Acridinium 6-carboxypyridine-2-carboxylate monohydrate

The title compound, C13H10N+·C7H4NO4 −·H2O or (acrH)+(pydcH)−·H2O, is a monohydrate of acridinium cations and a mono-deprotonated pyridine-2,6-dicarboxylic acid. The structure contains a range of non-covalent interactions, such as O—H⋯O, O—H⋯N and N—H⋯O hydrogen bonds, as well as π–π stacking [range of centroid–centroid distances = 3.4783 (5)–3.8059 (5) Å]. The N—H⋯O hydrogen bond between the donor acridinium cation and the carboxylate acceptor is particularly strong. The average separation between the π-stacked acridinium planes is 3.42 (3) Å.


Related literature
Hydrogen-bond geometry (Å , ). We have reported a number of crystal structures of protonated acridine and pyridine dicarboxylates (Derikvand et al., 2009(Derikvand et al., , 2010Aghabozorg et al., 2010;Attar Gharamaleki et al., 2010;Tabatabaee et al., 2009). Many other examples of acridinium salts are known, and they have π-π stacking of the acridinium ions and various types of hydrogen bonding in common. The molecular structure of the title compound, the 1:1 salt of acridinium and pydridine-2,6-dicarboylate is illustrated in Fig. 1.
The crystal structure shows one of the protons of the two carboxylic groups has been transferred to the nitrogen atom of the acridine molecule.
As expected, bond lengths of the -CO 2 groups reflect the presence or lack of an acidic H atom. At distances of 1.2403 (11) Å and 1.2806 (1)) Å, respectively, the O1-C19 and O2-C19 bond lengths are much closer to equality than O3-C20 and O4-C20, at 1.226 (11) Å and 1.3305 (11) Å. However, we can also point out that the O2-C19 bond is slightly longer than the O1-C19 bond, possibly due to there being two classical hydrogen bonds involving O2 and only one involving O1 (see Table 1). In fact, one of the hydrogen bonds for O2 can be classified as a very strong hydrogen bond, with an N···O distance of 2.5859 (9) Å. In a similar structure involving the acridinium salt of isophthalate (Shaameri et al., 2001), the analogous arrangement of cation and anion gives rise to a similar short hydrogen bond with N···O distance of 2.553 (2) Å.
A depiction of the hydrogen bonded motif involving anion and cation fragments and water molecules is presented in Fig. 2. The hydrogen bonds between the water molecule and O2 serve to link the anions into a chain along the a axis direction.
Additional noncovalent interactions cause the structure to form a self assembled system. In the structure π-π stacking interactions between the acridinium ions average 3.42[3]Å (average deviation in square brackets). Sideways strong hydrogen bonds between O2 and the the proton of acridine gather the π-stack and the anionic chain together as shown in Fig. 3.

Experimental
A solution of pyridine-2,6-dicarboxylic acid (167 mg, 1 mmol) in water (10 ml) was added to a solution of acridine(179 mg, 1 mmol) in methanol (5 ml) and stirring for 30 minutes, a clear solution was obtained (Scheme 1). Yellow-gold block crystals suitable for X-ray crystallography were produced by slow evaporation of the solvent at room temperature after a week.

Refinement
All hydrogen atoms were freely refined. 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.