A New Family of 3dâ4f Heterometallic Tetrazole-based Coordination

Article pubs.acs.org/crystal

A New Family of 3d−4f Heterometallic Tetrazole-based Coordination Frameworks: In Situ Tetrazole Ligand Synthesis, Structure, Luminescence, and Magnetic Properties Li Liang, Guo Peng, Li Ma, Lin Sun, Hong Deng,* Hong Li, and Weishan Li School of Chemistry & Environment and Key Laboratory of Electrochemical Technology on Energy Storage and Power Generation in Guangdong Universities, South China Normal University, Guangzhou 510006, People's Republic of China S Supporting Information *

ABSTRACT: A series of novel three-dimensional (3D) Ln(III)−Cu(I) heterometallic tetrazole-based coordination frameworks, namely, [LnCu(3-tzba)2(H2O)4] [Ln = Eu (1), Gd (2), Tb (3), Dy (4); 3-tzba = 3-(1H-tetrazol-5yl)benzoate], were successfully synthesized through in situ [2 + 3] cycloaddition reaction under hydrothermal conditions. Compounds 1−4 are isostructural 3D coordination frameworks with 1D anionic chains [Ln(3-tzba)2(H2O)4]− linking the adjacent Cu(I) ions possessing a uninodal 10-connected topology with the short (Schläfli) vertex symbol of (312.424.59). To the best of our knowledge, they are the first lanthanide− transition heterometal−organic coordination frameworks obtained through in situ tetrazole synthesis. In addition, the luminescence properties of 1 and 3 and the magnetic properties of 2 and 4 were also investigated.

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INTRODUCTION In recent years, the synthesis of 5-substituted 1H-tetrazoles is one of hottest fields in supramolecular chemistry and crystal engineering,1 due to their novel structural architectures and topologies, and potential applications in coordination chemistry, medicinal chemistry, and materials science.2 There was no effective method for synthesizing 5-substituted tetrazoles in high yield until Demko and Sharpless invented a safe, convenient, and environmentally friendly synthetic method through [2 + 3] cycloaddition reaction of azide anions and nitriles in water with the aid of Lewis acid catalysts.3 After which, Xiong and co-workers improved this method using Zn(II) to catalyze cycloaddition reactions of nitriles and azide to form zinc tetrazole coordination frameworks via in situ hydrothermal synthesis.4 With the development of this method, a great variety of tetrazole-based transition metal [Cd(II), Zn(II), Cu(II/I), Co(II), Mn(II), Hg(II), etc] coordination frameworks have been reported via in situ solvo/hydrothermal synthesis.5 For example, Long et al. synthesized a series of metal−organic frameworks (MOFs) mostly involving M4X (M = Mn, Mg, Cu; X = Cl or O) clusters as building blocks;6 Bu et al. obtained a uninodal 10-connected topology constructed from each tetranuclear Cd(II) cluster connecting to 10 adjacent clusters through two pairs of “double-bridges” ip ligands (isophthalic acid) and eight bridging tz ligands (tetrazolate).7 However, a careful review of the literature suggests that among the various strategies, most of the work has been focused on the assembly of d-block tetrazole-based metal− organic frameworks or lanthanide ion coordination frameworks;5,8 the lanthanide−transition heterometallic compounds obtained by in situ tetrazole-based reaction have never been © 2012 American Chemical Society

