Triclinic modification of diaquabis(5-carboxy-1H-imidazole-4-carboxylato-κ2 N 3,O 4)iron(II)

The title compound, [Fe(C5H3N2O4)2(H2O)2], is a triclinic modification of a monoclinic form recently reported by Du et al. [Acta Cryst. (2011) ▶, E67, m997]. The FeII ion lies at an inversion center and is coordinated by two N and two O atoms from two 5-carboxy-1H-imidazole-4-carboxylate ligands in trans positions, together with two water molecules, completing a slightly distorted octahedral coordination. Intermolecular N—H⋯O hydrogen bonding between the N—H group of the imidazole ring and the deprotonated carboxylate group builds a chain of 5-carboxy-1H-imidazole-4-carboxylate anions along the [101] direction. The water molecules form intermolecular hydrogen bonds to O—C and O=C sites of the carboxylate group in adjacent layers.

The title compound, [Fe(C 5 H 3 N 2 O 4 ) 2 (H 2 O) 2 ], is a triclinic modification of a monoclinic form recently reported by Du et al. [Acta Cryst. (2011), E67, m997]. The Fe II ion lies at an inversion center and is coordinated by two N and two O atoms from two 5-carboxy-1H-imidazole-4-carboxylate ligands in trans positions, together with two water molecules, completing a slightly distorted octahedral coordination. Intermolecular N-HÁ Á ÁO hydrogen bonding between the N-H group of the imidazole ring and the deprotonated carboxylate group builds a chain of 5-carboxy-1H-imidazole-4-carboxylate anions along the [101] direction. The water molecules form intermolecular hydrogen bonds to O-C and O C sites of the carboxylate group in adjacent layers.

Related literature
For the structural diversity of the coordination architecture of the metal complexes of 4,5-imidazoledicarboxylic acid, see Shimizu et al. (2004); Fang & Zhang (2006). For the isotypic Co analog, see: Li et al. (2011). For the coexisting phase, see Yakubovich et al. (1995). For the monoclinic form, see: .
complexes (Li et al., 2011). Iron(II) ion lies at the inversion center that is coordinated by two 1H-imidazole-4,5-dicarboxylate monoanionic ligands at the trans positions and two water molecules in a distorted octahedral geometry. A previous report described a monoclinic metal complex with a similar chemical composition . This structural diversity is attributed to the different types of coordination architectures of hydrogen bonding between molecules. The intermolecular hydrogen bonding between the N-H site of an imidazole ring and C═O site of a deprotonated carboxylate in the title compound builds a unique chain of 5-carboxy-1H-imidazole-4-carboxylate anion. Pale-green plate crystals were also crystallized in the present synthetic condition. This supports the hydrolysis of the 4,5imidazoledicarboxamide during the synthesis.

Refinement
H atoms attached to C and N atoms were placed at calculated positions (C-H = 0.93 Å, N-H = 0.86 Å) and refined as riding atoms with U iso (H) = 1.2U eq (C, N). The carboxy H was located at the idealized position (O-H = 0.82 Å) and refined as a riding atom with U iso (H) = 1.5 U eq (O). On the other hand, H atoms of water molecules were located in a difference map, and their positions were subsequently refined with U iso (H) = 1.5U eq (O).

Figure 1
The molecular structure with displacement ellipsoids drawn at the 50% probability level.

Diaquabis(5-carboxy-1H-imidazole-4-carboxylato-κ 2 N 3 ,O 4 )iron(II)
Crystal data Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. 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 R-factors (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.