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

The title compound, poly[(μ 3-formato)(μ 4-oxalato)terbium(III)], [Tb(CHO2)(C2O4)] n , is a three-dimensional coordination polymer, and is isotypic with the La(III), Ce(III) and Sm(III) analogues. The asymmetric unit contains one Tb(III) ion, one formate anion (CHO2 (-)) and half of an oxalate anion (C2O4 (2-)), the latter being completed by application of inversion symmetry. The Tb(III) ion is nine-coordinated in a distorted tricapped trigonal-prismatic manner by two chelating carboxyl-ate groups from two C2O4 (2-) ligands, two carboxyl-ate oxygen atoms from another two C2O4 (2-) ligands and three oxygen atoms from three CHO2 (-) ligands, with the Tb-O bond lengths and the O-Tb-O bond angles ranging from 2.4165 (19) to 2.478 (3) Å and 64.53 (6) to 144.49 (4)°, respectively. The CHO2 (-) and C2O4 (2-) anions adopt μ 3-bridging and μ 4-chelating-bridging coordination modes, respectively, linking adjacent Tb(III) ions into a three-dimensional 12-connected fcu topology with point symbol (3(24).4(36).5(6)). The title compound exhibits thermal stability up to 623 K, and also displays 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 EuIII and TbIII 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 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 (Xu et al., 2013 ▸; 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(CHO2)(C2O4)], (I), having a three-dimensional 12-connected fcu topology with point symbol (324.436.56). 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 LaIII, CeIII and SmIII analogues (Romero et al., 1996 ▸). The asymmetric unit contains one TbIII ion, one formate anion, and half of an oxalate anion. As shown in Fig. 1 ▸, each TbIII ion is nine-coordinated in a distorted tricapped trigonal prismatic manner (Fig. 1 ▸) by two chelating carboxyl­ate groups from two oxalate ligands, two carboxyl­ate oxygen atoms from another two oxalate ligands and three oxygen atoms from three formate ligands, with the O—Tb—O bond angles ranging from 64.53 (6) to 144.49 (4)°. The Tb—O bond lengths in (I) are in the range of 2.4165 (19) to 2.478 (3) Å (Table 1 ▸), which is in good agreement with the reported distances for other TbIII 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 ▸).

Figure 1

Coordination environment of the TbIII 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 ▸.

Table 1

Selected bond lengths (Å)

Tb1—O1	2.417 (3)	Tb1—O4iv	2.4370 (18)
Tb1—O1i	2.478 (3)	Tb1—O4v	2.4651 (17)
Tb1—O2ii	2.437 (3)	Tb1—O4vi	2.4370 (17)
Tb1—O3iii	2.4165 (19)	Tb1—O4vii	2.4651 (17)
Tb1—O3	2.4165 (19)

Symmetry codes: (i) ; (ii) ; (iii) ; (iv) ; (v) ; (vi) ; (vii) .

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—H⋯O hydrogen bonds (Table 2 ▸).

As shown in Fig. 2 ▸, each formate anion adopts a μ3-bridging coordination mode connecting three TbIII ions, forming a two-dimensional (2-D) layer in the ac plane. In the 2-D terbium-formate monolayer, the Tb1⋯Tb1 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 μ 4-chelating-bridging coordination mode, linking four TbIII ions along the a axis to form a three-dimensional (3-D) terbium–oxalate open framework (Fig. 3 ▸). The Tb1⋯Tb1 distance via the formate O1- and oxalate O4-bridging 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 Tb1⋯Tb1 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 (324.436.56) by linking each adjacent layer of TbIII atoms via formate and oxalate ligands.

Figure 3

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).

The infrared spectrum of (I) was collected from a polycrystalline sample pelletized with KBr, in the range 4000–400 cm−1. This spectrum indicates the presence of the carboxyl­ate groups of the ligands by appearance of the strong absorption bands at 1630 and 1315 cm−1 for the asymmetric (νasymCOO−) and the symmetric (νsymCOO−) 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 emission spectrum upon excitation at 305 nm exhibits the characteristic f–f transitions of TbIII ions (Bünzli, 2010 ▸). The emission peaks at 487, 543, 585, and 617 nm can be assigned to the 5 D 4 → 7 F (J = 6, 5, 4, 3) transitions, respectively. The most intense transition is 5 D 4 → 7 F 5, which implies the emitted light is green. The emission lifetime of (I) is 1.79 ms.

Figure 6

The solid-state emission spectrum of (I) at room temperature.

Supra­molecular features

The two-dimensional terbium-formate monolayers are stabilized by weak intra-layer C1—H1⋯O2viii 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 ▸).

