Dipyridinium diaquabis(pyrazole-3,5-dicarboxylato-κ2 N,O)cuprate(II) dihydrate

In the mononuclear title salt, (C5H6N)2[Cu(C5H2N2O4)2(H2O)2]·2H2O, the CuII ion is located on an inversion centre and is coordinated by two chelating pyrazole-3,5-dicarboxylate anions and two water molecules, forming a Jahn–Teller-distorted CuN2O4 octahedron. O—H⋯O and N—H⋯O hydrogen bonds are formed between water molecules, complex anions and the pyridine counter-cations, leading to the formation of layers parallel to (100). The layers are held together by weak C—H⋯O hydrogen bonds.

Data collection: SMART (Siemens, 1998); cell refinement: SAINT (Siemens, 1998); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: WinGX (Farrugia, 2012); software used to prepare material for publication: publCIF (Westrip, 2010 Pyrazole-3,5-dicarboxylic acid (H 3 dcp) is a versatile ligand with six potential coordinating sites, namely two N atoms of the pyrazole ring and four O atoms of the carboxyl groups. Together with π-π interactions between neighbouring pyrazole rings, the coordination mode of H 3 dcp can lead to different possibilities for creating supramolecular structures (King et al., 2004). When trying to synthesize a copper-containing coordination compound involving H 3 dcp, the title compound, (C 5 H 6 N) + 2 [Cu(C 5 H 2 N 2 O 4 ) 2 (H 2 O) 2 ] 2-. 2H 2 O, was obtained. The Cu II ion sits on an inversion center. One fully deprotonated pyrazole-3,5-dicarboxylic acid, one coordinating water molecule, one lattice water molecule and one pyridinium cation are also present in the asymmetric unit. The other half of the metal-containing moiety is generated by the inversion centre. Each pyrazole-3,5-dicarboxylate anion chelates the metal by one N atom and one O atom in the equatorial plane of an octahedron whereas the axial ligands are provided from water molecules at considerably longer distances ( Fig. 1), in agreement with the tetragonal Jahn-Teller distortion (Table 1).
The BVS calculation (Brown, 2002) of the Cu ion gave a value of 1.88 valence units, which indicates that the Cu ion is divalent. Because pyrazole-3,5-dicarboxylic acid is a rather strong acid, the terminal non-coordinating carboxyl group also loses its proton to make the solvent pyridine molecules protonated. Besides four protonated pyridine molecules, four lattice water molecules are present in one unit cell as solvent molecules.
Classical O-H···O and N-H···O hydrogen bonding occurs between water molecules, pyridinium cations and complex anions to form a layer in (100), in which N1, N3, O5, O6 act as donor atoms, and O1, O2, O3, O4, O6 are acceptors ( Table 2). The packing of adjacent layers along [100] is accomplished through non-classical weak C-H···O contacts, with the donor belonging to pyridine and acceptor being O atoms of the non-coordinating carboxylate groups.
Experimental 0.5 mmol H 3 dcp and 0.05 mmol CuCl were mixted in 10 ml H 2 O to give a suspension. After addition of 0.5 ml pyridine, the suspension turned to solution, which was then stirred for 4 h and filtered. After standing in ambient conditions for about 3 days, the filtrate yielded blue crystals suitable for X-ray diffraction.

Refinement
The H atoms on N3, O5 and O6 were found in the difference electron density map, and the corresponding N-H bond

Figure 1
The molecular structure of the title compound, with atom labels and 20% probability displacement ellipsoids for all non-  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 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.