Tetraaquabis[4-(pyrazin-2-ylsulfanylmethyl-κN 4)benzoato]cobalt(II)

In the title compound, [Co(C12H9N2O2S)2(H2O)4], the CoII ion, lying on an inversion center, has an octahedral coordination involving two N atoms of two 4-(pyrazin-2-ylsulfanylmethyl)benzoate ligands and four water molecules. In the crystal, O—H⋯O hydrogen bonds between the coordinated water molecules and uncoordinated carboxylate O atoms, and weak π–π interactions [centroid–centroid distance = 4.105 (2) Å] between the benzene and pyrazine rings lead to a three-dimensional supramolecular network.

In the title compound, [Co(C 12 H 9 N 2 O 2 S) 2 (H 2 O) 4 ], the Co II ion, lying on an inversion center, has an octahedral coordination involving two N atoms of two 4-(pyrazin-2-ylsulfanylmethyl)benzoate ligands and four water molecules. In the crystal, O-HÁ Á ÁO hydrogen bonds between the coordinated water molecules and uncoordinated carboxylate O atoms, and weakinteractions [centroid-centroid distance = 4.105 (2) Å ] between the benzene and pyrazine rings lead to a three-dimensional supramolecular network.
This work was supported financially by the College of Chemistry and Chemical Engineering, Pingdingshan University, China.
Tetraaquabis [4-(pyrazin-2-ylsulfanylmethyl-N 4 Crystal engineering based on metal-organic frameworks (MOFs) using asymmetric bridging ligands as building blocks has attracted much attention owing to their potential applications as second-order nonlinear optical (NLO) materials (Han et al., 2003;Zhao, Hong et al., 2002a,b). The combination of hydrogen bonding and π-π interactions has proved to be particularly useful for the assembly of MOFs (Zhao, Zou et al., 2004). Recently we have begun working on the architectures of polymeric structures containing a novel long and flexible monoanionic ligand with hybrid pyrazine and benzoate groups, namely 4-(2-pyrazinylthiomethyl)benzoic acid (Hpztmb). We report herein the synthesis and crystal structure of the title The title compound comprises of one Co II ion, two pztmb ligands and four coordinated water molecules (Fig. 1). The Co II ion lies on an inversion center in an octahedral coordination environment, with four O atoms from four coordinated water molecules in the equatorial positions and two N atoms from two different pztmb ligands in the axial sites (Table 1).
The uncoordinated carboxylate O atoms and the coordinated water molecules form abundant strong hydrogen bonds (Table   2). In addition, two neighboring pztmb ligands are parallel and inversely arranged so that there is a weak π-π interaction [centroid-centroid distance = 4.105 (2) Å] between the benzene ring of one pztmb ligand and the pyrazine ring of the other one. Consequently, the hydrogen bonds and weak π-π interactions lead to a three-dimensional supramolecular network (Fig.   2).

Experimental
The synthesis method of Hpztmb is similar to that of Hpmtmb (Han et al., 2006) except that 2-mercaptopyrazine was used instead of 2-mercaptopyrimidine. A mixture of Co(NO 3 ) 2 .6H 2 O (29 mg, 0.1 mmol) and Hpztmb (50 mg, 0.2 mmol) in 10 ml of H 2 O was sealed in a stainless-steel reactor with a Teflon liner and heated at 383 K for 72 h. A quantity of red single crystals were obtained after the solution was cooled to room temperature at a rate of 10 K h -1 .

Refinement
H atoms were positioned geometrically and refined using a riding model, with C-H = 0.93 (aromatic) and 0.97 (CH 2 ) Å and U iso (H) = 1.2U eq (C). Water H atoms were found in difference Fourier maps and initially included with a tight O-H restraint [0.85 (1) Å]. In the final refinement, the positions of the water H atoms were fixed, with U iso (H) = 1.5U eq (O). Fig. 1