Received 14 June 2006
In the title cocrystal, C6H9N3O2·C7H7NO2, the 2-amino-4,6-dimethoxypyrimidine molecule interacts with the carboxyl group of the 4-aminobenzoic acid molecule through N-HO and O-HN hydrogen bonds, forming a cyclic hydrogen-bonded motif [R22(8)]. This motif further self-organizes through N-HO hydrogen bonds to generate an array of six hydrogen bonds with the rings having the graph-set notation R23(6), R22(8), R42(8), R22(8) and R23(6). The 4-aminobenzoic acid molecules self-assemble via N-HO hydrogen bonds to form a supramolecular chain along the c axis.
Pyrimidine and aminopyrimidine derivatives are biologically important compounds as they occur in nature as components of nucleic acids. Some aminopyrimidine derivatives are used as antifolate drugs (Hunt et al., 1980; Baker & Santi, 1965). The adducts of carboxylic acids with 2-aminoheterocylic ring systems form a graph-set motif of R22(8) (Lynch & Jones, 2004). The crystal structure of 2-amino-4,6-dimethoxy pyrimidine has also been reported (Low et al., 2002). The crystal structure of 4-aminobenzoic acid (Lai & Marsh, 1967) is known. The interplay of strong N-HO and O-HN hydrogen bonds, and weak C-HO interactions, forms supramolecular motifs, involved in the molecular packing of organic solids. (Taylor & Kennard, 1982). In the present study, the hydrogen-bonding patterns in the 2-amino-4,6-dimethoxypyrimidine-4-aminobenzoic acid (1/1) cocrystal, (I), are investigated.
The asymmetric unit (Fig. 1) contains one 2-amino-4,6-dimethoxypyrimidine molecule and one 4-aminobenzoic acid molecule, which are linked by N2-H2BO3 and O4-H4N1 hydrogen bonds (Table 1), forming an eight-membered ring of graph-set notation R22(8) (Etter, 1990; Bernstein et al., 1995). This type of pairing has been observed in the crystal structure of 2-aminopyrimidine-fumaric acid (Goswami et al., 1999) and 2-aminopyrimidine-(+)-camphoric acid (Goswami et al., 2000). This motif further self organizes through N-HO hydrogen bonds (Fig. 2) to generate an array of six hydrogen bonds with the rings having the graph-set notations R23(6), R22(8), R42(8), R22(8) and R23(6). The 4-aminobenzoic acid molecules self-assemble via N-HO hydrogen bonds to form a supramolecular chain along the c axis, with the graph-set notation C(9); this is shown in Fig. 3. The pyrimidine ring is centrosymmetrically linked through a pair of C-HO hydrogen bonds involving a methyl group (C7) and methoxy atom O2. A - stacking interaction between two aminopyrimidine groups (at x, y, z and -x, 1 - y, -z), with a perpendicular separation of 3.306 Å, a centroid-centroid distance of 3.4129 (8) Å and a slip angle (the angle between the centroid vector and the normal to the plane) of 14.39° has also been observed. These are typical aromatic stacking values (Hunter, 1994).
| || Figure 1 |
A view of the asymmetric unit of (I), showing 50% probability displacement ellipsoids. Dashed lines indicate hydrogen bonds.
| || Figure 2 |
Hydrogen-bonding (dashed lines) patterns in compound (I).
| || Figure 3 |
Hydrogen-bonding (dashed lines) patterns in the supramolecular chain in compound (I) [symmetry code: (ii) 1 - x, + y, - z].
A hot methanol solution (20 ml) of 2-amino-4,6-dimethoxy pyrimidine (38 mg, Aldrich) and 4-aminobenzoic acid (34 mg, Loba Chemie) was warmed for half an hour over a water bath. The mixture was cooled slowly and kept at room temperature; after a few days, colourless plate-like crystals were obtained.
All H atoms were positioned geometrically and were refined using a riding model. The C-H, O-H and N-H bond lengths are 0.93-0.96, 0.82 and 0.86 Å, respectively [Uiso(H) = 1.2Ueq(parent atom)].
Data collection: DENZO (Otwinowski & Minor, 1997) and COLLECT (Hooft, 1998); cell refinement: DENZO and COLLECT; data reduction: DENZO and COLLECT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEPII (Johnson, 1976); software used to prepare material for publication: PLATON (Spek, 2003).
DL thanks the EPSRC National Crystallography Service (Southampton, England) for the X-ray data collection.
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