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Synthesis, characterization and photoluminescence properties of two new europium(III) coordination polymers with 3D open framework
Journal of Molecular Structure 796 (2006) 187–194 www.elsevier.com/locate/molstruc
Synthesis, characterization and photoluminescence properties of two new europium(III) coordination polymers with 3D open framework Guoqi Zhang, Qian Wang, Yan Qian, Guoqiang Yang *, Jin Shi Ma CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China Received 8 December 2005; received in revised form 25 January 2006; accepted 30 January 2006 Available online 29 March 2006
Abstract The 3D open framework coordination polymers, [Eu2(fum)3(H2O)4]n$3nH2O (1, fumZfumaric acid) and [Eu(pdc)(Hpdc)]n (2, pdcZpyridine2,5-dicarboxylic acid) have been synthesized via hydrothermal procedure. Their molecular structures were determined by single-crystal X-ray diffraction analysis. Polymer 1 adopts a 3D microporous structure filled with disordered free water molecules which interact by hydrogen bonds with terminal water bound to the metal centers. IR spectroscopy, TGA and powder X-ray diffraction analysis revealed that the remarkable dehydration–rehydration process of open-framework 1 is completely reversible. Polymer 2 forms a 3D framework composed of 2D grid-like inorganic subnetworks and pdc ligands as linkers. This solid is highly stable up to 405 8C. The solid-state photoluminescence measurements exhibited red light-emitting characteristic of two europium(III) coordination polymers. Moreover, the anhydrous products were found to be more strongly fluorescent and be of longer lifetime. q 2006 Elsevier B.V. All rights reserved. Keywords: Europium(III); Coordination polymer; Hydrothermal synthesis; Open-framework; Photoluminescence
1. Introduction The synthesis and assembly of metal–organic frameworks (MOFs) with open architectures or porous structures from small molecular building blocks have attracted high concern for some years and are still of considerable interests [1,2], owing to the observation of the large number of potential applications in catalysis, gas sorption and desorption, fluorescent sensing, opto-electronic devices, and molecular magnetism [3]. The wide combination of simple organic linkers (polycarboxylates, polyphosphonates, etc.) with inorganic metals involving transition metals and lanthanides may lead to a rich variation and modulation of both multidimensional framework structures and physical properties of the resultant solid materials [4–7]. In this field, most of the work were concentrated on the synthesis of coordination polymers containing transition-metal ions but few lanthanide ions, due to the predictability of the coordination geometry of transition metal ions compared to much flexible coordination geometry of lanthanide ions [8]. * Corresponding author. Tel.:C86 10 82617263; fax: C86 10 82617315. E-mail address: [email protected] (G. Yang).
0022-2860/$ - see front matter q 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2006.01.048
Hybrids incorporating lanthanides and aliphatic, unsaturated organic acids such as acetylenedicarboxylic or fumaric acid, and more rigid pyridinedicarboxylic acid may be promising multi-dimensional solid materials, according to the recent reports [6,9,10]. While a number of transition metal fumarate frameworks have been synthesized under hydrothermal condition, which displayed versatile structures and physical properties [9], lanthanide carboxylate complexes have been relatively less investigated to date [11]. In view of the performance of the versatile topologic structures in a variety of transition metals carboxylate, and the special luminescence properties of other lanthanides carboxylate [8,10–14], we report herein the hydrothermal synthesis, characterization and photoluminescence properties of one novel 3D open-framework europium(III)-fumarate coordination polymer, [Eu2(fum)3(H2O)4]n$(3nH2O) (1) containing 1D microporous channels filled with hydrogen-bonded water molecules, and another interesting 3D framework with 2D layer structure, [Eu(pdc)(Hpdc)]n (2). Two coordination polymers were characterized by TGA analysis, FT-IR spectroscopy, elemental analysis, powder and single-crystal X-ray diffraction. Reversible water absorption feature of openframework 1 was well determined. Solid-state photoluminescence measurement revealed that both the solid materials gave strong red fluorescence, with a characteristic of typical EuIII center emission. Moreover, the anhydrous products exhibited
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stronger fluorescence emission and longer lifetime, compared to the hydrous samples. 2. Experimental section 2.1. Materials and general methods Europium(III) oxide and two dicarboxylic acids were purchased from Aldrich Chemicals and used without further purification. The preparation of the coordination polymers 1 and 2 were performed by combining corresponding dicarboxylic acids and europium(III) oxide under hydrothermal condition in a Teflon-lined vessel (12 mL). The process was repeated three times and found to be fully reproducible. Compounds 1 and 2 are insoluble in water and organic solvents. Characterization of the sample was performed by using X-ray diffraction, TGA analysis, FT-IR, and elemental analysis. Powder X-ray diffraction (PXRD) data were collected ˚ ) on a using monochromated Cu Ka radiation (lZ1.54056 A Rigaku D/max-2500 diffractometer. TGA analysis was obtained on NETZSCH STA 409PC instrument at a temperature range of 30–700 8C (10 deg/min) with flowing N2(g). A BIO-RAD FT-165 IR spectrometer was used to gather IR spectra using KBr pellets. Elemental analysis was done with a Carlo Erba-1106 Instrument. Solid-state luminescence spectra were recorded on a Hitachi F-2500 fluorescence spectrophotometer. Fluorescence lifetimes were detected at 615 nm by a fluorescence lifetime analytical spectrometer (Erdinburgh FLS-920) and the samples were excited at 325 nm. 2.2. Synthetic procedure Synthesis of [Eu2(fum)3(H2O)4]n$3nH2O (1): Fumaric acid (0.174 g, 1.5 mmol) and Eu2O3 (0.176 g, 0.5 mmol) and water (5 mL), were added, respectively, to a Teflon-lined vessel, which was sealed and heated at 140 8C for 12 h. After slowly cooling the reaction vessel to room temperature, colorless block-like crystals of the products were collected by filtration, washed with deionized water and ethanol, and dried in air. Yield: 206 mg, 56% based on EuIII. The purity of bulk materials was confirmed by powder X-ray diffraction. IR (KBr pellet, cmK1): 3363w, 2354s, 1586w, 1435w, 1208s, 973s, 804m. Anal. Calcd (%) for C12H20O19Eu2: C, 18.66; H, 2.61. Found (%): C, 18.29; H, 2.25. Synthesis of [Eu2(fum)3(H2O)3.2]n (1a): Prepared by heating the crystalline samples of 1 in a furnace under vacuum at 110 8C for about 2 h. Anal. Calcd (%) for C12H12.4O15.2Eu2: C, 20.40; H, 1.76. Found (%): C, 20.28; H, 1.82. Synthesis of [Eu2(fum)3]n (1b): Prepared by heating the crystalline samples of 1 in a furnace under vacuum at 260 8C for about 10 h. Anal. Calcd (%) for C12H6O12Eu2: C, 22.31; H, 0.94. Found (%): C, 21.94; H, 1.05. Synthesis of [Eu2(fum)3(H2O)7]n (1c): Prepared by immersing the anhydrous samples 1b into water for 2 h, then dried in air for 3 days. Anal. Calcd (%) for C12H20O19Eu2: C, 18.66; H, 2.61. Found (%): C, 18.45; H, 2.51.
