Monoclinic polymorph of trans-tetraaquabis[(4-pyridylsulfanyl)acetato-κN]cobalt(II)

The crystal structure of the title compound, [Co(C7H6NO2S)2(H2O)4], is a polymorph of the structure first reported by Du, Zhao & Wang [(2004). Dalton Trans, pp. 2065–2072]. The asymmetric unit of the title compound contains one half-molecule; the CoII atom lies on an inversion centre in a distorted octahedral geometry coordinated by two N atoms of the pyridine rings of the 4-pyridylthioacetate anions and four O atoms of water molecules. In the crystal structure, intermolecular O—H⋯O hydrogen bonds link the molecules, forming a three-dimensional network.

The crystal structure of the title compound, [Co(C 7 H 6 N-O 2 S) 2 (H 2 O) 4 ], is a polymorph of the structure first reported by Du, Zhao & Wang [(2004). Dalton Trans, pp. 2065-2072. The asymmetric unit of the title compound contains one halfmolecule; the Co II atom lies on an inversion centre in a distorted octahedral geometry coordinated by two N atoms of the pyridine rings of the 4-pyridylthioacetate anions and four O atoms of water molecules. In the crystal structure, intermolecular O-HÁ Á ÁO hydrogen bonds link the molecules, forming a three-dimensional network.

Comment
Several transition metal coordination polymers that contain bridging 4-pyridylthioacetate ligands have been reported recently (Chiang et al., 1993;Du et al., 2004;Du & Li, 2006;Kondo et al., 2002). However, if the 4-pyridylthioacetate anions are coordinated only as terminal ligands there is a possibility that they may also be able to participate in a hydrogen-bonding network. As part of our efforts to investigate metal(II) complexes based on pyridyl-carboxylic acids, we report herein the crystal structure of the title compound, (I).
In the molecular structure of (I) (Fig. 1) the Co II atom lies on an inversion centre and adopts a distorted octahedral coordination geometry with the two N atoms of the pyridine rings of the 4-pyridylthioacetate anions and the four O atoms of the water molecules, where the two symmetry related 4-pyridylthioacetate ligands are in trans positions.
The bond lengths and angles may be compared with the corresponding values in the triclinic polymorph Du et al., 2004]. In (II), the Co II atom displays similar distorted octahedral coordination geometry, but the angle between the plane through the pyridine rings and that through the four water O atoms of 87.9° is closer to a right angle than the angle of 77.8° in (I). Correspondingly, the distance between the two planes of pyridine rings in In the crystal structure, intermolecular O-H···O hydrogen bonds (Table 1) link the molecules to form a three-dimensional network. The molecules of (I) lying in layers parallel to the ac plane are linked by O1W-H2W···O1 ii ; O2W-H3W···O2 ii and O2W-H4W···O2 iii [Symmetry codes: (ii) -x + 2, -y + 1, -z + 1; (iii) x -1, y, z] hydrogen bonds (Fig. 2). The hydrogen bonds between two coordinated water molecules O2W and two carboxylate groups through only one carboxylate O atom (O2) of the carboxylate group create R 4 2 (8) rings (Bernstein et al., 1995). On the other hand, both O atoms of the two carboxylate groups and two coordinated water molecules create R 4 4 (12) rings (Bernstein et al., 1995) in the triclinic polymorph (II).

Experimental
Well shaped red crystals of (I) suitable for X-ray analysis were prepared in an H-tube. An aqueous solution of the sodium salt of 4-pyridylthioacetic acid, was placed in the first part of the H-tube, and an aqueous solution of Co(II) sulfate in the second part. Crystals formed after two weeks, whereafter they were separated and dried at room temperature (yield 70%).

Refinement
All H atoms of C-H (aromatic and methylene) were placed in calculated positions (0.93 and 0.97 Å, respectively); isotropic displaced parameters were fixed [U iso (H) = 1.2 U iso (C) of C atoms to which they were attached] using a riding model. The

trans-tetraaquabis](4-pyridylsulfanyl)acetato-κN]cobalt(II)
Crystal data [Co(C 7   Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.