μ-Pyrazine-bis[tetraaquacadmium(II)] μ-pyrazine-bis[tetraacetatocadmium(II)]

In the title dinuclear ionic complex, [Cd2(C4H4N2)(H2O)8][Cd2(CH3CO2)8(C4H4N2)], the cation and anion are disordered equally over a site with symmetry mmm. The CdII ions and the N atoms of the bridging pyrazine ligand lie on the intersection of two crystallographic mirror planes. The C atoms of the bridging pyrazine ligand lie on one of these mirror planes, and the acetate groups and water molecules lie across the intersecting mirror planes. Each CdII atom in the cation is five-coordinated by four O atoms from four water molecules and one N atom from the bridging pyrazine ligand, whereas each CdII in the anion is nine-coordinated by four pairs of O atoms from the bidentate acetate ligands and one N atom from the bridging pyrazine ligand. In the crystal structure, each anion is surrounded by eight nearest-neighbour cations and vice versa. The crystal structure is stabilized by ionic interactions as well as by C—H⋯O interactions.

In the title dinuclear ionic complex, [Cd 2 (C 4 H 4 N 2 )(H 2 O) 8 ]-[Cd 2 (CH 3 CO 2 ) 8 (C 4 H 4 N 2 )], the cation and anion are disordered equally over a site with symmetry mmm. The Cd II ions and the N atoms of the bridging pyrazine ligand lie on the intersection of two crystallographic mirror planes. The C atoms of the bridging pyrazine ligand lie on one of these mirror planes, and the acetate groups and water molecules lie across the intersecting mirror planes. Each Cd II atom in the cation is five-coordinated by four O atoms from four water molecules and one N atom from the bridging pyrazine ligand, whereas each Cd II in the anion is nine-coordinated by four pairs of O atoms from the bidentate acetate ligands and one N atom from the bridging pyrazine ligand. In the crystal structure, each anion is surrounded by eight nearestneighbour cations and vice versa. The crystal structure is stabilized by ionic interactions as well as by C-HÁ Á ÁO interactions.

Comment
The investigations of coordination compounds between cadmium(II) and O and N donors atoms have attracted much attention due to their potential application in a number of areas (Inoue et al., 2000;Pons et al., 2007;Xia et al., 2004), including cytotoxic activities (Filipović et al., 2008). We report herein, the synthesis and crystal structure of the title compound which exhibits a mixed coordination for Cd II atom i.e nine-and five-coordination mode for Cd II , which is a rare case.
In the title compound, both [Cd 2 (H 2 O) 8 (C 4 H 4 N 2 )] 4+ cations and [Cd 2 (CH 3 CO 2 ) 8 (C 4 H 4 N 2 )] 4anions lie on the site symmetry mmm. The Cd II and the N atoms of the bridging pyrazine ligand lie on the intersection of two crystallographic mirror planes, one perpenidicular to the c axis (z = 0) and the other parallel to the c axis and passing through the mid points of the a and b axis. The C atoms of the bridging pyrazine ligand lie on the z = 0 mirror plane, and the acetate groups and water molecules lie across the intersecting mirror planes. There are two molecules of the title complex in the unit cell.
In the structure, the cation contains two Cd II ions, eight water molecules and one bridging pyrazine ligand whereas the anion contains two Cd II ions, eight acetate and one bridging pyrazine ligands. Each of the Cd II in the cation is five-coordin-  (Inoue et al., 2000;Pons et al., 2007;Xia et al., 2004).
In the crystal packing ( Fig. 2 and Fig.3), each anion is surrounded by eight nearest neighbour cations and vice-versa. The crystal structure is stablized by ionic interactions as well as by weak C-H···O interactions (Table 1).

Experimental
The title compound was synthesized by mixing Cd(CH 3 COO) 2 and pyrazine with a 2:1 molar ratio in a hot methanol-water (2:1 v/v) solution and stirred for 10 min at room temperature. The solution was then left at ambient temperature to allow the solvent to slowly evaporate. Colourless crystals of the title compound suitable for X-ray structure determination were obtained after a few weeks.

Refinement
Water H atoms were located in a difference map and refined isotropically, with a O-H distance restraint of 0.800 (1) Å.
C-bound H atoms were positioned geometrically and allowed to ride on their parent atoms, with C-H = 0.96 Å. The U iso value of H1A was constrained to 1.2U eq (C1) and for other H atoms the U iso values were refined. A rotating group model was used for the methyl group.