Synthesis and Molecular Structure of 6-Amino-3-benzyl-mercapto-1 , 2 , 4-triazolo [ 3 , 4-f ] [ 1 , 2 , 4 ] triazin-8 ( 7 H )-one

The title compound 6-amino-3-benzylmercapto-1,2,4-triazolo[3,4-f][1,2,4]triazin-8(7H)-one (4), molecular formula C11H10N6OS, was obtained by the reaction of 3-amino-2-benzyl-6-hydrazino-1,2,4-triazin-5(2H)-one (3) with carbon disulfide in a water/pyridine mixture. Compound 4 can also be synthesized by reacting 6-amino-3(2H)mercapto-1,2,4-triazolo[3,4-f][1,2,4]triazin-8(7H)-one (7) with benzyl bromide in methanolic ammonia water. The compound crystallizes in the monoclinic space group P21/c with a = 7.2926(15), b = 14.456(2), c = 11.436(2) Å, β = 105.30(2)°, V = 1162.9(4) Å and Z = 4, resulting in a density Dcalc of 1.567 g/cm. Molecules of 4 are linked by extensive intermolecular N-H···N and N-H···O hydrogen bonding [graph set R 2 (9)]. The structure is further stabilized by π-π stacking interactions.


Results and Discussion
When compound 3 was refluxed with carbon disulfide in a 1:1 water/pyridine mixture an unexpected ring closed product 4 was obtained (Scheme 2).An intense IR absorption at 1735 cm -1 suggested the carbonyl absorption of fused triazines and confirmed the occurrence of ring closure [3,4,18].The 1 H-NMR spectrum showed two singlets at δ H 4.50 and 6.50 ppm, corresponding to the benzyl group methylene protons and the 6-amino group, respectively.The remaining multiplets at δ H 7.30 to 7.37 ppm were attributed to the aromatic protons of the benzyl group, while the broad singlet at δ H 11.61 ppm corresponded to the NH proton of the 1,2,4-triazinone ring amide.The location of the 3-benzylmercapto group of 4 at C-3 was established by 2D-NMR.The long-range 1 H- 13 C heteronuclear correlation (HETCOR) NMR revealed the connectivity of the methylene protons (δ H 4.50 ppm) on the benzyl group to C-3 (δ C 144.86 ppm) in the 1,2,4-triazolo ring.Additional supporting evidence for 4 was found in the high-resolution mass spectrum showing the molecular ion at m/z 274.0635, confirming the precise molecular formula as C 11 H 10 N 6 OS (calcd.274.0637).

Scheme 2
Further confirmation of the structure of product 4 was obtained from its synthesis via an alternative approach, by reacting 6-amino-3(2H)mercapto-1,2,4-triazolo [3,4- (7) with benzyl bromide in methanolic ammonia (Scheme 2).The resulting nucleophilic substitution product was a material identical in every respect with compound 4. Finally, the structure of 4 was unambiguously confirmed by X-ray crystallography, revealing the structural framework as a 1,2,4-triazole five-membered ring fused at C(9)-N(4) with a 1,2,4-triazine six-membered ring (Figure 1).The most contributing prototropic tautomerism of 4 is the amino-oxo form 7H-tautomer (not the 5H-tautomer), similar to the tautomerism of 6-amino-3-ethyl-1,2,4-triazolo [3,4-   Analysis of the molecular packing in the unit cell reveals that each molecule is linked with two other molecules by intermolecular hydrogen bonds (Figure 3 and Table 1).Each title molecule is linked into two R  For notation and symmetry codes see Table 1.

Conclusions
We have synthesized the title compound 4, an unexpected ring closed product resulting from a reaction of a heterocyclic precursor (compound 3).The structure of 4 was clearly identified by 2D-NMR spectroscopy and unambiguously confirmed by X-ray crystallography.A reasonable mechanism for the ring cyclization of 3 to 4 is now being explored as a part of our ongoing research programs.

X-ray techniques
X-ray quality crystals of the colorless title compound 4 were obtained by crystallization from dimethyl sulfoxide.Crystal and experimental data are summarized in Table 2.The data were collected with a NONIUS CAD4 automated diffractometer equipped with a graphite-monochromatized Mo K α radiation (λ= 0.71073Å) at 295(2) K.The crystal structure data have been deposited at the Cambridge Crystallographic Data Centre [22].The unit-cell parameters were determined from 25 reflections with 8.75° ≤ 2θ ≤ 14.60°.Intensity data with 2θ ≤ 27.50° were collected with the ω-2θ scan technique at 2676 reflections.All reflections were corrected for Lorentz and polarization effects.Absorption corrections were made with the psi-scan method.The crystal structure was resolved by direct methods using SHELXS-86 [23] and refined by full-matrix least-squares methods on F 2 using SHELXL-93 [24].All non-H atoms were refined anisotropically.Hydrogen atoms were allowed as riding atoms with isotropic displacement parameters related to the non-H atoms on which they were riding.

Figure 1 .
Figure 1.ORTEP drawing and atom labelling scheme of the compound 4 with thermal ellipsoids drawn at the 50% probability level.

Figure 2 .
Figure 2. A perspective drawing of the packing arrangement of compound 4. Dashed lines are intermolecular N-H•••N and N-H•••O hydrogen bonds.

2 2 ( 9 )
graph set associations (Figure 3, notation [a], [d] and [b], [c]), each via one N-H•••N hydrogen bond and one N-H•••O hydrogen bond.Assignment of the H-bond descriptors is based on the graph-set theory [21].The structure is further stabilized by π-π stacking (Figure 3 notation [e]) interactions, which results in the centroid•••centroid distance (3.695 Å) being that between the benzene ring of the molecule at x, y, z and the 1,2,4-triazine ring of the molecule at 1-x, 0.5+y, 1.5-z (iii) ([e]).Molecular graphics and hydrogen bond geometry were obtained using the program Mercury (version 1.4, CCDC, Cambridge, UK).

Figure 3 .
Figure 3.A part of the crystal structure of the title compound, showing the molecule stacking (notation [e]) direction along the a-view.Broken lines indicate the intermolecular hydrogen bonding patterns.For notation and symmetry codes see Table1.