Bis[5-oxo-4,5-dihydro-8H-2-azonia-4,8,9-trizabicyclo[4.3.0]nona-2,6,9(1)-triene] sulfate

In the crystal structure of the title compound, 2C5H5N4O+·SO4 2−, N—H⋯O hydrogen bonds assemble the molecules into a two-dimensional network structure parallel to the cb plane. The S atom of the sulfate ion lies on a special position on a twofold axis.

In the crystal structure of the title compound, 2C 5 H 5 N 4 O + Á-SO 4 2À , N-HÁ Á ÁO hydrogen bonds assemble the molecules into a two-dimensional network structure parallel to the cb plane. The S atom of the sulfate ion lies on a special position on a twofold axis.
NVR thanks Birla Institute of Technology for financial support. NVR also thanks Prashant M. Bhatt and Professor Gautam R. Desiraju, University of Hyderabad, for help in X-ray diffraction, and Matrix Labs Ltd, Hyderabad, for a gift sample of allopurinol.

S1. Comment
Allopurinol is known to be a potent inhibitor of xanthine oxidase and is used extensively for treatment of gout (Rundles et al., 1966). It is used in conjunction with anticancer drugs which impede RNA biosynthesis and as adjunct therapy in conjunction with 6-mecaptopurine in treatment of leukemia (Elion et al., 1962). As part of our interest in salts and cocrystals of drugs, we have investigated the crystal structure of Allopurinol sulfate, (I) (Fig. 1). The least-squares plane of the six-membered ring makes an angle of 5.79 (11)° with the least-squares planes of the five-membered ring of the purine ring. The N3 nitrogen is protonated similar to the reported structure of chloride salt (Sheldrick & Bell, 1987). The molecules are linked via N-H···O hydrogen bonds,forming two-dimensional infinite chains along the cb plane. These sheets are linked together by the sulfate molecules which act as acceptors of H atoms, assembling the molecules in an infinite two-dimensional network (Fig. 2).

S2. Experimental
Allopurinol (0.1361 g, 1 mmol) and Sulfuric acid (0.27 ml, 5 mmol) were added to Dimethylformamide. The mixture was stirred and heated on a water bath. After solution was complete, it was filtered and allowed to evaporate at room temperature. Colorless plate like crystals, suitable for x-ray analysis grew over a period of two days when the solution was exposed to air.

S3. Refinement
Atoms H1, H3 and H5 on N1, N2 and N3 respectively were located in difference Fourier maps and refined isotropically.
All other H atoms were found in difference map but then placed in calculated positions with C-H = 0.93 Å and U iso (H) = 1.2Ueq(C) for all aromatic H atoms.  The molecular structure of (I), with atom labels and 50% probability displacement ellipsoids for non-H atoms. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.32 e Å −3 Δρ min = −0.38 e Å −3 Extinction correction: SHELXL97 (Sheldrick, 2008), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.0068 (13) Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. 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.