Supramolecular architecture in a co-crystal of the N(7)—H tautomeric form of N 6-benzoyladenine with adipic acid (1/0.5)

The supramolecular architecture in a co-crystal of N(7)—H tautomeric form of N 6-benzoyladenine-adipic acid (1/0.5) is reported. The typical C=O⋯π and C—H⋯π interactions are also present in this structure.

The asymmetric unit of the title co-crystal, C 12 H 9 N 5 OÁ0.5C 6 H 10 O 4 , consists of one molecule of N 6 -benzoyladenine (BA) and one half-molecule of adipic acid (AA), the other half being generated by inversion symmetry. The dihedral angle between the adenine and phenyl ring planes is 26.71 (7) . The N 6 -benzoyladenine molecule crystallizes in the N(7)-H tautomeric form with three nonprotonated N atoms. This tautomeric form is stabilized by intramolecular N-HÁ Á ÁO hydrogen bonding between the carbonyl (C O) group and the N(7)-H hydrogen atom on the Hoogsteen face of the purine ring, forming an S(7) ring motif. The two carboxyl groups of adipic acid interact with the Watson-Crick face of the BA molecules through O-HÁ Á ÁN and N-HÁ Á ÁO hydrogen bonds, generating an R 2 2 (8) ring motif. The latter units are linked by N-HÁ Á ÁN hydrogen bonds, forming layers parallel to (105Þ. A weak C-HÁ Á ÁO hydrogen bond is also present, linking adipic acid molecules in neighbouring layers, enclosing R 2 2 (10) ring motifs and forming a three-dimensional structure. C OÁ Á Á and C-HÁ Á Á interactions are also present in the structure.

Chemical context
Adipic acid has been widely used in controlled-release formulations of many drugs and food additives (Roew et al., 2009). N 6 -benzoyladenine is a synthetic analogue of a group of naturally occurring N 6 -substituted adenines having plantgrowth-stimulating activity (cytokinins) (McHugh & Erxleben, 2011). A number of co-crystals involving adipic acid have been reported in the literature (Lemmerer et al., 2012;Lin et al., 2012;Matulková et al., 2014;Thanigaimani et al., 2012). This paper deals with a co-crystal formed between N 6 -benzoyladenine and adipic acid (I).

Supramolecular features
Each of the two carboxyl groups of adipic acid interacts with the Watson-Crick face (atoms N1 and N6) of the corresponding BA through O-HÁ Á ÁN and N-HÁ Á ÁO hydrogen bonds, generating an R 2 2 (8) ring motif (Fig. 1). Thus each adipic acid molecule bridges two BA molecules. The latter units are linked by N7-H7Á Á ÁN9 iii hydrogen bonds (Table 1) forming layers parallel to plane (105). A weak C-HÁ Á ÁO hydrogen bond (C19-H19BÁ Á ÁO3A iv ) is also present (Table 1 and Fig. 2), linking adipic acid molecules in neighbouring layers, enclosing R 2 2 (10) ring motifs and forming a threedimensional structure. Thus atom O3A functions as a bifurcated hydrogen-bond acceptor whereas N7-H is a bifurcated hydrogen-bond donor.

Figure 2
A view of the sheet-like supramolecular architecture generated via C19-H19BÁ Á ÁO3A hydrogen bonds (black dotted lines). Phenyl rings are indicated as yellow balls. H atoms not involved in hydrogen bonding have been omitted for clarity. Symmetry codes are as given in Table 1. Table 1 Hydrogen-bond geometry (Å , ).

Synthesis and crystallization
The title co-crystal was synthesized by mixing a DMF solution of N 6 -benzoyladenine (30 mg) and adipic acid (19 mg) (total volume = 10 mL). The mixture was warmed in a water bath for 20 min. After cooling to room temperature, colourless platelike crystals were collected from the mother liquor after a few days (m.p. 438 K).