18 H 11 N 2 ClO 6 S]) and Pigment Red 52:2 (Mn[C 18 H 11 N 2 ClO 6 S]) (Czajkowski et al., 1980;Hunger & Schmidt, 2018). P.R.52:1 and P.R.52:2 are used for the colouration of printing inks and lacquers (Hunger & Schmidt, 2018). No crystal structures of P.R.52, or of its various metal salts, have previously been determined. Pigment Red 48 is an isomer of P.R.52, differing by mutual exchange of CH 3 and Cl substituents. Recently, the crystal structures of two hydrates of the monosodium salt of P.R.48 have been published (Tapmeyer et al., 2020). Correspondingly, similar monosodium hydrate phases could also be expected for P.R.52. Hitherto, nothing has been known about the existence of a monosodium salt of P.R.52 or its hydrates or solvates. In attempts to crystallize P.R.52 from dimethylsulfoxide, single crystals were obtained, which turned out to be a mono-DMSO solvate monohydrate of the monosodium salt of P.R.52:1. The crystal structure was determined by X-ray analysis.

Structural commentary
Pigment Red 52 monosodium salt DMSO monosolvate monohydrate crystallizes in the triclinic space group P1 with one pigment anion, one sodium cation, one molecule of DMSO and one water molecule in the asymmetric unit (Fig. 1).
The pigment exhibits the hydrazone tautomeric form, like all industrial hydrazone pigments (formerly known as 'azo pigments') (Gilli et al., 2005;Schmidt et al., 2008;Hunger & Schmidt, 2018). The N-H group forms two intramolecular [S 1 1 (6)] N-HÁ Á ÁO hydrogen bonds (Table 1). The sulfonate group is deprotonated, whereas the carboxylic group is protonated. The protonation site is unambiguously determined by the difference electron density, from the S-O and C-O bond lengths in the SO 3 À and COOH groups, and from the hydrogen-bond pattern. Intramolecular and intermolecular bond lengths and angles are in the usual ranges. The organic anion is nearly planar, with an RMSD of 0.553 Å for all non-hydrogen atoms, except for the oxygen atoms of the sulfonate group. The dihedral angle between the naphthyl moiety and the phenyl ring is 9.84 (16) .
The carboxylic acid group is coplanar with the naphthyl moiety [dihedral angle of 1.2 (5) , see Fig. 1]. This coplanarity is a peculiarity, as in most other related structures, the COOH group is rotated out of the naphthyl plane (Table 2).

Supramolecular features
The protonated carboxyl oxygen atom of the COOH group donates a hydrogen bond to the DMSO molecule (Table 1). The other carboxyl oxygen atom accepts a hydrogen bond from the water molecule and additionally coordinates to the sodium ion. The sodium ion is sixfold coordinated to one oxygen atom of the COOH group, the carbonyl group, an oxygen atom of the sulfonate group, and two water molecules (one belonging to the same asymmetric unit, the other one transformed by Àx, 1 À y, Àz). The sixth coordination site is occupied by an O atom of a sulfonate group of a neighbouring anion, generated by the symmetry operation À1 + x, y, z. The coordination polyhedron is a distorted octahedron. The crystal packing is characterized by chains built via Na-O coordinations, running along the a-axis direction (Fig. 2). Within this chain, the phenyl ring is -stacked above the O C-C N-N-H moiety of a symmetry-equivalent anion (1 + x, y, z) with A perspective view of the asymmetric unit of the title compound. Displacement ellipsoids are drawn at the 50% probability level.

Synthesis and crystallization
The title compound was obtained by recrystallization experiments of in-house synthesized P.R.52.

Synthesis of Pigment Red 52
2-Amino-5-chloro-p-toluenesulfonic acid (22.15 g, 0.1 mol) was dissolved with sodium hydroxide (6.4 g) in water (500 ml). The temperature was set at 278 K and concentrated hydrochloric acid (40 ml) as well as sodium nitrite (7.2 g) in water (100 ml) were added. The suspension was stirred for 30 min. The suspension was treated with amidosulfonic acid until all excess nitrous acid was destroyed. The suspension was then added dropwise to a solution of -oxynaphthoic acid (18.8 g, 0.1 mol) with NaOH (20.1 g) in water (550 ml). The pH was kept at alkaline conditions, around 11 to 9, maintained by the addition of 2 M NaOH solution as required, and the temperature was maintained at 278 K. When the dropwise addition of the suspension was finished, the solution was allowed to accommodate to room temperature and subsequently heated to 353 K for half an hour. The red suspension was then neutralized with 2 M HCl, filtered off and the obtained red powder was washed with water and dried at 323 K. The yield of the crude product was about 98%, but 404 Tapmeyer     X-ray powder diffraction revealed the presence of some sodium chloride as impurity.

Crystallization of the title compound
The crude in-house synthesized P.R.52 (0.59 g) was dissolved in DMSO (60 ml). The solution was transferred to a glass vessel, which in turn was placed into a further, larger vessel with water (100 ml). The outer vessel was closed with a plastic lid and stored for 20 days at room temperature, allowing the water to diffuse into the DMSO via the gas phase. Single crystals of the title compound were picked from the solution.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The H atoms bonded to C were refined using a riding model with C-H = 0.95 Å and with U iso (H) = 1.2U eq (C) or with C methyl -H = 0.98 Å and with U iso (H) = 1.5U eq (C). The methyl group attached to the phenyl ring was allowed to rotate but not to tip. The H atoms bonded to N and O were found in the difference-Fourier synthesis and freely refined.  SHELXL (Sheldrick, 2015b); molecular graphics: XP (Sheldrick, 2015b) and Mercury (Macrae et al., 2020); software used to prepare material for publication: publCIF (Westrip, 2010).

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