A polymorph of diaquabis(pyrazine-2-carboxylato-κ2 N 1,O)copper(II)

The title compound, [Cu(C5H3N2O2)2(H2O)2], is a new polymorph of the previously reported compound [Klein et al. (1982 ▶). Inorg. Chem. 21, 1891–1897]. The CuII atom, lying on an inversion center, is coordinated by two N atoms and two O atoms from two pyrazine-2-carboxylate ligands and by two water molecules in a distorted octahedral geometry with the water molecules occupying the axial sites. Intermolecular O—H⋯O, O—H⋯N and C—H⋯O hydrogen bonds connect the complex molecules into a two-dimensional layer parallel to (10), whereas the previously reported polymorph exhibits a three-dimensional hydrogen-bonded network.

The title compound, [Cu(C 5 H 3 N 2 O 2 ) 2 (H 2 O) 2 ], is a new polymorph of the previously reported compound [Klein et al. (1982). Inorg. Chem. 21, 1891-1897. The Cu II atom, lying on an inversion center, is coordinated by two N atoms and two O atoms from two pyrazine-2-carboxylate ligands and by two water molecules in a distorted octahedral geometry with the water molecules occupying the axial sites. Intermolecular O-HÁ Á ÁO, O-HÁ Á ÁN and C-HÁ Á ÁO hydrogen bonds connect the complex molecules into a two-dimensional layer parallel to (101), whereas the previously reported polymorph exhibits a three-dimensional hydrogen-bonded network.

Comment
Pyrazinecarboxylates have been extensively studied as excellent bridging ligands in the coordination chemistry research (Dong et al., 2000;Kubota et al., 2006;Luo et al., 2004;Ptasiewicz-Bak et al., 1995). The structure and magnetic properties of a copper(II) complex with the pyrazine-2-carboxylate (pzc) ligand (polymorph I) has been reported by Klein et al. (1982).
We report here the structure of a new polymorph (polymorph II) of the title compound.
The polymorph II crystallizes in the monoclinic space group P2 1 /n (polymorph I in P2 1 /c). The Cu II atom, lying on an inversion center, is six-coordinated in a distorted octahedral geometry, defined by two O atoms and two N atoms from two pzc ligands in the equatorial plane and two water molecules in the axial positions (Table 1 and Fig. 1). Weak coordination exists between the Cu II center and the coordinated water molecule, with a Cu-O distance of 2.6143 (14) Å, due to Jahn-Teller effects. The bond lengths and angles are in normal ranges (Chutia et al., 2009;Klein et al., 1982). The coordinated water molecule donates its two H atoms to an uncoordinated carboxylate O atom and a pyrazine N atom of the neighboring molecules (Table 2 and Fig. 2). One complex molecule is linked to four neighboring molecules through these O-H···O and O-H···N hydrogen bonds, forming a two-dimensional layer in the (1 0 1) plane. Weak C-H···O hydrogen bond (Table   2) stabilizes the layer structure. In the previously reported polymorph I, the water molecule forms two O-H···O hydrogen bonds with a coordinated carboxylate O atom and an uncoordinated carboxylate O atom. One complex molecule is linked to six neighboring molecules, leading to a three-dimensional network.

Experimental
Aqueous triethylamine (0.05 ml) was added to a suspending solution of Hpzc (0.012 g, 0.1 mmol) in H 2 O (7 ml), followed by dropwise addition of a solution of Cu(NO 3 ) 2 .3H 2 O (0.024 g, 0.1 mmol) in H 2 O (3 ml). The mixture was stirred and sealed in a 15 ml Teflon-lined stainless steel autoclave and heated at 413 K for 3 d under autogenous pressure. When the mixture was cooled to room temperature, blue block crystals of the title compound were obtained (yield 0.029 g, 85% based on Cu).