Synthesis of [Eu(pdc)(Hpdc)]n (2): Pyridine-2,5-dicarboxylic acid (0.334 g, 2 mmol) and Eu 2O 3 (0.176 g, 0.5 mmol) and water (5 mL), were added, respectively, to a Teflon-lined vessel, which was sealed and heated at 140 8C for 12 h. After slowly cooling the reaction vessel to room temperature, light yellow block-like crystals of the products were collected by filtration, washed with deionized water and ethanol, and dried in air. Yield: 0.358 g, 74% based on EuIII. The purity of bulk materials was confirmed by powder X-ray diffraction. IR (KBr pellet, cmK1): 3449w, 3128s, 1661m, 1488s, 1405s, 1366s, 1277w, 1160m, 1114s, 967m, 759s, 691s, 568s, 521s. Anal. Calcd (%) for C14H7N2O8Eu: C, 34.17; H, 1.56; N, 5.78. Found (%): C, 34.36; H, 1.62; N, 5.64. 2.3. Single-crystal X-ray diffraction Well-shaped single-crystals of 1 and 2 were selected and mounted in air onto thin glass fibers. Accurate unit cell parameters were determined by a least-square fit of 2q values, measured for 200 strong reflections, and intensity data sets were measured on a Rigaku Raxis Rapid IP diffractometer with ˚ ) at room temperature. The Mo Ka radiation (lZ0.71073 A intensities were corrected for Lorentz and polarization effects, but no corrections for extinction were made. All structures were solved by direct methods. The non-hydrogen atoms were located in successive difference Fourier synthesis. The final refinement was performed by full matrix least-squares methods with anisotropic thermal parameters for non-hydrogen atoms on F2. Crystallographic data and experimental details for structure analyses are summarized in Table 1. Relevant interatomic bond distances and bond angles for 1 and 2 are given in Table 2. CCDC 269990-269991 contain the supplementary crystallographic data for this paper. These data can be obtained free of Table 1 Crystal and structure refinement data for complexes 1 and 2
Empirical formula Formula wt Crystal system Space group T (K) ˚) l (A ˚) a (A ˚) b (A ˚) c (A b (8) ˚ 3) V (A Z F(000) dcalc (g/cm3) m (mmK1) Reflctns collected Unique reflections Variables R1a, wR2b (%) Goodness of fit a b
1
2
C12H20O19Eu2 773.90 monoclinic P21/n 293(2) 0.71073 9.4983(19) 14.764(3) 14.825(3) 91.57(3) 2078.1(7) 4 1424 2.423 6.075 4461 3812 298 2.73, 8.16 1.038
C14H7N2O8Eu 483.18 orthorhombic Pbcn 294(2) 0.71073 9.983(2) 8.685(2) 15.772(4) 90.00 1367.5(6) 4 928 2.347 4.641 7139 1121 114 1.65, 4.33 1.014
RZ ðFO KFC Þ=ðFO Þ. Rw Z ½SwðFo2 KFc2 Þ2 =wðFo2 Þ2 1=2 .
G. Zhang et al. / Journal of Molecular Structure 796 (2006) 187–194 Table 2 ˚ ) for complexes 1 and 2 Selected bond lengths (A 1 Eu(1)–O(1) Eu(1)–O(4) Eu(1)–O(7)#1 Eu(1)–O(11) Eu(1)–O(14) Eu(2)–O(3)#2 Eu(2)–O(9) Eu(2)–O(11) Eu(2)–O(15) 2 Eu(1)–O(1) Eu(1)–O(4)#4
2.483(3) 2.367(4) 2.464(4) 2.460(4) 2.462(3) 2.434(3) 2.456(3) 2.638(3) 2.409(4)
Eu(1)–O(2) Eu(1)–O(6) Eu(1)–O(8)#1 Eu(1)–O(13) Eu(2)–O(2) Eu(2)–O(5) Eu(2)–O(10) Eu(2)–O(12) Eu(2)–O(16)
2.623(4) 2.434(4) 2.541(4) 2.430(4) 2.407(4) 2.346(4) 2.538(4) 2.488(4) 2.443(4)
2.385(2) 2.410(2)
Eu(1)–O(2)#3 Eu(1)–N(1)
2.355(2) 2.591(2)
Symmetry transformations used to generate equivalent atoms for: #1Kx, Ky, K z; #2 xK1/2, KyK1/2, zK1/2; #3 xK1/2, yK1/2, KzK1/2; #4 x, Ky, zK1/2.
charge at www.ccdc.cam.ac.uk/conts/retrieving.html. [or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; Fax: (internat.) C44 1223/336 033; E-mail: [email protected]]. 3. Results and discussion 3.1. Crystal structures of 1 and 2 In the molecular structure of complex 1, EuIII ions have two different coordination environments and both of them are surrounded by nine oxygen atoms as shown in Fig. 1. The EuIII centers are both linked to seven oxygen atoms from five fum ligands, and the remaining two oxygen atoms belong to the coordinated water molecules. The Eu–O bond distances in two EuIII centers are obviously discrepant. (Table 2) The shortest ˚, and longest Eu(1)–O lengths are 2.367(4) and 2.623(4) A ˚ respectively, while they are 2.346(4) and 2.638(3) A for the Eu(2)–O lengths. Therefore, the centers of two EuO9 polyhedra are not symmetric. The EuO9 polyhedra are bridged with three
Fig. 1. The ORTEP plot of EuO9 polyhedron in 1 (30% probability), in which two EuIII centers adopt the same coordination mode but asymmetric coordination environments. Hydrogen atoms were omitted for clarity.
