Crystal structure of poly[diaqua(μ-2-carboxyacetato-κ3 O,O′:O′′)(2-carboxyacetato-κO)di-μ-chlorido-dicobalt(II)]

In the title coordination polymer, [Co(C3H3O4)Cl(H2O)]n, the sixfold coordination environment of the CoII atom consists of two O atoms from a chelating hydrogen malonate anion (HMal−), one O atom originating from a μ2-bridging malonate ligand (HMal−), one O atom from a water molecule and two μ2-bridging Cl− atoms, connecting neighbouring Co2Cl4 motifs into a two-dimensional polymer extending parallel to (001). Interlayer O—H⋯O hydrogen bonds link the layers into a three-dimensional network.

The asymmetric unit of the title polymer, [Co 2 (C 3 H 3 O 4 ) 2 Cl 2 (H 2 O) 2 ] n , comprises one Co II atom, one water molecule, one singly deprotonated malonic acid molecule (HMal À ; systematic name 2-carboxyacetate) and one Cl À anion. The Co II atom is octahedrally coordinated by the O atom of a water molecule, by one terminally bound carboxylate O atom of an HMal À anion and by two O atoms of a chelating HMal À anion, as well as by two Cl À anions. The Cl À anions bridge two Co II atoms, forming a centrosymmetric Co 2 Cl 2 core. Each malonate ligand is involved in the formation of six-membered chelate rings involving one Co II atom of the dinuclear unit and at the same time is coordinating to another Co II atom of a neighbouring dinuclear unit in a bridging mode. The combination of chelating and bridging coordination modes leads to the formation of a twodimensional coordination polymer extending parallel to (001). Within a layer, O-H water Á Á ÁCl and O-H water Á Á ÁO hydrogen bonds are present. Adjacent layers are linked through O-HÁ Á ÁO C hydrogen bonds involving the carboxylic acid OH and carbonyl groups.

Chemical context
Complexes with paramagnetic metal ions and extended structures are interesting due to their potential applications in molecular magnetism (Moroz et al., 2012;Pavlishchuk et al., 2010Pavlishchuk et al., , 2011Yuste et al., 2009). Malonic acid exhibits both chelating and bridging modes of coordination and is an efficient ligand for achieving two-or three-dimensional polymeric structures (Delgado et al., 2004). In the present communication we report on the structure of a two-dimensional coordination polymer, [Co(C 3 H 3 O 4 )Cl(H 2 O)] n , containing both chelating and bridging functions of singly deprotonated malonic acid ligands.

Structural commentary
The structure of the title compound is characterized by the presence of a two-dimensional coordination polymer extending parallel to (001). The monomeric fragment can be described as being composed of a centrosymmetric binuclear Co 2 Cl 4 motif with the Co II atoms having an overall distorted octahedral environment. The two octahedra are fused ISSN 2056-9890 together via two bridging Cl atoms with Co-Cl bond lengths of 2.4312 (12) and 2.4657 (16) Å .
In the octahedron, the Cl À atoms occupy equatorial positions, the other two equatorial positions being defined by the carboxylate O atom of a bridging hydrogenmalonate anion (HMal À ) and one O atom of a chelating HMal À anion, while one water O atom and the other O atom of the chelating HMal À anion are in axial positions (Fig. 1). The corresponding Co-O malonate bond lengths range from 2.051 (3) to 2.165 (3) Å which is similar to other structures containing this ligand in chelating and bridging modes (Delgado et al., 2004). The Co-O water bond has a length of 2.046 (3) Å . The C-O bond lengths in the carboxylic group differ significantly [1.225 (2) and 1.306 (4) Å ] while those in the carboxylate group [1.258 (4) and 1.267 (4) Å ] are more or less the same, which is typical for this functional group (Wö rl et al., 2005a,b).

Supramolecular features
The distribution of the dinuclear units within a coordination layer follows a chess-like pattern whereby each dinuclear coordination node is interconnected with each other through four bridging HMal À ligands (Fig. 2). The binuclear coordination nodes are additionally connected via intralayer O-H water Á Á ÁCl and O-H water Á Á ÁO hydrogen bonds (Table 1 and Table 1 Hydrogen-bond geometry (Å , ).

Database survey
A search of the Cambridge Structural Database (Groom & Allen, 2014) revealed a number of coordination polymeric structures containing cobalt(II) malonate moieties in different coordination modes. While the most typical coordination mode of malonate ligands in polymeric structures appears to be a 3 -bridging mode of the fully deprotonated acid involving all four oxygen atoms (usually two of them forming a chelating ring with one Co II atom) (Delgado et al., 2004;Xue et al., 2003;Lightfoot & Snedden, 1999;Walter-Levy et al., 1973;Zheng & Xie, 2004;Montney et al., 2008;Fu et al., 2006;Djeghri et al., 2006), there are also cases of less-common coordination modes in polymeric structures such as a 2 -bridging mode of the fully deprotonated ligand connecting two metal atoms (

Synthesis and crystallization
The title compound was synthesized by heating together 0.104 g (1 mmol) malonic acid dissolved in 15 ml of propanol and 0.238 g (1 mmol) of CoCl 2 Á6H 2 O dissolved in 5 ml of water. Violet crystals suitable for X-ray analysis were isolated after two weeks by slow evaporation of the solvent from the resulting mixture. Crystals were washed with small amounts of propanol and dried in air yielding 0.071 g (36%) of the title compound.

Special details
Experimental. The O-H H atoms were located from the difference Fourier map but constrained to ride it's parent atom, with U iso = 1.5 U eq (parent atom). Other H atoms were positioned geometrically and were also constrained to ride on their parent atoms, with C-H = 0.97 Å, and U iso = 1.2 U eq (parent atom). Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. 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 R-factors(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.