Bis(μ-5-nitro-2-oxidobenzoato)bis[triaquazinc(II)]

The title complex molecule, [Zn2(C7H3NO5)2(H2O)6], is a centrosymmetric dimer containing two zinc(II) cations with distorted octahedral geometries provided by the O atoms of three water molecules and the two bridging bidentate 5-nitrosalicylate ligands. The separation between the metal centres in the dimer is 3.1790 (11) Å. The crystal structure is stabilized by O—H⋯O hydrogen bonds, one of which intradimeric, linking the dimers into a three-dimensional network.

The title complex molecule, [Zn 2 (C 7 H 3 NO 5 ) 2 (H 2 O) 6 ], is a centrosymmetric dimer containing two zinc(II) cations with distorted octahedral geometries provided by the O atoms of three water molecules and the two bridging bidentate 5nitrosalicylate ligands. The separation between the metal centres in the dimer is 3.1790 (11) Å . The crystal structure is stabilized by O-HÁ Á ÁO hydrogen bonds, one of which intradimeric, linking the dimers into a three-dimensional network.
Although many complexes which use salicylate as ligand have been synthesized, two structures coming out from the reaction of the 5-nitrosalicylic acid with zinc salt are known to us: a tetrahydrate (Tahir et al., 1997), in an approximately octahedral geometry around the metal surrounding O atoms from four water ligands and two unidentate monoanionic 5nitrosalicylate ligands using one carboxylate oxygen and a pentahydrate (Morgant, et al., 2006), penta-aqua-(5-nitrosalicylato-O)-zinc(ii) 5-nitrosalicylate monohydrate, in which the metal is coordinated by five water ligands and one carboxylato O-atom from the 5-nitrosalicylato ligand. Interestingly, the title binuclear complex presents a third, different structure, being an hexahydrate dimer with its two zinc(II) atoms bridged by two carboxylate O atoms.
The structure of the title compounds is shown in Fig. 1. The distorted octahedral environment of each zinc(II) cation is defined by three O atoms from three water molecules, another two (the phenolate and a carboxylate one) from a chelating 5-nitrosalicylate and the centrosymmetric image of the latter. These two carboxylate O atoms bridge neighbouring zinc cations into a planar, four-membered matallacycle resulting in a Zn1···Zn1 i (see Fig 1 for symmetry codes) separation of 3.1790 (11) Å. It is the shortest of separation of Zn···Zn as reported previously in binuclear and tetranuclear zinc complex with salicylate ligands (Erxleben, 2001) It is worth mentioning that the use of a carboxylate oxygen as a bridging atom in salicylate metal complexes is rare; the title binuclear complex appears to be the first example of this behaviour in zinc complexes.
The carboxy group C1/O1/O2 as well as the nitro group N1/O4/O5 are effectively coplanar to the aromatic ring in the ligand as well as to the central Zn1/O2/C1/C2/C7/O3 six-membered ring generated upon coordination. The centrosymmetric character of the binuclear unit results in a large planar group composed of the two almost planar chelating ligands, the two zinc atoms and two O atoms from two aqua; the O atoms atoms from the remaining four aqua present Zn-O bonds almost orthogonal to this plane.
There are a number O-H···O hydrogen bonds stabilizing the structure (Table 1). The interaction involving O7-H7B and O1 i is intradimeric and coplanar to the dimer mean plane. The remainig ones define a three-dimensional framework.

S2. Experimental
The title complex was prepared by digesting a mixture of 5-nitrosalicylic acid (5 mmol) and fresh zinc hydroxide (10 mmol) in distilled water (30 ml) at 80 °C under stirring for 10 min. After filtration yellow-green crystals grew out of the solution by slow evaporation over a period of three days at room temperature. The starting zinc hydroxide was prepared from 50 ml aqueous solutions of 0.9 g of zinc chloride and 0.5 g of sodium hydroxide.

S3. Refinement
The H atoms of the water molecule were found in a difference Fourier map. However, during refinement, they were restrained to O-H = 0.85 (1) Å and their U iso values were set at 1.2 U eq (O). Other H atoms were treated as riding, with C-H = 0.93 Å, and with U iso (H) = 1.2 U eq (C).

Figure 1
The structure of the title compound, with the atom-numbering scheme, and 30% probability displacement ellipsoids. H atoms are shown as small spheres of arbitrary radii and hydrogen bonds are indicated by dashed lines. [Symmetry code: (i) -x + 1, -y + 1, -z + 1.]

Bis(µ-5-nitro-2-oxidobenzoato)bis[triaquazinc(II)]
Crystal data 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.