Supramolecular hydrogen-bonding patterns in a 1:1 co-crystal of the N(7)—H tautomeric form of N 6-benzoyladenine with 4-hydroxybenzoic acid

The asymmetric unit of the title co-crystal contains one molecule of N 6-benzoyladenine (BA) and one molecule of 4-hydroxybenzoic acid (HBA). The N 6-benzoyladenine (BA) has an N(7)—H tautomeric form with non-protonated N-1 and N-3 atoms. This tautomeric form is stabilized by a typical intramolecular N—H⋯O hydrogen bond on the Hoogsteen face of the purine ring. The primary robust (8) ring motif is formed in the Watson–Crick face via N—H⋯O and O—H⋯N hydrogen bonds.


Chemical context
Adenine is one of the major nucleobases and some of its N 6 -derivatives have plant hormone (kinetin) (Tr). They also offer a variety of hydrogen-bonding donor and acceptor sites (McHugh & Erxleben, 2011;Imaz et al., 2011). 4-Hydroxybenzoic acid is also a promising hydrogen-bond donor with the ability to form co-crystals with other organic molecules (Vishweshwar et al., 2003). It is used as an antimicrobial paraben (Barker & Frost, 2001). The present study investigates co-crystal formation between N 6 -benzoyladenine and 4-hydroxybenzoic acid.

Figure 1
The asymmetric unit of the title compound, showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. Dashed lines represent hydrogen bonds.

Synthesis and crystallization
The title co-crystal was prepared by mixing a hot ethanol solution of N 6 -benzoyladenine (30 mg) and 4-hydroxybenzoic acid (35 mg) in an equimolar ratio in a total volume of 30 mL. The mixture was warmed over a water bath for 30 min, filtered, and left aside for a few days. Colourless plate-shaped crystals were collected from the mother solution following slow cooling at room temperature.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 3. Hydrogen atoms were readily located in difference-Fourier maps and were subsequently treated as riding atoms in geometrically idealized positions, The formation of a supramolecular three-dimensional large ring structure in the title compound.

Figure 3
A view of the homo/hetero-stacking interactions in the title compound.  Extinction correction: SHELXL2014 (Sheldrick, 2015), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.0007 (2) Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.