supplementary materials

In the synthesis of crystal structure by design, the assembly of molecule unit in predefined arrangement is a key goal (Desiraju, 1997; Braga et al. 1998). The water soluble, proton transfer compounds can function as suitable ligands in the synthesis of metal complexes. In general, molecular association between carboxylic acid and a Lewis base results in more hydrogen bonding association with considerable stability upon a structure making process. This is because functionalized carboxylic acids and amines can enhance the intermolecular forces between the obtained cationic and anionic fragments, and these interactions can provide a large part of the stabilization energy of resulting self assembly systems (Moghimi et al., 2003, 2007). Proton transfer from carboxylic acid to different amines has been reported (Moghimi et al. 2002; Aghabozorg et al., 2008; Soleimannejad et al., 2008). Herein, we report a novel PTC that have been synthesized using pyridine-2,6-dicarboxylic acid and hydrazinecarboxamide at room temperature and its crystal structure.

In the crystal structure of the compound, intermolecular hydrogen bonds link the molecules to form a proton transfer supramolecular framework. These hydrogen bonds help in the stabilization of the resulting supramolecular structure of the compound (Table 1). The molecular structure of the title compound is shown in (Fig. 1). The crystal structure shows that a single proton from each of the carboxyl groups was transferred to the hydrazinecarboxamide. Thus, the negative charges of monoanionic pyridine-2,6-dicarboxylate groups, (pyH)-, are neutralized by a mono protonated hydraziniumcarboxamide fragment. The C–O distances for this compound support the existence of both ionic (COO) and non-ionic COOH group in the crystal structure of a new proton transfer system. The distance of ionic C—O in carboxylate ion is in the range of 1.250 Å due to resonance but in carboxylic acid group of pyridine-2,6-dicarboxylic acid has a deviation of 0.1 Å. The distance of C—O in carboxlate and carboxylic acid are quite similar to the report (Aghabozorg et al., 2008). The relatively short bond distances of C1–O2 [1.250 (18) Å] and C7–O7 [1.211 (19) Å] confirm the presence of double bonds. In the crystal structure of title compound, a number of N–H—O, N–H—N and O–H—O hydrogen bonds, with D—A ranging from 2.612 (2) to 2.994 (3) Å are observed (Fig. 2). The carboxylate group of one pyridine-2,6-dicarboxylic acid are bonded to OH of another pyridine-2,6-dicarboxylic acid through hydrogen bonding (Fig. 3) with distance of 1.791 Å. In the packing diagram, crystal structure shows hydrogen bonding, ion pairing, π-π stacking and van der Waals interactions. The π–π stacking interactions exist between the two pyridine rings and between the pyridine ring and the NH₂ group of the cation with a centroid-centroid distance of 4.626 Å and a π–HN distance of 3.693 Å, respectively (Fig. 4). These interactions result in the formation of a supramolecular structure.