Bis(4-aminopyridinium) hexaaquanickel(II) bis(sulfate)

In the title compound, (C5H7N2)2[Ni(H2O)6](SO4)2, the NiII cation is located on an inversion centre and is coordinated by six aqua ligands in a slightly distorted octahedral coordination environment. The [Ni(H2O)6]2+ ions are connected through an extensive network of O—H⋯O hydrogen bonds to sulfate anions, leading to the formation of layers parallel to (001). The 4-aminopyridinium cations are located between these layers and are connected to the anionic framework by N—H⋯O hydrogen bonds. Weak π–π interactions between the pyridine rings, with a centroid–centroid distance of 3.754 (9) Å, provide additional stability to the crystal packing.

In the title compound, (C 5 H 7 N 2 ) 2 [Ni(H 2 O) 6 ](SO 4 ) 2 , the Ni II cation is located on an inversion centre and is coordinated by six aqua ligands in a slightly distorted octahedral coordination environment. The [Ni(H 2 O) 6 ] 2+ ions are connected through an extensive network of O-HÁ Á ÁO hydrogen bonds to sulfate anions, leading to the formation of layers parallel to (001). The 4-aminopyridinium cations are located between these layers and are connected to the anionic framework by N-HÁ Á ÁO hydrogen bonds. Weakinteractions between the pyridine rings, with a centroid-centroid distance of 3.754 (9) Å , provide additional stability to the crystal packing.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: WM2787).

sup-1
Acta Cryst.  Schwid et al., 1997;Strupp et al., 2004). The combination of this amine with sulfuric acid leads to the formation of bis(4-aminopyridinium) sulfate monohydrate (Quah et al., 2010). In general, sulfates combined with transition metal and organic groups can lead to interesting materials as model compounds of ferroelectric and ferroelastic domains, but also to potentially applicable compounds with interesting magnetic properties (Rekik et al., 2008). For this reasons, we focused to study the crystal structure of the title compound, The crystal structure of compound (I) has an asymmetric unit consisting of one half of the cationic complex [Ni(H 2 O) 6 ] 2+ , an uncoordinating sulfate anion, and one 4-aminopyridinium cation, C 5 H 7 N 2 2+ (Fig. 1). The Ni(II) ion, located on an inversion centre, exhibits a distorted octahedral coordination environment with Ni-O distances ranging from 2.0388 (11) to 2.0701 (12) Å. The values of O-Ni-O angles are between 86.04 (5) and 180.00 (7). These values agree with those for other [Ni(H 2 O) 6 ] 2+ groups (Hajlaoui et al., 2011).
A proton transfer from sulfuric acid to atom N2 of 4-aminopyridine resulted in the formation of a 4-aminopyridinium cation. This protonation leads to the widening of the C3-N2-C4 angle of the pyridine ring to 121.02 (15)°, compared to 115.25 (13)° in the unprotonated 4-aminopyridine (Anderson et al., 2005). Such a protonation is observed in various 4aminopyridine acid complexes (Rotondo et al., 2009). The 4-aminopyridine ring is essentially planar with a maximum deviation from planarity of 0.002 (9) Å.
In the crystal structure, intermolecular O-H···O hydrogen bonds, established between the complex cations and sulfate anions, generate layers parallel to (001) (Fig. 2). The crystal packing of the title complex is stabilized by N-H···O hydrogen bonds between C 5 H 7 N 2 2+ cations and sulfate anions and by π···π interactions between parallel pyridine rings [ring centroid-to-centroid distance: 3.754 (9) Å], which link the different species into a three dimensional network (Fig.   3).

Experimental
4-Aminopyridine (0.19g, 2 mmol) and nickel sulfate hexahydrate (0.26g, 1 mmol) were dissolved in 15 ml of water. The resulting solution was added to an aqueous solution of sulfuric acid (0.1 M). The mixture was stirred for 20 min at room temperature. After slow evaporation during few days at ambient temperature, blue single crystals of the title compound appeared in the solution.

Refinement
The H atoms bonded to C and N2 atoms were positioned geometrically and treated as riding on their parent atoms, [N-H = 0.86, C-H =0.93 Å with U iso (H) = 1.2 U eq (C,N), but those attached to N1 were located in a difference Fourier map and refined freely. O-H bond lengths and distances between two H atoms of each water molecule were restrained to be 0.85 (2) and 1.37 (2) Å, with U iso (H) = 1.5 U eq (O).

Bis(4-aminopyridinium) hexaaquanickel(II) bis(sulfate)
Crystal data (C 5  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. 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 > 2sigma(F 2 ) is used only for calculating R-factors(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.