catena-Poly[[[diaqua[3-(pyridin-4-yl)benzoato-κ2 O,O′]gadolinium(III)]-bis[μ-3-(pyridin-4-yl)benzoato-κ2 O:O′]] monohydrate]

In the title coordination polymer, {[Gd(C12H8NO2)3(H2O)2]·H2O}n, the GdIII ion is ligated by one bidentate carboxylate group, four monodentate bridging carboxylate O atoms and two water molecules. The resulting GdO8 polyhedron approximates to a square antiprism. The bridging ligands link the metal ions into a [100] chain, with each pair of adjacent metal ions being bridged by two ligands. Inter-chain O—H⋯O and O—H⋯N hydrogen bonds help to establish the packing.


Dong-Ying Li and Guo-Ting Li Comment
Aromatic carboxylic ligands have been widely used to construct metal-organic frameworks (MOFs) with novel structures and unique properties (Li et al., 2010). Especially, lanthanide MOFs of aromatic carboxylic ligands have largely drawn current attention (Zhang, et al., 2010) owing to their potential applications in medical imaging, sensors and electrooptical devices. 3-Pyridin-4-ylbenzoic acid (HL) which possess a pyridyl group and a benzoic acid group is a typical unsymmetrical spacer, and up to now only a serial of its transition metal coordination complexes was synthesized and characterized (Wu, et al., 2011). Herein we report the synthesis and structure of a gadolinium(III) complex of deprotonated 3-pyridin-4-ylbenzoic acid (HL), namely, [Gd(L) 3 (H 2 O) 2 ] n (1).n(H 2 O).
In (1), the Gd atom is in an eight-coordinate environment of O 8 ligated by six carboxylato O atoms from five ligands L and two O atoms from water molecules (Fig. 1). The Gd-O bonds fall in the normal range from 2.290 (2) to 2.532 (2) Å. In (1), deprotonated ligands L act as bidentate ligands, and display two coordinating modes, namely, chelating and bridging coordination models of carboxyl groups without the nitrogen atoms of the pyridyl groups taking part in the coordination. Complex (1) is a two-stranded polymeric chain builted from two L simultaneously bridging adjacent two Gd side by side and two L chelating them up and down with the separations of the adjacent metal Gd ions bridged by the carboxylato O3 and O4 of one L, and by the carboxylato O5 and O6 of the other L being 4.833 and 4.895 Å, respectively ( Fig. 2). Apart from intrachain O-H···O hydrogen bonds, the chains of complex (1) are connected by interchain O-H···N hydrogen bonds between the coordinated water molecules (as donors) and the uncoordinated pyridyl groups of L (as acceptors), leading to the formation of a three-dimensional network structure with uncoordinated water molecules residing in the accessible void.

Experimental
A mixture of Gd(NO 3 ) 3 (0.045 g, 0.1 mmol), HL (0.060 g, 0.3 mmol), NaOH (0.012 0.3 mmol) and deionized water (10 ml) was sealed into a 25 ml Teflon-lined stainless autoclave. The autoclave was heated at 180 °C for four days. As cooled to room temperature gradually, colourless block crystals of (I) were obtained in 37% yield (based on Gd). Selected IR

Refinement
The H atoms of water were located from difference Fourier maps and included in the final refinement by using geometrical restrains, while the other hydrogen atom positions were generated geometrically and these H atoms were allowed to ride on their parent atoms.

Figure 1
Coordination environment of the Gd atom in (1). Displacement ellipsoids are drawn at the 30% probability level.

monohydrate]
Crystal data [Gd(C 12 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.