6-Amino-1,3-dimethyl-5-[(E)-2-(methylsulfanyl)benzylideneamino]pyrimidine-2,4(1H,3H)-dione–1,3,7,9-tetramethylpyrimido[5,4-g]pteridine-2,4,6,8-tetrone (1/1)

In the title co-crystal, C12H12N6O4·C14H16N4O2S, both molecules are essentially planar [maximum deviations = 0.129 (1) and 0.097 (1) Å, respectively]. The tricyclic and Schiff base molecules are alternately stacked along the a axis and are linked by π–π interactions with centroid–centroid distances of 3.5170 (16) and 3.6576 (17) Å. An intramolecular C—H⋯O hydrogen bond and a C—H⋯S contact occur in the Schiff base molecule. In the crystal, N—H⋯O, N—H⋯N and C—H⋯O hydrogen bonds lead to the formation of a three-dimensional network.

The tricyclic molecule consists of three fused rings, a central pyrazine ring and two terminal pyrimidine rings. The largest deviation from planarity is shown by the methyl carbon atom C10B, which is displaced by 0.129 (1) Å from the 22-atom mean plane defined by all non-hydrogen atoms. The bond lengths within the pyrazine ring suggest that they are part of a π-delocalized ring system. Thus the N4B and N5B atoms are sp 2 -hybridized which is confirmed by the bond angles C5B-N5B-C3B [117.2 (2)°] and C6B-N4B-C4B [115.6 (2)°].
The two molecules exhibit π-π interactions. Each tricyclic molecule is sandwiched between two Schiff base molecules.
The distance between the centroid of the pyrazine ring, Cg1, and the centroid of the Schiff base phenyl ring, Cg2, within the aysmmetric unit is 3.517 (1) Å. The distance between Cg1 and Cg2 of a symmetry related molecule is 3.658 (1) Å ( Figure 2). The crystal lattice is further stablized by a large number of both classical and non-classical hydrogen bonds.
These hydrogen bonds are both inter and inramolecular in nature and ultimately link the Schiff base and tricyclic molecules into an infinite, three-dimensional network. Hydrogen bond lengths and angles are summarized in Table 1. The hydrogen bonding pattern is illustrated in Figure 3.

Refinement
All non-hydrogen atoms were located in the difference Fourier map and refined anisotropically. The positions of all hydrogen atoms were calculated using the standard riding model of SHELXL97 with C-H(aromatic) distances of 0.93 Å and U iso = 1.2 U eq , and C-H(methyl) distances of 0.96 Å and U iso = 1.5 U eq . The amine hydrogen atoms were located in the difference Fourier map and allowed to refine isotropically.

Figure 1
The asymmetric unit of the title compound. Displacement ellipsoids have been rendered at the 50% probability level.  The tricyclic molecule sandwiched between two Schiff base molecules. π -π interactions are shown as dashed lines.

Figure 3
Hydrogen bonding between the tricyclic molecule and the Schiff base molecule.

Special details
Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'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 > 2σ(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.