Photoluminescent and Magnetic Properties of a Series of Lanthanide

DOI: 10.1021/cg100817s

Photoluminescent and Magnetic Properties of a Series of Lanthanide Coordination Polymers with 1H-Tetrazolate-5-formic Acid

2011, Vol. 11 372–381

Mei-Feng Wu,†,‡ Ming-Sheng Wang,† Sheng-Ping Guo,† Fa-Kun Zheng,*,† Hui-Fen Chen,† Xiao-Ming Jiang,†,‡ Guang-Ning Liu,†,‡ Guo-Cong Guo,*,† and Jin-Shun Huang† †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China, and ‡Graduate School, Chinese Academy of Sciences, Beijing 100039, P. R. China Received June 18, 2010; Revised Manuscript Received November 30, 2010

ABSTRACT: Reaction of lanthanide oxides and 1H-tetrazolate-5-ethyl formate in water at 50 °C in the presence of pyridine afforded a series of lanthanide coordination polymers, {(Hpy)2[Pr4(tzf)7(H2O)11] 3 10H2O}n (1) and {[Ln2(tzf)3(H2O)6] 3 4H2O}n, where Ln = Sm (2), Eu (3), Gd (4), Tb (5), Dy (6), Hpy = protonated pyridine, and H2tzf = 1H-tetrazolate-5-formate acid. Polymer 1 features a three-dimensional (3-D) anion framework built up by cross-linkage of tetrameric [Pr4(tzf)4]4þ units and tzf2- ligands, showing a six-connected primitive cubic (pcu) topology (412 3 63). Isomorphous polymers 2-6 present a twodimensional (2-D) neutral structure with a four-connected (4,4) topology. Four new coordination modes of the tzf2- ligand are first observed in tetrazolate-5-carboxylate complexes. The lanthanide contraction effect and coordination flexibility of tzf2ligand play a key role in governing the formation of coordination polymers with different polymeric architectures. The photoluminescent analyses for 2, 3, 5, and 6 suggest that ligand-to-Ln(III) energy transfer is efficient, especially for Eu(III) and Tb(III) ions, and the tzf2- ligand may act as a good “antenna molecule” to sensitize Ln(III) emission. The variable-temperature magnetic study shows that magnetic interactions between the Ln(III) ions in 1-6 are mainly ascribed to the antiferromagnetic coupling as well as the depopulation of the Stark levels. The spin-orbit coupling parameters (λ) for Sm(III) in 2 and Eu(III) in 3 are 259(2) and 344(1) cm-1, respectively, based on the free-ion approximation.

Introduction Much attention has been paid to exploration of lanthanide coordination polymers in recent years, which show characteristic luminescent emissions and generally possess a large anisotropic magnetic moment arising from a large number of spins and strong spin-orbit coupling, because of the unique nature of Ln(III) ions with partially filled 4f orbitals, large radii, and high and variable coordination numbers.1 The special properties have triggered wide applications of lanthanide complexes especially for the development of optical and magnetic materials.2,3 To a considerable extent, the structures and performances of lanthanide coordination polymers are influenced by the nature of organic bridging ligands, which can act as magnetic mediators and/or luminescent sensitizers via the “antenna effect”. Thus, a well-selected ligand is one of the key factors in building up the lanthanide coordination polymers with desired structures and expected properties.4 In general, some important requirements should be considered for the organic ligands selected, which can offer electronic transfer interactions between the Ln(III) ions and absorb the ultraviolet (UV) light to sensitize lanthanide emission by efficient energy transfer. Many carboxylic acids (aromatic acids, aliphatic acids, N-heterocyclic acids, etc.) have been successfully used to construct lanthanide coordination polymers5-7 and have been demonstrated to be a rational choice to support strong Ln(III)-centered luminescent emission as well as significant magnetic interactions between the Ln(III) ions.8 On the other hand, 5-substituted tetrazoles are also excellent *Author to whom correspondence should be addressed. E-mail: zfk@ fjirsm.ac.cn (F.-K.Z.); [email protected] (G.-C.G). pubs.acs.org/crystal

Published on Web 01/05/2011

bridging ligands for the formation of coordination polymers9 with interesting optical10 and magnetic11 properties as well as hydrogen storage ability.12 However, much of the research on the chemistry of 5-substituted tetrazoles has been focused on transition metal coordination polymers, and only a few isolated lanthanide complexes have been described in the literature.13 In this context, the carboxylate group deserves to be the preferred substituent for 5-substituted tetrazolate derivatives, denoted as tetrazolate-5-carboxylate, employed in synthesizing lanthanide coordination polymers. The tetrazolate-5-carboxylate ligand possesses at least four nitrogen and two oxygen electron-donating atoms and could exhibit versatile coordination modes upon metal complexation (Scheme S1, Supporting Information). However, the investigation on tetrazolate-5-carboxylate complexes remains limited.14-17 Recently, we have used 1H-tetrazolate-5-formic acid (H2tzf) and 1H-tetrazolate-5-acetic acid (H2tza) to synthesize transition metal coordination polymers, which display interesting magnetic or photoluminescent properties.17 It is worth noting that the lanthanide coordination polymers based on tetrazolate-5carboxylate ligands have not been explored. In this study, we chose the simplest tetrazolate-5-carboxylate ligand, H2tzf, as an organic linker for the preparation of lanthanide coordination polymers. A series of novel lanthanide coordination polymers were obtained, namely, three-dimensional (3-D) polymeric {(Hpy)2[Pr4(tzf)7(H2O)11] 3 10H2O}n (1) and twodimensional (2-D) polymeric {[Ln2(tzf)3(H2O)6] 3 4H2O}n, where Ln = Sm (2), Eu (3), Gd (4), Tb (5), Dy (6), and Hpy = protonated pyridine. Four new coordination fashions of tzf2- ligand are first found in tetrazolate-5-carboxylate r 2011 American Chemical Society

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Crystal Growth & Design, Vol. 11, No. 2, 2011

