Synthesis and crystal structure of a new coordination polymer based on lanthanum and 1,4-phenylenediacetate ligands

The crystalline structure of a new three-dimensional coordination polymer based on LaIII and 1,4-phenylenediacetate ligands is described.


Chemical context
In recent years, one of the most important fields of research in coordination chemistry and crystal engineering has been the design of metal-organic frameworks (MOFs), because of their intriguing network topologies and possible applications in gas storage (Eddaoudi et al., 2002;Reneike et al., 1999;Luo et al., 2011a,b;Kustaryono et al., 2010), catalysis (Lee et al., 2009), separation (Hamon et al., 2009), luminescence (Cui et al., 2012;Daiguebonne et al., 2008;Binnemans, 2009;) and molecular magnetism Sessoli et al., 2009). Our group has been involved in this field for more than a decade Fan et al., 2014;Luo et al., 2011a,b;Badiane et al., 2017a,b). The search for new ligands that can lead to new structural networks and/or new physical properties is a continuous concern (Qiu et al., 2007;Fan et al., 2015).
For the synthesis of MOFs, usually two complementary molecular precursors, a cation with vacant coordination sites and a bridging anion, are used to form the coordination polymer. This procedure offers the prospect of rationally designing extended solids with interesting properties. Most of the organic ligands used in MOF chemistry are rigid aromatic carboxylates (Luo et al., 2007;Huang et al., 2009). Compared to the rigid ligands, using flexible ligands such as 1,2- (Xin et al., 2011), 1,3- (Wang et al., 2012) or 1,4-phenylenediacetate (Fabelo et al., 2009a,b) to construct coordination polymers seems to be more difficult, and developing synthetic methodologies is still a challenge. However, flexibility of the ligand can promote structural and functional diversity.
Isomers of phenylenediacetic acid are flexible ligands and can therefore adopt different conformations in the crystal structure. 1,4-Phenylendiacetic acid is used as a readily available ligand that can coordinate two or more metal ions in bridging-mode, forming extended molecular networks (Pan et al., 2003;Chen et al., 2010a,b,c). The different coordination modes (Chen et al., 2010a,b,c;Rusinek et al., 2013;Ren et al., 2011;Pan et al., 2003;Singha et al., 2014;Singha et al., 2015) of the ligand with lanthanide ions that have been reported to date are shown in Fig. 1.
In this paper we report the synthesis and the crystal structure of a new coordination polymer with chemical formula [La 2 (p-pda) 3 (H 2 O) 4 Á8H 2 O] 1 .

Figure 1
Bonding modes in lanthanide-containing coordination polymers with 1,4phenylenediacetate ligands (p-pda 2À ) reported in the literature to date.
observed in lanthanide-based coordination polymers involving the p-pda 2À ligand. The monocapped square antiprisms are connected to each other by alternating L1 bridging carboxylate oxygen atoms (O5 and O6) and edge-sharing polyhedra through L2 oxygen atoms (O3), forming molecular chains along the a-axis direction (Fig. 4). These chains are connected to each other through ligands L1 and L2, which play the role of spacers, forming molecular layers that extend parallel to the ab plane (Fig. 4).

Figure 5
Perspective view along the a axis of [La 2 (p-pda) 3 Hydrogen atoms have been omitted for clarity.

