Poly[[nonaaquabis(μ-5-hydroxybenzene-1,3-dicarboxylato)(5-hydroxybenzene-1,3-dicarboxylato)dicerium(III)] hexahydrate]

In the title coordination polymer, {[Ce2(C8H4O5)3(H2O)9]·6H2O}n, the asymmetric unit is formed by two CeIII atoms, three 5-hydroxybenzene-1,3-dicarboxylate ligands, nine coordinating water molecules and six water molecules of crystallization. The two CeIII atoms are bridged by 5-hydroxybenzene-1,3-dicarboxylate ligands acting in a bis-bidentate coordination mode, generating infinite chains along [101]. Both independent metal atoms are nine-coordinated, one by four O atoms from the carboxylate groups of two bridging 5-hydroxybenzene-1,3-dicarboxylate ligands and five O atoms from water molecules, generating a tricapped trigonal–prismatic geometry. The coordination around the second CeIII atom is similar, except that one of the water molecules is replaced by an O atom from an additional 5-hydroxybenzene-1,3-dicarboxylate ligand acting in a monodentate coordination mode and forming a capped square-antiprismatic geometry.

In the title coordination polymer, {[Ce 2 (C 8 H 4 O 5 ) 3 (H 2 O) 9 ]Á-6H 2 O} n , the asymmetric unit is formed by two Ce III atoms, three 5-hydroxybenzene-1,3-dicarboxylate ligands, nine coordinating water molecules and six water molecules of crystallization. The two Ce III atoms are bridged by 5-hydroxybenzene-1,3-dicarboxylate ligands acting in a bis-bidentate coordination mode, generating infinite chains along [101]. Both independent metal atoms are nine-coordinated, one by four O atoms from the carboxylate groups of two bridging 5hydroxybenzene-1,3-dicarboxylate ligands and five O atoms from water molecules, generating a tricapped trigonalprismatic geometry. The coordination around the second Ce III atom is similar, except that one of the water molecules is replaced by an O atom from an additional 5-hydroxybenzene-1,3-dicarboxylate ligand acting in a monodentate coordination mode and forming a capped square-antiprismatic geometry.

Introduction
For more than a decade, our group has been involved in the synthesis of benzene-poly-carboxylate lanthanide-based coordination polymers: (Daiguebonne et al., 1998), (Qiu et al., 2007); because of their great interest in gas storage: (Eddaoudi et al. 2002), ; molecular magnetism: (Jeon et al., 2012), (Calvez et al., 2008) or luminescence: (Binnemans, 2009), . In the frame of this work we have recently proved that lanthanide-based coordination polymers can exhibit original luminescence properties when a donor group is present in the vicinity of the lanthanide ion: (Freslon et al., 2014). Therefore we have undertaken the study of lanthanide-based coordination polymers that involves 5-hydroxybenzene-1,3-dicarboxylate as ligand. This ligand has previously led to extended molecular networks in association with organic molecules: (Ermer & Neudörfl, 2001), transition metal ions: ( Lin et al., 2010) or lanthanide ions: (Xu & Li , 2004), (Chen et al., 2012), (Huang et al. , 2008). Previously reported lanthanide-based coordination polymers have been obtained by hydrothermal methods. The structure described here has been obtained on the basis of single crystals that have grown in gel medium.

Synthesis and crystallization
5-Hydroxybenzene-1,3-dicarboxylic acid was purchased from Alfa Aesar and used without further purification. Its disodium salt was prepared by addition of two equivalent of sodium hydroxide to an aqueous suspension of the acid. Then the obtained clear solution was evaporated to dryness. The resulting solid was suspended in a small amount of ethanol.
The mixture was stirred and refluxed for 1 hour. Upon addition of ethoxyethane, precipitation occurred. After filtration and drying the white powder of the di-sodium salt was obtained in 90% yield.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1.
H-atoms from water molecules have not been assigned and were thus not included in the refinement, but they were taken into account for the chemical formula sum, moiety, weight, as well as for the absorption coefficient and the number of electrons in the unit cell.

Results and discussion
The crystal structure of [Ce 2 (C 8 H 4 O 5 ) 3 (H 2 O) 9 ,6H 2 O] ∞ can be described on the basis of chains molecular motifs that spread in the (a+c) direction. Each chain is constituted by an alternation of cerium ions bridged by 5-hydroxybenzene-1,3-dicarboxylate ligands. There are two crystallographically independent cerium (III) ions in the asymmetric unit. Both are nine-coordinated. Ce1 is bound by four oxygen atoms from carboxylate groups and five oxygen atoms from water molecules that form a tricapped trigonal prism. On the other hand, Ce2 is bound by five oxygen atoms from carboxylate groups and four oxygen atoms from water molecules that form a capped square antiprism. There are three crystallographically independent ligands in the asymmetric unit. Two out of the three bridge the metal ions in a bisbidentate manner. A third ligand is only linked to the Ce2 atom in a monodentate fashion. Its second carboxylate clip is not bound and point toward the inter-molecular motifs space ( Figure 1). This is in agreement with the IR spectrum that shows no characteristic peak of any protonated carboxylate group.
The short distances ( in the range 2.7-2.8 Å) between some oxygen atoms allow to assume that neighboring chains are held together by strong intermolecular hydrogen bond interactions forming a double-chains molecular motif ( Figure 2).
Ligands that are bound in a unidentate fashion are pointing between the double-chains molecular motifs. Oxygen atoms from the free carboxylate clip are involved, with coordination and crystallization water molecules, in a complex Hydrogen-bonds network that ensure the stability of the crystal packing.

Figure 1
Extended asymmetric unit of the title compound. Displacement ellipsoids are drawn at a 50% probability level.
where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 1.46 e Å −3 Δρ min = −1.28 e Å −3 Absolute structure: Flack (1983), 4150 Friedel pairs Absolute structure parameter: 0.166 (19) 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 > 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.