Scheme 1. Four New Coordination Modes of the tzf2- Ligand Described in This Study

complexes (Scheme 1). The investigation indicates that tzf2ligand would be a promising emission sensitizer as well as magnetic coupling mediator for lanthanide ions. To the best of our knowledge, 1-6 are the first instances of lanthanide coordination polymers constructed by tetrazolate-5-carboxylate ligands. Experimental Section Materials and Instrumentation. All chemicals except Na2(tzf) were commercially available sources of analytical grade and used without further purification. Na2(tzf) used in the analysis of optical spectra was prepared by the reaction of 1H-tetrazolate-5-ethyl formate (Htzef) in dilute aqueous solution of NaOH. The elemental analyses were performed on an Elementar Vario EL III microanalyzer. The FT-IR spectra were obtained on a Perkin-Elmer spectrum using KBr disks in the range 4000-400 cm-1. The determination of photoluminescence (PL) and lifetime was conducted on a singlegrating Edinburgh EI920 fluorescence spectrometer equipped with a R928P PMT detector. Thermogravimetric analysis (TGA) experiments were done on a NETZSCH STA 449C Jupiter thermogravimetric analyzer under N2 atmosphere with the sample heated in an Al2O3 crucible at a heating rate of 10 K min-1. Powdered X-ray diffraction (PXRD) patterns were collected on a PANalytical X0 pert PRO diffractometer using Cu-KR radiation (λ = 1.540598 A˚) at 40 kV and 40 mA in the range of 5° e 2θ e 45°. The simulated patterns were derived from the Mercury Version 1.4 software (http: //www.ccdc.cam.ac.uk/products/mercury/). The variable-temperature magnetic susceptibilities were measured with a Quantum Design MPMS-XL superconducting quantum interference device (SQUID) magnetometer in the temperature range of 2-300 K at a field of 1 kOe, and diamagnetic corrections were made using Pascal’s constants. Syntheses of 1-6. All six new polymers 1-6 were prepared following a similar procedure. The reaction mixture of lanthanide oxide (0.25 mmol) (Ln2O3, Ln = Sm, Eu, Gd, and Dy; Pr6O11 or Tb4O7) and 1H-tetrazolate-5-ethyl formate (Htzef) (0.1775 g, 1.25 mmol) in 8.0 mL of distilled water at 50 °C was stirred for half an hour and gave rise to a concentrated solution. Then 1.6 mL of pyridine was added dropwise into the solution. The resulting solution was stirred continually at 50 °C for two days, and then filtered into a glass beaker for crystallization. The glass beaker was covered with a porous preservative film, and the filtrate was allowed to slowly evaporate at room temperature. After about 2-3 weeks, block or prismatic crystals suitable for X-ray analysis were obtained. Yield: 55% (based on Pr) for 1; 22% (based on Sm) for 2; 47% (based on Eu) for 3; 65% (based on Gd) for 4; 26% (based on Tb) for 5; 31% (based on Dy) for 6. Anal. Calcd for C24H54N30O35Pr4 (1): C, 15.28; H, 2.89; N, 22.27%. Found: C, 15.29; H, 2.81; N, 22.27%. Anal. Calcd for C6H20N12O16Sm2 (2): C, 8.82; H, 2.47; N, 20.57%. Found: C, 8.96; H, 2.44; N, 20.58%. Anal. Calcd for C6H20Eu2N12O16 (3): C, 8.79; H, 2.46; N, 20.49%. Found: C, 8.94; H, 2.37; N, 20.31%. Anal. Calcd for C6H20Gd2N12O16 (4): C, 8.67; H, 2.43; N, 20.23%. Found: C, 8.75; H, 2.33; N, 20.16%.