Figure 6
Projection view along the a axis of the molecular skeleton of [La 2 (ppda) 3 (H 2 O) 4 Á8H 2 O] 1 in space-filling mode. Hydrogen atoms and crystallization water molecules have been omitted.
These layers are further connected through the twisted ligand L3, leading to a three-dimensional molecular framework (Fig. 5). Ligand L3 acts as a spacer between the different polymeric layers because of its anti-anti conformation. The framework has channels along the a-axis direction in which the water molecules of crystallization are located. They are bound to the molecular skeleton via a hydrogen-bonded network ( Table 1). As can be seen from Fig. 6, the threedimensional crystal structure could potentially present some porosity properties. Indeed, removal of the water molecules of crystallization could create empty channels, as has been reported previously (Kustaryono et al., 2010;Kerbellec et al., 2008). For the coordination polymer in this study, the potential porosity is calculated to be 750 (20) m 2 g À1 for N 2 with a kinetic radius of 1.83 Å . The calculation was performed using a method described elsewhere (Kustaryono et al., 2010;Kerbellec et al., 2008).
Other crystal structures of lanthanide coordination polymers with the p-pda 2À ligand have been reported previously. This series of compounds, first described by Pan et al. (2003) has been widely studied because of potential applications in various fields such as explosives detection (Singha et al., 2014(Singha et al., , 2015, gas sorption (Pan et al., 2003) or catalysis (Ren et al., 2011). These compounds, with general chemical formula [Ln 2 (p-pda) 3 (H 2 O)Á2H 2 O] 1 with Ln = La-Ho have been obtained by hydrothermal synthesis and therefore present a lower hydration rate and a higher density than [La 2 (p-pda) 3 - Their three-dimensional crystal structures can be described on the basis of helicoidal molecular chains linked by p-pda 2À ligands.
The luminescent and porosity properties of these compounds are interesting, which suggests that the physical properties of compounds isostructural to [La 2 (ppda) 3 (H 2 O) 4 Á8H 2 O] 1 and involving other lanthanide ions (lanthanum is a diamagnetic non-luminescent ion) would be worth studying. Unfortunately, despite great synthetic efforts, no such compound has been obtained to date.
The compound reported here was obtained by crystallization in a gel (see next section; Luo et al., 2013), and as such is the first result from our group related to lanthanide-based coordination polymers with 1,4-phenylenediacetate ligands.

Synthesis and crystallization
Lanthanum oxide (La 2 O 3 ) was suspended in a small quantity of water. The suspension was then brought to about 323 K and concentrated hydrochloric acid was added dropwise under magnetic stirring, until a clear solution was obtained. The solution was then evaporated to dryness and the resulting solid was dissolved in absolute ethanol for removal of the residual hydrochloric acid. Crystallization of the salt was then obtained by adding diethyl ether (Et 2 O). The obtained microcrystalline solid was filtered and dried in the open air. The product LaCl 3 Á7H 2 O was obtained in close to 100% yield.
1,4-Phenylenediacetic acid, H 2 (p-pda), was purchased from Sigma-Aldrich and used without further purification. Its disodium salt was prepared by addition of two equivalents of sodium hydroxide to a suspension of the acid in de-ionized water. The obtained clear solution was evaporated to dryness and then refluxed in ethanol for one h. Addition of diethyl ether provoked precipitation of Na 2 (p-pda) in 90% yield. UVvis absorption spectrum of a 4.3 Â 10 À4 mol L À1 aqueous solution of the disodium salt of H 2 (p-pda) was measured with a Perkin-Elmer Lambda 650 spectrometer equipped with a 60 mm integrating sphere. It showed a maximum absorption at 225 nm. This short absorption wavelength, compared to other ligands in the literature (Badiane et al., 2017a,b;Freslon et al., 2016;Fan et al., 2015;Badiane et al., 2018), can be related to the -CH 2 -groups that cut conjugation.
Single crystals of the coordination polymer were obtained by slow diffusion of dilute aqueous solutions of lanthanum chloride (0.25 mmol in 10 mL) and of the sodium salt of paraphenylenediacetate (0.25 mmol in 10 mL) through an agaragar gel in a U-shaped tube. The gel was purchased from Acros Organics and jellified according to established procedures (Henisch, 1988;Daiguebonne et al., 2003). After several weeks, prismatic single crystals were obtained.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Hydrogen atoms bound to the organic ligands were placed at idealized positions (C-H = 0.93-0.97 Å ) and refined as riding with U iso (H) = 1.2U eq (C).  The water hydrogen atoms were localized and constrained. The thermal agitation of the two water molecules of crystallization was constrained. In order to stabilize the refinement several restraints (DANG, DFIX) were used for the hydrogen atoms bound to water oxygens.  (Otwinowski & Minor, 1997); data reduction: DENZO and SCALEPACK (Otwinowski & Minor, 1997); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL (Sheldrick, 2015b); molecular graphics: DIAMOND (Brandenburg, 2001); software used to prepare material for publication: WinGX (Farrugia, 2012).

Poly[[tetraaquatris(µ-1,4-phenylenediacetato)dilanthanum(III)] octahydrate]
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.