Tramadolium picrate

In the title salt {systematic name: [2-hydroxy-3-(3-methoxyphenyl)cyclohexylmethyl]dimethylazanium 2,4,6-trinitrophenolate}, C16H26NO2 +·C6H2N3O7 −, the cation is protonated at the N atom. The cyclohexane ring adopts a chair conformation with the hydroxy substituent in an axial position. In the crystal, O—H⋯O and N—H⋯O hydrogen bonds link the cations and anions into supramolecular chains along [100].

BPS thanks the University of Mysore for research facilities and HSY thanks R. L. Fine Chem, Bengaluru, India, for a gift sample of tramadol hydrochloride.
The protonation of tramadol molecule takes place at the sp 3 nitrogen atom (the hydrogen atom was found in difference Fourier map and successfully refined), and results in the quaternary ammonium cation. The cyclohexane ring adopts the typical chair conformation (Fig. 1). The deviations from the ideal D 3d symmetry are quite small, and the values of the asymmetry parameters (Duax & Norton, 1975) Table 1).

Experimental
Tramadol hydrochloride (2.84 g, 0.01 mol) was dissolved in 10 ml of methanol and picric acid (1.23 g, 0.01 mol) was dissolved in 10 ml of methanol. The solutions were mixed and stirred in a beaker at 333 K for 30 minutes. The mixture was kept aside for three days at room temperature. The product formed was recrystallized from dimethyl sulphoxide by slow evaporation (M.pt.: 491-493 K).

Refinement
Hydrogen atoms from C131 methyl groups were put in calculated positions and refined as a riding model with U iso = 1.5 times U eq (C131). All other hydrogen atoms were located in difference Fourier maps and refined isotropically.

Figure 1
Anisotropic ellipsoid representation of the ionic components of the salt 1, together with atom labelling scheme. The ellipsoids are drawn at 50% probability level, and hydrogen atoms are depicted as spheres with arbitrary radii.    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 R-factors(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 C1A 0.9734 (