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Anal. Calcd for C6H20N12O16Tb2 (5): C, 8.64; H, 2.42; N, 20.15%. Found: C, 8.84; H, 2.29; N, 20.11%. Anal. Calcd for C6H20Dy2N12O16 (6): C, 8.57; H, 2.40; N, 19.98%. Found: C, 8.69; H, 2.29; N, 19.88%. IR (KBr, cm-1) for 1: 3260 (br), 1599 (s), 1498 (s), 1439 (s), 1404 (s), 1335 (s), 1200 (m), 1167 (m), 1086 (m), 1052 (m), 838 (m), 823 (w), 751 (m), 680 (m), 493 (w); 2: 3240 (br), 1604 (s), 1519 (s), 1445 (s), 1412 (s), 1342 (s), 1201 (m), 1160 (m), 1084 (m), 1059 (m), 847 (m), 824 (w), 682 (m), 496 (w); 3: 3238 (br), 1608 (s), 1521 (s), 1447 (s), 1412 (s), 1343 (s), 1202 (m), 1161 (m), 1084 (m), 1059 (m), 848 (m), 824 (w), 682 (m), 496 (w); 4: 3243 (br), 1613 (s), 1524 (s), 1449 (s), 1413 (s), 1343 (s), 1202 (m), 1160 (m), 1084 (m), 1059 (m), 848 (m), 825 (w), 682 (m), 497 (w); 5: 3237 (br), 1608 (s), 1525 (s), 1452 (s), 1413 (s), 1345 (s), 1201 (m), 1161 (m), 1084 (m), 1059 (m), 849 (m), 824 (w), 683 (m), 497 (w); 6: 3221 (br), 1611 (s), 1526 (s), 1455 (s), 1413 (s), 1346 (s), 1202 (m), 1160 (m), 1084 (m), 1059 (m), 849 (m), 824 (w), 683 (m), 497 (w). The experimental PXRD patterns of 1-6 agree well with the simulated ones based on the single-crystal X-ray data (Figure S1, Supporting Information), implying that 1-6 are in a pure phase. Single-Crystal Structures Determination. The single-crystal X-ray diffraction measurement was performed on a Rigaku Saturn 70 CCD diffractometer for 1, Rigaku AFC7R for 2 and 4, Rigaku SCX mini for 3 and 6, and Rigaku Mercury CCD for 5, respectively, which were all equipped with Mo-KR radiation (λ = 0.71073 A˚). There was no evidence of crystal decay during data collection, denoting the obtained polymers are stable at ambient temperature. The intensity data sets were collected with the ω scan technique and corrected for Lp effects. The primitive structures were solved by the direct method and reduced by the CrystalClear software.18 The subsequent successive difference Fourier syntheses yielded the other non-hydrogen atoms. The final structure was refined using a fullmatrix least-squares refinement on F2. All non-hydrogen atoms except for the protonated pyridine molecules in 1 were refined anisotropically. The protonated pyridine molecules in 1 have been refined by applying the AFIX 66 command to impose a regular hexagonal geometry with all six atoms as carbon. And two disordered water molecules (O1W and O10W) in 1 have been treated as a diffuse contribution to the overall scattering without specific atom positions by SQUEEZE/PLATON.19 The hydrogen atoms of protonated pyridine molecules were calculated in idealized positions of the benzene ring and allowed to ride on their parent atoms. The hydrogen atoms of all lattice water molecules in 1-6 were not included, and those of coordinated water molecules were located in the idealized positions and refined with O-H distances restrained to a target value of 0.85 A˚ and Uiso (H) = 1.5Ueq (O). All of the calculations were performed by the Siemens SHELXTL version 5 package of crystallographic software.20 Notably, one of crystallographically independent tzf2- ligands in 1 is statistically disordered over two positions (A and B) in opposite orientations but in the same coordination mode (Scheme S2, Supporting Information). The occupancy factors for two positions were refined to be 0.80 versus 0.20 with the disordered O/N positions labeled as X. The tzf2- ligand in each disordered pair is related by sharing an O2C2N2 motif, hence leading to an approximate bitetrazolate model. The N-N distances of the tetrazolate ring in B position were constrained to 1.380, 1.300, and 1.400 A˚ for N31S-N32S, N32S-N33S, and N33S-N34S, respectively. For clarity, only one disordered component A is illustrated in the structural diagram of 1. The related crystal data and structure refinement results for 1-6 are given in Table 1. The selected bond distances are in the Supporting Information, Table S1. The CCDC reference numbers are 797452, 797453, 797454, 797455, 797456, and 797457 for 1-6, respectively.

Results and Discussion Syntheses. The synthetic route of six novel tzf2--based Ln(III) polymers 1-6 is shown in Scheme 2. All of these polymers exhibit good stability under ambient conditions for a long time and are insoluble in water and common organic solvents. The tzf2- ligand in 1-6 was generated by the in situ hydrolyzation of its ester, 1H-tetrazolate-5-ethyl formate (Htzef), in aqueous solution in the presence of pyridine. It is worthy to note that in situ decarboxylation of the tzf2-

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Table 1. Pertinent Crystal Data and Structure Refinement Results for 1-6 1

2

3

C6H20N12O16Sm2 C6H20Eu2N12O16 formula C24H54N30O35Pr4 fw 1886.61 817.04 820.26 crystal system monoclinic monoclinic monoclinic Pc Pc space group P21/c a (A˚) 17.086(7) 11.0765(19) 11.056(5) b (A˚) 21.121(8) 12.696(4) 12.677(5) c (A˚) 17.030(6) 9.0014(17) 8.965(4) R (deg) 90 90 90 β (deg) 99.158(6) 107.980(14) 107.981(4) γ (deg) 90 90 90 3 6067(4) 1204.0(5) 1195.1(8) V (A˚ ) Z 4 2 2 2.065 2.254 2.279 Dc (g cm-3) -1 3.272 4.923 5.294 μ (mm ) F (000) 3696 784 788 reflns (measured) 40380 2384 7391 reflns (unique) 11187 2384 4094 0.0515 0.0000 0.0258 Rint 1.011 1.065 1.057 GOF on F2 0.0640 0.0285 0.0257 R1a [I > 2σ(I)] 0.1778 0.0735 0.0584 wR2b (all data) P P P a b 2 2 2 P 2 2 1/2 R1 = (Fo - Fc) / Fo. wR2 = [ w(Fo - Fc ) / w(Fo ) ] .

4

5

6

C6H20Gd2N12O16 830.84 monoclinic Pc 11.062(5) 12.646(6) 8.947(3) 90 108.17(3) 90 1189.2(9) 2 2.320 5.623 792 4579 2346 0.1022 1.011 0.0425 0.1072

C6H20N12O16Tb2 834.18 monoclinic Pc 11.0653(14) 12.6079(17) 8.9055(13) 90 108.350(5) 90 1179.2(3) 2 2.349 6.044 796 7514 3761 0.0287 1.088 0.0268 0.0702

C6H20Dy2N12O16 841.34 monoclinic Pc 11.0182(7) 12.5780(8) 8.8819(6) 90 108.334(2) 90 1168.43(13) 2 2.391 6.442 800 7219 3691 0.0294 1.034 0.0304 0.0783

Scheme 2. The Synthetic Route of 1-6

ligand easily happens at higher temperature under solvothermal reaction.17c,21 Thus, in order to avoid decarboxylation of the tzf2- ligand, we succeeded in synthesizing 1-6 by means of the solution reaction in the air at 50 °C. Although weak basic pyridine does not take part in coordination with lanthanide ions, it can play an important role in adjusting the acidity of the reactant mixture and have a great impact on the formation of coordination polymers. Tetrazoles and their substituted derivatives have similar pKa values (≈4) to carboxylate acids,22 which results in protonation of pyridine to meet the charge balance in 1. When we tried to replace pyridine with a small amount of NaOH, powdered lanthanide hydrates easily precipitated. Hence, the choice of aciditycontrollable compound (pyridine in this study) is a crucial factor in assembling lanthanide coordination polymers. While we carefully manipulated the evaporation rate of the filtrate, single crystals with a large size of ca. 0.60 0.30 0.10 cm3, especially for Eu(III) and Gd(III) polymers, were obtained (Figure S2, Supporting Information). Description of the Structures. Single-crystal X-ray diffraction analyses revealed that 2-6 are isomorphous, so we chose 1 and 3 for detailed structural discussions. {(Hpy)2[Pr4(tzf)7(H2O)11] 3 10H2O}n (1). Polymer 1 crystallizes in the centrosymmetric space group P21/c and displays a complicated 3-D anion framework constructed by the linkage of tetrameric [Pr4(tzf)4]4þ units and tzf2- ligands. The asymmetric unit of 1 contains an anionic motif [Pr4(tzf)7(H2O)11]2-, ten lattice water molecules, and two protonated pyridine cations. As displayed in Figure 1, there are four crystallographically independent Pr(III) centers (Pr1, Pr2, Pr3 and Pr4) and seven crystallographically

