Crystal structure of poly[[di-μ3-acetato-tetraaquabis(μ2-cyclohexane-1,4-dicarboxylato)dilanthanum(III)] dihydrate]

The title compound is a binuclear lanthanum(III) complex having each metal ion in decacoordination with oxygen atoms from 1,4-chdc2− ligands, acetate groups and coordinated water molecules to form a distorted bicapped square antiprismatic geometry. The strong intermolecular O–H⋯O and weak C–H⋯O interactions lead to the construction of a three-dimensional supramolecular architecture.


Chemical context
1,4-Cyclohexanedicarboxyic acid (1,4-chdcH 2 ) is a flexible alicyclic, ditopic ligand having a chair-type backbone structure, which has been used for the construction of many coordination polymers (CPs) with remarkable architectures (Liu et al., 2010). It can exist in three different conformationstwo trans isomers, (a,a) and (e,e), and one cis (e,a) form. From a thermodynamical point of view, the trans (e,e) form is the most stable of the three different conformations as a result of the equatorial-equatorial -COOH groups and the trans (a,a) isomer is the least stable because of 1,3-diaxial hindrance (Yu et al., 2007;Gong et al., 2005;Bi et al., 2003;Du et al., 2005;Chen et al., 2014)ÁTheoretical calculations suggest that the isomers tend to cause conformational inversion within the ligand structure due to the flexibility of the C-C bond rotation and also because of the extremely low free energy change between them (Qiblawi et al., 2013;Lin & Tong, 2011;Liu et al., 2010). Furthermore, the isomeric separation of the organic ligand can be controlled by several factors such as the pH of the solution, the nature of the metal ion, the co-ligand, the reaction solvent and the temperature (Lin & Tong, 2011;Liu et al., 2010).

Structural commentary
The asymmetric unit of the title compound consists of one crystallographically unique La metal ion, a fully deprotonated 1,4-chdc 2À anion, an acetate moiety and three water molecules (two coordinated and one non-coordinated). From the molecular structure (Fig. 1), it is evident that each La III atom has a distorted bicapped square-antiprismatic coordination sphere defined by four oxygen atoms from two distinct 1,4-chdc 2À ligands (O1, O2, O7, O8), four oxygen atoms from three acetate groups (O5, O6, O5 0 , O6 0 ) and two oxygen atoms from coordinated water molecules (O3, O4) to form a [LaO 10 ] coordination polyhedron (Fig. 2). Of the three prevalent conformations of 1,4-chdcH 2 , low temperature usually favours the cis (e,a) and high temperature favours the trans (e,e) conformational compounds (Lin & Tong, 2011;Lu et al., 2008;Bi et al., 2004). Here, the bent structure of the organic linker possesses an L-shaped cis (e,a) conformation within the crystal structure. The corresponding La-O bond lengths are in the range 2.506 (8)-2.792 (7) Å and the O-La-O bond angles vary from 46.51 (19) to 170.7 (2) . The La-O bond distances are comparable with those in several reported structures in which 1,4-cyclohexanedicarboxylic acid exists in various coordination modes and conformations (Rao et al., 2007;Qi et al., 2008).
The bridging 3 -2 : 2 coordination mode (each oxygen atom connects two metal atoms) of the acetate group joins two [LaO 10 ] polyhedra by edge sharing to form a dimeric structure. The dimers are then interlinked by La-O-La bonding and as a consequence of this, infinite zigzag 1D [La 2 O 2 ] chains are formed. Within these chains, LaÁ Á ÁLa non-bonding distances are found to be 4.5835 (9) and 4.4125 (9) Å . Additionally, the bis-bidentate chelating 2 -1 : 1 : 1 : 1 coordination mode of the dicarboxylate group of 1,4-chdc 2À connects two metal atoms and hence converts it into a 2D coordination polymeric structure parallel to the ab plane. A perspective view of the packing along the c axis in a wireframe model (Fig. 3)     ORTEP view of the molecular structure of the title complex with the atom-numbering scheme and ellipsoids drawn at the 50% probability level..