Table 2

Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C1—H1⋯O2viii	0.93	2.15	3.051 (5)	164

Symmetry code: (viii) .

Database survey

A search of the Cambridge Structural Database (Groom & Allen, 2014 ▸) for lanthanide coordination polymers containing mixed oxalate and formate ligands gave four hits (RIFQIG, RIFRED, RIFRIH; Romero et al., 1996 ▸; RIFQIG01; Tan et al., 2009 ▸), which are isotypic with the title compound (I) as previously mentioned. The structures involving oxalate and acetate analogues have also been reported (AZOCIC; Di et al., 2011 ▸; Gutkowski et al., 2011 ▸; SOPPIX; Zhang et al., 2009 ▸; VORBUA; Koner & Goldberg, 2009 ▸).

Synthesis and crystallization

All reagents were of analytical grade and were used as obtained from commercial sources without further purification. Synthesis of (I): TbCl3·6H2O (0.187 g, 0.5 mmol), oxalic acid (0.045 g, 0.5 mmol), Na2CO3 (0.011 g, 0.1 mmol), and a mixture (1:1 v/v) of N,N′-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 TbIII source). Analysis (%) calculated for C3HO6Tb (291.96): C, 12.34; H, 0.35%. Found: C, 12.40; H, 0.33%. IR (KBr, cm−1): 2823 (w), 2491 (w), 1630 (s), 1440 (w), 1315 (s), 1022 (m), 914 (w), 795 (s), 611 (w), 492 (s), 408 (w).

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 3 ▸. The formate H atom was found in a difference electron-density map and was refined using a riding-model approximation, with C—H = 0.93 Å and with U iso(H) = 1.2U eq(C).

Table 3

Experimental details

Crystal data
Chemical formula	[Tb(CHO2)(C2O4)]
M r	291.96
Crystal system, space group	Orthorhombic, P n m a
Temperature (K)	296
a, b, c (Å)	7.0138 (3), 10.6077 (4), 6.6021 (2)
V (Å3)	491.20 (3)
Z	4
Radiation type	Mo Kα
μ (mm−1)	14.36
Crystal size (mm)	0.2 × 0.12 × 0.08
Data collection
Diffractometer	Bruker D8 QUEST CMOS
Absorption correction	Multi-scan (SADABS; Bruker, 2014 ▸)
T min, T max	0.655, 0.746
No. of measured, independent and observed [I > 2σ(I)] reflections	6517, 638, 594
R int	0.028
(sin θ/λ)max (Å−1)	0.666
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.012, 0.025, 1.10
No. of reflections	638
No. of parameters	52
H-atom treatment	H-atom parameters constrained
Δρmax, Δρmin (e Å−3)	0.75, −0.63

Computer programs: APEX2 and SAINT (Bruker, 2014 ▸), SHELXS2014 (Sheldrick, 2008 ▸), SHELXL2014 (Sheldrick, 2015 ▸), OLEX2 (Dolomanov et al., 2009 ▸), publCIF (Westrip, 2010 ▸) and enCIFer (Allen et al., 2004 ▸).

Crystal structure: contains datablock(s) I. DOI: 10.1107/S205698901502397X/zs2356sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S205698901502397X/zs2356Isup2.hkl

Click here for additional data file.

Supporting information file. DOI: 10.1107/S205698901502397X/zs2356Isup3.cdx

Click here for additional data file.

Supporting information file. DOI: 10.1107/S205698901502397X/zs2356Isup4.docx

CCDC reference: 1436132

Additional supporting information: crystallographic information; 3D view; checkCIF report

[Tb(CHO2)(C2O4)]	Dx = 3.948 Mg m−3
Mr = 291.96	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, Pnma	Cell parameters from 3952 reflections
a = 7.0138 (3) Å	θ = 3.6–28.3°
b = 10.6077 (4) Å	µ = 14.36 mm−1
c = 6.6021 (2) Å	T = 296 K
V = 491.20 (3) Å3	Block, colourless
Z = 4	0.2 × 0.12 × 0.08 mm
F(000) = 528

Bruker D8 QUEST CMOS diffractometer	638 independent reflections
Radiation source: microfocus sealed x-ray tube, Incoatec Iµus	594 reflections with I > 2σ(I)
Graphite Double Bounce Multilayer Mirror monochromator	Rint = 0.028
Detector resolution: 10.5 pixels mm-1	θmax = 28.3°, θmin = 3.6°
ω and φ scans	h = −9→9
Absorption correction: multi-scan (SADABS; Bruker, 2014)	k = −13→14
Tmin = 0.655, Tmax = 0.746	l = −8→8
6517 measured reflections

