Diaquabis(5-carboxy-1H-pyrazole-3-carboxylato-κ2 N 2,O 3)cobalt(II) dihydrate

In the title complex, [Co(C5H3N2O4)2(H2O)2]·2H2O, the CoII ion lies on an inversion center and is coordinated in a distorted octahedral environment. In the crystal structure, complex and water molecules are linked into a three-dimensional network by O—H⋯O and N—H⋯O hydrogen bonds.

In the title complex, [Co(C 5 H 3 N 2 O 4 ) 2 (H 2 O) 2 ]Á2H 2 O, the Co II ion lies on an inversion center and is coordinated in a distorted octahedral environment. In the crystal structure, complex and water molecules are linked into a three-dimensional network by O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds.
In the past few decades, self-assembly processes involving metal ions and organic ligands directed by either metal coordination or hydrogen bonds have received a great deal of attention in the field of supramolecular chemistry and crystal engineering. The 3,5-pyrazoledicarboxylic acid ligand is asymmetric and has six potential coordination sites which can act to link together metal centers through a number of bridging modes (King et al., 2004). A variety of complexes containing this ligand have been reported (Frisch et al., 2005;King et al., 2003King et al., , 2004Pan, Ching et al., 2001;Pan, Frydel et al., 2001;Li et al., 2005).
The molecular structure of the title complex , (I), is shown in Fig. 1. The Co II ion is located on an inversion center and is coordinated in a distorted octahedral environment. The axial sites are occupied by water molecules and the equatorial plane is fromed by two oxygen donors and two nitrogen donors from two chelating 5-carboxy-pyrazole-3-carboxylato ligands.
In the crystal structure complex and water molecules are linked into a three-dimensional network by O-H···O and N-H···O hydrogen bonds.

Experimental
A mixture of cobalt(II) nitrate (hexhydrate) (0.2 mmol, 58 mg), 3,5-pyrazoledicarboxylic acid (0.4 mmol, 62 mg) and H 2 O (18.0 ml) in a 1:2:5000 molar ratio was sealed in a 25 ml stainless steel reactor with a Teflon liner. The autoclave was kept at 423 K for 3 d, then cooled to room temperature at a rate of 4 K/h. Orange block-shaped crystals of the title complex were collected by filtration for the structural analysis.

Refinement
All H atoms bonded to C and N atoms were initially located in difference Fourier maps but were subsequently refined  Fig. 1. The molecular structure of (I), with atom labels and 35% probability displacement ellipsoids for non-H atoms [symmetry code: (A) -x+1, -y+2, -z]. Only the unique solvent water molecule is shown.  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.

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