Adenin-1-ium hydrogen isophthalate dimethylformamide monosolvate

In the title proton-transfer organic salt, C5H6.3N5 +·C8H4.7O4 −·C3H7NO, the adeninium moiety is protonated at the N atom in the 1-position of the 6-amino-7H-purin-1-ium (adeninium) cation. In the solid state, the second acidic proton of isophthalic acid is partially transferred to the imidazole N atom of the adeninium cation [refined O—H versus N—H ratio = 0.70 (11):0.30 (11)]. Through the partially transferred proton, the adeninium cation is strongly hydrogen bonded (N—H⋯O/O—H⋯N) to the isophthalate anion. This strong interaction is assisted by another N—H⋯O hydrogen bond originating from the adeninium NH2 group towards the isophthalate keto O atom, with an R 2 2(8) graph-set motif. This arrangement is linked via N—H⋯O hydrogen bonds to the O atoms of the carboxylate group of an isophthalate anion. Together, these hydrogen bonds lead to the formation criss-cross zigzag isophthalate⋯adeninium chains lying parallel to (501) and (50-1). The adeninium cations and the isophthalate anions are arranged in infinite π stacks that extend along the c-axis direction [interplanar distance = 3.305 (3) Å]. Molecules are inclined with respect to this direction and within the stacks they are offset by ca. half a molecule each. Combination of the N—H⋯O and O—H⋯N hydrogen bonds with the π–π interactions forms infinitely stacked isophthalate⋯adeninium chains, thus leading to a two-dimensional supramolecular structure with parallel interdigitating layers formed by the π stacked isophthalate⋯adeninium chains. The DMF molecules of crystallization are bonded to the adeninium cations through strong N—H⋯O hydrogen bonds and project into the lattice space in between the anions and cations. There are also C—H⋯O hydrogen bonds present which, combined with the other interactions, form a three-dimensional network. The crystal under investigation was found to be split and was handled as if non-merohedrally twinned.

In the title proton-transfer organic salt, C 5 H 6.3 N 5 + Á-C 8 H 4.7 O 4 À ÁC 3 H 7 NO, the adeninium moiety is protonated at the N atom in the 1-position of the 6-amino-7H-purin-1-ium (adeninium) cation. In the solid state, the second acidic proton of isophthalic acid is partially transferred to the imidazole N atom of the adeninium cation [refined O-H versus N-H ratio = 0.70 (11):0.30 (11)]. Through the partially transferred proton, the adeninium cation is strongly hydrogen bonded (N-HÁ Á ÁO/O-HÁ Á ÁN) to the isophthalate anion. This strong interaction is assisted by another N-HÁ Á ÁO hydrogen bond originating from the adeninium NH 2 group towards the isophthalate keto O atom, with an R 2 2 (8) graph-set motif. This arrangement is linked via N-HÁ Á ÁO hydrogen bonds to the O atoms of the carboxylate group of an isophthalate anion. Together, these hydrogen bonds lead to the formation crisscross zigzag isophthalateÁ Á Áadeninium chains lying parallel to (501) and (501). The adeninium cations and the isophthalate anions are arranged in infinite stacks that extend along the caxis direction [interplanar distance = 3.305 (3) Å ]. Molecules are inclined with respect to this direction and within the stacks they are offset by ca. half a molecule each. Combination of the N-HÁ Á ÁO and O-HÁ Á ÁN hydrogen bonds with theinteractions forms infinitely stacked isophthalateÁ Á Áadeninium chains, thus leading to a two-dimensional supramolecular structure with parallel interdigitating layers formed by the stacked isophthalateÁ Á Áadeninium chains. The DMF molecules of crystallization are bonded to the adeninium cations through strong N-HÁ Á ÁO hydrogen bonds and project into the lattice space in between the anions and cations. There are also C-HÁ Á ÁO hydrogen bonds present which, combined with the other interactions, form a three-dimensional network. The crystal under investigation was found to be split and was handled as if non-merohedrally twinned.