Figure 1. Molecular structure of [Pr4(tzf)7(H2O)11]2- in 1 with seven crystallographically independent tzf2- ligands marked as LI-LVII. The coordinated water molecules coordinated to Pr(III) centers were not labeled. Symmetry codes A: 1 - x, 1 - y, 1 - z; B: x, 0.5 - y, -0.5 þ z; C: -x, 1 - y, -z; D: -x, -0.5 þ y, 0.5 - z; E: x, 0.5 - y, 0.5 þ z; F: -x, 0.5 þ y, 0.5 - z.

independent tzf2- ligands (marked as LI-LVII). All the Pr(III) centers are nine-coordinated with the coordination geometry described as a distorted tricapped trigonal polyhedron depicted in Figure S3, Supporting Information. The Pr1 and Pr2 atoms are both surrounded by two chelating

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tzf2- ligands in a bidentate N,O-chelated mode, two terminal tzf2- ligand through a carboxylate oxygen atom, and three coordinated water molecules. The Pr3 atom is coordinated by three chelating and one terminal tzf2- ligands, and two coordinated water molecules. As for the Pr4 atom, it is bound to three chelating tzf2- ligands and three coordinated water molecules. The Pr-N bond has a longer average distance of 2.633(3) A˚ relative to that of the Pr-O bond (2.519(2) A˚). The coordination modes of seven crystallographically independent tzf2- ligands in 1 are divided into four types, resulting in the construction of intricate metal-organic framework. The LI ligand adopts a bidentate N,O-chelatedO-terminated mode, μ2-κN1,O1:κO1 (Scheme 1a), to link Pr1 and Pr3 centers. The LII, LIV and LVII ligands assume a tridentate N,O-chelated-O0 -terminated mode, μ2-κN1,O1:κO2 (Scheme 1b), to connect two metal centers (Pr4F and Pr1, Pr3 and Pr2E, Pr4 and Pr2C, respectively). The LIII and LVI ligands act as a tetradentate organic linker bound to Pr3 and Pr1A, Pr2 and Pr4 centers, respectively, in a bis(N,O-chelated) mode, μ2-κN1,O1:κN4,O2 (Scheme 1c), which resembles that of the oxalate group with an O2C2N2 bridge. Finally, the LV ligand displays a tetradentate bis(N,O-chelated)-Oterminated mode, μ3-κN1,O1:κN4,O2:κO1 (Scheme 1d), to connect Pr1, Pr2 and Pr3 centers. Interestingly, the tzf2ligand tends to chelate the Pr(III) center in an N,O-chelated mode to form a steady five-membered cyclic LnOC2N ring. Compared with the reported coordination modes (Scheme S1, Supporting Information), four coordination modes in 1 are first found in tetrazolate-5-carboxylate complexes to date. Furthermore, the coexistence of four different coordination modes in one tetrazolate-5-carboxylate complex is rarely observed. The most striking structural feature for 1 is that two kinds of tetrameric units [Pr4(tzf)4]4þ are interlinked via the tzf2ligands to generate a 3-D anionic framework of {[Pr4(tzf)7(H2O)11]2-}n. The LI and LIII ligands connect alternately to Pr1 and Pr3 centers, giving rise to a centrosymmetric tetrameric unit A of [Pr1LIPr3LIII]2. Similarly, Pr2 and Pr4 centers are joined together alternately by LVI and LVII ligands into another tetrameric unit B of [Pr2LVIPr4LVII]2. In tetrameric units A and B, four Pr(III) atoms display a quadrangular arrangement, which differs from the tetrahedral array in the reported tetranuclear lanthanide complexes.23 The major differences between tetrameric units A and B lie in the bridging fashions of LI and LVII ligands as well as the coordination environments of Pr3 and Pr4 centers, described in foregoing discussions. Each tetrameric unit A is linked to four tetrameric units B, and each tetrameric unit B is connected to four tetrameric units A, both via the bridges of LII and LIV ligands, which produces a 2-D layer structure parallel to the ab plane (Figure 2). The formed 2-D layer exhibits a (4,4)-connected topology with each tetrameric unit [Pr4(tzf)4]4þ as a four-connected node. Hence, all LI-LVII ligands except LV have accomplished their peripheral coordination to form tetrameric units [Pr4(tzf)4]4þ and a 2-D layer structure. Now taking the LV ligand into account, each LV ligand serves as a μ3-linker, chelating Pr2 center in the tetrameric unit B in an N,O-chelated mode, and connecting Pr3 and Pr1 centers in the tetrameric unit A from neighboring layer in an N0 ,O0 -chelated-O0 -terminated mode. Thus, tetrameric units A and B between adjacent layers are connected to each other through the LV ligand to generate a 3-D anionic {[Pr4(tzf)7(H2O)11]2-}n framework of 1 with small channels along the c direction with the dimensions about


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Figure 2. The 2-D network constructed by the linkage of μ2-bridging tzf2- ligands LI-LIV, LVI, and LVII with tetrameric units A and B.

Figure 3. View of the pcu 412 3 63 network topology in 1 built by tetrameric units A and B as six-connected nodes and LII, LIV, and LV ligands as linkers.

