Organically pillared layer framework of [Eu(NH2–BDC)(ox)(H3O)]

An organically pillared EuIII–oxalate–carboxylate framework structure with [Eu(NH2—BDC)(ox)(H3O)] topology is reported. The non-porous three-dimensional structure is constructed from two-dimensional layers of EuIII–carboxylate–oxalate, which are pillared by NH2—BDC2− pillars. The basic structural unit of the layer is an edge-sharing dimer of TPRS-{EuIIIO9}, which is assembled through the ox2− moiety. The intralayer void is partially occupied by TPR-{EuIIIO6} motifs.


Chemical context
Lanthanide coordination polymers (LnCPs) have emerged as authentic multifunctional materials finding potential in various applications, e.g. magnetism, optics, luminescence and in heterogeneous catalysis (Roy et al., 2014). In the crystal engineering of LnCPs, the judicious choice of organic ligands is critical. Among the widely employed dicarboxylates, 1,4-benzenedicarboxylic acid (H 2 BDC) tends to provide three-dimensional frameworks with permanent porosity. To enhance interactions with guest species, additional functional groups can be introduced onto the phenyl ring of BDC 2À , e.g. 2-amino-1,4-benzenediarboxylic acid (NH 2 -H 2 BDC) in NH 2 -MIL-53(Al) enhanced the carbon dioxide capture capacity (Stavitski et al., 2011;Flaig et al., 2017;Wang et al., 2012). The smallest dicarboxylic acid, i.e. oxalic acid (H 2 ox), on the other hand, may not facilitate the porous framework (Zhang et al., 2016;Xiahou et al., 2013) and its presence as a secondary ligand in the fabrication may lead to diversity in the framework structures.
Herein, NH 2 -H 2 BDC and H 2 ox were employed as mixed linkers in the synthesis of a new three-dimensional framework of europium, i.e. [Eu(NH 2 -BDC)(ox)(H 3 O)] (I). The crystal ISSN 2056-9890 structure of I, which exhibits site disorder at both the Eu III ion and the amino group, is reported. Weak intermolecular interactions and the framework topology are also described.

Supramolecular features
The structure of I features a three-dimensional framework, which can be regarded as being built up of two-dimensional layers of Eu III -carboxylate-oxalate connected by the NH 2 -BDC 2À organic pillars (Fig. 3). The basic building motif of the layer is the edge-sharing dimer of TPRS-{Eu III O 9 } (Fig. 4), which is fused together through two O8 atoms from two ox 2À groups and two O1-C1-O2 bridges of two NH 2 -BDC 2À .  Symmetry codes: (i) Àx þ 1; Ày þ 1; Àz; (ii) x þ 1; Ày þ 3 2 ; z þ 1 2 .

Figure 3
The three-dimensional framework structure of I.

Figure 2
Depictions of coordination modes adopted by NH 2 -BDC 2À and ox 2À ; (a) with (b) without Eu1B. groups and an O4 atom from NH 2 -BDC 2À . These layers are further connected by the NH 2 -BDC 2À organic pillars along the a-axis direction providing the non-porous threedimensional framework. The roles of ox 2À and NH 2 -BCD 2À in the framework of I are, therefore, to create the layer framework and to tether the layers, respectively.
The NH 2 -BDC 2À pillar is apparently organized through intramolecular hydrogen-bonding interactions from both strong N-HÁ Á ÁO and weak C-HÁ Á ÁO interactions (Table 1), and through the face-to-face antiparallel displacedinteractions (Banerjee et al., 2019) established between the phenyl rings of two adjacent NH 2 -BDC 2À pillars (Fig. 5). In addition to the intramolecular hydrogen-bonding interactions, two H atoms from the H 3 O + molecule are also involved in providing additional strong O-HÁ Á ÁO intermolecular hydrogen-bonding interactions.

Topology
The topology of the two-dimensional layer of I was analysed using TOPOS software (Blatov, 2004). If only the dominating motif, i.e. the edge-sharing dimer of Eu1A, is taken as a node, which is connected to the other equivalent dimer via the ox 2À linker, the two-dimensional layer of Eu1 can be simplified to a uninodal 4-connected sql/Shubnikov tetragonal plane net with a point symbol {4 4 .6 2 } (Blatov et al., 2014) (Fig. 6). The inclusion of the partially occupied TPR-{Eu III O 6 } motifs results in unknown topology. This is also the case for the three-dimensional framework with or without the TPR-{Eu III O 6 } motifs.

Photoluminescent property
A room temperature photoluminescent spectrum of I was collected (Jasco FB-8500 spectrofluorometer, excitation = 337 nm). It exhibits none of the characteristic f-f emission of Eu III . Even the broad emission characteristic of the ligandcenteredemission was not observed. This may be attributed to a proton-induced fluorescence-quenching mechanism facilitated by the presence of H 3 O + in close proximity to the phenyl ring of NH 2 -BDC 2À (Tobita & Shizuka, 1980;Shizuka & Tobita, 1982). The quenching consequently hinders the sensitization, which is important according to the antenna model (Einkauf et al., 2017). The simplified two-and three-dimensional topologies of I.

Figure 5
Views of (a) the hydrogen-bonding interactions and (b) theinteractions.

Synthesis and crystallization
To synthesize I, 2-aminoterephthalic acid (0.2 mmol, 0.0332 g), oxalic acid (0.2 mmol, 0.0180 g) and 1,4-diazabicyclo[2.2.2]octane (0.4 mmol, 0.0448 g) were dissolved in 8.0 mL of DMF/ H 2 O (1 m:7 mL) to prepare solution A. Separately, solution B was prepared by dissolving Eu 2 O 3 (0.1 mmol, 0.0180 g) in 1.0 mL of concentrated HNO 3 aqueous solution, which was then adjusted to pH 7 using a 10 M NaOH aqueous solution. Solution B was then gradually introduced into solution A, and the mixture was then transferred to a 22 mL Teflon-lined stainless-steel autoclave. The reaction was carried out under an autogenous pressure generated at 393 K for 7 days. Yellow crystals of I were then recovered by filtration. FT-IR of I (KBr; cm À1 ): 3361, 2987, 1617, 1313, 1053, 798.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The Eu III ion was refined as being disordered over two crystallographic sites resulting in refined occupancies of 0.855 (Eu1A) and 0.145 (Eu1B). The disorder of the amino group over three crystallographic sites could be clearly seen in the electron-density map and was refined using the SUMP command providing occupancies of 0.259 (N1), 0.440 (N2) and 0.305 (N3). EADP constraints were necessary to make the anisotropic refinements of the disordered N atoms stable. The three H atoms on the ligated H 3 O + (O9W) were evident in the electron-density map and therefore assigned as such. The SADI restraint was nonetheless applied on the refinements of the three O-H bonds. H atoms could be positioned from the electron-density maps and were refined as riding with U iso (H) = 1.2U eq (C, N) or 1.5U eq (O). Bond restraints o N-H and O-H were applied in the refunements.

Poly[(µ 6 -oxalato)(oxomium)(µ 5 -2-aminobenzene-1,4-dicarboxylato)europium(III)]
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.