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fum ligands, two in oxygen-sharing mode and one in carboxylate-bridging pattern, the two EuIII centers thus have a ˚ . With exception of the coordinated water separation of 4.618 A molecules, each asymmetric unit contains also three crystal water molecules in the crystal structure of 1. There are three types of bridging coordination modes in 1 each with distinct crystal engineering functions, two of them (1-L1 and 1-L2) consist of mixed bidentate bridging and chelating modes, while the third type (1-L3) is of bis-chelating mode (Fig. 2). The edgesharing EuO9 polyhedra centers are further connected through another carboxylate bridge to form a 1D infinite sinusoid-like inorganic chain (Fig. 3a), and this chain is also different from the fully edge-sharing inorganic chain in the structure of EuIIIcarboxylate frameworks reported recently[10a,12]. Again, this 1D chain can be regarded as the basic building block, all three types of ligands serve to link the building blocks and extend the structure along other two directions (a and b) to yield the resultant 3D open-framework architecture as shown in Fig. 3b. This framework is microporous and is of small 1D void channels. The dimensions of the pore are small with an ˚ 2. The channels are estimated free aperture of w3.77!8.98 A partially occupied by the centrosymmetrically related hydrogen-bonded water molecules including both disordered free water molecules and terminal water groups, bound to the europium atoms. It should be noted that two types of water molecules in the channels in fact behave as efficient hydrogen bonding moieties[15]. The water molecules strongly interact with each other through the O–H/O hydrogen bonds (the O/ ˚ ), contributing to O contacts are all in the range of 2.724–2.912 A the stabilization of the crystal structure. Complex 2 crystallizes in an orthorhombic space Pbcn and features a 3D structure. Fig. 4 shows a perspective view of the local coordination environment around the EuIII center in the molecular structure of 2. Unlike the case of 1, in the complex 2 each asymmetric unit contains only one metal center and the EuIII ion is octacoordinate with a N2O6 environment. Each EuIII ion is linked to six pdc ligands and each ligand coordinates to two EuIII ions in a cis fashion. Specifically, one pdc ligand chelates to one EuIII ion through the pyridyl nitrogen atom and an adjacent carboxylate oxygen atom, forming a five-member chelated ring, while one oxygen atom of the second carboxylate group bridges another EuIII ion in cis position. Overall in the structure there are one half of the pdc ligands are protonated with one of the two carboxylate groups in order to meet the charge equilibrium of the coordination polymer, and the protonated carboxylate groups are strongly hydrogen bonded to the unprotonated carboxlyate group (the O/O ˚ ). The Eu–N bond length and contact is short as 2.407(2) A ˚ and 120.92(10)8, N(1)–Eu–N(1A) bond angle are 2.591(2) A respectively. There are three kinds of symmetrically related Eu–O bonds, the distances are 2.385(2), 2.355(2) and ˚ , respectively, comparable to the Eu–O(fum) 2.410(2) A distances in the molecular structure of 1. The extended 3D structure in the crystal of 2 is exhibited in Fig. 5a. It is built up from octacoordinate europium polyhedra centers and pdc linkers. The 3D framework structure can be decomposed into 2D carboxylate bridging grid-like subnetworks in the [110]
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Fig. 2. Three distinct ligand conformations (1-L1, 1-L2 and 1-L3) observed in 1.
planes (Fig. 5b) and the organic linkers along c-axis. In the 2D ˚, subnetwork, the adjacent Eu/Eu separation is 6.579(1) A while the two diagonal Eu/Eu center distances of the Eu4 grid ˚ , respectively. are 8.625(2) and 9.938(2) A 3.2. FT-IR spectra The FT-IR spectrum of 1 shows a broad band in the region about 3000–3500 cmK1, indicative of the presence of hydrogen bonded water molecules in the crystal. This is in agreement with the results of the X-ray analysis. Apparently this large band
Fig. 3. (a) View of the 1D infinite sinusoid-like inorganic chain by both edgingsharing and carboxylate-bridging pattern in 1 along the c direction. (b) Perspective view of the 3D open-framework structure of 1 along the crystallographic a-axis. Europium polyhedra, carbon, and oxygen atoms are represented in gray, black ball, and hollow ball, respectively.
vanishes gradually when water molecules are removed partially and completely by heating the compound under vacuum. (Fig. 6) In the region of the stretching vibration of carboxylate groups (1300–1700 cmK1), the splitting of nsym in 1 reflects well the nonequivalent of the carboxylate oxygen donor atoms (see Table 2). It is well-known that the difference D, between the asymmetric stretching vibration and the symmetric one (DZ nasKnsym), of the carboxylate group depends on its coordination mode [16]. The broadness of the two peaks for 1 suggests that bridging, chelating bridging coordination mode are both present, consistent with the structural analysis. It is noted that two bands show considerable splitting and the value of D increases with the removal of water molecules from the framework. These observations indicate profound changes in the coordination environments of the metals, and correspondingly the distinct coordination framework in 1b. The FT-IR spectrum of 2 shows a broad band centered at 3449 cmK1, attributable to the vibration of O–H/O hydrogen bonds according to the X-ray structure analysis. Additionally, the presence of the characteristic bands around 1700 cmK1 in 2 can be assigned to the protonated carboxylate group as well [17].
Fig. 4. ORTEP plot of complex 2 with 30% thermal ellipsoid probability, showing the coordination environment of europium(III). Hydrogen atoms were omitted for clarity.
G. Zhang et al. / Journal of Molecular Structure 796 (2006) 187–194
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1b
1a
1
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Wavenumber (cm-1) Fig. 6. The FT-IR spectra of compound 1, the product of 1 after heating at 110 8C for 2 h (1a), and the product of 1 after heating at 260 8C for 10 h (1b).
The broad band at ca. 1620 cmK1 is indicative of the characteristic of carboxylate group for the antisymmetric stretching and at 1405, 1366 cmK1 for the symmetric stretching. The difference D indicates that the presence of only monodentate coordination mode in compound 2, which is consistent with the crystal structure of 2. 3.3. Thermal analyses and PXRD studies The TGA analyses were carried out in the interest of studying the thermal behaviors of two polymeric materials (Fig. 7) The experiment was performed on samples consisting of numerous single crystals of 1 or 2 under N2 atmosphere with a heating rate of 10 8C/min. In the curve of 1 the first weight loss of 16.42% corresponding to the departure of both the crystalline and coordinated water molecules (calculated 16.28%) occurred in stages beginning at 100 8C and completing above 255 8C. Then the residual was stable up to 350 8C, at which decomposition
1 2
100 90 Weight /%
Fig. 5. (a) The 3D framework of 2 viewed along the crystallographic b-axis. Eu, dark grey; N, grey; C, dark; O, dark circle; (b) The polyhedral representation of the 2D subnetwork in 2 viewed in the [110] plane.
continued. In contrast, complex 2 appeared rather stable up to 405 8C because of the absence of water molecules in the crystal, and the stronger coordination interactions in the structure, consistent with the X-ray structural analysis above. Powder X-ray diffraction (PXRD) patterns of complex 1 before and after water removal showed remarkable changes. (Fig. 8) Hence, the removal of water molecules from the lattice resulted in the obvious breakdown of the 3D framework, consistent with the results of infrared spectra and thermal analysis. From the diffraction mode, the dehydrated sample 1b still remained the crystallinity, probably due to the partial remaining of the framework structure of 1. However, the structure recovered almost completely when this dehydrated sample was immersed into water for 2h, as observed from the PXRD pattern, taken after rehydrating the heated sample (Fig. 8d), which is also in accordance with the results of elemental analyses. This remarkably reversible phenomenon of dehydration–rehydration is much significant and only observed in a few lanthanide coordination polymers with open-framework structures[11,18].
80 70 60 50 0
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300 400 500 Temperature /˚C
Fig. 7. TGA curves for complexes 1 and 2.
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Fig. 8. The X-ray powder diffraction patterns of (a) simulated result for compound 1, (b) experimental result for compound 1, (c) data for the dehydrated sample 1b, (d) data for rehydrated sample 1c.
3.4. Solid-state photoluminescence The solid-state photoluminescence studies of compound 1 and the dehydrated product 1b revealed the red-emitting characteristic of europium(III) complex from the transitions 5 D0–7FJ (JZ0–4) centered at 580, 592, 616, 652, and 698 nm,
respectively. (Fig. 9) While two samples showed similar luminescence spectra, the dehydrated product was more strongly luminescent than compound 1 consisting of water molecules, indicating the water molecules incorporated can partially quench the emission of 1 [19,20]. Indeed, it has been documented that vibrations of water molecules could
G. Zhang et al. / Journal of Molecular Structure 796 (2006) 187–194
the strong light-emitting characteristic of two europium(III) coordination polymers, and the anhydrous products were found to be more strongly fluorescent and be of longer lifetime.
8 7
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Acknowledgements
2 1 0 0.000 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008
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Financial support from NSFC (50221201, 20372065, 20573122) and the major state basic research development program of China is gratefully acknowledged. We thank Dr Y. Li and H.-W. Ma for X-ray diffraction analysis. Appendix A. Supplementary Material
800
Fig. 9. The solid-state photoluminescence spectrum of compound 1. The excitation wavelength was 325 nm, and the spectrum is taken at ambient temperature. Inset: the luminescence decay profiles of compound 1, and the dehydrated product 1b, monitored at 615 nm.
effectively remove the electronic energy of excited europium ions, and the extent of the quenching was directly related to the number of the coordinated water molecules [21]. Furthermore, the luminescence lifetimes of two samples are quite different seen from Fig. 9. The obtained lifetimes are 0.28 ms for 1 and 1.1 ms for the dehydrated sample 1b with single-exponential fitting after deconvolution, which is yet not surprising when considering that smaller hydration number in Eu(III) complexes would lead to longer luminescence lifetime [20,22]. Furthermore, complex 2 showed also strong characteristic luminescence of europium(III) centered at 580, 590, 614, 652 and 700 nm, respectively. The luminescence lifetime of complex 2 was found to be 1.0 ms with a single-exponential fitting after deconvolution, very similar to the case of the anhydrous sample 1b (see Supporting Information).
4. Conclusion In summary, we reported the hydrothermal syntheses, structures, thermal properties and photoluminescence of two 3D open-framework materials of europium(III)-fum and europium(III)-pdc complexes. The former open-framework solid contains microporous channels filled with crystal water molecules interacting with terminal water bound to the metal centers. We focused on the investigation of the physical properties of the original framework materials and dehydrated products. The complete removal of coordinated and crystal water molecules from the host channels resulted in a transformation of the crystal structure by altering the coordination modes of metal centers, but the rehydration process led to almost complete recovery of the original 3D open framework. This reversible transformation process provides an interesting phenomenon whose nature and potential applications deserve further investigation. Finally, the solid-state photoluminescence properties revealed
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.molstruc.2006.01.048
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[10] (a) C. Serre, J. Marrot, G. Fe´rey, Inorg. Chem. 44 (2005) 4576; (b) S. Skoulika, P. Dalla, M.G. Siskos, Y. Deligiannakis, A. Michaelides, Chem. Mater. 15 (2003) 4576; (c) N. Guillou, S. Pastre, C. Livage, G. Fe´rey, Chem. Commun. (2002) 2358. [11] Only one example involving ErIII-fumarate was reported: A. Michaelides, S. Skoulika, E.G. Bakalbassis, J. Mrozinski, Cryst. Growth Des. 3 (2003) 487. [12] Y. Kim, M. Suh, D.-Y. Jung, Inorg. Chem. 43 (2004) 245. [13] R. Baggio, M. Perec, Inorg. Chem. 43 (2004) 6965. [14] (a) S. Neogi, G. Savitha, P.K. Bharadwaj, Inorg. Chem. 43 (2004) 3771; (b) B. Zhao, X.-Y. Chen, P. Cheng, D.-Z. Liao, S.-P. Yan, Z.-H. Jiang, J. Am. Chem. Soc. 126 (2004) 15394; (c) S.K. Ghosh, P.K. Bharadwaj, Inorg. Chem. 43 (2004) 2293. [15] D. Braga, F. Grepioni, G.R. Desiraju, Chem. Rev. 98 (1998) 1375. [16] G.B. Deacon, R.J. Philips, Coord. Chem. Rev. 33 (1980) 227.
[17] L.J. Bellamy, The Infrared Spectra of Complex Molecules, Wiley, New York, 1958. [18] (a) L. Pan, X. Huang, J. Li, Y. Wu, N. Zheng, Angew. Chem., Int. Ed. 39 (2000) 527; (b) L. Pan, N. Zheng, Y. Wu, S. Han, R. Yang, X. Huang, Li, J. Inorg. Chem. 40 (2001) 828. [19] J.-C.G Bu¨nzli, in: J.-C.G. Bu¨nzli, G.R. Choppin (Eds.), Lanthanide Probes in Life, Chemical and Earth Sciences, Theory and Practice, Elsevier, New York, 1989, p. 219. [20] S. Neogi, P.K. Bharadwaj, Inorg. Chem. 44 (2005) 816. [21] (a) G.R. Choppin, D.R. Peterman, Coord. Chem. Rev. 174 (1998) 283; (b) W.D. Horrocks, Jr. D.R. Sudnick, Acc. Chem. Res. 14 (1981) 384. [22] D. Parker, Chem. Soc. Rev. 33 (2004) 156.
Synthesis, characterization and photoluminescence properties of two new europium(III) coordination polymers with 3D open framework Guoqi Zhang, Qian Wang, Yan Qian, Guoqiang Yang *, Jin Shi Ma CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China Received 8 December 2005; received in revised form 25 January 2006; accepted 30 January 2006 Available online 29 March 2006
Abstract The 3D open framework coordination polymers, [Eu2(fum)3(H2O)4]n$3nH2O (1, fumZfumaric acid) and [Eu(pdc)(Hpdc)]n (2, pdcZpyridine2,5-dicarboxylic acid) have been synthesized via hydrothermal procedure. Their molecular structures were determined by single-crystal X-ray diffraction analysis. Polymer 1 adopts a 3D microporous structure filled with disordered free water molecules which interact by hydrogen bonds with terminal water bound to the metal centers. IR spectroscopy, TGA and powder X-ray diffraction analysis revealed that the remarkable dehydration–rehydration process of open-framework 1 is completely reversible. Polymer 2 forms a 3D framework composed of 2D grid-like inorganic subnetworks and pdc ligands as linkers. This solid is highly stable up to 405 8C. The solid-state photoluminescence measurements exhibited red light-emitting characteristic of two europium(III) coordination polymers. Moreover, the anhydrous products were found to be more strongly fluorescent and be of longer lifetime. q 2006 Elsevier B.V. All rights reserved. Keywords: Europium(III); Coordination polymer; Hydrothermal synthesis; Open-framework; Photoluminescence
1. Introduction The synthesis and assembly of metal–organic frameworks (MOFs) with open architectures or porous structures from small molecular building blocks have attracted high concern for some years and are still of considerable interests [1,2], owing to the observation of the large number of potential applications in catalysis, gas sorption and desorption, fluorescent sensing, opto-electronic devices, and molecular magnetism [3]. The wide combination of simple organic linkers (polycarboxylates, polyphosphonates, etc.) with inorganic metals involving transition metals and lanthanides may lead to a rich variation and modulation of both multidimensional framework structures and physical properties of the resultant solid materials [4–7]. In this field, most of the work were concentrated on the synthesis of coordination polymers containing transition-metal ions but few lanthanide ions, due to the predictability of the coordination geometry of transition metal ions compared to much flexible coordination geometry of lanthanide ions [8]. * Corresponding author. Tel.:C86 10 82617263; fax: C86 10 82617315. E-mail address: [email protected] (G. Yang).
0022-2860/$ - see front matter q 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2006.01.048
Hybrids incorporating lanthanides and aliphatic, unsaturated organic acids such as acetylenedicarboxylic or fumaric acid, and more rigid pyridinedicarboxylic acid may be promising multi-dimensional solid materials, according to the recent reports [6,9,10]. While a number of transition metal fumarate frameworks have been synthesized under hydrothermal condition, which displayed versatile structures and physical properties [9], lanthanide carboxylate complexes have been relatively less investigated to date [11]. In view of the performance of the versatile topologic structures in a variety of transition metals carboxylate, and the special luminescence properties of other lanthanides carboxylate [8,10–14], we report herein the hydrothermal synthesis, characterization and photoluminescence properties of one novel 3D open-framework europium(III)-fumarate coordination polymer, [Eu2(fum)3(H2O)4]n$(3nH2O) (1) containing 1D microporous channels filled with hydrogen-bonded water molecules, and another interesting 3D framework with 2D layer structure, [Eu(pdc)(Hpdc)]n (2). Two coordination polymers were characterized by TGA analysis, FT-IR spectroscopy, elemental analysis, powder and single-crystal X-ray diffraction. Reversible water absorption feature of openframework 1 was well determined. Solid-state photoluminescence measurement revealed that both the solid materials gave strong red fluorescence, with a characteristic of typical EuIII center emission. Moreover, the anhydrous products exhibited
