Lanthanide metal–organic frameworks with 2,2′-bipyridine-polycarboxylic acid: Synthesis, crystal structures and fluorescent properties

Inorganica Chimica Acta 427 (2015) 112–117

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Lanthanide metal–organic frameworks with 2,20 -bipyridinepolycarboxylic acid: Synthesis, crystal structures and fluorescent properties Hui Bai, Zhenting Wu, Ming Hu ⇑ Inner Mongolia Key Laboratory of Chemistry and Physics of Rare Earth Materials, School of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, China

a r t i c l e

i n f o

Article history: Received 25 June 2014 Received in revised form 13 November 2014 Accepted 20 November 2014 Available online 20 December 2014 Keywords: Ln-MOFs Crystal structure Fluorescence

a b s t r a c t Four lanthanide metal–organic frameworks, namely, [{[Ln5(bptc)3(Hbptc)(H2O)12]4H2O}n (Ln = Eu, 1; Gd, 2; Tb, 3) and {[Eu(bptc0 )]}n (4), (H4bptc = 2,20 -bipyridine-3,30 ,6,60 -tetracarboxylic acid, H3bptc0 = 2,20 bipyridine-3,6,60 -tricarboxylic acid) have been hydrothermally synthesized. Compounds 1–4 were structurally characterized by IR spectra, thermogravimetric analyses and single-crystal X-ray diffractions. Complexes 1–3 are the isostructural 2D layered structures, five crystallographic independent Eu(III) ions display the two different kinds of coordination environments. Complex 4 exhibits a 3D framework structure, while H4bptc was decarboxylated to transform into the tricarboxylate group of bptc0 3 in the presence of Cu2+ ions under the hydrothermal conditions. The solid state fluorescent properties of 1, 3 and 4 were investigated at room temperature, respectively. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Metal–organic frameworks (MOFs) have been attracted considerable attentions in recent years because of their potential applications as functional materials in the areas of magnetism, sensors, gas adsorption, ion exchange, catalysis as well as photoluminescence [1–4]. The design and construction of lanthanide metal– organic frameworks (Ln-MOFs) based on judicious selections of the ligands and metal ions have become an extremely attractive research field of coordination chemistry and crystal engineering. As well known, lanthanide ions have the high affinity for hard donor-atoms in ligands, especially for multi-carboxylic acids containing pyridyl groups, which exhibit the higher coordination numbers with metal centers in diverse coordination modes to produce multidimensional intriguing networks, including 2D and 3D structures [5,6]. In addition, it is well known that trivalent lanthanide ions are fascinating luminescence sources for their color purity arising from electronic transitions within the partially filled 4fshell [7]. Therefore, the fluorescent Ln-MOFs have been considered for their unique monochromaticity, characteristic sharp emission peaks [8]. As a result, Ln-MOFs are the excellent candidates for development of fluorescent probes and so on [9].

The multidentate 2,20 -bipyridine-3,30 ,6,60 -tetracarboxylic acid (H4bptc) was chosen as the building block in favor of its versatile coordination modes in COO and N- donors to design the high dimensional structures. H4bptc has ten potential coordination sites in four carboxylate groups and two pyridyl N atoms, which can be rotated along the C–C bonds to meet the different coordination environments of metal ions. Until now, the two dinuclear transition-metal complexes (Fe(II) and Co(II)) and some lanthanide coordination polymers of H4bptc have been reported [10–13], but multidimensional Ln-MOFs of Eu(III), Gd(III) and Tb(III) and their luminescence properties have not been explored. In this paper, we describe the synthesis, characterization and crystal structures of four Ln-MOFs, {[{[Ln5(bptc)3(Hbptc)(H2O)12] 4H2O}n (Ln = Eu, 1; Gd, 2; Tb, 3) and {[Eu(bptc0 )]}n (4). The decarboxylated conversion from H4bptc to H3bptc0 has been achieved under hydrothermal conditions in the presence of Cu2+ ions. The solid-state photoluminescence of some compounds were also investigated.

2. Experimental 2.1. Materials and physical measurements

⇑ Corresponding author. E-mail address: [email protected] (M. Hu). http://dx.doi.org/10.1016/j.ica.2014.11.039 0020-1693/Ó 2014 Elsevier B.V. All rights reserved.

