Pyridine-2,5-diamine

In the title molecule, C5H7N3, intracyclic angles cover the range 117.15 (10)–124.03 (11)°. The N atoms of the amino groups have trigonal–pyramidal configurations deviating slightly from the pyridine plane by 0.106 (2) and −0.042 (2) Å. In the crystal, the pyridine N atom serves as an acceptor of an N—H⋯N hydrogen bond which links two molecules into a centrosymmetric dimer. Intermolecular N—H⋯N hydrogen bonds between the amino groups further consolidate the crystal packing, forming a three-dimensional network.

In the title molecule, C 5 H 7 N 3 , intracyclic angles cover the range 117.15 (10)-124.03 (11) . The N atoms of the amino groups have trigonal-pyramidal configurations deviating slightly from the pyridine plane by 0.106 (2) and À0.042 (2) Å . In the crystal, the pyridine N atom serves as an acceptor of an N-HÁ Á ÁN hydrogen bond which links two molecules into a centrosymmetric dimer. Intermolecular N-HÁ Á ÁN hydrogen bonds between the amino groups further consolidate the crystal packing, forming a three-dimensional network.

Timofeeva Comment
Polydentate ligands have found widespread use in coordination chemistry due to the increased thermodynamic stability of resultant coordination compounds as compared to that of such compounds with monodentate ligands. In this aspect, the title compound can be considered as a versatile polydentate ligand in metal-organic synthesis. Furthermore, owing to the presence of three donor atoms, the title compound might play a role of the building block in the formation of metalorganic frameworks as well as for cocrystals.
In this work, we determined the crystal structure of the title compound (I), C 5 H 7 N 3 (Figure 1), to enable comparative studies of its geometrical parameters in metal-organic complexes. Yet, the crystal structure of I can be helpful in the future investigations.
The geometry of aromatic molecules is known to be sensitive to the electronic effects of substituents. Based on the crystallographic analysis of monosubstituted arenes, it was concluded (Domenicano et al., 1975;Domenicano & Vaciago, 1979) that the endocyclic angle at the ipso-C atom is > 120° for a σ-electron-withdrawing substituent and < 120° for a σelectron-donating substituent. Moreover, in the nitrogen-containing heterocyclic aromatic molecules, the endocyclic angle at the nitrogen atom is < 120°, and those at the carbon atoms in ortho-positions to the heteroatom are > 120° (Mootz & Wussow, 1981;Crawford et al., 2009). Similarly to the related diaminopyridines (Schwalbe et al., 1987;Rubin-Preminger & Englert, 2007;Al-Dajani et al., 2009;Betz et al., 2011), these effects are also manifested in the investigated compound. So, the smallest endocyclic angle in I (117.13 (10)°) is observed at the C5 carbon atom bearing the σ-electron-donating amino group, whereas the largest endocyclic angle in I (124.03 (11)°) is observed at the C6 carbon atom in ortho-position to the N1 heteroatom. The value of the endocyclic angle at the second (C2) carbon atom in ortho-position to the N1 heteroatom (121.83 (11)°) is significantly smaller than that at the C6 carbon atom because the C2 carbon atom bears the σ-electron-donating amino group, i.e., the C2 carbon atom is subjected to the influence of the two opposed electronic effects. The value of the endocyclic angle at the N1 heteroatom (118.15 (10)°) is also in good agreement with the above mentioned electronic effects.
The N2 and N3 nitrogen atoms of the amino groups have the trigonal-pyramidal configurations. It is worthy to note that these atoms are slightly out of the plane defined the aromatic system (r.m.s. deviation is 0.003 Å) by 0.106 (2) and -0.042 (2) Å, respectively. Apparently, this fact is explained by the developed hydrogen bonding system with the participation of the both amino groups.
In the crystal of I, the amino groups act both as proton donors and proton acceptors upon formation of the intermolecular N-H···N hydrogen bonds (Table 1). The N1 heteroatom serves as the acceptor for a hydrogen atom of one of the two amino groups (Table 1). In total, the molecules of I are linked by the intermolecular N-H···N hydrogen bonds into a three-dimensional network (Figure 2). There are no π-π interactions between the aromatic rings.

Experimental
The compound I was obtained commercially (Aldrich) as a fine-crystalline powder and purified additionally by filtration.
Crystals suitable for the X-ray diffraction study were grown by slow evaporation from chloroform solution.

Refinement
The hydrogen atoms of the amino groups were localized in the difference-Fourier map and refined isotropically with fixed isotropic displacement parameters [U iso (H) = 1.2U eq (N)]. C-bound H atoms were placed in calculated positions [C-H = 0.93 Å], and refined as riding, with U iso (H) = 1.2U eq (C). program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: