Octaaquabis(μ2-1H-pyrazole-3,5-dicarboxylato)tricopper(II) tetrahydrate

In the trinucler CuII complex molecule of the title compound, [Cu3(C5HN2O4)2(H2O)8]·4H2O, the central CuII atom is located on an inversion centre and is coordinated in a distorted octahedral geometry. The equatorial sites are occupied by two N and two O atoms from two pyrazole-3,5-dicarboxylate ligands and the axial positions are occupied by two water molecules. The two other symmetry-related CuII atoms are pentacoordinated and assume a square-pyramidal geometry. In the crystal structure, coordinated and uncoordinated water molecules and carboxylate O atoms are linked by O—H⋯O hydrogen bonds.

In the trinucler Cu II complex molecule of the title compound, [Cu 3 (C 5 HN 2 O 4 ) 2 (H 2 O) 8 ]Á4H 2 O, the central Cu II atom is located on an inversion centre and is coordinated in a distorted octahedral geometry. The equatorial sites are occupied by two N and two O atoms from two pyrazole-3,5dicarboxylate ligands and the axial positions are occupied by two water molecules. The two other symmetry-related Cu II atoms are pentacoordinated and assume a square-pyramidal geometry. In the crystal structure, coordinated and uncoordinated water molecules and carboxylate O atoms are linked by O-HÁ Á ÁO hydrogen bonds.

Comment
The design and synthesis of novel coordination architectures is a fertile field due to the intriguing network topologies and potential a pplications as new classes of materials (Yaghi et al., 2003;Kitagawa et al., 2004). The ligand of pyrazole-3,5-dicarboxylic acid has several potential coordination sites involving both two N atoms of the pyrazole ring and four carboxylate O atoms. These multifunctional coordination sites are highly accessible to metal ions, as such, the ligand can coordinate as a mono-, bi-, or tetradentate ligand and can act to link together metal centers through a number of bridging modes (Li, 2005).
The divalent copper atoms are easily to precipitate with the OHwhen the pyrazole-3,5-dicarboxylic acids are deprotoned in base water solution, the mixed solution can obtain coordianted polymer single crystals in hydrothermal condition (King et al., 2003). Nevertheless, when the ammonia was added to the mixed solution, because of the complexing action between the copper atoms and NH 3 , the turbid soltuion became clear. After the ammonia slowly evaporated, we obtained the blue crystals, compound (I), as shown in Fig.1, a copper(II) trimer.
The central copper atom, Cu1, lies on a crystallographic inversion center. The Cu1 atom has a six-coordinate octahedral geometry, in which two O atoms and two N atoms from two pyrazole-3,5-dicarboxylate ligands occupy the equatorial plane, and the axial coordination sites are occupied two water molecules; the Cu-N/O bond distances range from 2.003 (2) to 2.437 (3) Å. The other two symmetry-related copper atoms, Cu2, have a pentacoordinate square-pyramidal geometry, where a pyrazole nitrogen N2 and a carboxylate oxygen O3 from one pyrazole-3,5-dicarboxylate ligand occupy two coordination sites and the remaining three positions are occupied by water molecules; the Cu-N/O bond distances range from 1.984 (2) to 2.237 (2) Å. The pyrazole-3,5-dicarboxylate ligand is not strictly planar. Deviation from the mean plane defined by the pyrazole ring is seen for both carboxylate groups with values ranging from 0.034 (1) to 0.205 (1) Å. The dihedral angle between the two carboxylate mean planes is 11.3 (3)°. It can be seen that the ligand bite angle at the two different copper centers Cu1 and Cu2 is similar, 74.8 (4) and 80.6 (4)°, respectively. This implies that the pyrazole-3,5-dicarboxylate ligand is a fairly rigid ligand and retains its integrity on metal chelation.

Experimental
The title complex was prepared by the addition of Cu(BF 4 ) 2 (20 mmol) and pyrazole-3,5-dicarboxylic acid (30 mmol) to 40 ml water. The mixture was stirred for 1 h, a blue precipitate was obtained. A minimum amount of ammonia (14 M) was supplementary materials sup-2 added to give a blue solution. Suitable crystals were obtained after standing at room temperature for several days (yield 51% based on Cu).

Refinement
Atom H2 was placed geometrically (C-H = 0.93 Å) and refined using a riding model, with U iso (H) = U eq (C). The H atoms bonded to O atoms of water molecules were located in a difference Fourier map and refined, with a bond distance restriction