Refinement on F2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.012	Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.025	H-atom parameters constrained
S = 1.10	w = 1/[σ2(Fo2) + (0.0092P)2 + 0.8666P] where P = (Fo2 + 2Fc2)/3
638 reflections	(Δ/σ)max = 0.001
52 parameters	Δρmax = 0.75 e Å−3
0 restraints	Δρmin = −0.63 e Å−3

Experimental. SADABS2014 (Bruker, 2014) was used for absorption correction. wR2(int) was 0.0566 before and 0.0416 after correction. The ratio of minimum to maximum transmission is 0.8789. The λ/2 correction factor is 0.00150.

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

	x	y	z	Uiso*/Ueq
Tb1	0.20226 (2)	0.7500	0.63323 (2)	0.00749 (6)
O1	0.5347 (4)	0.7500	0.5364 (4)	0.0132 (5)
O2	0.5527 (4)	0.7500	0.2000 (4)	0.0237 (7)
O3	0.2384 (3)	0.54490 (18)	0.4786 (3)	0.0186 (4)
O4	0.0873 (3)	0.37671 (16)	0.3522 (3)	0.0120 (4)
C1	0.6227 (6)	0.7500	0.3693 (6)	0.0197 (8)
H1	0.7551	0.7500	0.3761	0.024*
C2	0.0956 (4)	0.4788 (2)	0.4518 (4)	0.0124 (5)

	U11	U22	U33	U12	U13	U23
Tb1	0.00799 (8)	0.00750 (8)	0.00698 (9)	0.000	−0.00049 (7)	0.000
O1	0.0110 (13)	0.0209 (14)	0.0076 (13)	0.000	0.0013 (10)	0.000
O2	0.0235 (16)	0.0381 (18)	0.0096 (14)	0.000	−0.0010 (12)	0.000
O3	0.0133 (9)	0.0143 (9)	0.0282 (11)	−0.0028 (7)	0.0010 (8)	−0.0093 (9)
O4	0.0136 (8)	0.0093 (8)	0.0131 (9)	−0.0011 (7)	0.0027 (7)	−0.0041 (7)
C1	0.0141 (17)	0.029 (2)	0.016 (2)	0.000	−0.0009 (16)	0.000
C2	0.0151 (12)	0.0112 (11)	0.0109 (12)	0.0003 (10)	−0.0002 (10)	−0.0020 (10)

