Diammonium aqua(ethylenediaminetetraacetato)iron(II) trihydrate

In the title compound, (NH4)2[Fe(C10H12N2O8)(H2O)]·3H2O, the FeII center is in a distorted pentagonal-bipyramidal geometry. Two carboxylate O and two N atoms from the ethylenediaminetetraacetate (EDTA) ion and one O atom from coordinated water comprise the equatorial plane. Two other carboxylate O atoms from the EDTA ion occupy the apical sites. Both ammonium cations and all water molecules function as hydrogen-bond donors, and ten N—H⋯O and nine O—H⋯O hydrogen bonds form a three-dimensional network between the complex anions, cations and the water molecules.

In the title compound, (NH 4 ) 2 [Fe(C 10 H 12 N 2 O 8 )(H 2 O)]Á3H 2 O, the Fe II center is in a distorted pentagonal-bipyramidal geometry. Two carboxylate O and two N atoms from the ethylenediaminetetraacetate (EDTA) ion and one O atom from coordinated water comprise the equatorial plane. Two other carboxylate O atoms from the EDTA ion occupy the apical sites. Both ammonium cations and all water molecules function as hydrogen-bond donors, and ten N-HÁ Á ÁO and nine O-HÁ Á ÁO hydrogen bonds form a three-dimensional network between the complex anions, cations and the water molecules.

Comment
High concentrated EDTA ferric ammonium salts solution is an important oxidizer and bleacher in the photographic processing of film or paper because it can oxidize metallic silver in the film into silver ions that can be dissolved in the solution (Wang et al. 1999) and (Zheng et al. 2006). In the present study, we try to improve the production method by H4EDTA and ammonia reacted with ferric oxide.
In the title compound the iron atom adopts a distorted pentagonal bipyramidal geometry with a hexadentate EDTA ligand and an aqua ligand. The molecular structure unit ( Fig.1) consists of one iron(II) ion, one ethylenediamine tetraacetic acid ion, two ammonium ions, one coordinated water molecule and three lattice water molecules. Two carboxylate oxygen atoms and two nitrogen atoms from the EDTA ion and one oxygen atom from the coordinated water comprise the equatorial basal plane, and two other carboxylate oxygen atoms from the EDTA ion occupy the apical sites in the title structure (  Table 1).
The title structure is quite similar to those of [Fe(III)(EDTA)(H 2 O)] -, although the molecules have different cations (Na, Ag in a six-coordinate, and K and Tl in a seven coordinate fashion to oxygen atoms of water and C==O groups of EDTA), and the iron(III) atoms exhibit different distorted pentagonal bipyramidal coordination geometries (Solans et al. 1984). Similar coordination spheres were found in seven-coordinate EDTA-aqua-vanadate(III) complexes, and they were described as capped trigonal prisms (Shimoi et al. 1991). An eight-coordinate polymeric Eu(II)EDTA chain structure with guanidine cations and crystal water was discussed by Janicki et al. (2005), and the coordination figure described as a distorted dodecahedron.
In the title compound, there are many ammonium cations and water molecules between the complex anions ( Fig. 2).
Both ammonium cations and all water molecules function as hydrogen bond donors. Ten intermolecular N-H···O and nine O-H···O hydrogen bonds form a three-dimensional network (Table 2).

Experimental
The title compound was prepared by H4EDTA reacted with ferric oxide and ammonia in a water solution at 365 K over 3 hours, and recrystallized from water solution at room temperature yielding colorless crystals suitable for single-crystal X-ray diffraction. In the experimental process, it was found that a large number of precipitates were formed.

Refinement
The positions of water H and ammonium H atoms were located in a difference Fourier map and were allowed to ride with distances between 0.82 Å and 0.92 Å and with U iso (H) = 1.5 U eq (O,N). Methylene H atoms were positioned geometrically and refined as riding atoms, with C-H = 0.97 Å (CH2) and U iso (H) = 1.2Ueq(C).  Fig. 1. The molecular structure of the title compound with atom labelling of non-hydrogen atoms. Displacement ellipsoids are drawn at the 30% probability level.