3.9 4.4 A˚ (Figure S4, Supporting Information). To shed light on the complicated 3-D framework, each tetrameric unit is considered as one six-connected node to bound another six neighboring tetrameric units through LII, LIV, and LV ligands, resulting in the six-connected primitive cubic topology (pcu) (412 3 63) (Figure 3). Calculated from PLATON,19 1 has approximately 36% solvent accessible volume, and all pyridinium cations as counterions and lattice water molecules reside in the voids except small channels. {[Ln2(tzf)3(H2O)6] 3 4H2O}n (2-6). Isomorphous polymers 2-6 crystallize in the acentric monoclinic space group Pc and feature a 2-D network. Polymer 3 was chosen as a representative for their structural discussions. The asymmetric unit in 3 contains a neutral motif [Eu2(tzf)3(H2O)6] and four lattice water molecules. As shown in Figure 4, there are two crystallographically independent Eu(III) centers (nine-coordinated Eu1 and eight-coordinated Eu2) and three crystallographically independent tzf2- ligands (marked as Li-Liii). The nine-coordinated Eu1 atom is chelated by four tzf2- ligands in a bidentate N,O-chelated mode, filling up eight coordination sites of the Eu1 coordination polyhedron (O11/N11, O21/N21, O12B/N14B, O32D/N34D).

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Figure 4. Molecular structure of [Eu2(tzf)3(H2O)6] in 3 with three crystallographically independent tzf2- ligands marked as Li-Liii. Symmetry codes A: -1 þ x, y, -1 þ z; B: x, 1 - y, -0.5 þ z; C: x, 1 y, 0.5 þ z; D: 1 þ x, y, 1 þ z.

The remaining one site is taken up by one coordinated water molecules (O6W) to furnish a distorted tricapped trigonal prism (Figure S5, Supporting Information). The eightcoordinated Eu2 atom is coordinated by one terminal tzf2ligand through a carboxylate oxygen atom (O22), one chelating tzf2- ligand (O31/N31), and five coordinated water molecules (O1W, O2W, O3W, O4W, and O5W) to form a distorted square antiprism (Figure S5, Supporting Information). The crystallographically independent Li-Liii ligands adopt two different coordination modes to bind the Eu(III) centers. As shown in Figure 4, the Li and Liii ligands exhibit the same mode as LIII and LVI ligands in 1 (Scheme 1c), and the Lii ligand displays the same mode as the LII, LIV, and LVII ligands in 1 (Scheme 1d). The Eu-O bond distances range from 2.363(2) to 2.479(2) A˚, and the Eu-N bond lengths are a little longer ranging from 2.577(2) to 2.628(2) A˚. These values are comparable to those reported for other oxygen/nitrogen-donated Eu(III) complexes.24 Notably, the Eu2-O22 distance (2.363(2) A˚) is slightly shorter than other five Eu(III)-Ocarboxylate distances (2.421(2)-2.479(2) A˚) (Table S1, Supporting Information), which indicates that the terminal carboxylate oxygen atom presents a shorter interatomic distance than those involving the chelated carboxylate oxygen atoms.25 The highly unsymmetric coordination environment of the metal and ligand is responsible for a final acentrosymmetric 2-D structure. First, Li ligands link Eu1 centers to afford an infinite zigzag chain along the c direction with the shortest Eu1 3 3 3 Eu1 distance of 6.418(2) A˚ and a nearly right-angled Eu1 3 3 3 Eu1 3 3 3 Eu1 of 88.594(5)° (Figure 5a). Then, the Eu1 centers between adjacent chains are connected to each other through Lii-Eu2Liii bridging units, resulting in a 2-D neutral [Eu2(tzf)3]n puckered layer structure parallel to the ac plane (Figure 5b), differing from the 3-D anion {[Pr4(tzf)7]2-}n framework in 1. The shortest distance of Eu1 3 3 3 Eu1 between adjacent chains is 11.892(4) A˚. In view of structural topology, each Eu1 atom is considered as one four-connected node to link four neighboring Eu1 atoms through bridging Lii-Eu2-Liii units and Li ligands. As a result, a 2-D (4,4)-connected topology is presented as shown in Figure 5c. In the puckered 2-D layers in 3 a series of channels exists along the c direction with the dimensions about 5.2 7.1 A˚, in which lattice water molecules (O7W and O8W) reside (Figure S6, Supporting Information). The adjacent layers are stacked in an -AAA- sequence along the b direction (Figure S7, Supporting Information). The calculation by

Figure 5. (a) The zigzag chain along the c direction constructed by Eu1 atoms and Li ligands. (b) The 2-D framework of 3 parallel to the ac plane with the zigzag chains linked together through Lii-Eu2-Liii bridging units. (c) View of the 2-D (4,4)-connected topology of 3 with Eu1 centers as four-connected nodes and Lii-Eu2-Liii units and Li ligands as linkers.

PLATON suggests that there is approximately 209 A˚3 of solvent accessible volume (ca. 17.5% of the volume of the unit cell). The voids between the adjacent layers are occupied by O9W and O10W. On the basis of calculated O 3 3 3 N/O distances, there should be abundant strong hydrogen bonding interactions, which might contribute to stabilization of the whole structure. On the basis of the above-mentioned structural discussions, the tzf2- ligand can adopt diverse coordination modes upon complexation with Ln(III) (Scheme 1), differing from those found in tzf2--based transition metal complexes (modes a-g in Scheme S1, Supporting Information).15 Four and two coordination fashions of tzf2- ligand are observed in 1 and 2-6, respectively. The tzf2- ligand can hold Ln(III) centers in close proximity, tending to chelate the Ln(III) center in an N,O-chelated mode to form one or two steady five-membered cyclic LnOC2N ring(s). Furthermore, a structural comparison of two different types of polymeric architectures is shown in Table 2. Polymer 1 presents an anion 3-D framework with the centrosymmetric space group P21/c, while isostructural 2-6 exhibit a neutral 2-D layer structure with the acentric space group Pc. The lanthanide contraction effect has an influence on the lanthanide coordination number and bond lengths of Ln-O/N. The Pr(III) centers in 1 are all nine-coordinated, while the lanthanide centers with smaller atomic radii in 2-6 are both nine- and eight-coordinated. The average bond lengths of Ln-O/N slightly decrease from 1 to 6. The void in 1 and 2-6 is filled with different components. Protonated pyridine molecules as well as water molecules occupy the void in 1 to balance the

