catena-Poly[[[tetraaquairon(II)]-μ-5,5′-diazenediylditetrazolido] dihydrate]

In the title compound, {[Fe(C2N10)(H2O)4]·2H2O}n, the coordination geometry around the Fe(II) atom, which lies on a center of inversion, is distorted octahedral, with bonds to four O atoms and two N atoms. The azotetrazolate ligand displays a bridging coordination mode, forming an infinite zigzag chain. Intermolecular O—H⋯O and O—H⋯N hydrogen bonding and offset face-to-face π–π stacking interactions [centroid–centroid distance = 3.4738 (13) Å] lead to a three-dimensional network.

In the title compound, {[Fe(C 2 N 10 )(H 2 O) 4 ]Á2H 2 O} n , the coordination geometry around the Fe(II) atom, which lies on a center of inversion, is distorted octahedral, with bonds to four O atoms and two N atoms. The azotetrazolate ligand displays a bridging coordination mode, forming an infinite zigzag chain. Intermolecular O-HÁ Á ÁO and O-HÁ Á ÁN hydrogen bonding and offset face-to-facestacking interactions [centroid-centroid distance = 3.4738 (13) Å ] lead to a three-dimensional network.
We gratefully acknowledge the National Science Foundation of China (No. 20873100), the Natural Science Foundation of Shaanxi Province (2009JQ2015), the Special Foundation of the Education Department of Shaanxi Province (09 J K798) and the Research Foundation of Xi'an University of Arts and Science (kyc201026).
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: NG5036).

Comment
After Thiele prepared metallic salts of azotetrazole and claimed these for use in initiators, salts of AT 2-(AT = 5,5'-azotetrazolate) have been extensively investigated and have often been considered for practical use as a class of energetic materials. We report here the crystal structure of the title compound, [Fe(C 2 N 10 )(H 2 0) 4 .2H 2 0] n , (I).
Single-crystal analysis shows that (I) exists as a one-dimensional infinite chain. As shown in Figure 1, the coordination geometry around Fe 2+ cation can be described a disordered octahedral arrangement with coordination number of 6, where O1, O2, O1A and O2A form the equatorial plane, and axial positions are occupied by N1 and N1A. Additionally, each AT 2provides two terminal nitrogen atoms (N1 and N1A) acting as bridging ligand to connect two [Fe(H 2 O) 4 ] 2+ to form an infinite zigzag chain.
In the crystal structure, the interactions of hydrogen bonding between the water molecules and the N atoms in the terazole rings, the off-set face to face π-π stacking interactions of the terazole rings link the complex to a three dimensional structure, as shown in Figure 2.

Experimental
Brown block-like crystal for X-ray diffraction analysis was obtained from the mixture of (NH 4 ) 2 Fe(SO4) 2 .6H 2 O (0.392 g, 1 mmol), Na 2 AT.5H 2 O (0.304 g,1 mmol) and distilled H 2 O (20 ml), which was allowed to evaporate at room temperature for one week.   Fig. 1. A view of the molecular structure of (I) with the atom-labling scheme. Displacement ellipsoids are drawn at the 30% probability level and H atoms are shown as small spheres of arbitrary radii.  Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness 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 threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) 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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )