Tetraammonium μ-ethylenediaminetetraacetato-1κ3 O,N,O′:2κ3 O′′,N′,O′′′-bis[trioxidotungstate(VI)] tetrahydrate

The title compound, (NH4)4[W2(C10H12N2O8)O6]·4H2O, is a binuclear complex of tungsten with the edta4− ligand bridging two WO3 units, leading to a distorted octahedral coordination environment for the tungsten atoms. The supramolecular crystal structure is built up by the anion, four ammonium cations and four solvent water molecules and is established via hydrogen bonds of the N—H⋯O and O—H⋯O type.

The title compound, (NH 4 ) 4 [W 2 (C 10 H 12 N 2 O 8 )O 6 ]Á4H 2 O, was obtained from a mixture of tungstic acid, ammonia and ethylenediaminetetraacetic acid (H 4 edta) in a 2:4:1 ratio. The anion of the complex contains two WO 3 units and one bridging edta 4À ligand. Each central metal atom is tridentately coordinated by nitrogen and two carboxylate groups of the edta 4À ligand, together with the three oxido ligands, producing a distorted octahedral coordination environment around each tungsten atom. The center of the carbon-carbon bond of the ethylene bridge represents a crystallographic inversion center. The crystal structure consists of a three-dimensional supramolecular framework built up by the dinuclear cations, the ammonium counter-ions and the solvent water molecules via hydrogen bonds of the N-HÁ Á ÁO and O-HÁ Á ÁO type.

Structure description
Research on inorganic-organic framework materials is one of the fastest growing areas in materials chemistry because of their unique hybrid nature, which enables the combination of properties from both inorganic and organic materials (Cheetham & Rao, 2007). As organic ligands, polycarboxylates are multidentate chelating agents that are widespread in nature and industry because of their ability to coordinate with various transition metals in different ratios (Nicolau & Guy, 1995;Langer, 2000).
Nevertheless, tungsten has been reported to incorporate into several enzymes (Johnson et al., 1996). In fact, tungsten could be a useful probe for the active site of molybdenum enzymes. As a consequence, more effort has been put into tungsten chemistry by inorganic and bioinorganic chemists (Bagno & Bonchio, 2000;Enemark et al., 2004;Sung & Holm, 2001;Zhou et al., 2004).
In this study, the reaction of H 4 edta with tungstic acid has been investigated and a new binuclear 2:1 W-edta complex, (NH 4 ) 4 [W 2 (C 10 H 12 N 2 O 8 )O 6 ]Á4H 2 O, was isolated and structurally characterized.
As shown in Fig. 1, the dinuclear anion of the title compound shows one edta 4À ligand bonded to two tungstate WO 3 units. Each W atom is six-coordinate in a distorted octahedral environment built up by the tridentate facial coordination of one N and two O atoms of the edta 4À ligand as well as by three oxido ligands. The edta 4À ligand itself therefore acts as a bridge between the two WO 3 units, with the central carbon-carbon bond also representing a crystallographic center of inversion. The anion is accompanied by four ammonium cations and four solvent water molecules.
In the crystal structure, the complex anion, ammonium cations and solvent water molecules interact through medium-strong classical hydrogen bonds (Table 1). Two neighboring complexes are connected via hydrogen bonds of the N-HÁ Á ÁOw-HÁ Á ÁO, N-HÁ Á ÁO and Ow-HÁ Á ÁO types. These interactions lead to the supramolacular structure shown in Fig. 2.

Synthesis and crystallization
Tungstic acid (4 mmol, 0.999 g) and ammonia solution (8 mmol, 1.001 g) were mixed in 30 ml of water to solubilize the W VI source. To this mixture was slowly added ethylenediammine-tetraacetic acid (H 4 edta) (2 mmol, 0.584 g) under vigorous stirring. The solution was then stirred for two h at room temperature. The colorless solution thus obtained was left at room temperature for slow evaporation of water. After two weeks, colorless crystals (yield 11.6% based on W) were obtained from the solution.
The FT-infrared spectra of the title compound shows wellresolved absorption bands for the carboxylate of the coordinating edta 4À at 1651 cm À1 and 1402 cm À1 , which are attributed to the antisymmetric and symmetric stretching vibrations (COO-). The bands at 926, 857 and 666 cm À1 can be attributed to symmetric and asymmetric W Ot stretching vibrations (Lin et al., 2006;Li et al., 2007). The range of 3500-2800 cm À1 shows many bands ascribed to O-H stretching of water molecules, as well as N-H stretching vibrations of ammonium cations (Yaffa et al., 2020).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2.

Figure 2
Supramolecular arrangement of the title compound established by classical hydrogen-bonding interactions (dashed lines).  Special details 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. Refinement. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms bonded to carbon and oxygen were placed in idealized positions and refined using a riding model with isotropic displacement parameters calculated as U iso (H) = 1.2U eq (C) for ethylene and methylene hydrogen atoms and U iso (H) = 1.5U eq (O) for solvent water molecules. Hydrogen atoms of the ammonium cations were located in the difference-Fourier map and refined isotropically.