H4bptc was synthesized according to the literature method [14]. All other chemicals were commercially purchased and used without further purification. Elemental analysis (C, H and N) was deter-

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mined on Perkin-Elmer 2400 analyzer. The IR spectra were recorded as KBr pellets on a Nicolet Avatar-360 spectrometer in the 4000–400 cm1 region. Thermogravimetric analysis (TGA) was performed on a Perkin-Elmer TG-7 analyzer heated from 25 to 900 °C under air atmosphere. The luminescent spectra were recorded with a FLS 920 time resolve fluorescence spectrometer. 2.2. Preparation of complexes 1–4 The preparation for {[Eu5(bptc)3(Hbptc)(H2O)12]4H2O}n (1): A mixture of EuCl36H2O (0.1 mmol), H4bptc (0.2 mmol), bpe (0.2 mmol, bpe = 1,2-di(4-pyridyl)ethylene) and H2O (8 mL) was stirred for 30 min at room temperature and sealed in a 23 mL Teflon-lined stainless steel container, which was heated at 180 °C for 3 days and then slowly cooled to room temperature at the rate 5 °C h1. The yellowish crystal of 1 was isolated by filtration and washed with H2O several times, yield 52% (based on Eu). Anal. Calc. for C56H49Eu5N8O48 (Mr: 2361.87): C, 28.38; H, 2.03; N, 4.70; Found: C, 28.45; H, 2.07; N, 4.74%. IR (KBr pellet, cm1): 3406(s), 1627(s), 1593(s), 1419(m), 1366(m), 1231(m), 1087(m), 838(m), 772(m), 562(m). The preparation of {[Gd5(bptc)3(Hbptc)(H2O)12]4H2O}n (2) was similar to that of 1 except that GdCl36H2O (0.1 mmol) was used instead of EuCl36H2O. The pale yellowish crystal was washed with water and air-dried. Yield: 56% (based on Gd). Anal. Calc. for C56H49Gd5N8O48 (Mr: 2388.29): C, 28.04; H, 2.02; N, 4.64. Found: C, 28.14; H, 2.05; N, 4.69%. IR (KBr pellet, cm1): 3409(s) , 1632(s), 1590(s), 1461(m), 1425(s), 1360(m), 1228(m), 1090 (m), 835(m), 780(m). The preparation of {[Tb5(bptc)3(Hbptc)(H2O)12]4H2O}n (3) was similar to that of 1 except that TbCl36H2O (0.1 mmol) was used instead of EuCl36H2O. The yellowish crystal washed with water and air-dried. Yield: 61% (based on Tb). Anal. Calc. for C56H49Tb5N8O48 (Mr: 2396.29): C, 28.01; H, 2.01; N, 4.65. Found: C, 28.04; H, 2.04; N, 4.67%. IR (KBr pellet, cm1): 3402(s) , 1619(s), 1588(s), 1469(m), 1414(s), 1359(m), 1230(m), 1093(m), 840(m), 774(m). The preparation of {[Eu(bptc0 )]}n (4): EuCl36H2O (0.05 mmol), CuCl26H2O (0.05 mmol), H4bzptc (0.05 mmol) and H2O (15 mL) were mixed in a 23 mL Teflon-lined stainless steel container, The mixture was stirred for 30 min at room temperature and sealed, which was heated at 140 °C for 3 days and then slowly cooled to room temperature at the rate 5 °C h1. Finally, the pale brown crystal of 4 was obtained in 67% yield (based on Eu). Anal. Calc. for C13H5EuN2O6 (Mr: 437.16): C, 35.64; H, 1.12; N, 6.35. Found: C, 35.68; H, 1.14; N, 6.40%. IR (KBr pellet, cm1): 3421(s) , 1610(s) , 1394(s) , 1556(s), 1425(m), 1083(m), 750(m). 2.3. X-ray crystallography Single crystal X-ray diffraction data for 1–4 were collected on a Bruker Smart 1000 diffractometer equipped with the graphitemonochromatic Mo Ka radiation (k = 0.71073 Å) at room temperature. Semiempirical absorption corrections were applied using the SADABS program. The structure was solved by direct methods and refined on |F|2 by full-matrix least-squares using SHELXTL97 crystallographic software package [15]. All non-hydrogen atoms were refined anisotropically. The organic hydrogen atoms were geometrically generated, the hydrogen atoms of water molecules were located from difference Fourier maps and refined with the common isotropic thermal parameter. It should be noted that the hydrogen atoms of lattice water molecules in the complexes were not located by difference Fourier map and those of the lattice water molecules were disordered. The details of crystal parameters, data collection and refinement for 1–4 are summarized in Table S1. The selected bond lengths and angles of 1–4 are listed in Table S2–S5, Supporting Information.

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3. Results and discussion 3.1. IR Spectrum It is noted that the IR spectra of isostructural 1–3 are much alike. The characteristic bands of carboxylate groups in 1–3 are shown in about 1630 cm1 for asymmetric stretching and about 1360 cm1 for symmetric stretching vibrations of carboxylate groups. The strong peaks of in 4 are confirmed in 1610 cm1 for asymmetric stretching and 1394 cm1 for symmetric stretching vibrations of carboxylate groups, which are associated with different coordination modes [16]. 3.2. Crystal structures of 1–3 Single-crystal X-ray diffraction studies reveal complexes 1–3 are isostructural structures and crystallize in triclinic space group  The structure of complex 1 is described in detail here as the P1. representative example. In compound 1, H4bptc ligands are deprotonated to form trivalent Hbptc3 ions and tetravalent bptc4 ions. There are five crystallographic independent Eu(III) ion, three bptc4 ions, one Hbptc3 ion, twelve coordinated water molecules in the asymmetric unit of 1. Five Eu(III) ions show the two different kinds of coordination environments. The coordination environments of Eu2, Eu4 and Eu5 are same with that of Eu1 ion. It is noteworthy that bptc4 and Hbptc3 ligands link different numbers of Eu(III) ions and consequently the coordination environments of Eu1, Eu3 and Eu4 have been described for clarity of crystal structure of 1, respectively. In the Fig. 1a, Eu1 ion is eight-coordinated with four O atoms (O3, O22, O25 and O30) from three syn-conformation bptc4 ions and two O atoms (O1A and O2) from two coordinated water molecules, two bipyridyl nitrogen atoms (N1 and N2) belonging to one bptc4 ion. The bptc4 ligand adopts two types of coordination modes, three carboxylate groups employ the monodentate mode and one carboxylate group takes on the chelating bidentate mode to unite with three Eu(III) ions in synconformation. Two nitrogen atoms (N1 and N2) are in the chelating bidentate mode (Scheme 1a). As shown in Fig. 1b, it leads to an eight-coordination sphere of Eu1 ion, which resembles the distorted trigonal dodecahedron geometry. The coordination environments of Eu2 is same with that of Eu1 ion, but three carboxylate groups of Hbptc3 ligand employ the monodentate mode to connect with four Eu(III) ions in syn-conformation (Scheme 1b). As shown in Fig. 1c–d, Eu3 ion is nine-coordinated with two oxygen atoms (O19, O20) from a bptc4 ion, the O11 and O27 atoms belonging to another two Hbptc3, and O14, O15, O16, O17, O18 atoms from five coordinated water molecules, which is similar to the distorted tricapped trigonal geometry. As shown in Fig. 1e–f, the coordination environments of Eu4 is same with that of Eu1 ion, which resembles the distorted trigonal dodecahedron geometry, but four carboxylate groups of bptc4 ligand exploit the same monodentate mode to link four Eu(III) ions in syn-conformation (Scheme 1c). The Eu–O bond distances range from 2.306 to 2.672 Å and the Eu–N bond lengths are 2.558 and 2.589 Å in compound 1, respectively; the average distance of Eu–O and Eu–N bonds are 2.402 Å in compound 1, respectively. Both Eu–O and Eu–N bond lengths are well-matched to those observed in similar complexes [12]. The Eu(III) ions in compound 1 are linked by bptc4 ions and Hbptc3 ions to form a 2-D layer structure (Fig. 1g). 3.3. Crystal structure of 4 Complex 4 crystallizes in the orthorhombic space group P212121. The Cu2+ ions don0 t coordinate with ligands and probably

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a

b

c

e

d

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Fig. 1. (a) The coordination environment of Eu1 in compound 1; (b) The eight-coordination sphere of Eu1 in compound 1; symmetry transformations used to generate equivalent atoms: #1 x + 1, y, z; #2 x, y  1, z  1; #3 x + 2, y, z + 1; #4 x + 1, y + 1, z + 1; #5 x, y + 1, z + 1. (c) The coordination environment of Eu3 in compound 1. (d) The nine-coordination sphere of Eu3 in compound 1; symmetry transformations used to generate equivalent atoms: #1 x, y + 1, z + 1; #2 x + 1, y + 1, z + 2; #3 x + 1, y, z + 1. (e) The coordination environment of Eu4 in compound 1. (f) The eight-coordination sphere of Eu4 in compound 1. (g) The 2-D layer structure of 1; symmetry transformations used to generate equivalent atoms: #1 x, y, z; #2 x, y + 1, z + 1; #3 x + 1, y, z + 1.

