3,4-Diaminopyridinium 2-carboxy-4,6-dinitrophenolate

In the title salt, C5H8N3 +·C7H3N2O7 −, the pyridine N atom of the 3,4-diaminopyridine molecule is protonated. The 3,5-dinitrosalicylate anion shows whole-molecule disorder over two orientations with a refined occupancy ratio of 0.875 (4): 0.125 (4). In the crystal, the cations and anions are connected by intermolecular N—H⋯O and C—H⋯O hydrogen bonds, forming a three-dimensional network.

Since our aim is to study some interesting hydrogen-bonding interactions, the synthesis and structure of the title compound (I) is presented here.

Experimental
A hot methanol solution (20 ml) of 3,4-diaminopyridine (27 mg, Aldrich) and 3,5-dinitrosalicylic acid (57 mg, Merck) were mixed and warmed over a heating magnetic stirrer hotplate for a few minutes. The resulting solution was allowed to cool slowly at room temperature and crystals of the title compound appeared after a few days.

Refinement
All the H atoms were positioned geometrically [C-H = 0.93 Å; N-H = 0.8875-1.0684 Å] and were refined using a riding model, with U iso (H) = 1.2 U eq (C,N,O). The whole 3,5-dinitrosalicylate anion is disordered over two positions with a refined ratio of 0.886 (4): 0.114 (4). In the final difference Fourier map, the highest peak and the deepest hole are 1.24 Å and 0.62 Å from H1N2 and C5, respectively.

Special details
Experimental 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 > 2σ(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (