Bis(2-amino-6-methylpyridinium) trans-diaquabis(pyrazine-2,3-dicarboxylato)cuprate(II) hexahydrate

The title compound, (C6H9N2)2[Cu(C6H2N2O4)2(H2O)2]·6H2O, was obtained by the reaction of CuCl2·2H2O with pyrazine-2,3-dicarboxylic acid (pyzdcH2) and 2-amino-6-methylpyridine (2a-6mpy) in aqueous solution. The CuII atom is located on an inversion centre and has an overall octahedral coordination environment. Two N and two O atoms from (pyzdc)2− ligands define the equatorial plane and two water molecules are in axial positions, resulting in a typical tetragonally Jahn–Teller-distorted environment. Extensive classical O—H⋯O, O—H⋯N and N—H⋯O and non-classical C—H⋯O hydrogen bonds, as well as π–π stacking interactions between aromatic rings of the cations [centroid–centroid distance = 3.58 (9) Å], lead to the formation of a three-dimensional supramolecular structure.

The Ferdowsi University of Mashhad is gratefully acknowledged for financial support.

Comment
In recent years, supramolecular complexes have attracted extensive attention owing to their potential applications. In this context, our research group has made several attempts to prepare supramolecular crystalline coordination compounds based on proton-transfer mechanism between dicarboxylic acids and amines (Eshtiagh-Hosseini, et al., 2010a, 2010b, 2010c, 2011. Proton transfer mechanisms play a basic role in construction of supramolecular coordination compounds and water clusters (Aghabozorg et al., 2008(Aghabozorg et al., , 2010. In particular, pyrazine-2,3-dicarboxylic acid provides different modes of coordination to the metal ions (Che et al., 2009). Therefore, the anion of this acid is well-know to act as a suitable ligand, especially in the design and construction of supramolecular networks. Herein, we describe the molecular and supramolecular crystal structure of a new compound, 1, with chemical formula (2a-6mpyH) 2 [Cu(pyzdc) 2 (H 2 O) 2 ].6H 2 O, where pyzdcH 2 = pyrazine-2,3-dicarboxylic acid and 2a-6mpy = 2-amino-6-methylpyridine. Fig. 1 shows the coordination environment of the Cu II ion (site symmetry 1). The coordination sphere can be described as distorted octahedral, with two N and two O atoms from (pyzdc) 2ligands defining the equatorial plane and two water molecules in axial positions. The Jahn-Teller effect, as observed for numerous Cu II complexes, results in the elongation of the two axial Cu-O bonds towards a strong tetragonal distortion. The molecular entities of 1 consist of a [Cu(pyzdc) 2 (H 2 O) 2 ] 2anion, a (2a-6mpyH) + cation and uncoordinated water molecules in a 1:2:6 molar ratio.
For the three-dimensional supramolecular structural set-up, extensive X-H···O (X = O, N, and C) and O-H···N hydrogen bonding interactions as well as π-π stacking interactions between aromatic rings of the cations with a centroid-centroid distance of 3.589 Å are responsible (Fig. 2).

Experimental
A solution of pyzdcH 2 (0.6 mmol, 0.1 g) and 2a-6mpy (1.2 mmol, 0.13 g) in water (10 ml) was refluxed for 1 h, then a solution of CuCl 2 . 2H 2 O (0.2 mmol, 0.01 g) was added dropwise and refluxing was continued for 6 h at 343 K. The obtained blue solution yielded blue block-like crystals of the title compound after slow evaporation of the solvent at room temperature.

Refinement
The H atoms were generated geometrically and refined using a riding model, with C-H = 0.95-0.98 Å and U iso (H) = 1.2, 1.5 U eq (C). H atoms bonded to water molecules and nitrogen atoms were found from difference maps and than fixed. They were finally refined in the riding model approximation with riding model, with U iso (H) = 1.2U eq (N) and 1.5U eq (O).