Crystal structure of a mixed-ligand terbium(III) coordination polymer containing oxalate and formate ligands, having a three-dimensional fcu topology

The crystal structure of catena-[(μ 3-formato)(μ 4-oxalato)terbium(III)] features a three-dimensional 12-connected fcu topology with point symbol (324.436.56), exhibiting thermal stability up to 623 K and strong green photoluminescence in the solid state at room temperature.


Chemical context
Owing to their high colour purity, high luminescence quantum yields, narrow bandwidths, relatively long lifetimes and large Stokes shifts arising from 4f orbitals, coordination polymers of lanthanide(III) ions and organic linker ligands have received much attention from chemists during the past decade for the development of fluorescent probes and electroluminescent devices (Hasegawa & Nakanishi, 2015). In particular, polymeric Eu III and Tb III compounds with a range of organic linker ligands are the most intense emitters among the lanthanide(III) series, and they have been developed extensively as ion sensing and optical materials (Cui et al., 2014). Lanthanide(III) ions are known to have a high affinity and preference for hard donor atoms. Thus, dicarboxylic acid ligands containing aliphatic, aromatic and N-heterocyclic moieties have been widely employed in the construction of luminescent lanthanide coordination polymers (So et al., 2015). Among the ligands in this class, for instance, terephthalic acid is known to provide an efficient energy transfer to support strong lanthanide(III)-centered luminescent emission via the 'antenna effect' (Samuel et al., 2009). On the other hand, small rigid planar species with versatile coordination oxygen donor sites such as oxalate, carbonate, nitrate, and formate anions are also a very important class of ligands for the preparation of ISSN 2056-9890 lanthanide coordination polymers (Hong et al., 2014;Gupta et al., 2015). These small versatile ligands can bind to metals in different modes, resulting in the formation of multi-dimensional coordination networks with short intermetallic distances, which can aid the energy-transfer process between chromophoric antenna ligands and lanthanide(III) ions (Wang et al., 2012). In addition, the oxalate anion has proved to be an efficient sensitizer for lanthanide(III)-based emission (Cheng et al., 2007). Recently, many multi-dimensional luminescent lanthanide coordination polymers containing antenna and small rigid planar mixed ligands have been reported Wang et al., 2013). However, only a few compounds with mixed small rigid planar ligands alone have been described in the literature (Zhang et al., 2007;Huang et al., 2013;Tang et al., 2014).
Herein, we report the synthesis and structure of a terbium(III) coordination polymer containing formate and oxalate mixed ligands, [Tb(CHO 2 )(C 2 O 4 )] n , (I), having a three-dimensional 12-connected fcu topology with point symbol (3 24 .4 36 .5 6 ). The thermal stability and luminescent properties of compound (I) have also been investigated.

Structural commentary
Single crystal X-ray diffraction analysis revealed that (I) is isotypic in the orthorhombic Pnma space group with the La III , Ce III and Sm III analogues (Romero et al., 1996). The asymmetric unit contains one Tb III ion, one formate anion, and half of an oxalate anion. As shown in Fig. 1, each Tb III ion is ninecoordinated in a distorted tricapped trigonal prismatic manner ( Fig. 1)  Coordination environment of the Tb III ion in (I). Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii. For symmetry codes, see Table 1. Symmetry codes: (i) x À 1 2 ; y; Àz þ 3 2 ; (ii) x À 1 2 ; y; Àz þ 1 2 ; (iii) x; Ày þ 3 2 ; z; (iv)

