Poly[(μ5-5-carboxylatotetrahydrofuran-2,3,4-tricarboxylic acid)sodium]

The search for the novel metal-organic frameworks (MOFs) materials using tetrahydrofuran-2,3,4,5-tetracarboxylic acid (THFTCA) as a versatile multi-carboxyl ligand, lead to the synthesis and the structure determination of the title compound, [Na(H3THFTCA)] or [Na(C8H7O9)]n, which was obtained by a solution reaction at room temperature. The ligand is mono-deprotonated, coordinating five sodium ions through one furan oxygen atom and six carboxyl oxygen atoms. The sodium ion exhibits a distorted pentagonal-bipyramidal NaO7 geometry consisting of seven O atoms derived from five surrounding ligands. Two adjacent pentagonal bipyramids share an O—O edge, forming a dinuclear sodium cluster. Finally, these clusters are effectively linked by the carboxyl groups of THFTCA ligands, forming a firm metal organic framework and O—H⋯O hydrogen bonds contribute to the crystal packing.

The search for the novel metal-organic frameworks (MOFs) materials using tetrahydrofuran-2,3,4,5-tetracarboxylic acid (THFTCA) as a versatile multi-carboxyl ligand, lead to the synthesis and the structure determination of the title compound, [Na(H 3 THFTCA)] or [Na(C 8 H 7 O 9 )] n , which was obtained by a solution reaction at room temperature. The ligand is mono-deprotonated, coordinating five sodium ions through one furan oxygen atom and six carboxyl oxygen atoms. The sodium ion exhibits a distorted pentagonalbipyramidal NaO 7 geometry consisting of seven O atoms derived from five surrounding ligands. Two adjacent pentagonal bipyramids share an O-O edge, forming a dinuclear sodium cluster. Finally, these clusters are effectively linked by the carboxyl groups of THFTCA ligands, forming a firm metal organic framework and O-HÁ Á ÁO hydrogen bonds contribute to the crystal packing.

Comment
Metal organic frameworks (MOFs) have attracted a great deal of interest owing to the ability to tune their porosity and the functionalities that are incorporated within the framework scaffolds. As a result, numerous MOFs have been engineered for a number of potential applications, including gas storage, nonlinear optics, catalysis, and so on. (Moulton et al. 2001;Bradshaw et al. 2007) Usually, highly directional coordination bonds are adopted in the design of MOFs, and self-assembly of selected ligands around d-transition metal ions is now a widespread method for obtaining novel MOFs structure (Leininger et al. 2000). In contrast, the s-elements are more flexible of their coordination behaviour, and maybe present in more various structures (Lu et al. 2007). On the other hand, for the complex ligand with large numbers of potential binding sites, such as, tetrahydrofuran-2, 3, 4, 5-tetracarboxylic acid, it is difficult to predict the final structure. Therefore, the investigation of these complex ligands might provide novel MOFs with interesting structural topology. However, reports on THFTCA are rare (Hanson et al. 2004). Here, we report a three-dimensional MOFs compound Na(H 3 THFTCA) (I), which is assembled from THFTCA and sodium ion.
The title compound has a three-dimensional framework structure constructed by mono deprotonated THFTCA ligand; the asymmetric unit contains one full chiral THFTCA ligand and one sodium atom (Fig. 1). The THFTCA ligand coordinates the sodium ion with its furan oxygen atom and two adjacent carboxyl oxygen atoms, while its four carboxyl groups also grasp the neighbouring four sodium ions (Scheme 1). Thus, the sodium ion is located in a distorted pentagonal bipyramid NaO 7 , coordinated by seven O atoms from the five ligands. The two pentagonal bipyramids are fused via a common eadge O2-O2, generating a dinuclear sodium cluster with an an inversion centre at the midpoint of eadge O2-O2 (Fig. 2).
The title compound crystallizes in the centrosymmetric space group P21/c implying the presence of a racemate (1:1) in the crystal. The dinuclear sodium clusters are connected by carboxyl groups of THFTCA ligand. The crystal packing includes firm framework of multi-carboxyl ligand and sodium ion connected by hydrogen bonds of O-H···O (Table 1, Fig. 3).

Experimental
All chemicals were obtained from commercial sources and were used as obtained. The title compound was handily synthesized by a solution reaction from H4THFTCA. H 4 THFTCA (154 mg, 0.6 mmol) and NaOH (25 mg, 0.6 mmol) was dissolved in 10 ml of water. To this solution was added a 5 ml aqueous solution of Nd(NO 3 ) 3 . 6H 2 O (89 mg, 0.2 mmol) at room temperature. Amount of colourless crystals were obtained after the filtration was slowly evaporated at room temperature for several days.

Refinement
The structure was solved using direct methods and refined by full-matrix least-squares techniques. All non-hydrogen atoms were assigned anisotropic displacement parameters in the refinement. All hydrogen atoms were added at calculated positions and refined using a riding model. (Sheldrick et al., 2008). Fig. 1. Structure and labeling of the title compound, with displacement ellipsoids drawn at the 30% probability level and H atoms shown as small spheres of arbitrary radii.   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.