A second monoclinic polymorph of 6-amino-1,3-dimethyl-5-[(E)-2-(methylsulfanyl)benzylideneamino]pyrimidine-2,4(1H,3H)-dione

A new monoclinic form of the title compound, C14H16N4O2S, has been identified unexpectedly during an attempt to synthesize a coordination compound. The heterocyclic ring is essentially planar (r.m.s. deviation = 0.005 Å) and makes a dihedral angle of 8.77 (5)° with the benzene ring. This is in contrast to 12.24 (7)° reported for the first monoclinic polymorph [Booysen et al. (2011 ▶). Acta Cryst. E67, o1592]. An intramolecular N—H⋯S hydrogen bond is observed. In the crystal, intermolecular N—H⋯O hydrogen bonds link the molecules into zigzag chains along the b axis. The closest distance between the centroids of symmetry-related heterocyclic rings is 3.5161 (6) Å.


Data collection
Bruker APEXII CCD diffractometer 13030 measured reflections 3420 independent reflections 3112 reflections with I > 2(I) R int = 0.029 Refinement R[F 2 > 2(F 2 )] = 0.029 wR(F 2 ) = 0.081 S = 1.02 3420 reflections 201 parameters H atoms treated by a mixture of independent and constrained refinement Á max = 0.33 e Å À3 Á min = À0.24 e Å À3 Table 1 Hydrogen-bond geometry (Å , ). Data collection: APEX2 (Bruker, 2010); cell refinement: SAINT (Bruker, 2010); data reduction: SAINT; program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 (Farrugia, 1997) and Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXL97 and PLATON (Spek, 2009 (Gade, 1998). Combining different sets of donor atoms in one chelate ligand molecule, a probe for testing and accomodating metal centers of different Lewis acidities is at hand. Recently, we performed a coordination reaction applying the title compound as the ligand. A single-crystal X-ray analysis of the isolated reaction product did not show the expected compound but the organic starting material whose crystal structure had been determined earlier by us in the same spacegroup, however, with different cell constants (Booysen et al., 2011). Other crystal structures of Schiff-bases derived from ortho-(thiomethyl)-benzaldehyde are reported in the literature (Yan et al., 2007;Baidina et al. 1987).
The molecule is a Schiff-base featuring an ortho-(thiomethyl)phenyl moiety and a 6-amino-1,3-dimethylpyrimidine-2,4(1H,3H)-dione moiety. The double-bond is (E)-configured. A conformational analysis of the non-aromatic six-membered ring (Cremer & Pople, 1975) fails due to the low puckering amplitude. The molecule is almost planar, the least-squares planes defined by the respective atoms of both six-membered ring systems intersect at an angle of only 8.77 (5) °. It is pertinent to emphasize that the plane of the non-aromatic ring is tilted in a different direction with respect to the molecular structure of the title compound determined earlier by us ( Fig. 1 and Fig 2).
In the crystal structure, intra-as well as intermolecular hydrogen bonds can be observed, both supported by the amino group. While the intramolecular hydrogen bond is formed to the sulfur atom of the thiomethyl group, the intermolecular hydrogen bond uses one of the ketonic oxygen atoms as acceptor. The latter ones connect the molecules to zigzag chains along the crystallographic b axis. Unlike our earlier structure determination of the title compound, no C-H···O contacts are obvious. The descriptor for the hydrogen bonding system in terms of graph-set analysis (Etter et al., 1990;Bernstein et al., 1995) is DC 1 1 (6) on the unitary level (Fig. 3). The shortest distance between the centroids of symmetry related heterocyclic rings was found to be 3.5161 (6) Å. The packing of the title compound is shown in Fig. 4.

Experimental
The title compound was prepared as described in the literature (Booysen et al., 2011). Single crystals suitable for the X-ray analysis were obtained by reacting the title compound with ReOBr 3 (PPh 3 ) 2 in methanol and subsequent free evaporation of the solvent.

Refinement
H atoms bonded to aromatic C atoms were placed in calculated positions (C-H = 0.95 Å) and were included in the refinement in the riding model approximation, with U(H) set to 1.2U eq (C). The H atoms of the methyl groups were allowed to rotate with a fixed angle around the C-C bond to best fit the experimental electron density (HFIX 137 in the SHELX program supplementary materials sup-2 suite (Sheldrick, 2008)), with U(H) set to 1.5U eq (C). Both nitrogen-bound H atoms were located in a difference Fourier map and refined freely. Fig. 1. The molecular structure of the title compound, with atom labels and anisotropic displacement ellipsoids (drawn at the 50% probability level). Fig. 2. Comparison between the relative orientation of the planes of the non-aromatic sixmembered ring with respect to the plane of the aromatic system in the molecular structure of the title compound determined earlier by us (Booysen et al., 2011, top) and in the present study (bottom). Hydrogen atoms were omitted for reasons of clarity.