Poly[[diaquabis(μ-oxalato-κ4 O 1,O 2:O 1′,O 2′)bis(μ3-5-oxidopyridin-1-ium-3-carboxylato-κ3 O 3:O 3′:O 5)diholmium(III)] dihydrate]

In the title compound, {[Ho2(C6H4NO3)2(C2O4)2(H2O)2]·2H2O}n, the HoIII atom is coordinated by three O atoms from three 5-hydroxynicotinate ligands, four O atoms from two oxalate ligands, each lying on an inversion center, and one water molecule in a distorted square-antiprismatic geometry. The 5-hydroxynicotinate ligand is protonated at the N atom and deprotonated at the hydroxy group. The HoIII atoms are bridged by the carboxylate and phenolate O atoms, forming a three-dimensional framework. N—H⋯O and O—H⋯O hydrogen bonds, as well as π–π interactions between the pyridine rings [centroid–centroid distance = 3.794 (2) Å], are observed.

Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: HY2567).  Metal-organic frameworks (MOFs) remain nowadays one of the most studied topics in synthetic chemistry due to MOFs hold interesting structural features and properties. For example, they can be employed as effective heterogeneous catalysts (Dang et al., 2010;Lazare et al., 2010;Thallapally et al., 2010), selective adsorption of gases (Choi et al., 2008;Shimomura et al., 2010), photoluminescent (Allendorf et al., 2009) and magnetic properties (Ishikawa et al., 2005).
The title compound is isostructural with its Dy(III) and Er(III) analogues (Zhang et al., 2012). As shown in Fig. 1, the asymmetric unit is composed of one Ho III atom, one phenoxonicotinate ligand, two halfs of oxalate ligands, one coordinated and one solvent water molecules. The Ho III atom is coordinated by eight O atoms, exhibiting a distorted square-antiprismatic geometry. One basal square face of the antiprism is defined by two carboxylate O atoms, one oxalate O atom and one aqua O atom; the other base is completed by the other three oxalate O atoms and one phenolate O atom.
Adjacent Ho III atoms are bonded to the carboxylate and phenolate O atoms of the phenoxonicotinate ligand, forming dinuclear subunits, which are further extented at a syn-anti conformation into infinite ladder-like chains. The metal atoms are also bridged by oxalate ligands in a side-by-side manner, forming one-dimensional zigzag chains. Both the ladder-like and zigzag chains are finally linked together through the metal atoms into a three-dimensional framework with onedimensional microchannels (Fig. 2). The three-dimensional framework shows a topology of 3,5-connected {4 2 .6 5 .8 3 }.

Figure 2
The three-dimensional framework of the title compound. H atoms and solvent water molecules are omitted for clarity.  A schematic view of the three-dimensional framework for the title compound.

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.