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further with other four blocks through bptc0 3 ligands and extend to a 2D network structure (Fig. 2b). Further, the adjacent layers are interlinked by the bptc0 3 ions to form the 3D framework. (Fig. 2c). 4. Thermogravimetric analysis

g Fig. 1 (continued)

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5. Luminescence properties

O O

Thermogravimetric analyses of 1–4 were performed in the air atmosphere. The compounds 1–3 exhibit similar thermal stabilities since they are isostructural structures. The TGA studies reveal that the first-step weight losses between 50 and 200 °C are 12.3%, 12.2%, 11.8% for 1–3, respectively, which can be attributed to the release of four guest water molecule and twelve coordinate water molecules (Calcd. 12.2%, 12.1% and 12.0% for 1–3, respectively). The second-step weight losses form 350 to 800 °C, then the compounds are quickly decomposed after 400 °C, and the final residual Ln2O3(1–2) and Tb4O7 (3) are of 33.4%, 36.8% and 35.9%, respectively (Calcd. 33.2%, 33.0% and 33.2% for 1–3, respectively). As show in Fig. 3, the initial decomposition temperature for 1–3 are 350, 400 and 425 °C, respectively. It is noticed that the temperature of dehydration and decomposition rises following the increase in the atomic numbers of 1–3, this is also the results of lanthanide contractions in accordance with the decrease of Ln–O, Ln–N lengths [17,18]. Compound 4 maintains the considerable stability up to the temperature of 350 °C, the decomposition of 4 begins to decompose with rising temperature and the final residue is Eu2O3 of 36.8% (calcd. 39.7%). It indicates that the high coordination number and the coordination environment of lanthanide ions with ligands have profound effects on the framework rigidity and thermal stability [19,6].

Eu

d

Scheme 1. The coordination modes of Hbptc3, bptc4, bptc0 3 in complexes 1–4.

make a carboxylic group of H4bptc decarboxylation to transform into bptc0 3 ion since its strong oxidizing capability in the reaction condition. The temperatures of the reaction play a key role in structural conversion with hydrothermal decarboxylation of the ligand. There are a Eu(III) ion and a bptc0 3 ion in the asymmetric unit of 4. The coordination environment of Eu(III) ion is shown in Fig. 2a. Eu1 center is eight-coordinated, the six oxygen atoms (O1, O2, O3, O4, O5 and O6) from five syn-conformation bptc0 3 ions and two bipyridyl nitrogen atoms N1, N2 belonging to a bptc0 3 ligand, forming a [EuN2O6] unit. The [EuN2O6] unit can be best represented as triangular dodecahedron geometry, two oxygen atoms (O1 and O2) and two nitrogen atoms (N1 and N2) are from a bptc0 3 ligand, O3, O4, O5 and O6 from another four different bptc0 3 ligand. The three carboxylate groups of bptc0 3 ligand adopt the same bridging bidentate modes to link five Eu(III) ions, as shown in Scheme 1d. In the syn-conformation of bptc0 3, the two pyridyl rings are not coplanar with the dihedral angle of 20.32°, three carboxylate group are not in the same plane with the corresponding linked pyridyl ring of Hbptc3 ligand, and the dihedral angles are 6.62°, 24.64° and 86.73° respectively. The Eu–O bond lengths range from 2.276 to 2.394 Å and Eu–N bond lengths around Eu are 2.583 and 2.586 Å, respectively. The Eu–N bond lengths are slightly longer than Eu–O bond lengths. The bptc0 3 ions act in the diverse bridging fashions and connect each [EuN2O6] unit to create a 2D sheet arrangement in the bc plane. As a building block, the asymmetric [EuN2O6] unit links