reported before. It is well-known that lanthanide−transition heterometallic coordination frameworks not only possess fascinating structures and topologies but also have potential applications in catalysis, adsorption, and magnetic and optical materials.9,10 However, due to the variable and high coordination numbers of lanthanide ions, as well as the competition between lanthanide and transition ions, the preparation of extended 3d−4f coordination frameworks is still a challenging task.11 Therefore, the choice of appropriate ligand is crucial. On the basis of our recent studies on d−f heterometallic and tetrazole-based coordination frameworks,12 we think 3cyanobenzoic acid (3-Hcba) is a potential bifunctional precursor that can be used to construct heterometallic coordination frameworks due to the following reasons: (1) it can form 3-(1H-tetrazol-5-yl)benzoic acid (3-H2tzba) ligand through in situ solvo/hydrothermal synthesis; (2) the 3-H2tzba ligand contains nitrogen and oxygen donors, whereby the nitrogen atoms of tetrazole possess a strong tendency to coordinate to transition metal ions, while the oxygen atoms of carboxyl group prefer to bond to lanthanide metal ions according to the hard−soft acid base theory. Although several 3d or 3d−3d coordination frameworks have been obtained,13 no 3d−4f heterometallic coordination frameworks based on the 3-H2tzba ligand have been reported. With the above in mind, herein, we report the synthesis, structure, luminescence, and magnetic properties of four Received: July 7, 2011 Revised: December 19, 2011 Published: January 3, 2012 1151

dx.doi.org/10.1021/cg200856e | Cryst. Growth Des. 2012, 12, 1151−1158

Crystal Growth & Design

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Table 1. Crystallographic Data and Structure Refinement Summary for Compounds 1−4 compounds empirical formula formula wt. T (K) cryst. syst. space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρcalcd (g cm−3) M (mm−1) F(000) reflns collected/unique GOF Rint R1 [I > 2σ(I)]a wR2 (all data)b a

1 C16H16CuEuN8O8 663.89 296 monoclinic C2/c 15.991(10) 15.206(10) 9.272(6) 90 115.343(7) 90 2038(2) 4 2.164 4.159 1296 4810/1840 1.001 0.0182 0.0216 0.0528

2 C16H16CuGdN8O8 669.17 296 monoclinic C2/c 16.0425(19) 15.1600(18) 9.2883(11) 90 115.389(1) 90 2040.8(4) 4 2.178 4.330 1300 5088/1839 1.006 0.0240 0.0260 0.0691

3 C16H16CuTbN8O8 670.85 296 monoclinic C2/c 16.033(3) 15.155(2) 9.2818(15) 90 115.391(2) 90 2037.4(6) 4 2.187 4.553 1304 4973/1834 1.023 0.0172 0.0218 0.0556

4 C16H16CuDyN8O8 674.42 296 monoclinic C2/c 16.0231(17) 15.1138(16) 9.2701(10) 90 115.484(1) 90 2026.5(4) 4 2.211 4.775 1308 5125/1832 1.007 0.0207 0.0236 0.0622