Tb1—O1	2.417 (3)	Tb1—O4vii	2.4651 (17)
Tb1—O1i	2.478 (3)	O1—C1	1.265 (5)
Tb1—O2ii	2.437 (3)	O2—C1	1.221 (5)
Tb1—O3iii	2.4165 (19)	O3—C2	1.235 (3)
Tb1—O3	2.4165 (19)	O4—C2	1.268 (3)
Tb1—O4iv	2.4370 (18)	C1—H1	0.9300
Tb1—O4v	2.4651 (17)	C2—C2iv	1.551 (5)
Tb1—O4vi	2.4370 (17)
Tb1viii—Tb1—Tb1i	132.533 (9)	O4vi—Tb1—Tb1viii	138.32 (4)
O1—Tb1—Tb1viii	39.06 (6)	O4v—Tb1—Tb1viii	38.34 (4)
O1i—Tb1—Tb1i	37.94 (6)	O4vi—Tb1—Tb1i	38.87 (4)
O1—Tb1—Tb1i	171.60 (6)	O4vii—Tb1—Tb1i	108.19 (4)
O1i—Tb1—Tb1viii	94.59 (6)	O4iv—Tb1—Tb1i	38.87 (4)
O1—Tb1—O1i	133.65 (7)	O4iv—Tb1—Tb1viii	138.33 (4)
O1—Tb1—O2ii	100.16 (9)	O4v—Tb1—Tb1i	108.19 (4)
O1—Tb1—O4vii	65.01 (6)	O4vii—Tb1—Tb1viii	38.34 (4)
O1—Tb1—O4vi	144.49 (4)	O4iv—Tb1—O1i	64.53 (6)
O1—Tb1—O4v	65.01 (6)	O4vii—Tb1—O1i	76.57 (6)
O1—Tb1—O4iv	144.49 (4)	O4vi—Tb1—O1i	64.53 (6)
O2ii—Tb1—Tb1viii	139.22 (7)	O4v—Tb1—O1i	76.57 (6)
O2ii—Tb1—Tb1i	88.25 (7)	O4vi—Tb1—O2ii	71.16 (7)
O2ii—Tb1—O1i	126.19 (9)	O4iv—Tb1—O2ii	71.16 (7)
O2ii—Tb1—O4v	141.92 (5)	O4v—Tb1—O4vii	66.08 (8)
O2ii—Tb1—O4vii	141.92 (5)	O4vi—Tb1—O4v	140.95 (3)
O3—Tb1—Tb1viii	94.25 (5)	O4iv—Tb1—O4vi	66.94 (8)
O3iii—Tb1—Tb1i	105.42 (5)	O4iv—Tb1—O4v	100.09 (6)
O3iii—Tb1—Tb1viii	94.25 (5)	O4vi—Tb1—O4vii	100.09 (6)
O3—Tb1—Tb1i	105.42 (5)	O4iv—Tb1—O4vii	140.95 (3)
O3—Tb1—O1	77.72 (5)	Tb1—O1—Tb1viii	103.00 (9)
O3—Tb1—O1i	114.93 (5)	C1—O1—Tb1viii	122.4 (2)
O3iii—Tb1—O1i	114.93 (5)	C1—O1—Tb1	134.6 (2)
O3iii—Tb1—O1	77.72 (5)	C1—O2—Tb1ix	130.8 (3)
O3—Tb1—O2ii	70.35 (5)	C2—O3—Tb1	119.13 (17)
O3iii—Tb1—O2ii	70.35 (5)	Tb1iv—O4—Tb1x	102.79 (6)
O3—Tb1—O3iii	128.40 (10)	C2—O4—Tb1x	137.90 (16)
O3—Tb1—O4vii	132.53 (6)	C2—O4—Tb1iv	119.27 (16)
O3—Tb1—O4vi	126.90 (6)	O1—C1—H1	116.5
O3iii—Tb1—O4iv	126.90 (6)	O2—C1—O1	127.1 (4)
O3iii—Tb1—O4vi	66.88 (6)	O2—C1—H1	116.5
O3iii—Tb1—O4v	132.52 (6)	O3—C2—O4	126.6 (2)
O3iii—Tb1—O4vii	72.19 (6)	O3—C2—C2iv	118.5 (3)
O3—Tb1—O4v	72.19 (6)	O4—C2—C2iv	114.9 (3)
O3—Tb1—O4iv	66.88 (6)
Tb1viii—Tb1—O1—C1	180.0	O2ii—Tb1—O3—C2	−67.5 (2)
Tb1i—Tb1—O3—C2	14.9 (2)	O3iii—Tb1—O1—Tb1viii	112.87 (5)
Tb1viii—Tb1—O3—C2	151.3 (2)	O3—Tb1—O1—Tb1viii	−112.87 (5)
Tb1—O1—C1—O2	0.0	O3iii—Tb1—O1—C1	−67.13 (5)
Tb1viii—O1—C1—O2	180.0	O3—Tb1—O1—C1	67.13 (5)
Tb1ix—O2—C1—O1	180.0	O3iii—Tb1—O3—C2	−109.9 (2)
Tb1—O3—C2—O4	171.1 (2)	O4v—Tb1—O1—Tb1viii	−36.98 (5)
Tb1—O3—C2—C2iv	−9.4 (4)	O4vi—Tb1—O1—Tb1viii	108.29 (11)
Tb1x—O4—C2—O3	−6.7 (5)	O4vii—Tb1—O1—Tb1viii	36.98 (5)
Tb1iv—O4—C2—O3	170.9 (2)	O4iv—Tb1—O1—Tb1viii	−108.29 (11)
Tb1iv—O4—C2—C2iv	−8.7 (4)	O4v—Tb1—O1—C1	143.02 (5)
Tb1x—O4—C2—C2iv	173.74 (18)	O4vi—Tb1—O1—C1	−71.71 (11)
O1i—Tb1—O1—Tb1viii	0.0	O4vii—Tb1—O1—C1	−143.02 (5)
O1i—Tb1—O1—C1	180.0	O4iv—Tb1—O1—C1	71.71 (11)
O1i—Tb1—O3—C2	54.2 (2)	O4iv—Tb1—O3—C2	9.75 (19)
O1—Tb1—O3—C2	−173.1 (2)	O4vi—Tb1—O3—C2	−21.7 (2)
O2ii—Tb1—O1—Tb1viii	180.0	O4v—Tb1—O3—C2	119.5 (2)
O2ii—Tb1—O1—C1	0.0	O4vii—Tb1—O3—C2	148.72 (19)

D—H···A	D—H	H···A	D···A	D—H···A
C1—H1···O2xi	0.93	2.15	3.051 (5)	164