Comment
The title compound was obtained as a result of our studies on magnesium based metal-organic frameworks (MOFs) using solvothermal synthesis. In order to examine the interaction between metal salts and various organic ligands as linkers for possible formation of MOFs, we have extensively tested bis-and tris-carboxylic acid linkers such as isophthalic acid, cyclohexane dicarboxylic acid, and trimesic acid, but also nitrogen based ligands derived from DNA bases such as adenine, which was recently reported as a particularly effective linker and biomolecular building block  due to its rigidity and potential multiple coordination modes. We are currently interested in the synthesis of MOFs with Mg, isophthalate and adenine. Reaction of magnesium nitrate with isophthalic acid and adenine at 423 K did, however, not yield the desired extended framework. The presence of multiple Lewis basic sites (amino group) in adenine led to the formation of colourless plate like crystals of the title compound as a minor product along with a sand-like precipitate of  (11)]. Through the partially transferred proton the adeninium cation is strongly hydrogen bonded to the isophthalate anion (Gilli et al., 2009). This strong interaction is assisted by another N-H···O hydrogen bond originating from the adeninium NH 2 group towards the isophthalate keto oxygen atom, O1, forming an R 2 2 (8) graph set motif (Etter, 1990;Bernstein et al., 1995). This arrangement is linked via N-H···O hydrogen bonds to the O atoms of the carboxylate group of the isophthalate anion. Combined these hydrogen bonds lead to the formation of onedimensional zigzag isophthalate···adeninium chains that lie parallel to planes (501) and (50-1). The various hydrogen bonds are given in Table 1.
Mono-deprotonated isophthalate anions have also been observed in other related compounds, such as for dimethylammonium 3-carboxybenzoate (Siddiqui et al., 2012). In that structure the isophthalate monoanions are hydrogen bonded with neighbouring isophthalates as well as dimethylammonium cations resulting in the formation of a double-chain-like structure. In the title compound, the isophthalate monoanions are hydrogen-bonded with adeninium cations which leads to the formation of one-dimensional undulated isophthalate···adeninium chains. The adeninium cations and the isophthalate monoanions, from the zigzag chains, are in turn arranged in infinite π stacks that extend along the direction of the c axis [interplanar distances 3.305 (3) Å]. Molecules are inclined with respect to this direction and within the stacks they are offset by ca. half a molecule each, 1.736 (3) Å for the isophthalate anions and 1.827 (3) Å for the adeninium cations. The average centroid-centroid distances are thus substantially larger than the interplanar distances; 3.783 (4) Å for both the adeninium cations and the isophthalate anions. For the isophthalate anions, π-π stacking is thus mostly between the phenyl ring of one molecule and the carboxylate group of another. In the adeninium cations π-π supplementary materials sup-2 Acta Cryst. (2014). E70, o166-o167 stacking interactions are between the centers of the pyrimidine and imidazole rings.
Combination of the N-H···O and O-H···N hydrogen bonds with the π-π interactions forms infinitely stacked isophthalate···adeninium chains (Table 1 and Fig. 2), thus leading to a two-dimensional supramolecular structure with parallel interdigitating layers formed by the π stacked isophthalate···adeninium chains (Fig. 3). The DMF molecules are bonded to the adeninium cation through strong N-H···O hydrogen bonds and are projecting into the lattice space in between the anions and cations (Table 1). There are also C-H···O hydrogen bonds present (Table 1) which together with the other interactions form a three-dimensional network.

Experimental
The compound was synthesized under solvothermal conditions. In a typical synthesis, Mg(NO 3 ) 2 ·6H 2 O (0.128 g, 0.5 mmol) and adenine (0.068 g, 0.5 mmol) were dissolved in DMF (10.0 ml). Then isophthalic acid (0.166 g, 0.5 mmol) was added to the reaction mixture under continuous stirring. The mixture was stirred for 30 minutes before transferring the mixture into a 23 ml teflon-lined stainless steel autoclave. The final mixture, with a composition of 1:1:1, was heated to 423 K for 96 h. The autoclave was then cooled to room temperature, yielding colourless plate-like crystals of the title compound as a minor product, along with a sand like precipitate of Mg formate dihydrate, Mg(HCOO) 2 ·2(H 2 O), (ICSD #151330).

Refinement
The crystal under investigation was found to be split and was handled as if non-merohedrally twinned. The orientation matrices for the two components were identified using the program Cell Now (Sheldrick, 2004), which reports the second moiety to be related to the first by a 2.8 degree rotation around either the reciprocal a-axis, or around the real axis 1 0 -0.6. The two components were integrated using Saint, resulting in a total of 22391 reflections. 6713 reflections (1732   unique The data were corrected for absorption using TWINABS (Bruker, 2009), and the structure was solved using direct methods with only the non-overlapping reflections of component 1. The structure was refined using the hklf 5 routine with all reflections of component 1 (including the overlapping ones), resulting in a BASF value of 0.317 (5). In the absence of significant anomalous scatterers Friedel pairs were merged during correction for absorption effects with TWINABS.
The R int value given is for all reflections and is based on agreement between observed single and composite intensities and those calculated from refined unique intensities and twin fractions (TWINABS; Bruker, 2009).
Hydrogen atoms were placed in calculated positions with C-H = 0.95 Å (aromatic H), 0.99 Å (methyl H) or 0.88 Å (N -H). Methyl group H atoms were allowed to rotate freely around the C-C bond to best fit the experimental electron density. Carboxylic acid hydrogen atoms were located in difference electron density maps, but were placed in calculated positions with fixed C-O-H angles, but with the C-C-O-H dihedral angles and the O-H distances refined with the constraint AFIX 148 (Sheldrick, 2008). U iso (H) = 1.5U eq (C/O) for methyl and carboxylic acid H atoms, and = 1.2U eq (C/N) for aromatic and adenine H-atoms. One of the acidic hydrogen atoms was refined as disordered over a carboxylate versus a nitrogen bound site. The occupancy rate for the major O-bound site refined to 0.70 (9).

Figure 1
View of the title organic compound, with atom labelling. Displacement ellipsoids are drawn at the 50% probability level.
Green dots represent the partially transferred proton to atom N4.  View of one of the one-dimensional undulating chains lying parallel to (501) in the crystal packing of the title compound.
Hydrogen bonds are shown as green dashed lines (see Table 1 for details; the minor occupied hydrogen atoms, H4A, have been omitted for clarity).

Figure 3
View of the π-stacked chains in the crystal of the title compound. Pink spheres represent centroids of isophthalate benzene rings and adeninium cations; blue planes are weighted averages of each two adeninium and isophthalate ions as displayed. The green dotted lines represent centroid-centroid and interplanar distances. The minor occupied hydrogen atoms, H4A, and the DMF molecules have been omitted for clarity.