Poly[di-μ2-aqua-μ5-(pyridine-2,6-dicarboxylato)-μ3-(pyridine-2,6-dicarboxylato)-cobalt(II)disodium]

In the title compound, [CoNa2(C7H3NO4)2(H2O)2]n, the CoII atom is coordinated by two pyridine N atoms and four carboxylate O atoms from two doubly deprotonated pyridine-2,6-dicarboxylate ligands in a distorted octahedral geometry. One Na+ cation is coordinated by three carboxylate O atoms and two water molecules and the other is coordinated by five carboxylate O atoms and two water molecules in an irregular geometry. The bis(pyridine-2,6-dicarboxylato)cobalt complex units are connected by Na+ cations and bridging water molecules into a three-dimensional coordination network. O—H⋯O hydrogen bonds are formed between the water molecules and the carboxylate O atoms.

In the title compound, [CoNa 2 (C 7 H 3 NO 4 ) 2 (H 2 O) 2 ] n , the Co II atom is coordinated by two pyridine N atoms and four carboxylate O atoms from two doubly deprotonated pyridine-2,6-dicarboxylate ligands in a distorted octahedral geometry. One Na + cation is coordinated by three carboxylate O atoms and two water molecules and the other is coordinated by five carboxylate O atoms and two water molecules in an irregular geometry. The bis(pyridine-2,6-dicarboxylato)cobalt complex units are connected by Na + cations and bridging water molecules into a three-dimensional coordination network. O-HÁ Á ÁO hydrogen bonds are formed between the water molecules and the carboxylate O atoms.

Comment
Hydroxamic acids are widely used in synthesis of polynuclear compounds and coordination polymers. However, when the synthesis is conducted in alkaline conditions, hydrolytic decomposition of the hydroxamate function resulting in the formation of carboxylic groups sometimes occurs (Dobosz et al., 1999;Świątek-Kozłowska et al., 2000). Herein we report the crystal structure of the title compound obtained as a result of hydrolytic decomposition of pyridine-2,6-dihydroxamic acid in the course of formation of the anionic complex with cobalt(II) in the presence of sodium hydroxide.
In the title compound, the Co II atom is coordinated by two pyridine N atoms and four carboxylate O atoms from two doubly deprotonated pyridine-2,6-dicarboxylate ligands in a distorted octahedral geometry (Fig. 1). The Co-O bond lengths are in a range of 2.1235 (17)-2.2065 (14) Å, which clearly indicates that the central ion is in bivalent state (Sliva et al., 1997). The Na ions are coordinated by O atoms of pyridine-2,6-dicarboxylate ligands and two water molecules in irregular geometries. Na1 atom is in a strongly distorted square-pyramidal environment, while Na2 atom exhibits a coordination number 7 and its geometry approaches to a distorted pentagonal-bipyramidal. The Na-O bond lengths lie in a range of 2.2756 (11)-2.7557 (17) Å, which is normal for sodium ions (Mokhir et al., 2002;Świątek-Kozłowska et al., 2000). The C9-O3 and C9-O7 bond lengths [1.284 (2) and 1.240 (2) Å] are typical for a monodentately coordinated carboxylate (Wörl et al., 2005a,b). The C-N and C-C bond lengths in the pyridine rings are normal for 2-substituted pyridine derivatives (Fritsky et al., 2001;Krämer & Fritsky, 2000;Moroz et al., 2010;Sachse et al., 2008).
In the crystal packing (Fig. 2), the bis(pyridine-2,6-dicarboxylato)cobalt(II) complex units are connected by the sodium ions and water molecules into a three-dimensional coordination network. The two water molecules bridge the sodium ions and form intermolecular hydrogen bonds (Table 1).

Experimental
Cobalt(II) perchlorate hexahydrate (0.0365 g, 0.1 mmol) was dissolved in methanol (5 ml) and mixed with a solution of pyridine-2,6-dihydroxamic acid (0.0197 g, 0.1 mmol) synthesized according to Świątek-Kozłowska et al. (2002) in H 2 O, and then to the obtained mixture solution sodium hydroxide (0.1 M, 1 ml) was added. The mixture was stirred for 30 min and filtered. The insoluble material was dissolved in DMSO (3 ml) and set aside for crystallization by slow diffusion of isopropanol vapours into the formed solution. The greenish yellow crystals formed in 5-7 days were filtered off, washed with isopropanol and dried (yield: 78%).