Syntheses, structural characterizations and ...

Solid State Sciences 28 (2014) 61e66

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Syntheses, structural characterizations and ferroelectric properties of new Ce(III) coordination polymers via isomeric tartaric acid ligands Jin-Li Qi, Sheng-Liang Ni, Yue-Qing Zheng*, Wei Xu Crystal Engineering Division, Center of Applied Solid State Chemistry Research, Ningbo University, Ningbo 315211, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 October 2013 Received in revised form 6 December 2013 Accepted 9 December 2013 Available online 21 December 2013

Three new Ce(III) tartrate-based metal-organic frameworks (MOFs) [Ce2(H2O)3(D-tar)3]$3H2O 1, [Ce2(H2O)3(L-tar)3]$3H2O 2 and Ce(H2O)(meso-tar)1.5 3 have been hydrothermally synthesized and characterized by single-crystal and powder X-ray diffraction analyses, IR spectra, elemental analyses, as well as thermogravimetric analyses. 1 and 2 crystallize in polar space group P1, and 3 belongs to centrosymmetric 2þ one P21/n. The [Ce2(tar)2]2þ motifs in 1 and 2 rig up 2D 2N ½Ce2 ðH2 OÞ3 ðtarÞ2 networks and stacked via tar2 ligands into 3D 3N f½Ce2 ðH2 OÞ3 ðD tarÞ2 ðD tarÞg architectures with (42$64)(42$67$8) topology type. The 2þ [Ce(tar)]þ motifs in 3 assemble into 2D 2N ½CeðH2 OÞðmeso tarÞ layers and the resulting layers are linked through tartrato ligands into 3N f½CeðH2 OÞðmeso tarÞðmeso tarÞ0:5 g framework with (4$62)2(43$67) 2(44$62) topology. Both 1 and 2 exhibit ferroelectricity (Pr ¼ 0.009 and 0.015 mC cm2, Ec ¼ 0.570 and 0.505 kV cm1, Ps ¼ 0.233 and 0.171 mC cm2 for 1 and 2, respectively). Ó 2013 Published by Elsevier Masson SAS.

Keywords: Acentricity Ce(III) complexes Tartaric acid MOFs Topology Ferroelectric properties

1. Introduction The crystalline coordination polymers with various intriguing architectures and topologies have been extensively studied [1e4]. These substances often have diverse applications in the fields of catalysis, host-guest chemistry, gas storage and separation, ect [5e 8]. Acentric crystalline substances are also widely researched for possessing many important physical properties such as ferroelectricity, pyroelectricity, piezoelectricity, triboluminescence, as well as nonlinear optical (NLO) function [9e12]. As an outgrowth result by these two hot topic researches, design, synthesis and structural characterization of acentric coordination polymers become a study hotspot [13e16]. Flexible aliphatic polycarboxylic acids play an important role in the syntheses of metal-organic frameworks (MOFs) and show the diversity on compounds due to the multiple coordination modes [17e20]. Tartaric acid as a typical aliphatic dicarboxylic acid owns four ionizable protons from two carboxyl and two hydroxyl groups providing six coordinating sites, which is widely used in the construction of MOFs [21e25]. Chiral complexes are obtainable from the employing achiral and chiral building blocks, but voluminous researches indicated that chiral ligands such as tartaric acid which owns two chiral centers can bring the desired

* Corresponding author. Tel./fax: þ86 574 87600747. E-mail addresses: [email protected], [email protected] (Y.-Q. Zheng). 1293-2558/$ e see front matter Ó 2013 Published by Elsevier Masson SAS. http://dx.doi.org/10.1016/j.solidstatesciences.2013.12.006

complexes easier [14,26,27]. The four tartaric acid forms (D-(), L(þ), racemic mixture and achiral meso-varieties) can also make a contribution to the structural diversity. But it is difficult to predict the structure directing effects because of the facile s-bond rotation during the self-assembly, however that can be revealed via the resulting architectures which employing different configurational isomers of tartaric acid by using the same metal center [28,29]. The rare earth metals such as Ce(III) are always exhibiting a high coordination number which have advantages in the assembling of highdimensional structures [30e33]. In order to obtain acentric coordination polymers and explore the self-assembly regulation, we have engaged ourselves in the preparation of cerium(III) tartrate-based coordination polymers, and got three new MOFs, 3N f½Ce2 ðH2 OÞ3 ðD tarÞ2 ðD tarÞg$3H2O 1, 3N f½Ce2 ðH2 OÞ3 3 f½CeðH OÞðmeso tarÞ ðL tarÞ2 ðL tarÞg$3H2O 2 and 2 N ðmeso tarÞ0:5 g 3. 2. Experimental section 2.1. Materials and physical measurements All the chemicals of regent grade were purchased from commercial sources and used without purification. The Powder X-ray diffractions were carried out with a Bruker D8 Focus X-ray diffractometer using Cu target and Ni filter (l ¼ 1.54056 A) and recorded the diffraction pattern of the sample under room

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J.-L. Qi et al. / Solid State Sciences 28 (2014) 61e66

temperature with the range of 2q between 5 and 50 . The FI-IR spectra were recorded using KBr pellets in the range 4000e 400 cm1 with a Shimadzu FTIR 8900 spectrometer. Thermogravimetric measurements were performed under a flow of nitrogen gas from room temperature to 900 C at a heating rate of 10 C/min using a Seiko Extra 6300 TG/DTA apparatus. Single crystal data were collected on a Rigaku R-Axis Rapid IP X-ray diffractometer using Mo target (l ¼ 0.71073 A). The ferroelectric properties of the solid-state samples were measured by pellets of powdered samples using Premier II ferroelectric tester at room temperature with the samples immerged in insulating oil.

positions assigned fixed isotropic thermal parameter at 1.2 times the equivalent isotropic U of the atoms to which they were attached and allowed to ride on their respective parent atoms. The hydrogen atoms from water molecules were placed by difference Fourier map. A summary of the key crystallographic information is given in Table 1 and the main data of bond distances and bond angles are showed in Table S1eS3.

2.2. Synthesis of [Ce2(H2O)3(D-tar)3]$3H2O 1

The synthesis methods of 1 and 2 stated above result in poor yield proved by lots of experiments. As a result, a series of changes were tried to improve the production rate, such as utilization of Ce(NO3)3$6H2O in stead of CeO2, raising/reducing the reaction temperature or lengthen the hydrothermal time. But no obvious improvement was observed which requested for another synthesis method such as a solution based method replacing hydrothermal reactions. 0.120 g (3 mmol) NaOH and 0.434 g (1 mmol) Ce(NO3)3$6H2O were successively added to 18 mL aqueous solution containing 0.225 g (1.5 mmol) D-()-tartaric acid. The mixture was filtered after stirring for about 5 min, and the precipitation above and title compound 1 is the same phase proved by PXRD pattern. In order to obtain crystals, hydrothermal reaction was employed along with the solution based method. The precipitation in 3 mL H2O was added to a 25 mL Teflon-lined stainless-steel autoclave and heated at 140 C for 48 h. A large number of tiny colorless crystals were obtained after cooling to the room temperature. Those resulting crystals and polymer 1 are the same phase confirmed by PXRD pattern, I.R. spectra, thermal analyses and elemental analyses. 0.24 g products were obtained (58% on the basis of the initial CeO2 input). A synthesis method similar to 1 was employed except that L-(þ)-tartaric acid was used instead of the D enantiomer and yielded polymer 2 (47% on the basis of the initial CeO2 input).

0.306 g (1.8 mmol) CeO2 and 0.301 g (2 mmol) D-()-tartaric acid were successively added to a 25 mL Teflon-lined stainless-steel autoclave charged with an ethanolic aqueous solution composed of 2 mL EtOH and 16 mL H2O. Then the mixture (pH ¼ 1.96) was heated at 140 C for 1 day and subsequently cooled to room temperature at a rate of 5 C/h. After suction filtration, a few colorless plate-like crystals were obtained. The phase purity of the precipitation was confirmed by comparing an experimental powder X-ray diffraction (PXRD) pattern with the simulated one based on the single crystal data (Fig. S1). Anal. Calc. for C12H18Ce2O24 (826.50): C, 17.44; H, 2.20%. Found: C, 16.95; H, 2.51%. IR spectrum (KBr pellet, n/ cm1): 3421s, 3043m, 2818m, 2704m, 2536w, 1597vs, 1414s, 1321m, 1286m, 1121m, 1076m, 881m, 843m, 702m, 606m, 532m. 2.3. Synthesis of [Ce2(H2O)3(L-tar)3]$3H2O 2 Complex 2 was prepared analogously to 1 except that 0.301 g (2 mmol) L-()-tartaric acid was used in place of the D enantiomer, the pH of the mixture was 1.94, and yielded a few colorless platelike crystals. The product phase purity was checked according to the experimental PXRD. Anal. Calc. for C12H18Ce2O24 (826.50): C, 17.44; H, 2.20%. Found: C, 16.97; H, 2.67%. IR spectrum (KBr pellet, n/ cm1): 3423s, 3042m, 2820m, 2702m, 2542w, 1597vs, 1416s, 1321m, 1283m, 1117m, 1076m, 879m, 841m, 704m, 604m, 532m. 2.4. Synthesis of Ce(H2O)(meso-tar)1.5 3 A 25 mL Teflon-lined stainless-steel autoclave charged with 0.308 g (1.8 mmol) CeO2 and 0.304 g (2 mmol) meso-tartaric acid in 18 mL H2O was heated at 140 C for 5 days and cooled to room temperature at a rate of 5 C/h. After suction filtration, 0.31 g colorless needle-like crystals were obtained (46% on the basis of the initial CeO2 input). The product phase purity was also checked according to the experimental PXRD. Anal. Calc. for C8H6CeO10 (380.24): C, 25.27; H, 1.59%. Found: C, 24.87; H, 1.83%. IR spectrum (KBr pellet, n/cm1): 3416m, 3085m, 1628s, 1591vs, 1443w, 1384m, 1348m, 1251m, 1103w, 1107m, 1053m, 797w, 761w, 544w, 501w. 2.5. X-ray crystallography Suitable crystals of 1e3 were selected and the measurements were carried out on a Rigaku R-Axis Rapid IP X-ray diffractometer with graphite-monochromated Mo-Ka radiation (l ¼ 0.71073 A). The reflection intensities in an appropriate q range (3.12e27.46 for 1, 3.11e27.43 for 2 and 3.29e27.45 for 3) were collected at 295 K using the u scan technique. The data were corrected for Lp effects and empirical absorption. The structures were solved by direct methods using SHELXS-97 program [34] and refined through difference Fourier synthesis with SHELX-97 program [35]. All the nonhydrogen atoms were refined on F2 anisotropic by full-matrix leastsquares method. The hydrogen atoms were located in calculated

3. Results and discussion 3.1. Syntheses

3.2. Description of the crystal structures In the triclinic polymers 1 and 2, the asymmetric unit cell contains two Ce(III) ions, three anionic tartaric acid (D-tar2- for 1, L-tar2for 2), three aqua ligands and three lattice water molecules. As depicted in Fig. 1, three crystallographically independent tar2 anions display two types of coordination fashions, m4k2O,O0 :kO00 :k2O000 ,O0000 :kO00000 (tar-A) and m2-k2O,O0 :kO00 (tar-B). Two crystallographically distinct Ce3þ ions (Ce-A and CeeB) are both in CeO9 coordination environment. Ce-A atom is coordinated by two hydroxyl oxygen atoms and five carboxylate oxygen atoms from five tartrato ligands and two oxygen atoms from two aqua ligands. CeeB atom is coordinated by an aqua oxygen atom, two hydroxyl oxygen atoms and six carboxylate oxygen atoms from five tartrato ligands. The CeeO bond distances fall in the range 2.434(5)e 2.698(5) for 1, 2.431(5)e2.694(5) A for 2, and the OeCeeO bond angles are in the range 50.4(2)e149.4(2) for 1, 50.3(2)e149.4(2) for 2 (Table S1 for 1, Table S2 for 2), which are similar to the values of [Ce2(H2O)3(D/L-tar)3]$1.5H2O reported in literature [36]. The Ce atoms are connected to each other via tar-A forming 2D networks parallel (001) which can expressed in terms of 2 ½Ce ðH OÞ ðtar AÞ 2þ . Both metal centers and organic ions can 2 2 N 3 2 be regarded as 4-connected nodes with a (42$64) net and 4$6$4$63$63$64 long topological vertex in 2D layers which structure unfolds each Ce atoms interlink four tartrato ligands as well as the ligands in turn bond four central atoms. Hence, the 2D networks can be represented as 4-connected uni-nodal net with topological representation of (42$64) (Fig. 2 for 1, Fig. S2 for 2). The resulting


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Table 1 Summary of crystal data, data collection, structure solution and refinement details for 1, 2, 3 (T ¼ 293(2)). Param

1 (D)

2 (L)

3 (Meso)

Empirical formula C12H24Ce2O24 C12H24Ce2O24 C8H6CeO10 Formula weight 832.55 832.55 380.24 Crystal habit, color Colorless, block Colorless, block Colorless, needle Crystal system Triclinic Triclinic Monoclinic Space group P1 P1 P21/n a ( A) 6.119(1) 6.123(1) 5.952(1) b ( A) 7.459(1) 7.471(2) 10.165(2) c ( A) 13.061(2) 13.063(2) 15.606(3) a ( ) 88.22(1) 88.25(3) 90 b ( ) 81.16(1) 81.24(3) 91.83(3) g ( ) 89.29(1) 89.27(3) 90 3 588.75(3) 588.75(3) 943.7(3) Volume (A ) Z 1 1 4 3 Density (calculated, g cm ) 2.348 2.348 2.676 Measured reflections 5827 5782 9012 Independent reflections 4392 4372 2147 Reflection with I 2s(I) 4339 4288 2056 F(000) 404 404 728 Crystal size (mm) 0.300 0.140 0.130 0.280 0.160 0.110 0.370 0.130 0.100 q range for data collection ( ) 3.12e27.46 3.11e27.43 3.29e27.45 Rint 0.0332 0.0354 0.0535 Number of parameters 344 343 160 Goodnesseofefit on F2 1.035 1.090 1.063 R1, wR2 [I 2s(I)]a 0.0337, 0.0843 0.0280, 0.0714 0.0244, 0.0576 R1, wR2 (all data)a 0.0340, 0.0848 0.0287, 0.0732 0.0269, 0.0585 drmax, drmin (e$ A3) 2.390, 2.365 2.040, 1.549 1.350, 1.678 P P P a R1 ¼ (jFoj jFcj)/ jFoj, wR2 ¼ [ w (F2o F2c )2/Sw (F2o)2]1/2, and w ¼ [s2(F2o) þ (aP)2 þ bP]1 where P ¼ (F2o þ 2 F2c )/3. For 1, a ¼ 0.0474 and b ¼ 0.8808. For 2, a ¼ 0.0395 and b ¼ 0.2238. For 3, a ¼ 0.0168 and b ¼ 1.8630.

layers are held together by tar-B ligands building up 3D 3 f½Ce ðH OÞ ðtar AÞ ðtar BÞg metal-organic framework 2 2 N 3 2 (MOF). Two Ce atoms from adjacent layers were linked by tar-B which acts as two-connected linker, leading to Ce atoms become

5-connected nodes with (42$67$8) net and a long schläfli notation 4$4$6$6$6$62$63$63$64$812. The tar-A ligand retain 4-connectivity with schläfli symbol (42$64) and circuits 4$6$4$64$64$64. As a result, the whole topology can be described as (4,5)-connected

Fig. 1. Ortep view of the coordination environments of Ce3þ ions and tartrato ligands with displacement ellipsoids (45% probability) and atomic labeling in 1, 2 and 3 (Symmetry transformations used to generate equivalent atoms: see Table S1eS3).

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Fig. 2. 2D

Fig. 3. 2D

2 ½Ce ðH OÞ ðD 2 2 N 3

2 ½CeðH OÞðmeso 2 N

þ

tarÞ2 network, 3D

þ

tarÞ network, 3D

3 f2 ½Ce ðH OÞ ðD 2 2 N N 3

3 f2 ½CeðH OÞðmeso 2 N N

tarÞ2 ðD tarÞg framework along with the corresponding topological nets in 1.

tarÞðmeso tarÞ0:5 g framework along with the corresponding topological nets in 3.


dinodal net topology of (42$64)(42$67$8). The lattice water molecules are located in tunnels between layers and contributed themselves to the stabilization of 3D framework through the extensive hydrogen bonds (Table S1eS2). The asymmetric unit cell of 3 is composed of a Ce(III) ion, one (tarA) and a half (tar-B) anionic meso-tartaric acid and one coordinated water molecule (Fig. 1). Tar-A shows a m3-k2O,O0 :kO00 :k2O000 ,O00000 coordination fashion. The tar-B center is located in wyckoff 2b site with m4-k2O,O0 :kOPrime;:k2O000 ,O00000 :kO000000 binding mode. The Ce atoms are each coordinated by an aqua oxygen atom, three hydroxyl oxygen atoms and five carboxylic oxygen atoms of five tar2 anions to form CeO9 chromophore. All the CeeO bond lengths are very close (2.434(5)e2.698(5) A) and the OeCeeO bond angles have a significant difference (59.9(1)e144.7(1) ) (Table S3). Both metal centers and organic anions are 3-connected nodes lead to (63) nets with a 2þ long topological vertex symbols 6$6$6 in 2N ½CeðH2 OÞðtar AÞ 2D layers parallel (001) which structure unfolds each Ce atom interconnects each other by three tar2 ions (tar-A), as well as every tar-A ion in turn links three metal atoms, and the topological representation of 2D layers can be describe as (63). The other crystallographically distinct anionic tartaric acid (tar-B) functions as pillar and links the resulting 2D layers into 3D 3 f2 ½CeðH OÞðtar AÞðtar BÞ 2 N N 0:5 g framework. Each pillar tar-B joins four Ce atoms from two layers which seems like the character ‘X’, acting as 4-connected nodes with Schläfli symbol (44$62) and circuits 4$4$4$4$62$62. As a result, the metal centers turn out to be 5-connectivity with (43$67) net and a long schläfli notation 4$4$4$6$6$6$6$6$6$62. The anionic tartaric acid in layers remain 3connectivity with schläfli symbol (42$64) and circuits 4$62$2. Consequently, the 3D metal-organic framework topologically can be describes as a (3,4,5)-connected net of (4$62)2(43$67)2(44$62) (Fig. 3). The carboxylic groups, hydroxyl groups and water molecules are engaged in OeH/O hydrogen bonds, which obviously strengthen the stabilization of 3D MOF.

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polymers 1 and 2 undergo two extremely similar TG movements while 3 displays different thermal decomposition course. The TG behaviors of 1 and 2 show a first weight loss from 45 to 260 C in good agreement with the removal of three lattice water molecules and three coordinated water molecules (observed value: 12.6%, calc value: 13.1%). Upon heating, the dehydrate intermediate “Ce2(tar)3” subsequently undergoes a framework collapse, and simultaneously anionic tartaric acid decompose. Finally, the remnant (48.4%) collected at 900 C is assumed to be a mixture of Ce2O3 and C. In comparing with 1 and 2, compound 3 is found to be stable from room temperature to 310 C which indicates the strong coordination interactions leading no coordination water molecule losing. When further heated, the losing of H2O molecules and the decomposition of anionic tartaric acid in continuous steps. The final residue of 51.9% at 900 C is assumed to be Ce2O3 and C. 3.5. Ferroelectric properties As we all know, the complexes belonging to ten polar point groups (C1, Cs, C2, C2n, C3, C3n, C4, C4n, C6 and C6n) may possess ferroelectric properties. Fortunately, both enantiomorphs 1 and 2 crystallize in space group P1 associated with point group C1, thus, the ferroelectric behaviors were investigated. The electrical hysteresis loops of 1 and 2 were recorded at room temperature, using powder samples in pellets (Fig. 4). Experimental results indicate

3.3. Infrared spectra The infrared spectra of complexes 1e3 are shown in Fig. S3. As the influence of OeH/O hydrogen bonds, eOH groups on water molecules and tartaric acids in 1e3 are observed at broad dispersion bands in the range 2400e3500 cm1, leading to the CeH stretch submersion in this range simultaneously. The spectra of compounds 1 and 2 are exactly similar which is in good agreement with the X-ray structural analyses, so describe the spectra features of 1 that also reflects analogous discussion of 2. Two strong sharp peaks centered at 1597 and 1414 cm1 correspond to the asymmetric and symmetric eCOO vibrations of the tartrato ligands, respectively. The coupling of stretching and in-plane bending vibration of CeOH on anionic tartaric acid brings about splitting in two peaks centered at 1321 and 1286 cm1. The present relatively board multiplets in the range 1058e1121 cm1 can be attributed to the linear CeCeCeC skeleton vibration of the tartrato ligands. Coordination polymer 3 exhibits an infrared spectrum different from 1 and 2. The doublet absorption peaks at 1628 and 1591 cm1 result from the asymmetric vibrations of two crystallographically distinct tartarto ligands and the symmetric vibrations of the carboxylate groups cause a middle absorption peaks at 1348 cm1 and two shoulders at 1384 and 1443 cm1. The middle relatively board bands in the range 1012e1103 cm1 can also be assigned to the linear CeCeCeC skeleton vibration of the tartarto ligands. 3.4. Thermal analyses Thermogravimetric analyses for compounds 1e3 have been measured from 30 to 900 C. As shown in Fig. S4, the two isomer

Fig. 4. Electric hysteresis loops for a pellet obtained from powdered samples of 1 and 2, respectively.

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typical ferroelectric feature with a remnant polarization (Pr) is of ca. 0.009 mC cm2 for 1 (ca. 0.015 mC cm2 for 2) and coercive field (Ec) of ca. 0.570 kV cm1 for 1 (ca. 0.505 kV cm1 for 2). The spontaneous saturation polarization (Ps) is of ca. 0.233 mC cm2 for 1 (ca. 0.171 mC cm2 for 2), which are slightly smaller than that of the typical ferroelectric Rochelle salt (NaKC4H4O6$4H2O, Ps ¼ 0.25 mC cm2) [37]. Many different kinds of intramolecular and intermolecular hydrogen bonds are the important factors of the ferroelectric properties. 4. Conclusion Hydrothermal reactions of cerium(III) cation with three tartaric acid isomers yielded three new coordination polymers [Ce2(H2O)3(D-tar)3]$3H2O 1, [Ce2(H2O)3(L-tar)3]$3H2O 2, and Ce(H2O)(meso-tar)1.5 3. Compound 1 and 2 can be described as (4,5)-connected topology network with schläfli symbol of (42$64)(42$67$8), the metal-organic framework topologically of 3 is a (3,4,5)-connected net of (4$62)2(43$67)2(44$62). The results above illustrate that employing different configurational isomers of one substance as ligands in a specific metal environment could obtain different MOFs, also different topology. Furthermore, compound 1 and 2 contain chiral ligands, and crystallize in polar triclinic space group P1, while 3 is prepared by achiral meso-tartrate, and crystallizes in centrosymmetric space group P21/n. These above also reveal that using chiral building blocks is a significant effective way to prepare acentric coordination complexes. The preliminary measurements show 1 and 2 are potential candidates for ferroelectric materials. Acknowledgments This project was supported by the Open Foundation from Application of Nonlinear Science and Technology in the Most Important Subject of Zhejiang (Grant No. xkzl2006). The honest thanks are also extended to K.C. Wong Magna Fund in Ningbo University. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.solidstatesciences.2013.12.006. References [1] X.D. Zhu, Z.J. Lin, T.F. Liu, B. Xu, R. Cao, Cryst. Growth Des. 12 (2012) 4708e 4711.

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