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stronger fluorescence emission and longer lifetime, compared to the hydrous samples. 2. Experimental section 2.1. Materials and general methods Europium(III) oxide and two dicarboxylic acids were purchased from Aldrich Chemicals and used without further purification. The preparation of the coordination polymers 1 and 2 were performed by combining corresponding dicarboxylic acids and europium(III) oxide under hydrothermal condition in a Teflon-lined vessel (12 mL). The process was repeated three times and found to be fully reproducible. Compounds 1 and 2 are insoluble in water and organic solvents. Characterization of the sample was performed by using X-ray diffraction, TGA analysis, FT-IR, and elemental analysis. Powder X-ray diffraction (PXRD) data were collected ˚ ) on a using monochromated Cu Ka radiation (lZ1.54056 A Rigaku D/max-2500 diffractometer. TGA analysis was obtained on NETZSCH STA 409PC instrument at a temperature range of 30–700 8C (10 deg/min) with flowing N2(g). A BIO-RAD FT-165 IR spectrometer was used to gather IR spectra using KBr pellets. Elemental analysis was done with a Carlo Erba-1106 Instrument. Solid-state luminescence spectra were recorded on a Hitachi F-2500 fluorescence spectrophotometer. Fluorescence lifetimes were detected at 615 nm by a fluorescence lifetime analytical spectrometer (Erdinburgh FLS-920) and the samples were excited at 325 nm. 2.2. Synthetic procedure Synthesis of [Eu2(fum)3(H2O)4]n$3nH2O (1): Fumaric acid (0.174 g, 1.5 mmol) and Eu2O3 (0.176 g, 0.5 mmol) and water (5 mL), were added, respectively, to a Teflon-lined vessel, which was sealed and heated at 140 8C for 12 h. After slowly cooling the reaction vessel to room temperature, colorless block-like crystals of the products were collected by filtration, washed with deionized water and ethanol, and dried in air. Yield: 206 mg, 56% based on EuIII. The purity of bulk materials was confirmed by powder X-ray diffraction. IR (KBr pellet, cmK1): 3363w, 2354s, 1586w, 1435w, 1208s, 973s, 804m. Anal. Calcd (%) for C12H20O19Eu2: C, 18.66; H, 2.61. Found (%): C, 18.29; H, 2.25. Synthesis of [Eu2(fum)3(H2O)3.2]n (1a): Prepared by heating the crystalline samples of 1 in a furnace under vacuum at 110 8C for about 2 h. Anal. Calcd (%) for C12H12.4O15.2Eu2: C, 20.40; H, 1.76. Found (%): C, 20.28; H, 1.82. Synthesis of [Eu2(fum)3]n (1b): Prepared by heating the crystalline samples of 1 in a furnace under vacuum at 260 8C for about 10 h. Anal. Calcd (%) for C12H6O12Eu2: C, 22.31; H, 0.94. Found (%): C, 21.94; H, 1.05. Synthesis of [Eu2(fum)3(H2O)7]n (1c): Prepared by immersing the anhydrous samples 1b into water for 2 h, then dried in air for 3 days. Anal. Calcd (%) for C12H20O19Eu2: C, 18.66; H, 2.61. Found (%): C, 18.45; H, 2.51.
Synthesis of [Eu(pdc)(Hpdc)]n (2): Pyridine-2,5-dicarboxylic acid (0.334 g, 2 mmol) and Eu 2O 3 (0.176 g, 0.5 mmol) and water (5 mL), were added, respectively, to a Teflon-lined vessel, which was sealed and heated at 140 8C for 12 h. After slowly cooling the reaction vessel to room temperature, light yellow block-like crystals of the products were collected by filtration, washed with deionized water and ethanol, and dried in air. Yield: 0.358 g, 74% based on EuIII. The purity of bulk materials was confirmed by powder X-ray diffraction. IR (KBr pellet, cmK1): 3449w, 3128s, 1661m, 1488s, 1405s, 1366s, 1277w, 1160m, 1114s, 967m, 759s, 691s, 568s, 521s. Anal. Calcd (%) for C14H7N2O8Eu: C, 34.17; H, 1.56; N, 5.78. Found (%): C, 34.36; H, 1.62; N, 5.64. 2.3. Single-crystal X-ray diffraction Well-shaped single-crystals of 1 and 2 were selected and mounted in air onto thin glass fibers. Accurate unit cell parameters were determined by a least-square fit of 2q values, measured for 200 strong reflections, and intensity data sets were measured on a Rigaku Raxis Rapid IP diffractometer with ˚ ) at room temperature. The Mo Ka radiation (lZ0.71073 A intensities were corrected for Lorentz and polarization effects, but no corrections for extinction were made. All structures were solved by direct methods. The non-hydrogen atoms were located in successive difference Fourier synthesis. The final refinement was performed by full matrix least-squares methods with anisotropic thermal parameters for non-hydrogen atoms on F2. Crystallographic data and experimental details for structure analyses are summarized in Table 1. Relevant interatomic bond distances and bond angles for 1 and 2 are given in Table 2. CCDC 269990-269991 contain the supplementary crystallographic data for this paper. These data can be obtained free of Table 1 Crystal and structure refinement data for complexes 1 and 2
Empirical formula Formula wt Crystal system Space group T (K) ˚) l (A ˚) a (A ˚) b (A ˚) c (A b (8) ˚ 3) V (A Z F(000) dcalc (g/cm3) m (mmK1) Reflctns collected Unique reflections Variables R1a, wR2b (%) Goodness of fit a b
1
2
C12H20O19Eu2 773.90 monoclinic P21/n 293(2) 0.71073 9.4983(19) 14.764(3) 14.825(3) 91.57(3) 2078.1(7) 4 1424 2.423 6.075 4461 3812 298 2.73, 8.16 1.038
C14H7N2O8Eu 483.18 orthorhombic Pbcn 294(2) 0.71073 9.983(2) 8.685(2) 15.772(4) 90.00 1367.5(6) 4 928 2.347 4.641 7139 1121 114 1.65, 4.33 1.014
RZ ðFO KFC Þ=ðFO Þ. Rw Z ½SwðFo2 KFc2 Þ2 =wðFo2 Þ2 1=2 .