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Table 2. The Structural Comparison for 1-6

lanthanide atom polymeric dimensions coordination types of tzf2- ligand Ln(nine-coordinated)-N a Ln(nine-coordinated)-O a Ln(eight-coordinated)-N a Ln(eight-coordinated)-O a a

1

2

3

4

5

6

Pr 3-D four 2.633(3) 2.519(2)

Sm 2-D two 2.602(2) 2.454(1) 2.613(2) 2.418(1)

Eu 2-D two 2.599(2) 2.446(2) 2.591(2) 2.408(2)

Gd 2-D two 2.590(2) 2.431(2) 2.586(2) 2.384(2)

Tb 2-D two 2.574(1) 2.421(1) 2.588(1) 2.377(1)

Dy 2-D two 2.565(2) 2.401(1) 2.559(2) 2.372(1)

Average bond length (A˚).

Figure 6. The steady-state emission spectra of 2 (a), 3 (b), 5 (c), and 6 (d).

charge,26 whereas only water molecules locate in vacancies in 2-6. Thermogravimetric Analysis. The thermal stability for 1 and 3-6 was investigated under a nitrogen atmosphere in the temperature range of 30-1000 °C (Figure S8, Supporting Information). For 1, a weight loss of 10.00% up to 116 °C is in accordance with the release of 10 lattice water molecules (calc. 9.55%) and the framework remained stable (Figure S9, Supporting Information). Further heating led to the framework collapse with the decomposition of organic ligands. For isomorphous 3-6, which exhibit similar thermal behavior, only the thermal stability of 3 was discussed in detail. A first weight loss of 8.91% occurred below 116 °C, which corresponds to the loss of four lattice water molecules (calc. 8.79%). When the temperature continued to rise up to 168 °C, a weight loss 6.65% is consistent with the departure of three coordinated water molecules (calc. 6.59%), and then the rapid breakdown of the framework took place above 168 °C. Optical Spectroscopy. The IR spectra of 1-6 as well as the sodium salt of 1H-tetrazolate-5-formic acid (Na2tzf) are present in Figure S10, Supporting Information. The characteristic bands of tetrazolate and carboxylate groups in

1-6, which are sensitive to their coordination environments, are red-shifted to the lower frequencies relative to the values of Na2tzf. This suggests that both tetrazolate and carboxylate groups are involved in metal coordination. The broad bands centered at about 3240 cm-1 are ascribed to the vibration of water molecules in 1-6. To examine the ability of the tzf2- ligand as “antenna molecule” for sensitizing Ln(III) emission, the steady-state emission spectra of 2 (Sm), 3 (Eu), 5 (Tb), and 6 (Dy) in the solid state were measured at room temperature upon photoexcitation at 346, 362, 350, and 352 nm, respectively, as shown in Figure 6. Typically, due to the low extinction coefficients of the parity-forbidden f-f transitions, lanthanide ions are excited indirectly with the aid of chromophores with a reasonably large molar absorption cross section (socalled “antenna effect”). When an organic chromophore binds to the lanthanide center and absorbs ultraviolet radiation, the absorbed energy is then efficiently transferred to the excitation state level of the lanthanide ion, which emits its characteristic luminescence.27 The tzf2- ligand possesses the delocalized π-electron conjugated system and may provide a strongly absorbing chromophore to sensitize Ln(III) emission. As observed in our earlier work,17c the Na2tzf ligand

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exhibits a maximum emission at 424 nm upon excitation at 356 nm, which can be attributed to ligand-centered π-π* transitions. Notably, the ligand-centered emission almost disappears when tzf2- ligands are bound to lanthanide centers for 2 and 6, and even vanishes completely for 3 and 5. Meanwhile, the luminescence of the corresponding Ln(III) ion appears as sharp emission lines in the visible region. Polymer 2 displays characteristic emission bands centered at 561, 595, 642, and 705 nm, which originate from the characteristic 4G5/2 f 6HJ (J = 5/2, 7/2, 9/2, 11/2, respectively) transitions of the Sm(III) ion. For 3, the maximum emissions at 580, 593, 615, 650, and 695 nm were observed, which are assigned to the characteristic 5D0 f 7FJ (J = 0, 1, 2, 3, 4) transitions of Eu(III). The local-ligand field splitting is particularly evident for the 5D0 f 7F4 transition, presenting at least three Stark components. The appearance of the weak symmetry-forbidden 5D0 f 7F0 emission at 580 nm indicates that the Eu(III) ions features a low-symmetry coordination environment, in agreement with the result of X-ray structural analysis. This is also reflected by the fact that the intensity ratio (5D0 f 7F2/5D0 f 7F1) in the solid state is high up to about 3.10, much higher than the value (0.67) for a centrosymmetric Eu(III) complex.28 Polymer 5 exhibits seven emission bands around 489, 544, 582, 621, 650, 669, and 679 nm, corresponding to the characteristic 5D4 f 7FJ (J = 6, 5, 4, 3, 2, 1, 0) transitions of the Tb(III) ion. Among these transitions, 5D4 f 7F5 green emission is the strongest. The emission peaks at 479, 572, 661, and 749 nm for 6 can be assigned to 4F9/2 f 6HJ (J = 15/2, 13/2, 11/2, 9/2) of the Dy(III) ion. On the basis of the observed luminescent performances, tzf2- f Ln(III) energy transfer is efficient for tzf2--based Eu(III), Tb(III), Sm(III), and Dy(III) coordination polymers, and more efficient for Eu(III) and Tb(III) ions than Sm(III) and Dy(III) ions under the experimental conditions. This behavior can be understood mainly in terms of the energetic factor. When the triplet-state energy of ligand is a little greater than the energy gap (ΔE) between the lowestlying excited state and ground state of the Ln(III) ion, efficient luminescent emission could take place.29 Thus, we could presume that the triplet state energy of the tzf2- ligand is comparable with the energy gap (ΔE) of Sm(III), Dy(III), Eu(III), and Tb(III) ions, and ΔE of Eu(III) and Tb(III) (12150 and 14200 cm-1 for Eu(III) and Tb(III), respectively) have greater values than those of Sm(III) and Dy(III) ions (7400 and 7850 cm-1 for Sm(III) and Dy(III), respectively).29b The ΔE value has also an effect on the solid-state luminescent lifetimes for the strongest maximum emission of isostructural 2, 3, 5, and 6 at room temperature, corresponding to decay of the 4G5/2 luminescence at 595 nm, 5 D0 luminescence at 615 nm, 5D4 luminescence at 544 nm, 4 F9/2 luminescence at 572 nm, respectively (Figure S11, Supporting Information). The emission decay curves are characterized by a monoexponential function with the lifetime values of around 4.62 μs for 2, 266.7 μs for 3, 623.4 μs for 5, and 2.60 μs for 6, comparing with those reported values for Eu(III) (1.25 ms), Tb(III) (0.78 ms), Sm(III) (13 μs), and Dy(III) (1.2 μs) complexes.30 It was found that the extent of deactivation by O-H vibronic coupling from water molecules was inversely proportional to the energy gap.29b,31 The relatively small energy gap of Sm(III) and Dy(III) is easily matched by a lower overtone of an O-H vibration mode (νOH ∼ 3300-3500 cm-1), leading to possible quenching of the excited state 4G5/2 of Sm(III) and 4F9/2 of Dy(III).29b

