1-Methyl-4-(4-nitrobenzoyl)pyridinium perchlorate

In the main molecule of the title compound, C13H11N2O3 +·ClO4 −, the two aromatic rings are twisted by 56.19 (3)° relative to each other and the nitro group is not coplanar with the benzene ring [36.43 (4)°]. The crystal packing is dominated by infinite aromatic stacks in the a-axis direction. These are formed by the benzene units of the molecule featuring an alternating arrangement, which explains the two different distances of 3.3860 (4) and 3.4907 (4) Å for the aromatic units (these are the perpendicular distances of the centroid of one aromatic ring on the mean plane of the other other aromatic ring). Adjacent stacks are connected by π–π stacking between two pyridinium units [3.5949 (4) Å] and weak C—H⋯O interactions. The perchlorate anions are accomodated in the lattice voids connected to the cation via weak C—H⋯O contacts between the O atoms of the anion and various aromatic as well as methyl H atoms.


Comment
The development of nonlinear optical materials has attracted much attention in the last years. A typical representative consists of conjugated π-systems in which nonlinearities can be achieved by introduction of donor and acceptor substituents. A compound family with such properties relates to appropriately derivatized 4-benzoylpyridine and its respective pyridinium ions (Kolev et al., 2006), with the title compound (I) as an example. Its electrochemical (Leventis et al., 2004a,b) and host/guest (Rawashdeh et al., 2008) characteristics have already been reported earlier. However, considering research on the behaviour in the crystal state, only 4-benzoylpyridine as a mother compound was described so far in complexes with transition metals (Araki et al., 2005;Mautner & Gohera, 1998;Gohera & Mak, 1998;Escuer et al., 2000;Gohera & Mautner, 1999;Drew et al., 1985;Gotsis & White, 1987) as well as in co-crystals with various benzoic acids (Sugiyama et al., 2002a,b) and being derivatized with chlorine in the para position of the benzene ring (Syed et al., 1984). Structural studies on the respective benzoyl-pyridinium species are rather rare both featuring a squaric acid group at the nitrogen atom (Kolev et al., 2001;Kolev et al., 2005). As an extension to the literature, we present in this paper the synthesis and structure characteristics of N-methyl-4-(4-nitrobenzoyl)pyridinium perchlorate (I).
Compound (I) crystallizes from a mixture of ethanol and perchloric acid (20:3 v/v) as colourless crystals in the triclinic space group P-1 with one cation and one anion in the asymmetric unit ( [108.89 (6)-110.12 (6)°] confirm a tetrahedral configuration. Considering the cation, the aromatic and the pyridinium ring are more or less planar with atoms C3 and C8 deviating as much as 0.0115 (6) and 0.0183 (6) Å from their respective meanplanes. Furthermore, the nitro group is not completely coplanar to the corresponding benzene ring [36.43 (4)°]. As anticipated, the central carbonyl part of the structure shows a high degree of planarity, though the overall cation adopts a twisted conformation to minimalize the repulsion between its two rings: torsion angles C5-C4-C7-C8 and C4-C7-C8-C9 are -23.70 (11) and -42.06 (11)°, respectively, and we observed a dihedral angle for the two rings of 56.19 (3)°.
The title compound lacks of donors for strong hydrogen bonds, thus the crystal packing is dominated by aromatic stacks in direction of the crystallographic a axis. These are formed by the slightly displaced and tilted benzene units of the molecule featuring an alternating arrangement, which explains the two different distances of 3.3860 (4) and 3.4907 (4) Å for the aromatic units (Fig. 2). Similar to the dimeric structure of the 4-benzoylpyridine in its monoprotonated form (Mautner & Gohera, 1998), adjacent stacks are connected by π-π-stacking between two pyridinium units [d = 3.5949 (4) Å] and weak C-H···O interactions [d(H···O) = 2.43-2.63 Å] involving two of the aromatic H atoms (H10, H11) and two methyl H atoms (H13A, H13C) on the one hand and the carbonyl oxygen (O3) as well as the two nitro O atoms (O1, O2) on the other hand.
The perchlorate anions are accommodated in the lattice voids connected to the cation via weak C-H···O contacts between the O atoms of the anion and various aromatic as well as methyl H atoms.
In conclusion, the title compound, similar to the related compounds, shows a twisted conformation in the crystalline state [56.19 (3)°], in order to avoid sterical clash. It is interesting to note that another substituent at the benzene unit [p-chlorobenzoylpyridine (Syed et al., 1984)] produces a more similar dihedral angle (52.2°) than observed for the squaric acid derivat-supplementary materials sup-2 ive of benzoyl pyridinium (82.6°) (Kolev et al., 2005). Compared to them, the torsion angles of the title compound reveal a much higher twist of the carbonyl group and the two adjacent rings. Further investigation on the influence of different substituents at the benzene and pyridine entities will deliver more information about this interesting class of compounds.

Experimental
Following a procedure for the synthesis of N-methyl-4-(4-nitrobenzyl)pyridinium iodide described by Fischer (1973), we obtained the respective benzoyl species in a two-step synthesis.
To a stirred solution of 2.14 g (10 mmol) 4-(4-nitrobenzyl)pyridine in 20 ml toluene, 2.50 g (17.6 mmol) methyl iodide were added. While heating under reflux for 30 min, the colour of the solution changed from yellow to purple, and a solid

Refinement
H atoms were positioned geometrically and allowed to ride on their respective parent atoms, with C-H = 0.98 Å and U iso (H) = 1.5U eq (C) for methyl, and C-H = 0.95 Å and U iso (H) = 1.2U eq (C) for aryl H atoms. Fig. 1. Molecular structure of the title compound with 50% probability displacement ellipsoids.   Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.