Crystal structure of the 1:1 co-crystal 4-(dimethylamino)pyridin-1-ium 8-hydroxyquinoline-5-sulfonate–N,N-dimethylpyridin-4-amine

The asymmetric unit of the title compound consists of two independent ion pairs of 4-(dimethylamino)pyridin-1-ium quinolin-8-ol-5-sulfonate (HDMAP+·HqSA−) and neutral N,N-dimethylpyridin-4-amine (DMAP), forming a 1:1:1 cation:anion:neutral molecule co-crystal. The compound has a layered structure, including cation layers of HDMAP+ with DMAP and anion layers of HqSA− in the crystal. The cation and anion layers are linked by intermolecular C—H⋯O hydrogen bonds and C—H⋯π interactions.


Structural commentary
The title compound is composed of two independent HDMAP + •HqSA À ion pairs and neutral DMAP molecules, co-crystallized in the monoclinic system, space group Pc as shown in Fig. 1.The phenolic H atoms (H6, H10) in the HqSA À moieties are not dissociated.

Supramolecular features
In the title co-crystal, both the cation layers of [HDMAP•DMAP] + and the anion layers of HqSA À run parallel to the ab plane.The hydrogen-bond geometry is summarized in Table 1.The pyridine rings in the cation layer are stacked along the ab plane as shown in Fig. 2. In the cation layer, two independent cation units of [HDMAP•DMAP] + are formed by intermolecular N-H� � �N hydrogen bonds (N14-H14� � �N15 and N18-H18� � �N19).The

Figure 2
The layer structure of the [HDMAP•DMAP] + cationic unit in the ab plane.The intermolecular N-H� � �N hydrogen bonds are shown as dashed lines.

Figure 1
The molecular structure of the title compound with atom labeling.Displacement ellipsoids are drawn at the 50% probability level.H atoms are represented by spheres of arbitrary radius.

Database survey
A search of the Cambridge Structural Database (CSD, Version 2024.1.0,update of March 2024; Groom et al., 2016) for compounds containing the 4-aminopyridine skeleton with hydrogen atom bound at the 2, 3, 5, 6-positions of the pyridine ring gave 5687 hits.Among those, a search for the containing DMAP molecule gave 1794 hits and for those of protonated DMAP gave 360 hits.A search for compounds containing a pyridine-protonated pyridine skeleton gave 15 hits.In these compounds, the dihedral angles between two pyridine rings are close to 0 � in seven structures, which are essentially coplanar due to unique hydrogen-bonding networks stemming from the substituents on the pyridine rings (BAYBIN; Kobayashi et al., 2003;BECHOG;Glidewell et al., 1982;KIFBIO;Vladiskovic et al., 2023;WAZNET;Lackova et al., 2014;WEVHOX;Zhang et al., 2018;XACFOW;Mautner & Goher, 1998;XOHWAT;Santra et al., 2008).In single crystals of salts of the mellitate anion, which is obtained by deprotonation of mellitic acid (benzene hexacarboxylic acid), with substituted pyridinium derivatives, the triangular hydrogen-

Figure 3
The S( 5 bonded unit between the anions induces a two-dimensional sheet self-organizing structure (BAYBIN, Kobayashi et al., 2003).On the other hand, ferrocene derivatives substituted with pyridine form cationic dimers via a hydrogen bond between two pyridine rings (WOFGII; Braga et al., 2008).A search for containing both of protonated DMAP and the other neutral DMAP gave 14 hits.There are five hits having the proton between two N-(4-pyridyl)dimethylamine skeletons (2, 3, 5, 6-carbon atoms are bound to hydrogen atoms).In these compounds, the dihedral angles between two pyridine rings are close to 0 � in three structures, which are essentially coplanar structures [1.et al., 2004).According to the crystal structures of BAYBIN, EFAQUZ, EYIYOA and ISUTAR, these compounds form layered structures by constructing 2D layers of the cationic and anionic moieties with these layers arranged sterically.

Synthesis and crystallization
To a solution of DMAP (611 mg, 5.0 mmol) in H 2 O (5 mL) at 353 K, an ethanol (1 mL) solution of H 2 qSA (450 mg, 2.0 mmol) was added and then stirred for 30 min.Orange single crystals of the title compound suitable for X-ray diffraction were grown by slow evaporation of the aqueous ethanol solution mentioned above for a week at ambient temperature.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2.The title compound was refined as an inversion twin in Pc whose twin component mass ratio refined to 0.522 (18):0.478(18).The hydroxy H atoms, H6 and H10, were located in a difference-Fourier map and freely refined.The N-bound H atoms, H14 and H18, were located in difference-Fourier maps but were refined with a distance restraint of N-H = 0.86 � 0.02 A ˚.All H atoms bound to carbon were positioned geometrically and refined using a riding model, with C-H = 0.95 or 0.98 A ˚and U iso (H) = 1.2 or 1.5U eq (C).
) ring motifs formed by intramolecular O-H� � �N hydrogen bonds involving the hydroxy groups and quinoline N atoms of the HqSA À anionic units.The intramolecular O-H� � �N hydrogen bonds are shown as dashed lines.The sheet structure of the HqSA À anionic units is formed by the planar intermolecular hydrogen-bond networks in the ab plane.The intermolecular O-H� � �O, C-H� � �O, O-H� � �N hydrogen bonds are also shown as dashed lines.[Symmetry codes:

Table 2
Experimental details.