catena-Poly[bis(4-aminopyridinium) [[tetraaquanickel(II)]-μ-benzene-1,2,4,5-tetracarboxylato] dihydrate]

The asymmetric unit of the title compound, {(C5H7N2)2[Ni(C10H2O8)(H2O)4]·2H2O}n, contains an NiII atom, two water molecules of coordination, one half of a benzene-1,2,4,5-tetracarboxylate (btec) anionic ligand, one 4-aminopyridinium cation (papy) and an uncoordinated water molecule. The metal center lies on an inversion center and adopts an octahedral geometry with the carboxylate groups tilted out of the mean plane formed by the btec. In the crystal, molecules are linked into one-dimensional coordination polymers running along the ac diagonal. The crystal structure is consolidated by N—H⋯O and O—H⋯O hydrogen bonds.

The asymmetric unit of the title compound, {(C 5 H 7 N 2 ) 2 -[Ni(C 10 H 2 O 8 )(H 2 O) 4 ]Á2H 2 O} n , contains an Ni II atom, two water molecules of coordination, one half of a benzene-1,2,4,5tetracarboxylate (btec) anionic ligand, one 4-aminopyridinium cation (papy) and an uncoordinated water molecule. The metal center lies on an inversion center and adopts an octahedral geometry with the carboxylate groups tilted out of the mean plane formed by the btec. In the crystal, molecules are linked into one-dimensional coordination polymers running along the ac diagonal. The crystal structure is consolidated by N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonds.

Experimental
Crystal data (C 5 Table 1 Hydrogen-bond geometry (Å , ). The 1,2,4,5-benzene-tetracarboxylate (btec) is assessed to be a very versatile ligand able to achieve several coordination modes with different degrees of deprotonation (Du et al. 2007). The specific features of btec prompted researchers to explore different possible coordination polymers bearing specific features (Loiseau et al. 2005;Ruiz-Pérez et al. 2004), expecially in the presence of amines acting as ligands (Stephenson & Hardie, 2006;Wang et al. 2005) and as templates (Fang et al. 2008). Our attempts confirmed that the combination under standard conditins around pH 5 of btec, amines and metals, strictly depended on metals but also on the ancillary amines (yet to be published). The combination of equimolar solutions of nickel chloride, p-aminopyridine and di sodium dihydrogen btec under standard conditions gave two different kinds of green crystals, the rhomboidal-shaped ones (still under study) and the block-shaped ones which were found to be made up of the title compound, (I). The structure of (I) is isomorphous with the already described Co and Cu analogous (Majumder et al., 2006).
The asymmetric unit of (I) is made by half of the metal centre with two coordinated water molecules and half btec ligand, one p-amino pyridinium cation (papy) and another uncoordinated water molecule. The metal centers present the octahedral coordination geometry, whereas carboxylate moieties are tilted out of the btec mean plane in order to develop specific inter-strand and intrastrand hydrogen bonding interactions. The crystallographic symmmetry results in monodimensional coordination polymers running along the diagonal of a and b crystallographic axes. These polymers are kept together by arrays of hydrogen bound papy cations (Fig. 2). The overal paking is obviously stabilized by a crowded hydrogen bonding network (Table 1).

S2. Experimental
An aqueous solution of disodium-dihydrogen 1,2,4,5-benzene tetracarboxylate (25 mmol) was slowly added to an equimolar aqueous solution of NiCl 2 (50 mmol). Then an equimolar aqueous solution of p-aminopyridine was added to the mixture. The clear green solution at pH = 5.15 was left covered. After few days a white solid was separed from two different kind of green crystals. The rhomboidal-shaped crystals are under study (Bruno & Rotondo, to be pubblished), whereas the block-shaped ones were identified as I.

S3. Refinement
All hydrogen atoms were located in the difference map and were included in the refinements at geometrically idealized ORTEP view of I. Displacement ellipsoids are drawn at the 60% probability level. Symmetry codes for the dotted atoms: * -x, -y, -z + 2 and # -x + 1, -y + 1, -z + 2.

Figure 2
Molecular packing view of I along the diagonal of a and b axes. The crowded hydrogen bonding network is represented by dotted lines.

catena-Poly[bis(4-aminopyridinium) [[tetraaquanickel(II)]-µ-benzene-1,2,4,5-tetracarboxylato] dihydrate]
Crystal data (C 5   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 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq  (2)