G. Zhang et al. / Journal of Molecular Structure 796 (2006) 187–194 Table 2 ˚ ) for complexes 1 and 2 Selected bond lengths (A 1 Eu(1)–O(1) Eu(1)–O(4) Eu(1)–O(7)#1 Eu(1)–O(11) Eu(1)–O(14) Eu(2)–O(3)#2 Eu(2)–O(9) Eu(2)–O(11) Eu(2)–O(15) 2 Eu(1)–O(1) Eu(1)–O(4)#4
2.483(3) 2.367(4) 2.464(4) 2.460(4) 2.462(3) 2.434(3) 2.456(3) 2.638(3) 2.409(4)
Eu(1)–O(2) Eu(1)–O(6) Eu(1)–O(8)#1 Eu(1)–O(13) Eu(2)–O(2) Eu(2)–O(5) Eu(2)–O(10) Eu(2)–O(12) Eu(2)–O(16)
2.623(4) 2.434(4) 2.541(4) 2.430(4) 2.407(4) 2.346(4) 2.538(4) 2.488(4) 2.443(4)
2.385(2) 2.410(2)
Eu(1)–O(2)#3 Eu(1)–N(1)
2.355(2) 2.591(2)
Symmetry transformations used to generate equivalent atoms for: #1Kx, Ky, K z; #2 xK1/2, KyK1/2, zK1/2; #3 xK1/2, yK1/2, KzK1/2; #4 x, Ky, zK1/2.
charge at www.ccdc.cam.ac.uk/conts/retrieving.html. [or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; Fax: (internat.) C44 1223/336 033; E-mail: [email protected]]. 3. Results and discussion 3.1. Crystal structures of 1 and 2 In the molecular structure of complex 1, EuIII ions have two different coordination environments and both of them are surrounded by nine oxygen atoms as shown in Fig. 1. The EuIII centers are both linked to seven oxygen atoms from five fum ligands, and the remaining two oxygen atoms belong to the coordinated water molecules. The Eu–O bond distances in two EuIII centers are obviously discrepant. (Table 2) The shortest ˚, and longest Eu(1)–O lengths are 2.367(4) and 2.623(4) A ˚ respectively, while they are 2.346(4) and 2.638(3) A for the Eu(2)–O lengths. Therefore, the centers of two EuO9 polyhedra are not symmetric. The EuO9 polyhedra are bridged with three
Fig. 1. The ORTEP plot of EuO9 polyhedron in 1 (30% probability), in which two EuIII centers adopt the same coordination mode but asymmetric coordination environments. Hydrogen atoms were omitted for clarity.
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fum ligands, two in oxygen-sharing mode and one in carboxylate-bridging pattern, the two EuIII centers thus have a ˚ . With exception of the coordinated water separation of 4.618 A molecules, each asymmetric unit contains also three crystal water molecules in the crystal structure of 1. There are three types of bridging coordination modes in 1 each with distinct crystal engineering functions, two of them (1-L1 and 1-L2) consist of mixed bidentate bridging and chelating modes, while the third type (1-L3) is of bis-chelating mode (Fig. 2). The edgesharing EuO9 polyhedra centers are further connected through another carboxylate bridge to form a 1D infinite sinusoid-like inorganic chain (Fig. 3a), and this chain is also different from the fully edge-sharing inorganic chain in the structure of EuIIIcarboxylate frameworks reported recently[10a,12]. Again, this 1D chain can be regarded as the basic building block, all three types of ligands serve to link the building blocks and extend the structure along other two directions (a and b) to yield the resultant 3D open-framework architecture as shown in Fig. 3b. This framework is microporous and is of small 1D void channels. The dimensions of the pore are small with an ˚ 2. The channels are estimated free aperture of w3.77!8.98 A partially occupied by the centrosymmetrically related hydrogen-bonded water molecules including both disordered free water molecules and terminal water groups, bound to the europium atoms. It should be noted that two types of water molecules in the channels in fact behave as efficient hydrogen bonding moieties[15]. The water molecules strongly interact with each other through the O–H/O hydrogen bonds (the O/ ˚ ), contributing to O contacts are all in the range of 2.724–2.912 A the stabilization of the crystal structure. Complex 2 crystallizes in an orthorhombic space Pbcn and features a 3D structure. Fig. 4 shows a perspective view of the local coordination environment around the EuIII center in the molecular structure of 2. Unlike the case of 1, in the complex 2 each asymmetric unit contains only one metal center and the EuIII ion is octacoordinate with a N2O6 environment. Each EuIII ion is linked to six pdc ligands and each ligand coordinates to two EuIII ions in a cis fashion. Specifically, one pdc ligand chelates to one EuIII ion through the pyridyl nitrogen atom and an adjacent carboxylate oxygen atom, forming a five-member chelated ring, while one oxygen atom of the second carboxylate group bridges another EuIII ion in cis position. Overall in the structure there are one half of the pdc ligands are protonated with one of the two carboxylate groups in order to meet the charge equilibrium of the coordination polymer, and the protonated carboxylate groups are strongly hydrogen bonded to the unprotonated carboxlyate group (the O/O ˚ ). The Eu–N bond length and contact is short as 2.407(2) A ˚ and 120.92(10)8, N(1)–Eu–N(1A) bond angle are 2.591(2) A respectively. There are three kinds of symmetrically related Eu–O bonds, the distances are 2.385(2), 2.355(2) and ˚ , respectively, comparable to the Eu–O(fum) 2.410(2) A distances in the molecular structure of 1. The extended 3D structure in the crystal of 2 is exhibited in Fig. 5a. It is built up from octacoordinate europium polyhedra centers and pdc linkers. The 3D framework structure can be decomposed into 2D carboxylate bridging grid-like subnetworks in the [110]
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Fig. 2. Three distinct ligand conformations (1-L1, 1-L2 and 1-L3) observed in 1.
planes (Fig. 5b) and the organic linkers along c-axis. In the 2D ˚, subnetwork, the adjacent Eu/Eu separation is 6.579(1) A while the two diagonal Eu/Eu center distances of the Eu4 grid ˚ , respectively. are 8.625(2) and 9.938(2) A 3.2. FT-IR spectra The FT-IR spectrum of 1 shows a broad band in the region about 3000–3500 cmK1, indicative of the presence of hydrogen bonded water molecules in the crystal. This is in agreement with the results of the X-ray analysis. Apparently this large band
Fig. 3. (a) View of the 1D infinite sinusoid-like inorganic chain by both edgingsharing and carboxylate-bridging pattern in 1 along the c direction. (b) Perspective view of the 3D open-framework structure of 1 along the crystallographic a-axis. Europium polyhedra, carbon, and oxygen atoms are represented in gray, black ball, and hollow ball, respectively.
vanishes gradually when water molecules are removed partially and completely by heating the compound under vacuum. (Fig. 6) In the region of the stretching vibration of carboxylate groups (1300–1700 cmK1), the splitting of nsym in 1 reflects well the nonequivalent of the carboxylate oxygen donor atoms (see Table 2). It is well-known that the difference D, between the asymmetric stretching vibration and the symmetric one (DZ nasKnsym), of the carboxylate group depends on its coordination mode [16]. The broadness of the two peaks for 1 suggests that bridging, chelating bridging coordination mode are both present, consistent with the structural analysis. It is noted that two bands show considerable splitting and the value of D increases with the removal of water molecules from the framework. These observations indicate profound changes in the coordination environments of the metals, and correspondingly the distinct coordination framework in 1b. The FT-IR spectrum of 2 shows a broad band centered at 3449 cmK1, attributable to the vibration of O–H/O hydrogen bonds according to the X-ray structure analysis. Additionally, the presence of the characteristic bands around 1700 cmK1 in 2 can be assigned to the protonated carboxylate group as well [17].
