trans-Diaquabis(pyridazine-3-carboxylato-κ2 N 2,O)copper(II)

In the title compound, [Cu(C5H3N2O2)2(H2O)2], the CuII ion, located on an inversion center, exhibits an octahedral coordination geometry. The equatorial plane is defined by two trans-related N,O-bidentate pyridazine-3-carboxylate ligands and the axial positions are occupied by two water molecules. In the crystal, molecules are connected by O—H⋯O hydrogen bonds between the water molecules and the noncoordinating carboxylate O atoms, forming layers parallel to the bc plane. The layers are stacked along the a axis by further O—H⋯O hydrogen bonds between the water molecules and the coordinating carboxylate O atoms. Weak C—H⋯O hydrogen bonds are also observed between the pyridazine rings and the water molecules and between the pyridazine rings and the non-coordinating carboxylate O atoms.

In the title compound, [Cu(C 5 H 3 N 2 O 2 ) 2 (H 2 O) 2 ], the Cu II ion, located on an inversion center, exhibits an octahedral coordination geometry. The equatorial plane is defined by two trans-related N,O-bidentate pyridazine-3-carboxylate ligands and the axial positions are occupied by two water molecules. In the crystal, molecules are connected by O-HÁ Á ÁO hydrogen bonds between the water molecules and the noncoordinating carboxylate O atoms, forming layers parallel to the bc plane. The layers are stacked along the a axis by further O-HÁ Á ÁO hydrogen bonds between the water molecules and the coordinating carboxylate O atoms. Weak C-HÁ Á ÁO hydrogen bonds are also observed between the pyridazine rings and the water molecules and between the pyridazine rings and the non-coordinating carboxylate O atoms.

Related literature
For the isotypic zinc complex, see: Gryz et al. (2004). For a related cobalt(II) complex which contains two non-coordinating water molecules, see: Artetxe et al. (2013).
This work was supported financially by Eusko Jaurlaritza/ Gobierno Vasco (grant IT477-10). AP also thanks EJ/GV for predoctoral fellowships. The metal ion, the pyridazine ring and carboxylate atoms are coplanar. As expected, the Cu-O and Cu-N distances (Table 1) are similar to the Zn(II) and Co(II) analogue compounds (Gryz et al., 2004;Artetxe et al., 2013). Table 2 summarizes the geometrical parameters of the O-H···O and C-H···O hydrogen bonding interactions.

Experimental
To a solution of CuCl 2 .2H 2 O (34 mg, 0.2 mmol) in water (10 mL) 3-pyridazine carboxylic acid (48 mg, 0.4 mmol) was dropwise added and the resulting solution was stirred for 1 h at 80 °C. Blue prismatic crystals suitable for single-crystal X-ray diffraction were obtained by slow evaporation of the resulting solution after six days.

Refinement
H atoms of the water molecules were located in a Fourier difference map and refined isotropically with O-H bond lengths restrained to 0.88 (1) Å. All H atoms of the pyridazine ring were positioned geometrically and refined using a riding model with standard SHELXL parameters.

Figure 1
Molecular structure of the title compound, showing atom labelling and 50% probability displacement ellipsoids.

trans-Diaquabis(pyridazine-3-carboxylato-κ 2 N 2 ,O)copper(II)
Crystal data [Cu(C 5   Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s involving l.s. planes. 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 > 2σ(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.