cis-Bis(azido-κN)bis(pyridine-2-carboxamide-κ2 N 1,O)nickel(II)

The title compound, [Ni(N3)2(C6H6N2O)2], was obtained as the first crystalline product from the reaction of Ni(NO3)2·6H2O, picolinamide and NaN3 in aqueous media. After a few days in the mother liquor, crystals of the cis isomer transformed into the trans isomer [Đaković & Popović (2007 ▶). Acta Cryst. C63, m507–m509]. The Ni atom exhibits a distorted octahedral environment and contains two azide ions and two planar N,O-chelating picolinamide ligands, all cis related. The dihedral angle between the two chelate rings is 82.43 (7)°. Pairs of molecules are linked by N—H⋯N hydrogen bonds into cyclic R 2 2(16) dimers, which are further packed into a three-dimensional framework by C(6) and C(8) chains by N—H⋯N hydrogen bonds.

The title compound, [Ni(N 3 ) 2 (C 6 H 6 N 2 O) 2 ], was obtained as the first crystalline product from the reaction of Ni(NO 3 ) 2 Á-6H 2 O, picolinamide and NaN 3 in aqueous media. After a few days in the mother liquor, crystals of the cis isomer transformed into the trans isomer [Ðaković & Popović (2007). Acta Cryst. C63, m507-m509]. The Ni atom exhibits a distorted octahedral environment and contains two azide ions and two planar N,O-chelating picolinamide ligands, all cis related. The dihedral angle between the two chelate rings is 82.43 (7) . Pairs of molecules are linked by N-HÁ Á ÁN hydrogen bonds into cyclic R 2 2 (16) dimers, which are further packed into a three-dimensional framework by C(6) and C(8) chains by N-HÁ Á ÁN hydrogen bonds.

S1. Comment
This research is a part of our wider interest of the structural role of azide ions and of its metal complexes in metabolic processes of mitohondria (Yuwen et al., 2000).
In the title compound Ni II atom lies in a general position and exhibits distorted octahedral environment (Fig. 1). The coordination sphere is composed by two cis-related N,O-chelating picolinamide and two azide ligands. The picolinamide ligands are bonded more tightly (Table 1) than in its trans-isomer (Đaković & Popović, 2007). All other bond lengths are comparable to the values reported for similar compounds (Allen et al., 1987). The azide ligands are coordinated to the central metal ion in non-linear mode (123.7 (1) and 123.0 (1)°) with the azide bond angles being 178.2 (2) and 176.9 (3)°.
The crystals of the title compound (I) are turquoise-green apart from the crystals of its trans-isomer which are olivegreen.
The crystal structure (Fig 2) is stabilized by N-H···N hydrogen bond network between carboxamide groups and azide ligands. Typical amide N-H···O carboxamide dimers of R 2 2 (8) found in trans-isomer are not observed in the cis-isomer.
Instead, the amide N atoms, N2 and N4, are involved in two hydrogen bonds, forming R 2 2 (16) rings, between two neighbouring molecules whereas C(8) chains along the axis c and C(6) chains along the axis b complete the network (Bernstein et al., 1995;Etter, 1990).
The slightly smaller density of (I), and the fact that it is formed first and then transformed into its trans-isomer, suggests that (I) is the thermodinamically less stable isomer.

S2. Experimental
The title compound was obtained by in situ reaction from Ni II nitrate hexahydrate, sodium azide and picolinamide in a 1: 2: 2 molar ratio. All starting substances were dissolved in water. The sodium azide solution was added in small portions with stirring into the solution mixture of the picolinamide and Ni II nitrate. In a few h the dark-green crystals of (I) were isolated. If the crystals are left in a mother liquor for a few days the dark-green crystals of (I) were transformed into the olive-green trans-isomer.

S3. Refinement
Aromatic H atoms were fixed in geometrically idealized positions and refined using a riding model with [C-H = 0.93 Å and U iso (H) = 1.2U eq (C)]. The amide H atoms were placed in the positions indicated by difference electron-density maps and their positions were allowed to refine together with individual isotropic displacement parameters.  The ORTEP-3 drawing of (I) with the atom numbering scheme. Displacement ellipsoids for non-H atoms are drawn at the 30% probability level.  Crystal packing of (I) showing the hydrogen bonds as dashed lines.

Special details
Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and torsion angles Refinement. Refinement on F 2 for ALL reflections except those flagged by the user for potential systematic errors. Weighted R-factors wR and all goodnesses of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The observed criterion of F 2 > σ(F 2 ) is used only for calculating -R-factor-obs etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.