Hexa­aqua­iron(II) dipicrate dihydrate

Picric acid (2,4,6-trinitrophenol) was used as a military explosive during the latter part of the nineteenth century and the early part of the twentieth century (Kaye, 1978), and a large number of warheads were abandoned and buried, especially after World War II. It is thought that picric acid reacts with metal ions such as iron, zinc and copper from the corroded warheads, resulting in the formation of metal salts, some of which are known to exhibit higher sensitivities to impact than the free acid (Urbański, 1964; Kaye, 1978). Thus, to establish a safe technology with which to treat abandoned warheads, it is both crucial and urgent to investigate the chemical and physical properties of metal picrates. In particular, information about the three-dimensional molecular arrangements is essential if we are to understand other properties. We describe here the crystal structure of an iron picrate, (I).

Fig. 1 depicts the molecular structures of the hexaaquairon cation and picrate anion. Selected bond lengths and bond angles are summarized in Table 1. The Fe atom lies on a crystallographic inversion center. Six water molecules are coordinated to the central metal atom, and no picrate species ligate to the metal atom. This situation is similar to the crystal structure of hexaaquamagnesium dipicrate trihydrate (Harrowfield et al., 1995) and is unique among the metal picrate crystals in which picrate anions tend to coordinate to metal cations from the main-group, transition, lanthanoid and actinoid metals (Harrowfield, 1996; Harrowfield et al., 1995).? The picrate anion adopts a keto-form structure. The O1—C1 bond distance of 1.256 (2) Å indicates that the O—C bonding is a double bond rather than a single one. The C1—C2 and C1—C6 bond lengths of 1.448 (3) and 1.452 (3) Å, respectively, are considerably larger than the standard average C—C bond lengths of 1.395 Å in a phenyl ring. The averaged value of the corresponding O—C and C—C bond lengths are respectively 1.25 (2) and 1.45 (1) Å for 244 picrate anions in 198 non-disordered crystal structures in the Cambridge Structural Database (CSD; Version 5.23; Allen, 2002). This suggests that the picrate anion is likely to prefer the keto form to the enolate form in the crystal structure, which is the case with most metal picrate crystals, regardless of whether the phenolate group ligates directly to the metal cation. The only obvious exception is tris(µ-hydroxo)-hexaaqua- triberyllium(II) tris(picrate) hexahydrate (Cecconi et al. 1998), in which all of the three independent anions seem to adopt enolate forms (1.320–1.334 and 1.389—1.391 Å for O—C and C—C bonds, respectively). The nitro groups of the anion are twisted slightly out of the molecular plane defined by C1–C6, O1 and N1–N3; the distances from the plane range from 0.215 (2) (O5) to 0.539 (2) (O6) Å.

The unit cell contains four iron cations and eight picrate anions (Fig. 2). The cation complexes and picrate anions form separate stacks extending along the b axis. The water molecules of crystallization are linked via hydrogen bonds (Table 2) to the surrounding O atoms of the nitro groups in the picrate anions and the aqua ligands of the iron complexes. Fig. 3 illustrates the molecular stacking of the picrate anions, viewed along the c axis. The successive anions of the stack are not parallel to one another, the dihedral angle between their phenyl planes being 24.67 (7)°. The anions in the stack are linked via short intermolecular C···C and O···C contacts of 3.083 (4), 3.055 (4), 2.994 (2) and 2.962 (3) Å for C3···C3ⁱ, C5···C5ⁱⁱ, O4···C2ⁱ and O5···C6ⁱⁱ, respectively [symmetry codes: (i) 1/2 − x, 1/2 − y, z; (ii) 1/2 − x, −1/2 − y, z]. These contacts are shorter than those found amongst 16 non-disordered crystal structures of metal picrates in the CSD, all of which have structures contained stacked picrate anions. The picric acid crystal does not posses such a structure (Duesler et al., 1978).

If the crystal structure exhibits a phase transition, it could potentially affect the explosive sensitivity of the metal picrate. However, variable temperature measurements of X-ray diffraction from 326 to 93 K showed a linear decrease of the lattice parameters. The thermal expansion coefficients were 6.85 × 10⁻⁵, 4.78× 10⁻⁵, 1.44× 10⁻⁵ and 1.30× 10⁻⁴ K⁻¹ for a, b, c, and V, respectively. Diffraction photographs at 93 K did not show any new appearance or diminution of the diffraction spots compared with those observed at 299 K. These results imply that the crystal is isomorphous between room temperature and 93 K, and IR spectrometry also did not indicate any phase transition between 294 and 79 K. The O—H stretching mode peaks in the 3300–3600 cm⁻¹ region shifted to lower frequencies with decreasing the temperature, suggesting a monotonous shortening of the hydrogen-bond distances. The rate of the shift for the strongest peak near 3500 cm⁻¹ was −0.035 cm⁻¹K⁻¹. The single-crystal decomposed and lost diffraction spots above 326 K. A differential scanning calorimetry and thermal gravity analysis confirmed that the solvate water molecules begin to be eliminated at approximately that temperature (Matsukawa et al., 2002).