Bis(μ-hydroxido-κ2 O:O)bis[bis(5-carboxypyridine-2-carboxylato-κ2 N,O 2)iron(III)] dihydrate

The complete binuclear complex in [Fe2(C7H4NO4)4(OH)2]·2H2O, is generated by the application twofold symmetry. The FeIII atom is coordinated by the O atoms of two bridging hydroxyl groups and by two N and two O atoms from two pyridine-2,5-dicarboxylato ligands, forming a distorted octahedral geometry. The Fe⋯Fe separation within the dinuclear complex is 3.0657 (4) Å. In the crystal, O—H⋯O and C—H⋯O hydrogen-bonding interactions connect the molecules into a three-dimensional supramolecular network.

The complete binuclear complex in [Fe 2 (C 7 H 4 NO 4 ) 4 (OH) 2 ]Á-2H 2 O, is generated by the application twofold symmetry. The Fe III atom is coordinated by the O atoms of two bridging hydroxyl groups and by two N and two O atoms from two pyridine-2,5-dicarboxylato ligands, forming a distorted octahedral geometry. The FeÁ Á ÁFe separation within the dinuclear complex is 3.0657 (4) Å . In the crystal, O-HÁ Á ÁO and C-HÁ Á ÁO hydrogen-bonding interactions connect the molecules into a three-dimensional supramolecular network.

Comment
In the past few decades, pyridine-2,5-dicarboxylic acid (H 2 pydc) has attracted considerable attention for its ability to coordinate to different metal centres. It can display different kinds of coordination modes, and the relative position of the coordinative moieties is adequate to form supramolecular structures of varied structural features (Zhang et al., 2006;Liang et al., 2000;Wibowo et al., 2011;Zhang et al., 2005). A number of compounds based on pyridine-2,5-dicarboxylic acid and transition metals have been reported, few of them containing Fe ions (Shi et al., 2011;Xu et al., 2004;Gao et al., 2005). Herein, the synthesis and crystal structure of a novel binuclear iron(III) derivative is reported.
As shown in Fig. 1, the metal is coordinated by two O atoms (O1, O5) and two N atoms (N3, N5) from two Hpydcligands and two µ 2 -OH groups (O9, O9A) to form a slightly distorted octahedral geometry. Two iron metals related by a two-fold axis are bridged by the OH groups to form a binuclear complex molecule. The mean Fe-O and Fe-N distances are 1.971 (9) Å and 2.114 (2) Å, respectively. In the crystal, the title compound features two kinds of hydrogen interactions (O-H···O and C-H···O; Table 1), which connect the binuclear units into a three-dimensional supramolecular network (Fig. 2).

Refinement
Water and hydroxy H atoms were located in a difference Fourier map and refined as riding, with O-H = 0.85-0.88 Å and U iso (H) = 1.5 U eq (O). All other H atoms were positioned geometrically and refined as riding with C-H = 0.93 Å and U iso (H) = 1.2 U eq (C).

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq