DOI: 10.1021/cg9011668
New Family of Porous Lanthanide-Containing Coordination Polymers: [Ln2(C2O4)3(H2O)6,12H2O]¥ with Ln=La-Yb or Y
2010, Vol. 10 775–781
Doddy Kustaryono, Nicolas Kerbellec, Guillaume Calvez, Stephane Freslon, Carole Daiguebonne, and Olivier Guillou* Universit e europ eenne de Bretagne, France INSA, UMR 6226 “Sciences Chimiques de Rennes”, F-35043 Rennes Received September 23, 2009; Revised Manuscript Received October 14, 2009
ABSTRACT: Reactions at low temperature in a mixture benzene/water between dimethyloxalate and a lanthanide chloride lead to a family of 3D-coordination polymers with general chemical formula [Ln2(C2O4)3(H2O)6 3 12H2O]¥, where Ln=La-Yb (except Pm) or Y. All these compounds are isostructural to the already described [Er2(C2O4)3(H2O)6 3 12H2O]¥. Their crystal structure can be described as the juxtaposition of channels with hexagonal cross-section. The channels are filled with crystallization water molecules. Their thermal dehydration produces a collapse of the molecular skeleton leading to amorphous compounds. On the opposite, their dehydration by freeze-drying is possible and the twelve crystallization water molecules can be removed without destruction of the molecular skeleton. The porosity of the dehydrated compounds has been estimated by computational methods to 483 m2 g-1. The luminescence properties of the Eu-containing and Tb-containing compounds are also briefly described.
Introduction For almost a decade the number of reported molecular open frameworks is steadily increasing.1-6 This renewal of interest is mainly the result of their potential interest for depollution or gas storage purposes. Despite the huge number of reported compounds, there are still relatively few example of lanthanide containing MOFs.7-20 Some of us are currently studying lanthanide containing coordination polymers aiming materials exhibiting porosity21-25 or interesting luminescent properties.26-30 Some years ago, in the frame of these studies we have synthesized, by diffusion in a gel medium, and structurally characterized a 3-D coordination polymer with chemical formula [Er2(C2O4)3(H2O)6 3 12H2O]¥.31 This compound crystallizes in the trigonal system, space group R3 (No. 148) with a=30.8692(10) A˚, c=7.2307(2) A˚, and Z=12. This compound exhibits large channels with hexagonal cross-section spreading along the c axis (see Figure 1). These channels are filled with crystallization water molecules. Unfortunately, despite its promising molecular structure this compound exhibits no porosity or interesting luminescent properties. Actually, Er(III) ions are well-known for their NIR luminescent properties,32-34 but it is also well established that high energy vibrators,35,36 such as O-H vibrators, are very efficient inhibitors of NIR luminescence. In this compound, each Er(III) ion is surrounded by three coordination water molecules, and there are six additional crystallization water molecules per Er(III) ion. This high hydration rate makes the Er(III) NIR luminescent inefficient. Furthermore, the thermal dehydration of the compound provokes the collapse of the molecular structure and leads to the well-known 2-D [Er2(C2O4)3(H2O)4 3 2H2O]¥.37,38 At last, the compound, obtained by slow diffusion of the reactants through a gel in a U-shaped tube, was not available for technological applications. However, a careful reinvestigation
of the literature revealed that compounds with chemical formula Ln2(C2O4)3 3 18H2O with Ln = Dy-Yb plus Y had already been evoked in 1971.39 In this paper, the existence of these compounds was suggested on the basis of chemical analysis and TGA measurements. The experimental X-ray diffraction diagram of the Er containing powder was reported. This experimental diagram was very similar to the calculated X-ray diffraction diagram of [Er2(C2O4)3(H2O)6 3 12H2O]¥. We have thus decided to reinvestigate this system aiming the expansion of the family to lanthanide ions exhibiting visible luminescence (such as Eu3þ or Tb3þ) and the study of the potential porosity. In this paper, we describe the optimized synthesis of coordination polymers with general chemical formula [Ln2(C2O4)3(H2O)6 3 12H2O]¥ with Ln = La-Yb or Y. Their dehydration by freeze-drying leads to a new family of porous compounds with general chemical formula [Er2(C2O4)3(H2O)6]¥ with Ln = La-Yb or Y. The porosity of the Y-containing compounds has been evaluated by computational methods and the luminescence properties of the Eu3þ- and Tb3þ-containing hydrated compounds have been explored. Experimental Section
*To whom correspondence should be addressed. E-mail: Olivier.guillou@ insa-rennes.fr.
Synthesis of the Microcrystalline Powders [Ln2(C2O4)3(H2O)6 3 12H2O]¥ with Ln=La-Yb or Y. Dimethyloxalate was purchased from Acros Organics and used without further purification. Hydrated lanthanide chlorides were prepared from the corresponding oxides according to literature methods40. Lanthanide oxides were purchased from STREM Chemicals and used without further purification. On one hand, 2 mmol of hydrated lanthanide chloride are dissolved in 40 mL of deionized water. On the other hand, 4.8 mmol of dimethyloxalate are dissolved in benzene. The two clear solutions are mixed and kept under vigorous magnetic stirring for several hours. The reaction pathway (see Scheme 1) implies the dimethyloxalate hydrolysis and occurs at the water/benzene interphases. A white precipitate progressively appears. Once the reaction is completed, the white precipitate is filtered off and dried at room temperature. The product of the reaction is highly temperature
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Kustaryono et al. Table 1. Chemical Analysis Results for [Ln2(C2O4)3(H2O)6 3 12H2O]¥ with Ln = La-Yb or Y anal. calcd (found) -1
Ln
MW(gmol )
Ln (%)
O (%)
C (%)
H (%)
Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb
766.14 866.14 868.56 870.15 876.82 889.05 892.26 902.83 906.18 913.33 918.19 922.85 934.41
23.2 (23.2) 32.0 (32.2) 32.2 (32.0) 32.4 (32.4) 32.9 (32.8) 33.8 (33.8) 34.0 (34.2) 34.8 (34.8) 35.1 (35.0) 35.6 (35.5) 35.9 (35.7) 36.2 (36.1) 37.0 (37.0)
62.6 (62.4) 55.4 (55.4) 55.3 (55.5) 55.1 (55.2) 54.7 (54.8) 53.9 (54.0) 53.8 (54.0) 53.2 (53.2) 52.9 (53.0) 52.5 (52.5) 52.3 (52.4) 52.0 (52.1) 51.4 (51.5)
9.4 (9.6) 8.3 (8.2) 8.3 (8.3) 8.2 (8.3) 8.2 (8.2) 8.1 (8.1) 8.1 (8.0) 7.9 (8.0) 7.9 (8.0) 7.8 (7.8) 7.8 (7.9) 7.8 (7.8) 7.7 (7.7)
4.7 (4.8) 4.2 (4.2) 4.2 (4.2) 4.1 (4.1) 4.1 (4.2) 4.1 (4.1) 4.0 (3.8) 4.0 (4.0) 4.0 (4.0) 3.9 (4.2) 3.9 (4.0) 3.9 (4.0) 3.8 (3.8)
Table 2. Cell Parameters for [Ln2(C2O4)3(H2O)6 3 12H2O]¥ with Ln = Gd or Er
Figure 1. Projection view along the c axis of [Er2(C2O4)3(H2O)6 3 12H2O]¥.
Scheme 1. Schematic Representation of the Reaction Pathway
dependent. Therefore the temperature of the reaction media have been rigorously controlled ((1 C). Reactions under 15 C have been carried out in a cold room. Reactions at higher temperature have been carried out in thermostatic reactors. The products of the different synthesis have been characterized on the basis of their X-ray powder diffraction diagrams and on their elemental analysis (Table 1). All the obtained compounds present similar IR spectra (cm-1): 3550-3200 (s) ν(O-H), 1686-1560 (s) ν(C-O); δ(H-O-H), 1370 (s) ν(C-O); ν(C-C), 1330 (s) ν(C-O), 817 (s) ν(Ln-O); δ(O-C-O), 602 (w) ν(Ln-OH2), 519 (m) ν(Ln-O), 490 (m) δ(O-C-O), 416 (m) ν(Ln-O).41,42 Synthesis of [Ln2(C2O4)3(H2O)6 3 12H2O]¥ with Ln=Gd or Er as Single Crystals. The potassium oxalate salt has been purchased from Acros Organics and used without further purification. The gel of TMOS (tetramethylorthosilicate) has been purchased from Acros Organics and gelified according established procedures.43,44 It has been prepared by mixing 3 mL of TMOS and 17 mL of water. Dilute aqueous solutions of hexa-hydrate Ln(III) chloride (0.25 mmol in 20 mL) and dihydrate potassium salt of oxalate (0.25 mmol in 20 mL) were allowed to slowly diffuse through the gel medium in an U-shaped tube. After few weeks, single crystals suitable for X-ray diffraction were obtained. Elemental analysis of [Gd2(C2O4)3(H2O)6 3 12H2O]¥ (MW = 902.83 g mol-1) calculated (found): C 7.9% (7.8%), H 4.0% (4.1%), O 53.2% (53.2%), Gd 34.8% (34.8%). Elemental analysis of [Er2(C2O4)3(H2O)6,12H2O]¥ (MW = 922.85 g mol-1) calculated (found): C 7.8% (7.7%), H 3.9% (4.0%), O 52.0% (52.0%), Er 36.2% (36.3%). The two compounds were assumed to be isostructural on the basis of their cell parameters (Table 2). Synthesis of the Microcrystalline Powders [Ln2(C2O4)3(H2O)6]¥ with Ln = La-Yb or Y. The hydrated compounds [Ln2(C2O4)3(H2O)6 3 12H2O]¥ with Ln = La-Yb or Y have all been partially dehydrated by freeze-drying leading to a new family of compounds with general chemical formula [Ln2(C2O4)3(H2O)6]¥ with Ln = La-Yb or Y (Table 3).
Ln
space group
a (A˚)
c (A˚)
V (A˚3)
Gd Er
R3 (No. 148) R3 (No. 148)
30.8532(7) 30.8692(10)
7.2177(4) 7.2307(2)
5950.2(6) 5967.1(3)
Table 3. Chemical Analysis Results for [Ln2(C2O4)3(H2O)6]¥ with Ln = La-Yb or Y Anal. Calculated (found) Ln MW(gmol-1) Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb
549.96 649.96 652.38 653.96 660.63 672.87 676.08 686.65 690.00 697.15 702.01 706.67 718.23
Ln (%)
O (%)
C (%)
H (%)
32.3 (32.0) 42.7 (42.5) 42.9 (43.0) 43.1 (43.2) 43.7 (43.5) 44.7 (44.7) 44.9 (45.0) 45.8 (45.6) 46.1 (46.1) 46.6 (46.7) 46.9 (47.0) 47.3 (47.3) 48.2 (48.0)
52.4 (52.5) 44.3 (44.4) 44.1 (44.0) 44.0 (44.2) 43.6 (43.6) 42.8 (42.9) 42.6 (42.5) 41.9 (42.0) 41.7 (41.9) 41.3 (41.3) 41.0 (41.0) 40.7 (40.7) 40.1 (40.0)
13.1 (13.2) 11.1 (11.2) 11.0 (11.0) 11.0 (10.8) 10.9 (10.9) 10.7 (10.6) 10.7 (10.7) 10.5 (10.5) 10.4 (10.3) 10.3 (10.2) 10.3 (10.4) 10.2 (10.2) 10.0 (10.3)
2.2 (2.3) 1.9 (1.9) 1.8 (2.0) 1.8 (1.8) 1.8 (2.0) 1.8 (1.8) 1.8 (1.8) 1.7 (1.9) 1.7 (1.7) 1.7 (1.8) 1.7 (1.6) 1.7 (1.8) 1.6 (1.7)
All compounds present similar IR spectra. These spectra are exactly the same than those observed for the corresponding hydrated compounds. Thermal Analysis. Thermogravimetric and thermal differential analyses were performed in a platinum crucible under a nitrogen atmosphere between room temperature and 1000 C with a heating rate of 5 C min-1 using a Perkin-Elmer Pyris-Diamond thermal analyzer. The thermal dependence X-ray Diffraction experiments have been performed by a Panalytical X’Pert Pro diffractometer with an X’celerator detector diffractometer using Cu KR1 radiation in the range 5-70 in 2θ. The heating of the samples (from room temperature to 1000 C) was performed by an Anton Paar HTK 1200 furnace under nitrogen atmosphere. X-ray Powder Diffraction. The diagrams have been collected using a Panalytical X’Pert Pro diffractometer with a X0 celerator detector. The typical recording conditions were 40 kV, 40 mA for Cu KR (λ=1.542 A˚), the diagrams were recorded in θ-θ mode in 60 min between 5 and 75 (8378 measurements) with a step size of 0.0084 and a scan time of 50 s. The calculated patterns were produced using the Powdercell and WinPLOTR software programs45-47. Solid State Luminescence Measurements. Solid state emission spectra were measured on a Perkin-Elmer LS-55 fluorescence spectrometer with a pulse Xe lamp. Slit width was 5 nm for excitation and 5 nm for emission. The excitation wavelength has been fixed at 266 nm because it corresponds to a maximum of absorption of the oxalate ligand (Figure 2). Luminescence spectra were all recorded at room temperature between 400 and 800 nm in identical operating conditions without
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turning the lamp off to ensure a valid comparison between the emission spectra. Reproducibility of the measurements has been carefully checked by reproducing several times. The data were collected, at 100 nm min-1, in phosphorescence mode with 0.3 ms delay time between the excitation pulse and the emission measurement.
Results and Discussion Synthesis. For more than a decade, some of us are interested in the synthesis and the structural characterization of lanthanide-containing coordination polymers.48-51 Thanks to the crystal growth techniques we have developed,52 we
Figure 2. Absorption Spectrum of [Y2(C2O4)3(H2O)6 3 12H2O]¥.
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have succeeded, some years ago, in synthesizing a 3D oxalate-based Er(III)-containing coordination polymer31 with chemical formula [Er2(C2O4)3(H2O)6 3 12H2O]¥. This compound crystallizes in the trigonal system, space group R3 (CCDC No 199964). Its crystal structure can be described as the juxtaposition of channels with hexagonal cross-section spreading along the cB axis (see Figure 1). These channels are filled with crystallization water molecules. Two channel out of three are actually helixes spreading along the cB axis (Figure 3.). Each such channel is connected to three other helical channels in such a way that if one turns right, adjacent others turn left. (Figure 4). The connections between helixes are ensured by common Er(III)-C2O4-Er(III) bridges. Six connected helixes form another hexagonal cross-section channel. In this crystal structure the Er(III) ion is surrounded by six oxygen atoms from three bidentate oxalate groups and three oxygen atoms from coordination water molecules. These nine oxygen atoms form a distorted tricapped trigonal prism. The simulated pattern of the X-ray powder diffraction matches rather well the experimental X-ray diffraction diagram reported previously by Watanabe et al.39. By reproducing the synthesis described in this paper we confirmed that the single crystal obtained in gel medium and the microcrystalline powder obtained by reacting dimethyloxalate and ErCl3 3 6HO in a mixture water/benzene were the same compound. In this paper, it is noticed that isostructural compounds involving Ho(III), Tm(III) or Y(III) can also be obtained. As we had obtained the Gd(III) containing analogous compound we thought it was possible to further expand the family. With this aim we have reinvestigated and optimized the reaction pathway described by Watanabe. Our study reveals that this reaction is very temperature dependent. The results of this synthetic study are summarized in Figure 5.
Figure 3. Projection view along B c axis (left) and along aB þ bB direction (right) of a helical molecular channel in [Er2(C2O4)3(H2O)6 3 12H2O]¥.
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Figure 4. Left: Extended asymmetric unit for [Er2(C2O4)3(H2O)6 3 12H2O]¥. Right: Projection view along the aB - bB direction of two adjacent molecular motifs. Table 4. Yields of the Synthesis of [Y2(C2O4)3(H2O)6 3 12H2O]¥ at 2C [Y3þ]/[(C2O4)2-] yield
Figure 5. Schematic summary of the syntheses. Ionic radii are for nine-coordinated ions53.
All the synthesis reported in this figure have been carried out with exactly the same operating mode. Only the reaction medium temperature and the involved lanthanide ion have been changed from a trial to another. All the obtained microcrystalline powders have been characterized on the basis of their elemental analysis and X-ray diffraction diagram. Figure 5 reveals the existence of three domains. Domain I is encountered for highest temperature and lightest lanthanide ions. In this domain the obtained compounds have general chemical formula [Ln2(C2O4)3(H2O)6,4H2O]¥ with Ln=La-Er or Y 54-56. Domain II corresponds to the
1/1.5 14%
1/2 17%
1/2.2 21%
1/2.4 24%
1/3 25%
1/4 25%
heaviest lanthanide ions and the highest temperature. In this domain, the products of the synthesis have general chemical formula [Ln2(C2O4)3(H2O)6]¥ with Ln=Ho-Yb or Y 37,38. Finally, the domain III contains the compounds with general chemical formula [Ln2(C2O4)3(H2O)6,12H2O]¥ with Ln = La-Yb or Y. They are generally obtained at low temperature (