Fig. 4. ORTEP plot of complex 2 with 30% thermal ellipsoid probability, showing the coordination environment of europium(III). Hydrogen atoms were omitted for clarity.
G. Zhang et al. / Journal of Molecular Structure 796 (2006) 187–194
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Wavenumber (cm-1) Fig. 6. The FT-IR spectra of compound 1, the product of 1 after heating at 110 8C for 2 h (1a), and the product of 1 after heating at 260 8C for 10 h (1b).
The broad band at ca. 1620 cmK1 is indicative of the characteristic of carboxylate group for the antisymmetric stretching and at 1405, 1366 cmK1 for the symmetric stretching. The difference D indicates that the presence of only monodentate coordination mode in compound 2, which is consistent with the crystal structure of 2. 3.3. Thermal analyses and PXRD studies The TGA analyses were carried out in the interest of studying the thermal behaviors of two polymeric materials (Fig. 7) The experiment was performed on samples consisting of numerous single crystals of 1 or 2 under N2 atmosphere with a heating rate of 10 8C/min. In the curve of 1 the first weight loss of 16.42% corresponding to the departure of both the crystalline and coordinated water molecules (calculated 16.28%) occurred in stages beginning at 100 8C and completing above 255 8C. Then the residual was stable up to 350 8C, at which decomposition
1 2
100 90 Weight /%
Fig. 5. (a) The 3D framework of 2 viewed along the crystallographic b-axis. Eu, dark grey; N, grey; C, dark; O, dark circle; (b) The polyhedral representation of the 2D subnetwork in 2 viewed in the [110] plane.
continued. In contrast, complex 2 appeared rather stable up to 405 8C because of the absence of water molecules in the crystal, and the stronger coordination interactions in the structure, consistent with the X-ray structural analysis above. Powder X-ray diffraction (PXRD) patterns of complex 1 before and after water removal showed remarkable changes. (Fig. 8) Hence, the removal of water molecules from the lattice resulted in the obvious breakdown of the 3D framework, consistent with the results of infrared spectra and thermal analysis. From the diffraction mode, the dehydrated sample 1b still remained the crystallinity, probably due to the partial remaining of the framework structure of 1. However, the structure recovered almost completely when this dehydrated sample was immersed into water for 2h, as observed from the PXRD pattern, taken after rehydrating the heated sample (Fig. 8d), which is also in accordance with the results of elemental analyses. This remarkably reversible phenomenon of dehydration–rehydration is much significant and only observed in a few lanthanide coordination polymers with open-framework structures[11,18].
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Fig. 7. TGA curves for complexes 1 and 2.
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Fig. 8. The X-ray powder diffraction patterns of (a) simulated result for compound 1, (b) experimental result for compound 1, (c) data for the dehydrated sample 1b, (d) data for rehydrated sample 1c.
3.4. Solid-state photoluminescence The solid-state photoluminescence studies of compound 1 and the dehydrated product 1b revealed the red-emitting characteristic of europium(III) complex from the transitions 5 D0–7FJ (JZ0–4) centered at 580, 592, 616, 652, and 698 nm,
respectively. (Fig. 9) While two samples showed similar luminescence spectra, the dehydrated product was more strongly luminescent than compound 1 consisting of water molecules, indicating the water molecules incorporated can partially quench the emission of 1 [19,20]. Indeed, it has been documented that vibrations of water molecules could
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the strong light-emitting characteristic of two europium(III) coordination polymers, and the anhydrous products were found to be more strongly fluorescent and be of longer lifetime.
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Financial support from NSFC (50221201, 20372065, 20573122) and the major state basic research development program of China is gratefully acknowledged. We thank Dr Y. Li and H.-W. Ma for X-ray diffraction analysis. Appendix A. Supplementary Material
800
Fig. 9. The solid-state photoluminescence spectrum of compound 1. The excitation wavelength was 325 nm, and the spectrum is taken at ambient temperature. Inset: the luminescence decay profiles of compound 1, and the dehydrated product 1b, monitored at 615 nm.
effectively remove the electronic energy of excited europium ions, and the extent of the quenching was directly related to the number of the coordinated water molecules [21]. Furthermore, the luminescence lifetimes of two samples are quite different seen from Fig. 9. The obtained lifetimes are 0.28 ms for 1 and 1.1 ms for the dehydrated sample 1b with single-exponential fitting after deconvolution, which is yet not surprising when considering that smaller hydration number in Eu(III) complexes would lead to longer luminescence lifetime [20,22]. Furthermore, complex 2 showed also strong characteristic luminescence of europium(III) centered at 580, 590, 614, 652 and 700 nm, respectively. The luminescence lifetime of complex 2 was found to be 1.0 ms with a single-exponential fitting after deconvolution, very similar to the case of the anhydrous sample 1b (see Supporting Information).
4. Conclusion In summary, we reported the hydrothermal syntheses, structures, thermal properties and photoluminescence of two 3D open-framework materials of europium(III)-fum and europium(III)-pdc complexes. The former open-framework solid contains microporous channels filled with crystal water molecules interacting with terminal water bound to the metal centers. We focused on the investigation of the physical properties of the original framework materials and dehydrated products. The complete removal of coordinated and crystal water molecules from the host channels resulted in a transformation of the crystal structure by altering the coordination modes of metal centers, but the rehydration process led to almost complete recovery of the original 3D open framework. This reversible transformation process provides an interesting phenomenon whose nature and potential applications deserve further investigation. Finally, the solid-state photoluminescence properties revealed
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.molstruc.2006.01.048
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