Figure 2
A view of the two-dimensional terbium-formate network in (I), showing the monolayer structure projected in the ac plane. The dashed lines indicate the intralayer C-HÁ Á ÁO hydrogen bonds (Table 2).
complexes containing oxygen donor ligands (Cheng et al., 2007;Zhu et al., 2007). All of the bond lengths and bond angles in the formate and oxalate anions are also within normal ranges (Rossin et al., 2012;Hong et al., 2014;Gupta et al., 2015). The coordination modes of the formate and oxalate ligands in (I) (Fig. 2) are commonly observed in lanthanide coordination polymers (Zhang et al., 2007;Rossin et al., 2012). As shown in Fig. 2, each formate anion adopts a 3 -bridging coordination mode connecting three Tb III ions, forming a twodimensional (2-D) layer in the ac plane. In the 2-D terbiumformate monolayer, the Tb1Á Á ÁTb1 separations along the formate ligands in syn-anti and anti-anti O1,O2-bridging coordination modes (Rossin et al., 2012) are 6.1567 (3) and 6.6021 (2) Å , respectively. The adjacent 2-D monolayers are stacked in an -ABA-sequence running perpendicular to the b axis with an interlayer spacing of ca 5.3 Å (Fig. 3). The oxalate ligand adopts a 4 -chelating-bridging coordination mode, linking four Tb III ions along the a axis to form a threedimensional (3-D) terbium-oxalate open framework (Fig. 3). The Tb1Á Á ÁTb1 distance via the formate O1-and oxalate O4bridging ligands is 3.8309 (2) Å with the Tb1-O1-Tb1 and Tb1-O4-Tb1 bond angles being 103.00 (9) and 102.79 (6) , respectively. On the other hand, the channels in the 3-D open framework have an approximate rhombic shape with a Tb1Á Á ÁTb1 separation of 6.2670 (2) Å , and are cross-linked parallel to the c axis by bridging formate ligands as shown in Fig. 4. The presence of guest molecules in the lattice as well as the formation of interpenetrated networks of (I) are thus prevented. Furthermore, the topology of the network in (I) was analysed using TOPOS (Blatov et al., 2000). As schematically depicted in Fig. 5, the overall framework can be defined as a 12-connected fcu topology with point symbol (3 24 .4 36 .5 6 ) by linking each adjacent layer of Tb III atoms via formate and oxalate ligands.
The infrared spectrum of (I) was collected from a polycrystalline sample pelletized with KBr, in the range 4000-400 cm À1 . This spectrum indicates the presence of the carboxylate groups of the ligands by appearance of the strong absorption bands at 1630 and 1315 cm À1 for the asymmetric ( asym COO À ) and the symmetric ( sym COO À ) carboxylate vibrations, respectively (Deacon & Phillips, 1980). To examine the thermal stability of (I), thermogravimetric analysis was performed on a polycrystalline sample under a nitrogen atmosphere in the temperature range of 303-1273 K. There is no weight loss before 623 K due to the stability of the fcu-type 3-D frameworks. The decomposition of the framework, however, occurred rapidly at temperatures above 628 K.
The photoluminescence properties of (I) were investigated in the solid state at room temperature. The emission spectrum is shown in Fig. 6 The terbium-formate layered structure viewed along the c axis.

Figure 4
A perspective view along the a axis of the three-dimensional framework.

Figure 5
Schematic representation of the 12-connected fcu topology in (I).
D 4 ! 7 F 5 , which implies the emitted light is green. The emission lifetime of (I) is 1.79 ms.

Supramolecular features
The two-dimensional terbium-formate monolayers are stabilized by weak intra-layer C1-H1Á Á ÁO2 viii hydrogen bonds giving S(7) graph-set motifs (Bernstein et al., 1995), in which each formate anion acts as a donor and acceptor for one hydrogen bond (Table 2, Fig. 2).

Synthesis and crystallization
All reagents were of analytical grade and were used as obtained from commercial sources without further purification. Synthesis of (I): TbCl 3 Á6H 2 O (0.187 g, 0.5 mmol), oxalic acid (0.045 g, 0.5 mmol), Na 2 CO 3 (0.011 g, 0.1 mmol), and a mixture (1:1 v/v) of N,N 0 -dimethylformamide (DMF) and water (6 ml) was sealed in a 23 ml Teflon-lined stainless steel vessel and heated under autogenous pressure at 463 K for two days. After the reactor was cooled to room temperature, colorless block-shaped crystals were filtered off and dried in air. Yield: 0.118 g (63% based on the Tb III source). Analysis