Wu et al.

However, efficient coupling occurs to the third vibrational overtone of proximate O-H oscillators for Eu(III), and to the fourth harmonic in the case of Tb(III), where the Franck-Condon overlap factor is less favorable.31 Thus, 3 and 5 display longer lifetimes than do 2 and 6. The observation and analysis of these luminescent performance allow the conclusion that the tzf2- ligand can efficiently sensitize emission of Sm(III), Dy(III), Eu(III), and Tb(III) ions as well as protect the Ln(III) ions from nonradiative deactivations caused by the coordinated water, especially for Eu(III) and Tb(III) ions. The tzf2- ligand would be a very promising and versatile sensitizer for emission of lanthanide ions. Magnetic Properties. The temperature-dependent magnetic susceptibility for 1-6 was measured at a field of 1 kOe in the 2-300 K temperature range. In general, the magnetic analysis of lanthanide coordination polymers is very difficult because of exchange-coupling and large orbital contributions as well as the crystal field perturbation.32 For most of the Ln(III) ions, the energy separation between the 2Sþ1 LJ ground state and the first excited state is usually so large that only the ground state is thermally populated at room and low temperature, while in the cases of Sm(III) and Eu(III), the first excited states may be thermally populated due to the weak energy separation.33 Therefore, the crystal field effects would be taken into account, and the possible thermal population of higher states should be also taken into consideration for 2 and 3. For 1, variable-temperature magnetic susceptibilities, χM and χMT (χM is the molar magnetic susceptibility per Pr4 unit) are shown in Figure 7a. At 300 K, χMT is equal to 6.159 cm3 K mol-1, which is comparable with the expected value of 6.40 cm3 K mol-1 for four independent Pr(III) ions in the 3H4 ground state (g = 4/5). As the temperature is lowered, the χMT value decreases continuously to a value of 0.411 cm3 K mol-1 at 2 K. The feasible fit of the experimental data for the plot of χM vs T over the temperature range 100-300 K, following the Curie-Weiss law [χM=C/(T - θ) þ χ0], yielded the Curie constant C = 7.24(5) cm3 K mol-1, Weiss constant θ = -47.6(8) K, and the background susceptibility χ0 = -3.8 10-4 cm3 mol-1. It is very difficult to strictly analyze the magnetic properties for 1 because of the large anisotropy and strong spin-orbit coupling of Pr(III) ions. The slight decrease of χMT within the higher temperature range is mainly ascribed to the splitting of the 9-fold degenerate 3 H4 ground state into Stark levels by the crystal-field effect and the progressive depopulation of the higher energy when the temperature is lowered. The similar trend of χMT vs T has also been observed in other 3-D Pr(III) complexes.34 As we have known, the respective ground term of free Sm(III) and Eu(III) ions, 6H and 7F state, is split into six and seven states by spin-orbit coupling, respectively. Because of the weak energy separation, both the possible thermal population of the higher states and crystal field effects have influences on the magnetic properties of 2 and 3. As can be seen from Figure 7b for 2, the χMT value (χM is the molar magnetic susceptibility per Sm2 unit) is nearly linear over the whole temperature range, which is similar to those reported for homodinuclear and 3-D Sm(III) complexes.35 As the temperature is lowered, the χMT value of 0.671 cm3 K mol-1 at 300 K decreases rapidly to a value of 0.071 cm3 K mol-1 at 2 K, obviously smaller than the value of 0.178 cm3 K mol-1 predicted by theory for two Sm(III) ions. The χMT vs T data have been analyzed on the basis of the equations deduced from the monomeric Sm(III) system with free-ion

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Figure 7. Plots of χM and χMT vs T for 1-6 (a-f). The red lines represent the fitting results.

