Crystal structures of N′-aminopyridine-2-carboximidamide and N′-{[1-(pyridin-2-yl)ethylidene]amino}pyridine-2-carboximidamide

In the crystal structures of N′-aminopyridine-2-carboximidamide (C6H8N4), 1, and N′-{[1-(pyridin-2-yl)ethylidene]amino}pyridine-2-carboximidamide (C13H13N5), 2, molecules are linked by intermolecular N—H⋯N hydrogen-bonding interactions, forming a two-dimensional network in 1 and a chain in 2.


Chemical context
The preparation of hydrazidines with the general formula RC( NH)NHNH 2 is accomplished by the action of hydrazine on the corresponding thioamide, imido ether or nitrile (Case, 1965). A pyridine-2-carboxamidrazide co-crystal form has previously been crystallized as a pyridine-2-carboxamidrazonium hydrogenoxalate salt, obtained by the reaction of pyridine-2-carboxamidrazide with oxalic acid (Wang et al., 2007). Related molecules with diazine (N-N) bridges, obtained by condensation of hydrazidines with ketones can bring two metal centres into close proximity and provide an intramolecular exchange pathway for spin-exchange interactions via the p-orbital system ( pathway) of the heterocyclic ligand (Xu et al., 1997(Xu et al., , 2000. The latter type of molecules present an unusual arrangement of potential donor sites, with many possible mononucleating and dinucleating coordination modes (Xu et al., 1997). Semi-empirical structural calculations demonstrate that the N-N bond in these azines is rotationally soft, thereby allowing significant twisting at little energy cost (Kesslen et al., 1999). Copper azine and imine complexes possess a significant antimalarial and antitumor action (Gokhale et al., 2001a(Gokhale et al., ,b, 2003. Coordination complexes of 2-acetylpyridine-pyridine-2-carboxamidrazone have been obtained with cadmium(II), copper(II), nickel(II) and manganese(II) ions. The organic molecule behaves as a mono-and bis(bidentate) chelator (Xu et al., 2000;Gokhale et al., 2001a;Yue et al., 2004Yue et al., , 2006. A polymorph of 2-acetylpyridine-pyridine-2-carboxamidrazone as been obtained with two crystallographically independent molecules included in the asymmetric unit (Yue et al., 2006).

Structural commentary
The molecular structure of 1 is shown in Fig. 1. The molecule is close to planar; the r.m.s. deviation of non-hydrogen atoms from planarity is 0.0108 Å with atom N2 displaying the largest deviation from the mean plane of 0.016 (3) Å . The geometry about N2 and N4 is not planar. H2A and H2B lie 0.12 (6) and 0.24 (6) Å out of the mean plane of non-hydrogen atoms. For H4A and H4B, the deviation is even greater at 0.37 (5) and 0.54 (5) Å from the mean plane. Rotation of the non-planar NH 2 group, particularly for N4, facilitates hydrogen bonding to other molecules. The N-N single bond length in 1 [1.424 (5) Å ] is slightly shorter than that in the free hydrazine (1.449 Å ).
The molecular structure of 2 is shown in Fig. 2. The molecule is not planar, perhaps as a result of conjunction of supramolecular interactions and packing effects. Each of the two ring systems is essentially planar (r.m.s. deviations for the two six-membered rings are 0.0162 and 0.0057 Å for N1/C1-C5 and N5/C9-C13, respectively). The hydrazidine group N3/ C8/N4 is rotated slightly away from the plane of the sixmembered ring along the C8-C9 bond by 8.6 (3) . The imine group N2/C6/C7 is rotated from the plane of the adjacent sixmembered ring by rotation about C5-C6 by 14.5 (2) . The molecule is further distorted away from planarity by rotation of 17.8 (2) about the central N2-N3 bond.
The bond lengths indicate that within the central chain of the molecule, the C6-N2 and C8-N3 linkages have largely double-bond character. The azine linkages are in the E,E conformation, suggesting conjugation throughout the systems. The C6-N2-N3 and C8-N3-N2 angles of 115.5 (2) and 110.57 (19) , respectively are significantly below the ideal sp 2 value of 120 , a consequence of the repulsion between the nitrogen lone pair and the adjacent bonds. The C6-N2-N3-C8 torsion angle is À162.2 (2) . This large deviation from planarity has two consequences. First, there is a loss of conjugation between the imine bonds across the azine bond, reflected in the shorter imine bond length. The torsion also leads to a shorter N2-N3 bond length [1.408 (3) Å ] compared to that observed for 1 [1.424 (5) Å ]. Finally, a short intramolecular contact between N3 i and H4B, 2.42 (3) Å , may add a favorable electrostatic contribution to the stability of this conformation. Notably, there is minimal change in the bond lengths within the ligands when a first row transition metal ion is bound. When the ligand chelates to a metal ion through both N3 and N5, only the bond length C8-N4 changes significantly, becoming shorter on binding.

Supramolecular features
There are two molecules of 1 in each unit cell and these are related by the screw axis. Curiously, N1 does not act as a hydrogen-bond acceptor. H2A is also not involved with the formation of any (short) classical hydrogen bonds. H2B forms a hydrogen bond to N4 i [symmetry code: (i) 1 -x, y + 1 2 , 1 -z]. ORTEP representation of the asymmetric unit of 2, with displacement ellipsoids drawn at the 50% probability level.
This is augmented by the longer hydrogen bond N4-H4BÁ Á ÁN4 i . N4-H4A forms a hydrogen bond to N3 ii [symmetry code: (ii) -x, y + 1 2 , -z + 1]. These three sets of hydrogen bonds (Table 1) are sufficient to hold pairs of molecules together within the unit cell and to knit these dimers together to form sheets in the xy plane (see Fig. 3). These sheets then stack parallel to the [001] direction, presumably held together by van der Waals interactions.

Figure 3
A portion of the hydrogen-bonded sheet present in 1. Hydrogen bonds are shown as dashed lines.

Figure 5
The synthesis of 1 and 2.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. There is no significant anomalous dispersion at this wavelength so the Flack parameter is meaningless and this is not reported.
For compound 1, hydrogen atoms of the aromatic ring were placed using a riding model with the C-H bond length allowed to refine subject to the restraint that all these bond lengths were equal within a estimated standard deviation of 0.02 Å . These C-H bond lengths lie in the range 0.97 (3) (Blessing, 1987(Blessing, , 1989, SHELXT (Sheldrick, 2015a) andSHELXL2014 (Sheldrick, 2015b

(1) N′-Aminopyridine-2-carboximidamide
Crystal data 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq C1 0.3776 (8)  ( where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.13 e Å −3 Δρ min = −0.17 e Å −3 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.