Comment

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].

Experimental

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.

Crystal data

C6H9N3O2·C7H7NO2 Mr = 292.30 Monoclinic, P 21 /c a = 6.6358 (4) Å b = 7.5560 (5) Å c = 27.4226 (16) Å = 94.418 (2)° V = 1370.89 (15) Å3 Z = 4 Dx = 1.416 Mg m-3 Mo K radiation = 0.11 mm-1 T = 293 K Plate, colourless 0.44 × 0.32 × 0.08 mm

Data collection

Bruker-Nonius KappaCCD area-detector diffractometer and scans Absorption correction: none 14577 measured reflections 3130 independent reflections 2469 reflections with I > 2(I) Rint = 0.032 max = 27.5°

Refinement

Refinement on F2 R[F2 > 2(F2)] = 0.043 wR(F2) = 0.127 S = 1.03 3130 reflections 194 parameters H-atom parameters constrained w = 1/[2(Fo2) + (0.0746P)2 + 0.4081P] where P = (Fo2 + 2Fc2)/3 (/)max < 0.001 max = 0.45 e Å-3 min = -0.30 e Å-3 Extinction correction: SHELXL97 Extinction coefficient: 0.016 (4)

Table 1 Hydrogen-bond geometry (Å, °) D-HA D-H HA DA D-HA N2-H2AO3i 0.86 2.07 2.8546 (17) 152 N2-H2BO3 0.86 1.96 2.8180 (17) 172 O4-H4N1 0.82 1.83 2.6426 (16) 171 N4-H4AO2ii 0.86 2.47 3.0621 (18) 127 N4-H4AO4ii 0.86 2.45 3.1566 (18) 140 C7-H7CO2iii 0.96 2.60 3.4578 (18) 150 Symmetry codes: (i) -x+1, -y+2, -z; (ii) ; (iii) -x, -y+1, -z.

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 [U_iso(H) = 1.2U_eq(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).