Figure 3
Perspective view of the packing along the c axis. the formation of infinite 2D lanthanide-carboxylate layers. The [La 2 O 2 ] chains are then further interconnected by a dicarboxylate anion from two 1,4-chdc 2À units to form a 24membered macrocyclic ring as shown in Fig. 4. A series of organotin complexes of the cis and trans isomers of 1,4-chdcH 2 show similar 2D networks containing 26-and 36-membered tetratin macrocyclic rings (Ma et al., 2009).

Supramolecular features
From the polyhedral view along the a axis (Fig. 5), it is clear that the two lattice water molecules residing in the voids of the 1,4-chdc 2À units are responsible for the development of hydrophilic channels within the crystal structure. The hydrogen-bonding interactions (Table 1) shown in Fig. 6 play a vital role in increasing the stability and higher dimensionality of the crystal packing. Here, the oxygen atom O9 of the lattice water molecule acts as a donor for hydrogen bonds with oxygen atoms O1 and O2 of the carboxylate group of the 1,4chdc 2À ligand [O9-H9AÁ Á ÁO2 = 2.786 (12) Å and O9-H9BÁ Á ÁO1 iii = 2.846 (11) Å ]. It also acts as the hydrogen-bond acceptor for oxygen atoms O3 and O4 of the coordinated water molecules [O3-H3CÁ Á ÁO9 = 2.858 (12) Å and O4-H4DÁ Á ÁO9 ii 2.812 (11) Å ]. Similarly, oxygen atom O7 of the carboxylate group of 1,4-chdc acts as an acceptor to atoms O3 and O4 of the coordinated water molecules [O3-H3DÁ Á ÁO7 ii = 2.750 (11) Å and O4-H4CÁ Á ÁO7 i = 2.771 (10) Å ]. Apart from this strong intermolecular hydrogen bonding, there are also weak C-HÁ Á ÁO interactions between the carbon atom C10 of the coordinated acetate group and the O1 oxygen atom of a carboxylate group of the organic linker [C10-H10CÁ Á ÁO1 = 3.295 (14) Å ].

Figure 6
Hydrogen-bonding interactions (dashed lines) in the structure of the title compound. For symmetry operations, see Table 1.

Synthesis and crystallization
Single crystals of the title compound were prepared by the geldiffusion technique at ambient temperature using sodium metasilicate nonahydrate (Na 2 S 2 O 3 Á9H 2 O) as the gel medium. The optimum condition for crystal growth was obtained by dissolving 0.75 g of 1,4-H 2 chdc in 25 ml of 1.04 g cm À3 dense gel medium. 5 ml of the above solution was poured into glass tubes and the pH of the solution was set to 7.0 by adding glacial acetic acid drop by drop. On completion of the gel-setting process, 3 ml of 0.5 M concentration of aqueous lanthanum nitrate solution was added as the upper reagent. The whole arrangement was kept undisturbed at room temperature and was covered to protect it from the foreign matter present in the atmosphere. Within seven days, transparent, colourless block-shaped crystals were observed at the gel interface. The diffusion of La 3+ ions and 1,4-chdcH 2 through the fine pores of the gel media lead to the expected chemical reaction as shown below:

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Carbon-bound hydrogen atoms were placed in calculated positions and included in the refinement in the riding-model approximation with C-H distances of 0.96-0.98 Å and with U iso (H) = 1.2U eq (C) for methyl hydrogen atoms and U iso (H) = 1.2U eq (C) for all others. Water hydrogen atoms were located from difference-Fourier maps and refined with an O-H distance restraint of 0.90 (2) Å and an HÁ Á ÁH separation of 1.39 (2) Å . The isotropic displacement parameters of the hydrogen atoms attached to atoms O3, O4 and O9 were made equal by using an EDAP instruction. The crystal studied was refined as a twocomponent twin (BASF = 0.4203).  (Sheldrick, 2015); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 2010); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015) and publCIF (Westrip, 2010)′.