3-Hydroxy-2-(hydroxymethyl)pyridinium chloride

The cation of the title compound, C6H8NO2 +·Cl−, is essentially planar (r.m.s. deviation = 0.0104 Å). Intermolecular O—H⋯Cl and N—H⋯Cl hydrogen bonds, as well as C—H⋯O contacts, connect the molecules in the crystal structure. A short C⋯C distance of only 3.3930 (19) Å between C atoms of neighbouring rings is indicative of π-stacking. The corresponding centroid–centroid distance between the two aromatic systems is 4.2370 (7) Å due to the small overlap of the adjacent rings.

The cation of the title compound, C 6 H 8 NO 2 + ÁCl À , is essentially planar (r.m.s. deviation = 0.0104 Å ). Intermolecular O-HÁ Á ÁCl and N-HÁ Á ÁCl hydrogen bonds, as well as C-HÁ Á ÁO contacts, connect the molecules in the crystal structure. A short CÁ Á ÁC distance of only 3.3930 (19) Å between C atoms of neighbouring rings is indicative of -stacking. The corresponding centroid-centroid distance between the two aromatic systems is 4.2370 (7) Å due to the small overlap of the adjacent rings.

Comment
Chelate ligands have found widespread use in coordination chemistry due to the enhanced thermodynamic stability of resultant coordination compounds in relation to coordination compounds exclusively applying comparable monodentate ligands (Gade, 1998). Combining different donor atoms, a molecular set-up to accomodate a large variety of metal centers of variable Lewis acidity is at hand. In this aspect, the title compound seemed of interest due to its possible use as a strictly neutral or, depending on the pH value, as an anionic or cationic ligand. In addition, due to the set-up of its functional groups, it may act as mono-or bidentate ligand offering the possibility to create five-or six-membered chelate rings. To enable comparative studies in terms of bond lengths and angles in the envisioned coordination compounds, we determined the molecular and crystal structure of the title compound. Structural information about 3-hydroxy-2-hydroxymethyl-6-methylpyridine is available in the literature (Casas et al., 2007).
Protonation took place on the nitrogen atom. Intracyclic angles span a range from 118.46 (10) ° to 123.99 (10) ° with the largest angle found on the nitrogen atom and the smallest angle on the carbon atom bearing the hydroxymethyl group.
The non-hydrogen atoms of the organic cation essentially lie in one common plane (r.m.s. of fitted non-hydrogen atoms = 0.0104 Å). The hydroxymethyl group adopts a conformation in which the aliphatic hydroxyl group is bent away from the aromatic hydroxyl group (Fig. 1).
In the crystal structure, hydrogen bonds involving all hydroxyl groups and the protonated nitrogen atom as donors are present. In every case, the chloride anion serves as acceptor (Fig. 2). In addition, a C-H···O contact is observed whose range falls by more than 0.2 Å below the sum of van-der-Waals radii of the atoms participating. The latter contact involves the CH group in ortho position to the hydroxyl group bound to the aromatic system and the oxygen atom of the aliphatic hyxdroxyl group. A description of the classical hydrogen bonding system in terms of graph-set analysis (Etter et al., 1990;Bernstein et al., 1995) necessitates a DDD descriptor on the unitary level while the C-H···O contacts can be described by a C(6) descriptor at the same level. A C···C distance of only 3.3930 (19) Å between carbon atoms of neighbouring rings is indicative of π-stacking with the shortest intercentroid distance between two aromatic systems measured at 4.2370 (7) Å due to the small overlap of adjacent rings. In total, the components of the title compound are connected to a three-dimensional network with the C-H···O contacts forming chains along the crystallographic c axis.
The packing of the compound in the crystal structure is shown in Figure 3.

Experimental
The compound was obtained commercially (Aldrich). Crystals suitable for the X-ray diffraction study were obtained upon slow evaporation of an aqueous solution of the compound at room temperature.
supplementary materials sup-2 Refinement Carbon-bound H atoms were placed in calculated positions (C-H 0.95 Å for aromatic carbon atoms and C-H 0.99 Å for the methylene groups) and were included in the refinement in the riding model approximation, with U(H) set to 1.2U eq (C).
The H atom of the hydroxyl groups as well as the H atom of the protonated nitrogen atom were located on a difference Fourier map and refined with individual thermal parameters. Fig. 1. The molecular structure of the title compound, with atom labels and anisotropic displacement ellipsoids (drawn at 50% probability level). . Blue dashed lines indicate classical hydrogen bonds while green dashed lines indicate C-H···O contacts. Symmetry operators: (i) x, y, z + 1; (ii) -x + 1, -y + 1, -z + 1; (iii) -x + 1, -y + 2, -z; (iv) x -1, y, z;