R1 = ∑||Fo| − |Fc||/∑|Fo|. bwR2 = {∑[w(Fo2 − Fc2)2]/∑(w(Fo2)2}1/2. H, 2.43; N, 16.88. Found: C, 28.74; H, 2.33; N, 16.90. IR data (KBr, cm−1): 3375(s), 1681(s), 1599(s), 1538(s), 1503(s), 1467(s), 1411(s), 1183(m), 923(w), 789(w), 744(w), 691(w). Synthesis of [GdCu(3-tzba)2(H2O)4] (2). A mixture of Gd(NO3)3·6H2O (0.225 g, 0.5 mmol), CuBr2 (0.112 g, 0.5 mmol), 3cyanobenzoic acid (0.074 g, 0.5 mmol), NaN3 (0.033 g, 0.5 mmol), and H2O (10 mL) (pH = 5) was stirred for 30 min at room temperature and kept in a 23 mL Teflon-lined autoclave at 150 °C for 3 days. The mixture was cooled to room temperature at 5 °C/h (pH = 6). Yellow block single crystals of 2 were obtained (yield: 28% based on Gd). Elemental Anal. Calcd (%) for 2, C16H16CuGdN8O8: C, 28.72; H, 2.41; N, 16.75. Found: C, 28.49; H, 2,70; N, 16.90. IR data (KBr, cm−1): 3376(s), 1682(s), 1603(s), 1541(s), 1504(s), 1468(s), 1410(s), 1182(m), 921(w), 787(w), 742(w), 691(w). Synthesis of [TbCu(3-tzba)2(H2O)4] (3). A mixture of Tb(NO3)3·6H2O (0.227 g, 0.5 mmol), CuBr2 (0.112 g, 0.5 mmol), 3cyanobenzoic acid (0.074 g, 0.5 mmol), NaN3 (0.033 g, 0.5 mmol), and H2O (10 mL) (pH = 5) was stirred for 30 min at room temperature and kept in a 23 mL Teflon-lined autoclave at 150 °C for 3 days. The mixture was cooled to room temperature at 5 °C/h (pH = 6). Yellow block single crystals of 3 were obtained (yield: 36% based on Tb). Elemental Anal. Calcd (%) for 3, C16H16CuTbN8O8: C, 28.65; H, 2.40; N, 16.70. Found: C, 28.89; H, 2.81; N, 16.54. IR data (KBr, cm−1): 3373(s), 1679(s), 1598(s), 1537(s), 1501(s), 1466(s), 1411(s), 1185(m), 924(w), 788(w), 742(w), 691(w). Synthesis of [DyCu(3-tzba)2(H2O)4] (4). A mixture of Dy(NO3)3·6H2O (0.228 g, 0.5 mmol), CuBr2 (0.112 g, 0.5 mmol), 3cyanobenzoic acid (0.074 g, 0.5 mmol), NaN3 (0.033 g, 0.5 mmol), and H2O (10 mL) (pH = 5) was stirred for 30 min at room temperature and kept in a 23 mL Teflon-lined autoclave at 150 °C for 3 days. The mixture was cooled to room temperature at 5 °C/h (pH = 6). Yellow block single crystals of 4 were obtained (yield: 29% based on Dy). Elemental Anal. Calcd (%) for 4, C16H16CuDyN8O8: C, 28.50; H, 2.39; N, 16.62. Found: C, 28.58; H, 2.45; N, 16.43. IR data (KBr, cm−1): 3371(s), 1680(s), 1597(s), 1542(s), 1505(s), 1463(s), 1409(s), 1183(m), 922(w), 785(w), 740(w), 690(w). Crystal Structure Determination. Single-crystal X-ray diffraction data collections of 1−4 were performed on a Bruker Apex II CCD diffractometer operating at 50 kV and 30 mA using Mo Kα radiation (λ = 0.71073 Å). Data collection and reduction were performed using the APEX II software.14 Multiscan absorption corrections were applied

unprecedented 3D heterometallic Ln(III)−Cu(I) compounds, namely, [LnCu(3-tzba)2(H2O)4] [Ln = Eu (1), Gd (2), Tb (3), Dy (4); 3-tzba = 3-(1H-tetrazol-5-yl)benzoate]. To the best of our knowledge, they are the first examples of lanthanide−transition heterometallic compounds through in situ tetrazole synthesis.

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EXPERIMENTAL SECTION

Materials and Physical Measurements. All the materials and reagents were obtained commercially and used without further purification. Elemental (C, H, N) analyses were performed on a Perkin−Elmer 2400 element analyzer. Infrared (IR) samples were prepared as KBr pellets, and spectra were obtained in the 4000−400 cm−1 range using a Nicolet Avatar 360 FT-IR spectrophotometer. Thermogravimetric analysis (TGA) experiments were carried out on a Perkin−Elmer TGA 7 thermogravimetric analyzer with the heating rate of 10 °C/min from 35 to 800 °C under dry air atmosphere. Powder XRD investigations were carried out on a Philips PW-1830 Xray diffractometer with Cu Kα radiation. Fluorescence spectra were recorded with an Edinburgh FLS920 spectrophotometer analyzer. The data of magnetic susceptibility were collected using the Quantum Design SQUID MPMS-XL magnetometer from polycrystalline samples at an external field of 1000 Oe with the temperature range from 1.8 to 300 K. The magnetic data were corrected for diamagnetic contributions of the sample holder and calculated for the polymers by Pascal constants. The products of all measurements were handled as follows: (1) pick crystals out one by one under the microscope; (2) wash them with water and alcohol three times; (3) place them in drying oven at least 10 h at 70 °C. CAUTION! Azido and tetrazolate compounds are potentially explosive under hydrothermal reaction conditions. Only a small amount of compounds should be prepared, and they should be handled with care. Synthesis of [EuCu(3-tzba)2(H2O)4] (1). A mixture of Eu(NO3)3·6H2O (0.223 g, 0.5 mmol), CuBr2 (0.112 g, 0.5 mmol), 3cyanobenzoic acid (0.074 g, 0.5 mmol), NaN3 (0.033 g, 0.5 mmol), and H2O (10 mL) (pH = 5) was stirred for 30 min at room temperature and kept in a 23 mL Teflon-lined autoclave at 150 °C for 3 days. The mixture was cooled to room temperature at 5 °C/h (pH = 6). Yellow block single crystals of 1 were obtained (yield: 35% based on Eu). Elemental Anal. Calcd (%) for 1, C16H16CuEuN8O8: C, 28.95; 1152



