Crystal structure of aqua(1H-pyrazole-κN 2)(pyridine-2,6-dicarboxylato-κ3 O 2,N,O 6)copper(II) dihydrate

The synthesis and crystal structure of tridentate pyridine-2,6-dicarboxylate CuII complex with a heterocyclic pyrazole ligand, a potential candidate for metal catalysts, are reported. The CuII atom is coordinated by three O atoms and two N atoms, provided by a tridentate pyridine-2,6-dicarboxylate, one pyrazole and one water ligand, forming a slightly distorted square-pyramidal geometry.


Chemical context
Metal complexes with the tridentate ligand 2,6-bis[(1Hpyrazol-1-yl)methyl]pyridine are known to be catalysts of polyethylene polymerization (Singh et al., 2003;Watson et al., 1987;Son et al., 2014;Kim & Kang, 2015). 2,6-Bis[(1Hpyrazol-1-yl)methyl]pyridine was oxidized to pyridine-2,6-dicarboxylate (pdc) by metal nitrate (Unuigboje & Anyile, 2007). The pdc molecule has been recognized as a component of bacterial spores, and is also useful in a variety of processes as an enzyme inhibitor, plant preservative and food sanitizer (Cui et al., 2011). The pdc molecule has been selected as a primary dibasic tridentate ligand and a metal complex with pdc was reported to be a new chemical sensor (Mistri et al., 2013). Attention has been paid to the design of various Ndonor ligands with special structural properties in order to investigate the specific stereochemical requirements of a particular metal-binding site (Mukherjee, 2000). Various substituted N-donor heterocyclic ligands such as imidazole and pyrazole have been selected as a second ligand, so that the structural and electronic effects on the biologically important Cu-N bond could be probed (Ang et al., 1991;Chen et al., 2011;Lin et al., 2009;Liu et al., 2005). As part of these continuing studies, the title complex has been synthesized and characterized by single crystal X-ray diffraction.

Structural commentary
The molecular structure of the title compound is shown in Fig. 1. The Cu II atom is coordinated by three O atoms and two N atoms from tridentate pyridine-2,6-dicarboxylate (pdc), pyrazole and water ligands. The coordination geometry around the Cu II atom is a distorted square pyramid as indi- ISSN 2056-9890 cated by the value of 0.113 (Addison et al., 1984). The Cu II atom lies in the center of the basal plane defined by two nitrogen atoms (N2 from pdc and N14 from pyrazole) and two oxygen atoms (O9 and O12 from pdc). The plane including the Cu II atom is almost planar, with an r.m.s. deviation of 0.0847 Å from the corresponding least-squares plane defined by the five constituent atoms. The pyrazole ring is twisted by 66.61 (10) from the basal plane. The apical Cu1-O19 bond length of 2.217 (2) Å is much longer than those of the basal Cu-O lengths [Cu1-O9 = 2.026 (2) Å and Cu1-O12 = 2.058 (2) Å ].

Database survey
A search of the Cambridge Structural Database (Version 5.37 with two updates, Groom et al., 2016) returned 1448 entries for crystal structures related to the name pyridine-2,6-dicarboxylato. Most of them are crystal structures of metal complexes. However, there are only four entries with a secondary ligand of a pyrazolyl derivative bonded to a transition metal, viz. a Cu complex (Lin et al., 2009;Wang et al., 2014) and Zn and Co complexes  1876 Kim and Kang [Cu(C 7 Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
Part of the packing diagram of the title compound, showing molecules linked by intermolecular O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds (dashed lines).

Figure 1
The molecular structure of the title compound, showing the atomnumbering scheme and 30% probability ellipsoids for non-H atoms. H atoms are drawn as small spheres of arbitrary radii. The O-HÁ Á ÁO hydrogen bonds are indicated by dashed lines.

Synthesis and crystallization
A solution of copper nitrate trihydrate (0.072 g, 0.3 mmol) in acetonitrile (5 ml) was added to a solution of 2,6-bis[(1Hpyrazol-1-yl)methyl]pyridine (0.072 g, 0.3 mmol) in acetonitrile (5 ml) in a high-pressure vessel. After sealing the highpressure vessel, the resulting solution was stirred for three days at 403 K. The precipitate formed was removed by filtration, and the filtrate was washed with acetonitrile and dichloromethane to get a dark-green powder. Single crystals of the title compound were obtained from its aqueous solution by slow evaporation of the solvent at 333 K within five days.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms of the water molecules and the NH group were located in a difference-Fourier map and refined freely [refined distances; O-H = 0.70 (5)

Funding information
This work was supported by research fund of Chungnam National University.  program(s) used to solve structure: SHELXS2013 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2013 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: WinGX (Farrugia, 2012).  Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.