approximation over the whole temperature range.36 The best agreement between the experimental and calculated data corresponds to the spin-orbit coupling parameter, λ = 259(2) cm-1, and the Weiss-like constant, θ = -14.3(7) K, with an agreement factor R (R = Σ[(χMT)obsd - (χMT)calcd ]2/ Σ(χMT)obsd2) of 2.7 10-4. The λ value is slightly larger than those of the reported Sm(III) complexes (λ=216, 214.5 cm-1, respectively)35 but lower than the value deduced from the spectroscopic data (λ = 281 cm-1 with J = 7/2). The deviation of χMT with respect to the equation for the Sm(III) ion with free-ion approximation (θ = -14.3(7) K) is due to the crystal field effect, together with the possible antiferromagnetic interactions between Sm(III) ions through tzf2bridges at lower temperature. Interestingly, the whole profile of χM and χMT vs T curves for 3 (Figure 7c) is similar to those of the mononuclear Eu(III) complex.36 The χM value smoothly increases over the 300-100 K temperature range and then tends to a plateau. Below 30 K, χM starts to increase again and more rapidly at low temperature, reaching a value of 0.020 cm3 mol-1 at 2 K. At 300 K, χMT is equal to 2.776 cm3 K mol-1, obviously smaller than the theoretical high-temperature limit (χMT)HT = 4.50 cm3 K mol-1) and decreases continuously because of the depopulation of Stack levels, reaching a value close to zero (0.040 cm3 K mol-1) at 2 K, which corresponds to a nonmagnetic ground state of 7F0 for Eu(III) ions. Below 100 K, the thermal variation of χMT is rigorously linear, with the slope (χM)LT = 0.012 cm3 mol-1, tending to two times that for the mononuclear complex ((χM)LT = 5.99 10-3 cm3 mol-1). Similar to 2, the χMT vs T data for 3 could also been analyzed on the basis of the equations deduced from the monomeric Eu(III) system with the free-ion approximation over the whole temperature range. The least-squares fitting of the χMT versus T curve leads to the spin-orbit coupling parameter, λ = 344(1) cm-1, and the Weiss-like constant, θ = 0.88(17) K, with an agreement factor R (R = Σ[(χMT)obsd (χMT)calcd ]2/Σ(χMT)obsd2) of 2.7 10-4. The λ value is comparable with that of the mononuclear Eu(III) complex (λ = 362 cm-1).36 So the magnetic behavior of 3 is mainly caused by single-ion properties, which is also reflected in the small value of the Weiss-like constant θ (θ = 0.88(17) K). For 4, as the temperature is lowered from 300 to 20 K, the χMT value per Gd2 unit is almost constant with a small scale

of 15.38 to 15.61 cm3 K mol-1, which is slightly smaller than the theoretical value for two isolated Gd(III) (15.86 cm3 K mol-1). Below 20 K, χMT shows a sharp decrease with decreasing temperature to reach a value 14.70 cm3 mol-1 K at 2 K. The global feature supports the existence of a weak antiferromagnetic interaction between Gd(III) ions mediated by tzf2- ligand. An attempt to fit the experimental data for the plot of χM vs T via χM = C/(T - θ) þ χ0 over the temperature range 50-300 K revealed a Curie-Weiss law behavior with C = 15.55(6) cm3 K mol-1, θ = -0.42(19) K and the background susceptibility χ0 = 1.4 10-4 cm3 mol-1. Polymers 5 and 6 present a similar behavior, where the χMT value decreases slowly from 300 to 2 K. The χMT values for 5 and 6 are equal to 23.10 and 27.78 cm3 K mol-1 at 300 K, respectively, which are slightly lower than the expected 23.50 and 28.34 cm3 K mol-1 for two isolated Tb(III) and Dy(III) ions. The lowering of χMT values with decreasing temperature can be ascribed to the depopulation of the Stark levels together with weak antiferromagnetic interactions between the lanthanide ions in 5 and 6.37 The magnetic susceptibility data for the plot of χM vs T follow the Curie-Weiss behavior in the 50-300 K temperature range, with C = 23.52(6) cm3 K mol-1 and θ = -5.75(14) K for 5 and C = 28.32(11) cm3 K mol-1 and θ = -5.96(21) K for 6. Conclusions In summary, six novel tzf2--based Ln(III) coordination polymers 1-6 have been synthesized via common solution reaction, and their luminescent and magnetic properties have been investigated. These polymers present two different structural types, 3-D anionic framework for 1 and 2-D neutral layer net for isomorphous 2-6. The tzf2- ligand in 1-6 exhibits four unprecedented coordination modes in tetrazolate-5-carboxylate complexes. The effects of radii of lanthanide ions and coordination diversities of tzf2- ligand on polymeric architectures have been discussed. The photoluminescent analyses for 2, 3, 5, and 6 indicate that tzf2- ligand could efficiently sensitize Ln(III) ion emission, especially for Eu(III) and Tb(III), which display longer lifetimes than Sm(III) and Dy(III) due to the effect of energy gap between the lowest-lying excited state and ground state of the Ln(III) ion. The variable-temperature magnetic study for 1-6 suggests

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that magnetic interactions between the Ln(III) ions are mainly ascribed to the antiferromagnetic coupling as well as the depopulation of the Stark levels. The spin-orbit coupling parameters, λ, for Sm(III) (259(2) cm-1) in 2 and Eu(III) (344(1) cm-1) in 3, have been deduced through free-ion approximation. To the best of our knowledge, lanthanide coordination polymers with carboxylate-introduced 5-substituted tetrazolate ligand have been investigated for the first time. The piezoelectric, ferroelectric, and nonlinear properties of the bigger and better single crystals of 3 and 4 deserve further investigation due to their acentric structures, which are likely to provide new multifunctional materials. Acknowledgment. This work was financially supported by 973 Program (2006CB932900 and 2007CB936703), Key Technologies R&D Program of China (2007BAE08B01), and National Nature Science Foundation of China (20871115). Supporting Information Available: Additional structural figures and tables; luminescent decay curves for 2, 3, 5, and 6; PXRD patterns and IR spectra for 1-6; and X-ray crystallographic files in CIF format of 1-6. This information is available free of charge via the Internet at http://pubs.acs.org/.

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