Acridinium 3-carboxypyrazine-2-carboxylate

The title ion pair, C13H10N+·C6H3N2O4 −, contains a protonated acridine cation and a 3-carboxypyrazine-2-carboxylate monoanion, which are linked together through O—H⋯O, N—H⋯O and weak C—H⋯O hydrogen bonds. These hydrogen bonds generate a C(10) chain graph-set motif. The crystal structure is further stabilized by extensive π–π stacking interactions between nearly parallel [dihedral angle = 1.21(2)°] acridine systems. The shortest distance between the centroids of the six-membered rings within the cations is 3.6315 (8) Å. In addition, C—H⋯π edge-to-face interactions are present.


Experimental
Cg1 is the centroid of the N1,N2,C1-C4 ring. (17 HSE thanks the staff of the X-ray Application LAB, CSEM, Neuch\^atel for access to the X-ray diffraction equipment.

D-HÁ
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: OM2349).  (1974). Acta Cryst. B30, 1044-1047. Pyrazine-2,3-dicarboxylic acid, pz-2,3-dcH 2 , has proved to be well suited for the construction of the multi-dimensional frameworks due to the presence of two adjacent carboxylic acid groups.
There have been several attempts to prepare proton transfer compounds involving carboxylic acids and amines. For example, ion pairs have been reported between pz-2,3-dcH 2 and various organic bases such as 8-hydroxy quinoline (Smith et al., 2006a) and guanidine (Smith et al., 2006b). The crystal structure of a proton-transfer compound of acridine and benzene-1,3,5-tricarboxylic acid has been reported (Derikvand et al., 2009). In this work, we report a new proton transfer compound obtained from pyrazine-2,3-dicarboxylic acid as a proton donor and acridine as an acceptor.
The molecular structure of the title compound ( Fig. 1), confirmed the full proton transfer, i.e. protonation of the acridine N atom and deprotonation of one of the carboxylic acid groups in the pyrazine-2,3-dicarboxylic acid. The carboxylate groups of the anion are twisted by 46.09 (7) and 37.34 (7)° with respect to the aromatic ring of pyrazine.
Non-covalent interactions cause the structure to form a self-assembled system. A hydrogen bonded motif is found involving the anion and cation fragments. The (pz-2,3-dcH)units are linked to each other by O-H···O hydrogen bonds to form one-dimensional chains with a C(10) graph-set motif (Bernstein et al., 1995). In contrast, the N-H···O and C-H···O hydrogen bonds link the (acrH) + cations to these chains (Fig. 2, Table 1). In addition, interactions consisting of π-π stacking with centroid-to-centroid distances of 3.6315 (8) to 3.7202 (9) Å between two acridine parallel rings are also present. Furthermore, C-H···π edge-to-face interactions are present involving the CH group of acridine with an aromatic ring of (pz-2,3-dcH) -, with a H···π distance of 2.95 Å for C18-H18···Cg1 i [symmetry code: (i) 1 -x, 1/2 + y, 1/2 -z; Cg1= centroid of ring N1,N2,C1-C3; Fig. 3)]. The sum of these weak non-covalent interactions seems to play an important role in the crystal packing. The unit cell packing diagram of the title compound, showing the N-H···O and O-H···O hydrogen bonding, is shown in Fig. 4 and details are given in Table 1.

Experimental
The reaction between a solution of pyrazine-2,3-dicarboxylic acid (160 mg, 1 mmol) in 20 ml water and acridine (180 mg, 1 mmol) in 10 ml methanol, in a 1:1 molar ratio, gave brown rod-like crystals after slow evaporation of the solvent at room temperature.

Refinement
The OH and NH H-atoms were located in a difference electron-density map and were freely refined: O-H = 0.948 (7) Å, N-H = 0.944 (16) Å. The C-bound H-atoms were included in calculated positions and treated as riding atoms: C-H = 0.94 Å with U iso (H) = 1.2U eq (parent C-atom). Fig. 1. Molecular structure of the title ion pair, with ellipsoids drawn at the 50% probability level.  Table 1 for details).