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for all the data sets using the APEX II program.14 The structures were solved by direct methods and refined by full-matrix least-squares on F2 using the SHELXL program package.14 All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms attached to carbon were placed in geometrically idealized positions and refined using a riding model. Hydrogen atoms on water molecules were located from difference Fourier maps and were also refined using a riding model. Crystallographic data for compounds 1−4 are listed in Table 1, and selected bond lengths and angles are given in Table 2 for compound 4 and Table S1 for 1−3 (Supporting Information). CCDC numbers are 808464−808467 for compounds 1−4.

methods. Taking into account these factors, it prompted us to choose 3-cyanobenzoic acid as raw material for in situ ligand synthesis of 5-substituted 1H-tetrazoles and further in the construction of novel lanthanide−transition heterometallic compounds. The synthetic route of four new Ln(III)−Cu(I) heterometallic tetrazole-based coordination frameworks 1−4 is shown in Scheme 1. All compounds are stable under ambient Scheme 1. In Situ Hydrothermal Syntheses of Compounds 1−4

Table 2. Selected Bond Distances (Å) and Angles (deg) for Compound 4a Cu(1)−N(2)#1 Dy(1)−O(3)#2 Dy(1)−O(3) Dy(1)−O(4)#3 Dy(1)−O(4)#4 N(2)#1−Cu(1)− N(2) O(3)#2−Dy(1)− O(3) O(3)#2−Dy(1)− O(4)#3 O(3)−Dy(1)−O(4) #3 O(3)#2−Dy(1)− O(4)#4 O(3)−Dy(1)−O(4) #4 O(4)#3−Dy(1)− O(4)#4 O(3)#2−Dy(1)− O(1W) O(3)−Dy(1)− O(1W) O(4)#3−Dy(1)− O(1W) O(4)#4−Dy(1)− O(1W) O(3)#2−Dy(1)− O(1W)#2 O(3)−Dy(1)− O(1W)#2 O(4)#3−Dy(1)− O(1W)#2 O(4)#4−Dy(1)− O(1W)#2

1.867(4) 2.296(3) 2.296(3) 2.312(3) 2.312(3) 180.00(1)

Cu(1)−N(2) Dy(1)−O(1W) Dy(1)−O(1W)#2 Dy(1)−O(2W)#2 Dy(1)−O(2W)

1.867(4) 2.431(3) 2.431(3) 2.497(3) 2.497(3)

82.35(16)

O(1W)−Dy(1)− O(1W)#2 O(3)#2−Dy(1)− O(2W)#2 O(3)−Dy(1)−O(2W) #2 O(4)#3−Dy(1)− O(2W)#2 O(4)#4−Dy(1)− O(2W)#2 O(1W)−Dy(1)− O(2W)#2 O(1W)#2−Dy(1)− O(2W)#2 O(3)#2−Dy(1)− O(2W) O(3)−Dy(1)−O(2W)

133.22(16)

141.68(11) 109.97(11) 109.97(11) 141.68(11) 83.16(15) 76.74(11) 145.79(11) 73.62(11) 71.83(11) 145.79(11) 76.74(11) 71.83(11) 73.62(11)

