Research Article: Crystal structure of hydrazine iron(III) phosphate, the first transition metal phosphate containing hydrazine

Date Published: December 01, 2015

Publisher: International Union of Crystallography

Author(s): Renald David.


The structure of the title compound can be described as a three-dimensional network of FePO4N2 octa­hedra resulting from corner-sharing with four PO4 tetra­hedra and bonding with two trans-arranged hydrazine mol­ecules.

Partial Text

During the last century, transition metal phosphates have been studied intensively not only for their rich crystal- and magneto-chemistry (Kabbour et al., 2012 ▸), but also for their various potential applications. For example, NH4MIIPO4·H2O phases, where M is a transition metal, are used as pigments for protective paint finishes on metals, as fire retardants in paints and plastics but may also be applied as catalysts, fertilizers and magnetic devices (Erskine et al., 1944 ▸; Bridger et al., 1962 ▸; Barros et al., 2006 ▸; Ramajo et al., 2009 ▸). More recently, it was demonstrated by Goodenough and co-workers that in electrodes the presence of PO4 groups results in higher positive potentials (Padhi et al., 1997 ▸), leading to an intensive research on LiFePO4, one of the most promising materials for the new generation of Li batteries (Ouvrard et al., 2013 ▸).

The structure of the title compound, [Fe(PO4)(N2H4)], is isotypic with the sulfates [Co(SO4)(N2H4)] and [Mn(SO4)(N2H4)] (Jia et al., 2011 ▸). The FeIII atom is bound to four PO4 tetra­hedra and to two N atoms of hydrazine ligands, resulting in a slightly distorted FeO4N2 octa­hedron (Fig. 1 ▸). The crystal structure consists of a three-dimensional network made up of FeIII atoms which are inter­connected through neutral hydrazine (N2H4) ligands and phosphate (PO43−) anions (Fig. 2 ▸). If the phosphate and sulfate structures are isotypic, the presence of phosphate implies an oxidation state of +III for the transition metal compared to +II for the sulfate analogues. The replacement of sulfate for phosphate leads to a change in the coordination sphere of the metal. These differences are mainly associated with the metal–oxygen bond lengths. The average FeIII—O bond length is 1.97 Å for [Fe(PO4)(N2H4)] and the average CoII—O bond length is 2.12 Å for [Co(SO4)(N2H4)], whereas the average M—N bond lengths involving the N atom of the hydrazine ligand are similar, with values of 2.17 and 2.12 Å, respectively. As a consequence, the FeN2O4 octa­hedron is more distorted, appearing like an FeO4 square additionally bound by two trans hydrazine ligands in axial positions.

The three-dimensional framework structure of [Fe(PO4)(N2H4)] is consolidated by N—H⋯O inter­actions between the hydrazine ligands and phosphate O atoms (Fig. 3 ▸). One of the two hydrogen bonds is bifurcated. Considering the N⋯O distances and the values of the N—H⋯angles (Table 1 ▸), this type of hydrogen bonding can be considered as moderately strong.

Iron(II) chloride tetra­hydrate (>99.0%, Sigma–Aldrich), hydrazine monohydrate (99+%) and KH2PO4 (both VWR Inter­national) were used as received without further purification. Iron(II) chloride tetra­hydrate (2 g) was dissolved in water (20 ml) before adding hydrazine monohydrate (2 ml). The obtained solution was stirred for 5 min. Then, KH2PO4 (11.5 g) was added. After 10 min of stirring for homogenization, the obtained solution (15 ml) was incorporated in a 23 ml autoclave. The autoclave was then heated at 433 K for 10 h before being cooled to room temperature at a rate of 10 K h−1. The obtained mixture, consiting of orange crystals of the title phase and yellow crystals of an additional phase, was washed with water. The obtained crystals were very small (around 20 µm) and well isolated from the others. Details of the composition and structure of the yellow crystals will be described in a forthcoming article.

Crystal data, data collection and structure refinements are summarized in Table 2 ▸. All H atoms were located in a difference Fourier map and were refined freely with isotropic displacement parameters.