Poly[[tri-μ3-hydroxido-tris(μ4-pyridine-2,5-dicarboxylato)trineodymium(III)] monohydrate]

In the title compound, {[Nd3(C7H3NO4)3(OH)3]·H2O}n, the NdIII atom is eight-coordinated by the three O atoms of three asymmetrically μ3-bridging hydroxide groups, by four carboxylate O atoms of four different pyridine-2,5-dicarboxylate (2,5-pydc) ligands, and by the N atom of a 2,5-pydc ligand. Six Nd atoms are connected by six hydroxide groups, forming an [Nd6(μ3-OH)6] cluster unit of symmetry -3 and a slightly compressed octahedral geometry. Adjacent [Nd6(μ3-OH)6] clusters are connected by the 2,5-pydc ligands, via O and N atoms, forming chains along the c axis. The remaining O atoms of the 2,5-pydc ligands link these chains into a three-dimensional framework. A disordered water molecule, located on a threefold rotation axis at the opposite side of the [Nd6(μ3-OH)6] cluster and exposed to each of the three Nd atoms, completes the structure.

In the title compound, {[Nd 3 (C 7 H 3 NO 4 ) 3 (OH) 3 ]ÁH 2 O} n , the Nd III atom is eight-coordinated by the three O atoms of three asymmetrically 3 -bridging hydroxide groups, by four carboxylate O atoms of four different pyridine-2,5-dicarboxylate (2,5-pydc) ligands, and by the N atom of a 2,5-pydc ligand. Six Nd atoms are connected by six hydroxide groups, forming an [Nd 6 ( 3 -OH) 6 ] cluster unit of symmetry 3 and a slightly compressed octahedral geometry. Adjacent [Nd 6 ( 3 -OH) 6 ] clusters are connected by the 2,5-pydc ligands, via O and N atoms, forming chains along the c axis. The remaining O atoms of the 2,5-pydc ligands link these chains into a threedimensional framework. A disordered water molecule, located on a threefold rotation axis at the opposite side of the [Nd 6 ( 3 -OH) 6 ] cluster and exposed to each of the three Nd atoms, completes the structure.
Data collection: SMART (Bruker, 2002); cell refinement: SAINT (Bruker, 2002); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL. In recent years, much attention has been paid to the research on the coordination chemistry of 2,5-pyridinedicarboxylic acid (2,5-pydc), including complexes of lanthanide (Qin et al. 2005;Song et al. 2005;Huang et al. 2007;Huang, Jiang et al. 2008). The 2,5-pydc ligand acts as a good O donor as well as a N donor, owing to the two carboxylate groups and the pyridine ring, which may help to increase the dimensionality of the assembled covalent network (Aghabozorg et al. 2008;Xu et al. 2008;Colak et al. 2010). In addition, the construction of multidimensional M-O-M frameworks has been shown to produce materials with effective cooperation and has also lead to improvements in thermal stabilities (Huang et al. 2007;Price et al. 2001;Huang, Song et al. 2008;Zhang et al. 2009) The title compound crystallizes in a trigonal lattice of space group symmetry R3. The neodymium atom is trivalent and is eight-coordinated by three oxygen atoms (O5, O5 iv and O5 v ) of three µ 3 -bridging hydroxyls, four carboxylate oxygen atoms (O1 iii , O2 i , O3 ii and O4) of four different 2,5-pydc ligands, and a nitrogen atom (N1) of a 2,5-pydc ligand as shown in Fig. 1. Each six Nd atoms are connected by six hydroxide groups to form a cluster unit [Nd 6 (µ 3 -OH) 6 ] of symmetry 3 and with the shape of an octahedron slightly compressed along the threefold crystallographic axis (Fig. 2). The Nd-O bond lengths in this cluster vary from 2.482 (3) to 2.502 (3) Å, and the internal Nd···Nd distances are 4.018 (inclinded to the threefold axis) and 4.530 Å (perpendicular to the threefold axis). Such cluster units are linked by 2,5-pydc ligands via their O3, O4, and N1 atoms to form an extended single-chain structure as shown in Fig. 3. Neighbouring single chains are then connected by the O1 and O2 atoms of the 2,5-pydc ligands to form a three-dimensional network (Fig. 4). Each 2,5pydc ligand acts as a µ 4 -bridge to link four Nd atoms, in which the nitrogen N1 and the oxygen O4 of the 2-carboxylate group chelate one Nd, while its other oxygen O3 ligates another Nd atom in monodentate mode. The 5-carboxylate group ligates two Nd atoms in dimonodentate fashion. The crystal structure is completed by a water molecule O1w, which is located on a threefold axis and has a pyramidal environment by three Nd at a distance of 2.984 (3) Å, which is about 0.5 Å longer than that of the coordination partners of Nd. The relatively large anisotropic displacement parameters of O1w indicate, that this molecule is disordered and that it probably deviates somewhat from the average position on the threefold axis.

Experimental
All reagents were commercially available and used without any further purification. A mixture of 2,5-pyridine dicarboxylic acid (0.0167 g, 0.1 mmol), Nd(NO 3 ) 3 .6H 2 O (0.0661 g, 0.2 mmol), 13 drops of 1 mol/L NaOH and distilled water (10 mL) was placed in a 25 mL Teflon-lined stainless steel autoclave, and heated at 453 K for 3 days. Cooling slowly to room temperature, the pink prism crystals of title complex were obtained.

Refinement
All H atoms on C atoms were positioned geometrically (C-H = 0.93Å) and refined as riding, with U iso (H)= 1.2U eq (C).
The H atom of the bridging hydroxy ligand O5 was located in a difference Fourier map and refined independently with U iso (H) = 1.5U eq (O). The hydrogen atoms of the water molecule O1w, which is located on a threefold axis, could not be located. According to an extra refinement, O1w is fully occupied but may deviate slightly from the threefold axis, as indicated by the relatively large displacement parameters of this atom.

Poly[[tri-µ 3 -hydroxido-tris(µ 4 -pyridine-2,5-dicarboxylato)trineodymium(III)] monohydrate]
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 > 2sigma(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.