Co3(PO4)2·4H2O

Single crystals of Co3(PO4)2·4H2O, tricobalt(II) bis[orthophosphate(V)] tetrahydrate, were obtained under hydrothermal conditions. The title compound is isotypic with its zinc analogue Zn3(PO4)2·4H2O (mineral name hopeite) and contains two independent Co2+ cations. One Co2+ cation exhibits a slightly distorted tetrahedral coordination, while the second, located on a mirror plane, has a distorted octahedral coordination environment. The tetrahedrally coordinated Co2+ is bonded to four O atoms of four PO4 3− anions, whereas the six-coordinate Co2+ is cis-bonded to two phosphate groups and to four O atoms of four water molecules (two of which are located on mirror planes), forming a framework structure. In addition, hydrogen bonds of the type O—H⋯O are present throughout the crystal structure.

We gratefully acknowledge the Brain Korea 21 programme and the Australian Research Council for support.

Comment
The synthesis and investigation of open-framework transition-metal phosphates has been a growing area of research over recent times. This is not only because of the rich structural chemistry involved, but is also due to many potential applications such as for catalysis, as alternatives for zeolites in separation and storage applications and, in particular, as potential gas storage materials (Kuzicki et al., 2001;Chen et al., 2006;Jiang & Gao, 2007;Cheetham et al., 1999). For example, microporous nickel phosphates incorporating 24-membered rings such as VSB-5 (Versailles/Santa Barbara-5) have been demonstrated to exhibit hydrogen uptake at low temperatures (Forster et al., 2003). Over the past couple of decades, a considerable number of metal phosphates/phosphites with open molecular architectures have also been synthesized incorporating organic units (see, for example: Jiang et al., 2001) and ionic liquids (Cooper et al., 2004) as structure-directing agents, often under hydrothermal or solvothermal conditions. One of the best known families of this type consists of zinc phosphate structures; individual materials of this type can exist as one dimensional (chain and ladder), two dimensional (layer) and three dimensional framework arrangements (Choudhury et al., 2000).
We are currently investigating the synthesis of a variety of similar functional materials through templation effects under hydrothermal conditions. The title compound, Co 3 (PO 4 ) 2 .4H 2 O, (I), and the related compound Co 3 (PO 4 ) 2 .H 2 O (Lee et al., 2008) were synthesized and structurally characterized as a part of these studies.
The structure of (I) is isotypic with the mineral hopeite, Zn 3 (PO 4 ) 2 .4H 2 O (Liebau, 1965) and contains two different Co 2+ centres bridged by orthophosphate anions (Fig. 1). The coordination environment of Co1 is slightly distorted tetrahedral while that of Co2 is close to octahedral (Table 1). Co1 is bonded to the O atoms of four different phosphate ligands, while Co2 is bonded to the O atoms of two orthophosphate ligands in a cis-arrangement. The other coordination sites are occupied by O atoms of the water ligands. A mirror plane passes through Co2 and two of the water molecules (O4 and O5). This coordination geometry leads to the formation of a three-dimensional framework (Fig. 2). A number of hydrogen bonding interactions O-H···O are present and stabilize the structure (Table 2).
A mixture of plate-like and prismatic purple crystals had formed and was filtered off. The crystals were washed with water, dried under vacuum and were manually separated under a microscope. The yields were approximately 0.4 g of the plate-like crystals of the compound Co 3 (PO 4 ) 2 .H 2 O (Lee et al., 2008) and and 0.2 g of the prismatic crystals of compound (I).

Refinement
Water H atoms were located in difference Fourier maps and were refined with U iso (H) values fixed at 1.5U eq of the parent O atoms. O-H bond length restraints of 0.89 (1) Å were also employed. The highest peak and the deepest hole in the final Fourier map are located 0.49 Å from Co1 and 0.33 Å from P2, respectively. Fig. 1. The asymmetric unit of compound (I), drawn with displacement parameters at the 50% probability level. H atoms are given as spheres of arbitrary radius.

Special details
Experimental. The crystal was coated in Exxon Paratone N hydrocarbon oil and mounted on a thin mohair fibre attached to a copper pin. Upon mounting on the diffractometer, the crystal was quenched to 150(K) under a cold nitrogen gas stream supplied by an Oxford Cryosystems Cryostream and data were collected at this temperature.
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.