The solid state luminescence emission spectra of compounds 1, 3 and 4 were measured at room temperature, and the characteristic emission bands for the corresponding Ln(III) ions are shown in Figs. 4–6. As shown in Fig. 4, the emission spectrum of 1 was measured (kex = 394 nm) and peaks occurring at 578, 591, 613 and 695 nm are assigned to 5D0 ? 7F0, 5D0 ? 7F1, 5D0 ? 7F2, 5D0 ? 7F4 transitions. The most intense emission peak is corresponding with the hypersensitive transition 5D0 ? 7F2. The intensity of the 5 D0 ? 7F2 transition is stronger than that of the 5D0 ? 7F1 transition, the intensity ratio [I(5D0 ? 7F2)/I(5D0 ? 7F1) = 1.94] indicates that Eu3+ ions are not in an inversion center. Meanwhile, the 5 D0 ? 7F2 transition is splitting, and it is also demonstrating that the environment of Eu(III) ion possesses the low symmetry [20,21]. This is in agreement with the result of the single crystal X-ray diffraction. Compound 3 emits the green luminescence upon excitation wavelength at 318 nm. The photoluminescence spectra are shown in Fig. 5. The emission spectrum of 3 was measured (kex = 318 nm) and shows four characteristic peaks of Tb3+ ions, which are assigned to the 5D4 ? 7FJ (J = 6,5,4,3) [20,22] transitions, namely, 5 D4 ? 7F6 (488 nm), 5D4 ? 7F5 (543 nm), 5D4 ? 7F4 (585 nm), 5 D4 ? 7F3 (620 nm), respectively. The most intense emission peak corresponds to the hypersensitive transition 5D4 ? 7F5 [23]. Compound 4 shows the characteristic emission bands of Eu(III) ions upon excitation at 348 nm as shown in Fig. 6. The characteristic emission peaks of 4 at 591, 610, and 697 nm are assigned to 5 D0 ? 7FJ transitions (J = 1, 2 and 4, respectively). At the same time, the 5D0 ? 7F2 transition split into two peaks at 610 and 616 nm, and it indicates the low symmetry of Eu(III) ion. Moreover, the 5 D0 ? 7F2 transition is much more intense than the 5D0 ? 7F1 transition. The intensity ratio I(5D0 ? 7F2)/I(5D0 ? 7F1) is equal to 3.91, which indicates that the environment of Eu(III) ions of 4 are more asymmetric than that of 1 [21,24,25].

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a

b

c Fig. 2. (a) The coordination environment of Eu1 in compound 4. (b) The 2-D layer structure of 4. (c) The 3-D net structure of 4; symmetry transformations used to generate equivalent atoms: #1 x, y + 1/2,z + 3/2; #2 x + 1, y, z; #3 x + 1/2, y + 1/2, z + 1; #4 x  1/2, y  1/2,z + 1; #5 x, y  1/2,z + 3/2; #6 x  1, y, z; #7 x  1/2, y + 1/2, z + 1; #8 x + 1/2, y  1/2, z + 1.

Fig. 3. TGA curves of the compound 1–4.

Fig. 4. The emission spectrum of 1.

Moreover, the strong characteristic emission peaks in compounds 1, 3 and 4 indicate that the ligand-to-metal energy transfers are efficient under the experimental conditions [26–29],

and they may be candidates for the fluorescence-emitting materials.

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5000000 Em Ex=318

Intensity(a.u.)

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3000000

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can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ica.2014.11.039. References

2000000

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Wavelength(nm) Fig. 5. The emission spectrum of 3.

Fig. 6. The emission spectrum of 4.

6. Conclusions Four lanthanide metal–organic frameworks based on H4bptc ligand have been synthesized under hydrothermal conditions. The compounds 1–3 display the isostructural 2-D layered structures and 4 exhibits a 3-D framework structure. The decarboxylation of H4bptc in the presence of Cu2+ ions under the hydrothermal conditions was found in compounds 4. The fluorescent properties of compounds 1, 3 and 4 were investigated further, which may have potential applications in luminescent materials. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (No. 21361017), Inner Mongolia Natural Science Foundation of China (No. 2012MS0214), and the Scientific and Technological Key Research Project of Inner Mongolia Colleges & Universities (NJZZ12012). Appendix A. Supplementary material CCDC Nos. 978418, 978419, 978420, 978417 contain the supplementary crystallographic data for 1–4, respectively. These data

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