Poly[[aqua(μ2-oxalato)(μ2-2-oxidopyridinium-3-carboxylato)holmium(III)] monohydrate]

In the title complex, {[Ho(C2O4)(C6H4NO3)(H2O)]·(H2O)}n, the HoIII ion is coordinated by three O atoms from two 2-oxidopyridinium-3-carboxylate ligands, four O atoms from two oxalate ligands and one water molecule in a distorted bicapped trigonal-prismatic geometry. The 2-oxidopyridinium-3-carboxylate and oxalate ligands link the HoIII ions into a layer in (100). These layers are further connected by intermolecular O—H⋯O hydrogen bonds involving the coordinated water molecules to assemble a three-dimensional supramolecular network. The uncoordinated water molecule is involved in N—H⋯O and O—H⋯O hydrogen bonds within the layer.

In the title complex, {[Ho(C 2 O 4 )(C 6 H 4 NO 3 )(H 2 O)]Á(H 2 O)} n , the Ho III ion is coordinated by three O atoms from two 2oxidopyridinium-3-carboxylate ligands, four O atoms from two oxalate ligands and one water molecule in a distorted bicapped trigonal-prismatic geometry. The 2-oxidopyridinium-3-carboxylate and oxalate ligands link the Ho III ions into a layer in (100). These layers are further connected by intermolecular O-HÁ Á ÁO hydrogen bonds involving the coordinated water molecules to assemble a three-dimensional supramolecular network. The uncoordinated water molecule is involved in N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonds within the layer.

S1. Comment
Molecular self-assembly of supramolecular architectures has received much attention during recent decades (Deng et al., 2008;Zeng et al., 2007). The structures and properties of such systems depend on the coordination and geometric preferences of both the central metal ions and the bridging building blocks, as well as the influence of weaker noncovalent interactions, such as hydrogen bonds and π-π stacking interactions. As a building block, 2-oxynicotinic acid and oxalic acid are good ligands with multifunctional coordination sites providing a high likelihood for the generation of structures with high dimensions (Huang et al., 2009). Recently, we obtained the title coordination polymer, which was synthesized under hydrothermal conditions.
In the title compound ( Fig. 1), the Ho III centre is eight-coordinated by seven O atoms from two 2-oxynicotinate ligands and two oxalate ligands, and by one water molecule in a distorted bicapped trigonal prismatic geometry, with Ho-O distances and O-Ho-O angles ranging from 2.279 (5) to 2.406 (4) Å and 67.30 (17) to 148.26 (17)°, respectively (Table 1). The carboxylate groups of the 2-oxynicotinates and oxalates act as bridging ligands, linking the Ho centres into a layer parallel to the (100) plane (Fig. 2). The layers are further connected by intermolecular O-H···O hydrogen bonds involving the coordinated water molecules into a three-dimensional supramolecular network ( Table 2). The uncoordinated water molecule is involved in N-H···O and O-H···O hydrogen bonds within the layer (Table 2).

S3. Refinement
Water H atoms were tentatively located in difference Fourier maps and refined with distance restraints of O-H = 0.84 (1) and H···H = 1.35 (1) Å, and with a fixed U iso (H) = 0.064 Å 2 . H atoms attached to C and N atoms were placed at calculated positions and treated as riding on their parent atoms, with C-H = 0.93 and N-H= 0.86 Å and with U iso (H) = 1.2U eq (C,N). The highest residual electron density was found 0.94 Å from Ho1 and the deepest hole 0.91 Å from Ho1.  The asymmetric unit of the title compound. Displacement ellipsoids are drawn at the 30% probability level. H atoms have been omitted for clarity. [Symmetry codes: (i) -x, -y, 1 -z; (ii) -x, 1 -y, -z; (iii) -x, 1 -y, 1 -z.]

Figure 2
View of a layered network of the title compound.