Crystal structure studies and Hirshfeld surface analysis of 4-(dimethylazaniumyl)-2-hydroxyanilinium dichloride monohydrate at 90 K

The low-temperature (90 K) crystal structure of 4-(dimethylazaniumyl)-2-hydroxyanilinium dichloride monohydrate is presented along with a Hirshfeld surface analysis of the organic cation.


Chemical context
Aniline is an important industrial feedstock chemical, broadly utilized throughout the chemical industry.For example, as a precursor to indigo, it is of paramount importance in the manufacture of dyes.Indeed, the modern synthetic dyestuffs industry traces its origin to mauveine, a product of William Henry Perkin's attempts to synthesize quinine by oxidation of aniline (see e.g.Perkin, 1896).Aniline and its derivatives find extensive use in the rubber industry for processing materials used in products such as car tires, balloons, and gloves.In addition, aniline plays a crucial role in the production of numerous pharmaceutical drugs, including such well-known medications as paracetamol (aka, acetaminophen/Tylenol) and the fenamate family of NSAIDs (anthranilic acid derivatives).Within this context, a concise review of aniline and its derivatives was presented by Anjalin et al. (2020).The hydrogen-bonding behavior of aniline derivatives has been investigated using FT-IR spectroscopy by Meng-Xia & Yuan (2002).The application of anilinium salts in polymer networks, resulting in materials with superior mechanical stability and mild thermally induced dynamic properties was reported by Chakma et al. (2019).
Given the industrial and pharmaceutical significance of anilinium salts, this paper presents the crystal structure and Hirshfeld-surface analysis of 4-(dimethylazaniumyl)-2-hy-

Structural commentary
The asymmetric unit of I (see Fig. 1) consists of a single 4-(dimethylazaniumyl)-2-hydroxyanilinium dication, two chloride anions and a water of crystallization.The cation is largely planar.Aside from the two methyl groups, the r.m.s.deviation from the mean plane passing through the ring carbons, two nitrogens and phenolic oxygen atom is 0.0045 A ˚, with the largest deviation being only 0.0096 (7) A ˚, for C5.The two methyl carbons lie 1.3125 (12) A ˚and 1.1278 (12) A ˚(for C7 and C8 respectively) either side of this mean plane.The water oxygen (O1W), at 0.1059 (14) A ˚, is also almost coplanar with the cation, while the chloride anions deviate by 0.4827 (12) A ˚(Cl1) and 0.4443 (12) A ˚(Cl2) to either side.The only internal degree of freedom involves rotation of the dimethylaminium group about the C4-N2 bond, leading to torsion angles C3-C4-N2-C7 = 108.41(9) � , C3-C4-N2-C8 = À 125.32 (9) � and C3-C4-N2-H2N = À 8.3 (8) � .There are no unusual bond lengths or angles in the structure.
Two-dimensional fingerprint plots (Fig. 4) derived from a Hirshfeld surface analysis mapped over d norm for the cation in I were obtained using CrystalExplorer (Spackman et al., 2021).These show that atom-atom contacts for the cation are dominated by hydrogen, either to other H atoms (51.3%) or to

Database survey
A search of the Cambridge Structural Database (CSD, v5.43 with all updates to November 2022; Groom et al., 2016) for a molecular fragment composed of a benzene ring with any Nbound group at the 1-and 4-positions and an O-bound group at the 2-position yielded 471 matches.With the O-bound group defined as hydroxyl there were 62 hits.The further restriction of having two C-bound groups attached to the 4-N nitrogen returned 15 entries (13 unique), but with the Cbound groups both specified as methyl there were no matches.Of the 13 unique structures only one, XAVKAJ [(C 30 H 32 N 6 O 2 ) 4+ •4Cl À •4H 2 O; Stylianou et al., 2017] is a salt or a hydrate, but it has little else in common with I. Two other anilinium salts not returned in the above search but that share similar features to I are POMXUL (Smirani & Rzaigui, 2009), or 2,5-dimethylanilinium chloride monohydrate (C 8 H 12 N + •Cl À •H 2 O) and PAXXIX (Devi et al., 2012), which is 4-[(E)-(hydroxyimino)methyl]-N,N-dimethyl anilinium chloride (C 9 H 13 N 2 O + •Cl À ).

Synthesis and crystallization
The sample of I was obtained as a gift from Honeychem Pharma, Bengaluru, India.Crystals suitable for X-ray struc-

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2.All hydrogen atoms were present in difference-Fourier maps.Carbon-bound hydrogens were subsequently included in the refinement using riding models, with constrained distances of 0.95 A ˚(R 2 CH) and 0.98 A (RCH 3 ) and U iso (H) parameters set to either 1.2U eq (R 2 CH) or 1.5U eq (RCH 3 ) of the attached carbon.Nitrogen and oxygen-bound hydrogens were fully refined (x, y, z, and U iso ).

Special details
Experimental.The crystal was mounted using polyisobutene oil on the tip of a fine glass fibre, which was fastened in a copper mounting pin with electrical solder.It was placed directly into the cold gas stream of a liquid-nitrogen based cryostat (Hope, 1994;Parkin & Hope, 1998).Diffraction data were collected with the crystal at 90K, which is standard practice in this laboratory for the majority of flash-cooled crystals.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.Refinement.Refinement progress was checked using Platon (Spek, 2020) and by an R-tensor (Parkin, 2000).The final model was further checked with the IUCr utility checkCIF.

Figure 1
Figure 1An ellipsoid plot (50% probability) of I. Hydrogen atoms are shown as small circles.Hydrogen bonds are drawn as dashed lines.

Figure 2 A
Figure 2 A partial packing plot of I showing R 2 4 (12) hydrogen-bonded (dotted lines) ring motifs that link to form chains that propagate parallel to [101].

Figure 3
Figure 3 Partial packing plots of I showing: (a) hydrogen-bonded (dotted lines) layers that extend parallel to (101) and (b) the same layers viewed side-on to highlight the corrugation.

Table 2
Experimental details.