(μ-4,4′-Bipyridine-κ2 N:N′)bis[triaqua(4,4′-bipyridine-κN)(3-nitrophthalato-κO 2)cobalt(II)]

The title binuclear complex, [Co2(C8H3NO6)2(C10H8N2)3(H2O)6], has been synthesized hydrothermally from 3-nitrophthalic acid (H2NPA), Co(NO3)2·6H2O and 4,4′-bipyridine (4,4′-bipy). The molecule of the complex occupies a special position on an inversion centre. The CoII atom has a slightly distorted octahedral environment formed by two N atoms from two 4,4′-bipy ligands, one carboxylate O atom from NPA, and three O atoms of water molecules. An extensive O—H⋯O and N—H⋯O hydrogen-bonding system links molecules of the complex into a three-dimensional network.

Complex (I) occupies a special position in the inversion centre; the asymmetric unit consists of one cobalt(II) atom, one NPA ligand, one terminal and one-half of a bridging 4,4′-bipy groups, as well as three metal-coordinated water molecules.
( Fig. 1 and Table 1). The Co1 atom has a a slightly distorted octahedral environment formed by two N atoms from two different bipy ligands, one carboyxlate O atom of the NPA ligand, and three water molecules. The µ 2 -4,4′-bipyridine ligand bridges two [Co(NPA)(bipy)(H 2 O) 3 ] units of the binuclear complex.
The extensive system of O-H···O hydrogen bonds links molecules of the complex into a three-dimensional network ( Fig. 2; Table 1).

S2. Experimental
A solution of Co(NO 3 ) 2 .6H 2 O (0.0291 g, 0.1 mmol) in 5 ml of water was added dropwise under continuous stirring to an aqueous solution (5 ml) of 3-nitrophthalic acid (0.0211 g, 0.1 mmol) and 4,4′-bipyridine (0.0156 g, 0.1 mmol). The resulting mixture was then transferred into a Teflon-lined stainless steel vessel, which was sealed and kept at 393 K for 72 h. The vessel was then cooled to room temperature, the reaction mixture was filtered, and single crystals were obtained from the filtrate after a few days of slow evaporation at room temperature.

S3. Refinement
The aromatic H atoms were positioned geometrically and allowed to ride during subsequent refinement, with C-H = 0.93 Å and U iso (H) = 1.2U eq (C). Water H atoms were located in a difference map and refined with O-H and H···H distance restraints of 0.85 (1) and 1.39 (1) Å, respectively and U iso (H)= 1.2U eq (O).   Crystal packing of the title compound viewed down the a axis; H-bonds are shown as dashed lines.

(µ-4,4′-Bipyridine-κ 2 N:N′)bis[triaqua(4,4′-bipyridine-κN)(3-nitrophthalato-κO 2 )cobalt(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.

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