Poly[di-μ9-citrato-cobalt(II)tetrasodium]

The title compound, [CoNa4(C6H5O7)2]n, was obtained under hydrothermal conditions as a minor product. The Co2+ cation is located on a crystallographic inversion center and is coordinated by six O atoms from two different citrate units, forming a [Co(C6H5O7)2]4− building unit with Co—O bond lengths between 2.0578 (17) and 2.0813 (16) Å. The structure features two crystallographically independent Na+ ions. The first Na+ cation is five-coordinated by O atoms of five carboxylate groups from four different citrate anions. The second Na+ cation is surrounded by six O atoms of five carboxylate groups from five different citrate anions. The carboxylate groups of the citrate are completely deprononated, the hydroxyl group, however, is not. It is coordinated to the Co2+ cation, and through an O—H⋯O hydrogen bond connected to a neighboring [Co(C6H5O7)2]4− building unit. The coordination modes of the carboxylate O atoms vary, with one O atom being coordinated to three different Na+ cations, three are bridging O atoms bound to two Na+ cations and two are connected to a Co2+ cation and a Na+ cation, respectively. Through these interconnections, the basic [Co(C6H5O7)2]4− building units are linked with each other through coordination of their carboxylate groups to the Na+ cations, forming a three-dimensional framework.

The title compound, [CoNa 4 (C 6 H 5 O 7 ) 2 ] n , was obtained under hydrothermal conditions as a minor product. The Co 2+ cation is located on a crystallographic inversion center and is coordinated by six O atoms from two different citrate units, forming a [Co(C 6 H 5 O 7 ) 2 ] 4À building unit with Co-O bond lengths between 2.0578 (17) and 2.0813 (16) Å . The structure features two crystallographically independent Na + ions. The first Na + cation is five-coordinated by O atoms of five carboxylate groups from four different citrate anions. The second Na + cation is surrounded by six O atoms of five carboxylate groups from five different citrate anions. The carboxylate groups of the citrate are completely deprononated, the hydroxyl group, however, is not. It is coordinated to the Co 2+ cation, and through an O-HÁ Á ÁO hydrogen bond connected to a neighboring [Co(C 6 H 5 O 7 ) 2 ] 4À building unit. The coordination modes of the carboxylate O atoms vary, with one O atom being coordinated to three different Na + cations, three are bridging O atoms bound to two Na + cations and two are connected to a Co 2+ cation and a Na + cation, respectively. Through these interconnections, the basic [Co(C 6 H 5 O 7 ) 2 ] 4À building units are linked with each other through coordination of their carboxylate groups to the Na + cations, forming a three-dimensional framework.

Comment
The design and synthesis of coordination polymers have attracted increasing attention in recent years because of their potential applications in drug delivery, shape-selective sorption/separation, and catalysis (Chen et al., 2007, Zeng et al., 2009). Architectures of coordination polymers described vary between one-dimensional and three-dimensional (Qiu et al., 2009, Du et al., 2009. Citric acid and its anions have been widely used as ligands for the construction of coordination polymers, because of their versatility and ability to bind metals with diverse connection modes. In this paper, we present a new three-dimensional coordination polymer [Na 4 Co(C 6 H 5 O 7 ) 2 ] n with citric acid as the ligand.
As shown in Fig. 1, the asymmetric unit of the crystal structure of the title compound consists of half a Co 2+ center, two Na + cations, and a citrate anion. The Co 2+ center, located on a crystallographic inversion center, is coordinated by six O atoms from two different citrate units with the Co-O bond distances in the range of 2.0578 (17) to 2.0812 (16) Å, resulting in a slightly distorted octahedral coordination geometry. For the citrate unit, all the carboxylic acid groups are completely deprononated, however the hydroxyl group is not. Three O atoms of each citrate unit are bonded to the Co 2+ center, one of which is the hydroxy O atom of the citric acid, the other two are from two different carboxylate groups of the citrate ligands. In such a way, two citrate anions and one Co 2+ cation form a [Co(C 6 H 5 O 7 ) 2 ] 4building unit such as described previously by Matzapetakis et al. (2000). The citrate anions also coordinate to the Na + cations. Oxygen atom O1 of the hydroxyl group is connected only to the Co cation. Carboxylate oxygen atom O5 is linked to three different Na cations. The other five O atoms are bridging O atoms bound to two Na cations (O3, O4, and O7 atoms) or a Co cation and a Na cation (O2 and O6 atoms), respectively. The two Na + cations display different coordination modes by the O atoms. The Na1 + cation is five-coordinated by five O atoms of five carboxylic acid groups from four different citrate units. The Na2 + cation, on the other hand, is surrounded by six O atoms of five carboxylic acid groups from five different citrate units. The Na-O bond distances are in the range of 2.286 (2)-2.562 (2) Å, which is in the usual range expected for such a compound (see for example: Pan et al., 2011). In this way, the [Co(C 6 H 5 O 7 ) 2 ] 4building units are linked with each other through coordination of their carboxylate groups to the Na + cations to form a three-dimensional framework (see Fig. 2).

Refinement
All H atoms were positioned geometrically (C-H = 0.97 Å and O-H = 0.85 Å) and allowed to ride on their parent atoms, with U iso (H) = 1.2U eq (parent atom).

Computing details
Data collection: RAPID-AUTO (Rigaku, 1998); cell refinement: RAPID-AUTO (Rigaku, 1998); data reduction: CrystalStructure (Rigaku/MSC, 2002); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).    where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.29 e Å −3 Δρ min = −0.31 e Å −3 Special details 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.  (2)