Tetraaquadiazidocobalt(II) 3,3′-dicarboxylato-1,1′-ethylenedipyridinium

The asymmetric unit of the title compound, [Co(N3)2(H2O)4]·C14H12N2O4, comprises half of the cobalt(II) complex molecule and a half of the 3,3′-dicarboxylato-1,1′-ethylenedipyridinium molecule. The CoII atom is located on an inversion centre and hence the complex molecule adopts a centrosymmetric trans-octahedral geometry. The zwitterionic organic molecule is also centrosymmetric with the centre of the C—C bond of the ethylene moiety coinciding with an inversion centre. The adduct of metal complex and organic molecule is associated into a three-dimenional network through O—H⋯O hydrogen bonds.

The asymmetric unit of the title compound, [Co(N 3 ) 2 (H 2 O) 4 ]Á-C 14 H 12 N 2 O 4 , comprises half of the cobalt(II) complex molecule and a half of the 3,3 0 -dicarboxylato-1,1 0 -ethylenedipyridinium molecule. The Co II atom is located on an inversion centre and hence the complex molecule adopts a centrosymmetric trans-octahedral geometry. The zwitterionic organic molecule is also centrosymmetric with the centre of the C-C bond of the ethylene moiety coinciding with an inversion centre. The adduct of metal complex and organic molecule is associated into a three-dimenional network through O-HÁ Á ÁO hydrogen bonds.
We are thankful for financial support from the Shanghai Leading Academic Discipline Project (B409).

Comment
Hydrogen bonds play a key role in biological systems and materials, and they have been widely used as a putative supramolecular tool for engineering organic and metal-organic solids (Fabbiani et al., 2010;Salitros et al., 2010;Schultheis et al., 2010;Braga & Grepioni, 2000). In this paper, we report the structure of the title compound, (I), which contains a neutral metal complex molecule, [Co(N 3 ) 2 (H 2 O) 4 ], and a zwitterionic dicarboxylate, 1,2-bis(3-carboxylatopyridinium)ethane ( Fig. 1). The metal complex molecule is centrosymmetric, with the octahedral-coordinated Co II by two azide anions and four water molecules in a trans arrangement ( Fig. 1, Table 1). Two opposite Co-O distances are longer than the Co-N and other Co-O ones, defining an axially elongated geometry. The zwitterionic molecule is also centrosymmetric (Fig.1 ). The inorganic complex molecules and the carboxylate groups are associated into a sheet through O-H···O hydrogen bonds involving the coordinated aqua ligands (O3 and O4) and the carboxylate oxygen atoms (O1 and O2) (Fig. 2, Table  2). The two O4 aqua ligands from symmetry related complex molecules and two O2 atoms from symmetry related organic molecules form a hydrogen-bonded ring which can be denoted by the graph set R 4 2 (8) (Bernstein et al., 1995;Etter, 1990).
Similar hydrogen-bonded rings are formed by O1 and O3. The carboxylate group forms a R 2 2 (8) hydrogen-bonded ring with two aqua ligands from the same complex molecule. Besides, a large hydrogen-bonded ring [R 4 4 (16)] is formed by two carboxylate groups and four aqua ligands from two complex molecules. The organic ligands interlink the hydrogen-bonded sheets of the metal complexes into the three-dimensional structure (Fig. 3).

Experimental
The zwitterionic ligand ([H 2 L 1 ]Br 2 ) was synthesised from 1,2-dibromoethane and ethyl nicotinate according to the published procedure (Loeb et al., 2006). An aqueous solution (4 mL) of [H 2 L 1 ]Br 2 (0.1 mmol) and NaN 3 (1 mmol) was added to a DMF solution (1.5 mL) of Co(ClO 4 ) 2 .6H 2 O (0.2 mmol) with stirring. The resulting solution was allowed to evaporate slowly at room temperature, yielding light-red block crystals of (I) in three days. Yield: 75%. Anal. calcd (found) (%) for    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 Rfactors(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.