Poly[di-μ9-citrato-tetrasodiumzinc]

In the title compound, [Na4Zn(C6H5O7)2]n, the ZnII ion lies on an inversion center and is coordinated by six O atoms from two citrate ligands, forming a distorted octahedral geometry. There are two crystallographically independent Na+ cations in the asymmetric unit. One Na+ cation exhibits a distorted square-pyramidal geometry defined by five O atoms from four citrate ligands. The other Na+ cation is surrounded by six O atoms from five citrate ligands in a distorted octahedral geometry. The Na+ cations are bridged by citrate carboxylate groups, forming a layer parallel to (100). The layers are further assembled into a three-dimensional network with the [Zn(citrate)2]4− building units as ‘pillars’; O—H⋯O hydrogen bonds also stabilize the structure.

In the title compound, [Na 4 Zn(C 6 H 5 O 7 ) 2 ] n , the Zn II ion lies on an inversion center and is coordinated by six O atoms from two citrate ligands, forming a distorted octahedral geometry. There are two crystallographically independent Na + cations in the asymmetric unit. One Na + cation exhibits a distorted square-pyramidal geometry defined by five O atoms from four citrate ligands. The other Na + cation is surrounded by six O atoms from five citrate ligands in a distorted octahedral geometry. The Na + cations are bridged by citrate carboxylate groups, forming a layer parallel to (100). The layers are further assembled into a three-dimensional network with the [Zn(citrate) 2 ] 4À building units as 'pillars'; O-HÁ Á ÁO hydrogen bonds also stabilize the structure.

Comment
Citric acid has been widely used for the construction of coordination polymers due to their diverse coordination modes (Liu et al., 2012). Here, we report a new three-dimensional coordination polymer, [Na 4 Zn(C 6 H 5 O 7 ) 2 ] n , based on citric acid.
As shown in Fig. 1, the asymmetric unit of the title compound consists of half a Zn II ion, two Na + cations and a citrate anion. The Zn II ion lies on a crystallographic inversion center and is coordinated by six O atoms from two different citrate ligands, forming a distorted octahedral geometry. Three O atoms of each citrate ligand are bonded to the Zn II ion, one of which is the hydroxy O atom and the other two are from different carboxylate groups. Thus, two citrate ligands and one Zn II ion form a [Zn(C 6 H 5 O 7 ) 2 ] 4building unit. This unit bridges sixteen Na + cations (Fig. 2). Na1 exhibits a distorted square-pyramidal geometry, defined by five O atoms from four different citrate ligands. Na2 is surrounded by six O atoms from five different citrate ligands, building a distorted octahedral geometry. The Na + cations are bridged by carboxylate groups from the citrate ligands into a two-dimensional layer parallel to (100) (Fig. 3). The layers are further assembled into a three-dimensional network through [Zn(C 6 H 5 O 7 ) 2 ] 4building units as 'pillars′ ( Fig. 4).

Experimental
A mixture of citric acid (0.2 mmol), NaOH (0.2 mmol) and zinc nitrate hexahydrate (0.1 mmol) was dissolved in DMAC/H 2 O solvent (5 ml, v/v = 1:4) (DMAC = N,N′-dimethylacetamide) and placed in a capped vial (10 ml), which was heated to 363 K for three days and then cooled to room temperature. The crystals obtained were washed with water and dried in air.

Refinement
C-bound H atoms were placed at calculated positions and refined as riding atoms, with C-H = 0.97 Å and with U iso (H) = 1.2U eq (C). The hydroxy H atom was located in a difference map and refined isotropically.     A view of the three-dimensional network in the title compound.

Poly[di-µ 9 -citrato-tetrasodiumzinc]
Crystal data 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.