Bis[(E)-4-(hydroxyiminomethyl)pyridinium] oxalate

The formula unit of the title compound, 2C6H7N2O+·C2O4 2−, comprises two symmetry-equivalent 4-(hydroxyiminomethyl)pyridinium cations on general positions, linked through hydrogen bonding via an oxalate anion that resides on a crystallographic centre of symmetry. The crystal structure consists of infinite chains of cations and oxalate anions directed by O—H⋯O and multicentre N—H⋯O intermolecular hydrogen-bonding interactions.

The formula unit of the title compound, 2C 6 H 7 N 2 O + ÁC 2 O 4 2À , comprises two symmetry-equivalent 4-(hydroxyiminomethyl)pyridinium cations on general positions, linked through hydrogen bonding via an oxalate anion that resides on a crystallographic centre of symmetry. The crystal structure consists of infinite chains of cations and oxalate anions directed by O-HÁ Á ÁO and multicentre N-HÁ Á ÁO intermolecular hydrogen-bonding interactions.

RWS thanks Dr Tobias van Almsick for helpful discussions.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: GG2061).
This leads to the formation of infinite chains in the crystal structure, where one molecule is connected to a neighbouring molecule by an O-H···N hydrogen bond. Its protonated analoque, the pyridinium-4-carbaldehyde oxime cation is able to act as a twofold hydrogen bonding donor. Oxalate anions are well known as multiple hydrogen bonding acceptors. Herein we report the crystal and molecular structure of the oxalic acid salt of (E)-pyridinium-4-carbaldehyde oxime oxalate (Scheme 1).
An ORTEP diagram of the title compound is given in Fig. 1: selected geometric parameters are in table 1. The molecular geometry parameters are of usual order of magnitude and for the cation similar to its parent freebase compound (Martínez-Ripoll & Lorenz, 1976a). The bond length of the oxime group N42-C42 and O43-N42 are 1.274 (2) and 1.380 (2) Å, respectively. The C4-C41 bond is relatively short with 1.468 (2) Å due to partial delocalization of the oxime group with the pyridinium ring. Like the freebase (E)-pyridin-4-carbaldehyde oxime the cation of the title compound is not planar. The torsion angle of the N atom N42 of oxime group out of the plane of the pyridinium ring is 20.0 (2) °. The bond length of C7-C7i is relatively long with 1.563 (3) Å as expected for an oxalate anion.
The crystal structure of the title compound is given in Fig. 2 as a projection along the a axis direction. The O atom O43 of the oxime group of the pyridinium-4-carbaldehyde oxime cation is connected to O72 of the oxalate anion by a single hydrogen bond. The O43-H···O72 distance is 2.571 (2) Å indicating a moderate strong hydrogen bond. The N atom of the pyridinium ring is directed to the oxalate anion due to electrostatic interactions. The N1-H···O71 and N1-H···O72 is 2.660 (2) Å and 2.897 (2) Å, respectively. An increased hydrogen bond distance is indicative for multicentre bonding.

S2. Experimental
Pyridin-4-carbaldehyde oxime (Acros) was allowed to react with oxalyl dichloride (2:1 molar ratio) in dry tetrahydrofuran at ambient temperature. The resulting precipitate was filtered off, dried on air and dissolved in a mixture of dimethylformamide and water (1:1). This solution was allowed to evaporate slowly at room temperature. Single crystals of the title compound suitable for X-ray diffraction were obtained after two weeks.

S3. Refinement
The structure was refined by full-matrix least-squares refinement against F 2 using SHELXL97 (Sheldrick, 1997). All nonhydrogen atoms were refined freely by using anisotropic displacement parameters. Hydrogen atoms were placed on geometrically calculated positions and refined with riding models in all cases with U iso (H) = 1.2 (1.5 for hydroxy groups) supporting information sup-2 Acta Cryst. (2008). E64, o181 times U eq (C, N, O).

Figure 1
ORTEP diagram of the title compound. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms are drawn at arbitrary size and symmetry generated atoms are labelled with a.