O(4)#3−Dy(1)− O(2W) O(4)#4−Dy(1)− O(2W) O(1W)−Dy(1)− O(2W) O(1W)#2−Dy(1)− O(2W) O(2W)#2−Dy(1)− O(2W)

environment for a long time and insoluble in water as well as common organic solvents. Control experiments show that if some other Cu(II) or Cu(I) salts such as CuCl2, Cu(NO3)2, or CuCl were employed to replace CuBr2 as the copper source in the reaction system, the final products did not present any crystallinity. However, when lanthanide oxide was introduced in place of Ln(NO3)3 in the synthetic system, we did not obtain any crystals as well. If LnCl3 was used to replace Ln(NO3)3, the yield of crystal product is extremely low (yield 5% based on Ln). Our own experimental evidence has led us to believe that CuBr2 and Ln(NO3)3 jointly play a critical role in crystal formation. However, due to the complexities involved in the in situ ligand syntheses and supramolecular assemblies, it is difficult to explain the exact reaction mechanism in the one-pot, black-boxlike hydrothermal metal/ligand reactions.15 Description of Crystal Structure of [LnCu(3tzba)2(H2O)4] [Ln = Eu (1), Gd (2), Tb (3), Dy (4); 3-tzba = 3-(1H-tetrazol-5-yl)benzoate]. X-ray analysis reveals that compounds 1−4 are isomorphic, all crystallizing in the monoclinic group C2/c and exhibiting 3D unique frameworks so only the structure of 4 is described in detail here. Compound 4 represents a 3D coordination framework with 1D anionic chains [Dy(3-tzba)2(H2O)4]− linking the adjacent Cu(I) ions. The asymmetric unit of compound 4 contains one unique Dy(III) ion, one Cu(I) ion, one 3-tzba ligand, and two coordinated water molecules. It should be noted that monovalent copper is generated from the reduction of divalent copper under hydrothermal conditions.16 The Dy(III) ion is eight-coordinated and has a bicapped trigonal prismatic coordination geometry with four oxygen atoms from four bridging 3-tzba ligands and four oxygen from four coordinated water molecules. The bond lengths of Dy−O are 2.296(3)− 2.497(3) Å, comparable to those of other dysprosium− carboxylate compounds.17 The O−Dy−O bond angles range from 68.6(11)° to 147.06(14)°. As far as the Cu(I) ion in the coordination framework is concerned, it is coordinated in a linear fashion by two nitrogen atoms from two different 3-tzba ligands with Cu−N distances of 1.867(4) Å and bond angle of 180.00(1)° (Figure 1a). The 3-tzba ligands in 4 only adopt one coordinated mode (μ3) with the nitrogen atom of tetrazole coordinating to one Cu(I) center and the carboxylate connecting to two different Dy(III) ions via a μ2-η1:η1 fashion (Figure 1b). On the basis of these connections, an anionic chain, [Dy(3-tzba)2(H2O)4]−, is generated via one Dy(III) ion linked by two 3-tzba ligands and four coordinated water

80.11(11) 75.22(10) 137.58(11) 71.70(11) 126.15(10) 68.61(11) 75.22(10) 80.11(11) 71.70(11) 137.58(11) 68.61(11) 126.15(10) 147.06(14)

Symmetry codes: (#1) −x + 1/2, −y + 1/2, −z + 2; (#2) −x, y, −z + /2; (#3) −x, −y, −z; (#4) x, −y, z + 1/2.

a

1

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RESULTS AND DISCUSSION

Synthesis. It is well-known that the selection or design of organic ligands containing appropriate coordination sites is crucial to constructing high-dimensional heterometallic coordination frameworks. 3-H2tzba is a potential bridging ligand with oxygen (from carboxyl) and nitrogen (from tetrazole) donors on the metal positions of the molecule. When using such a bifunctional ligand with both a carboxylate and an N-donor, lanthanide ions have a strong preference to bond to the Odonor atoms forming lanthanide carboxylate subunits and the transition metal ions such as Cu(I)/(II) or Ag(I) ions bind more easily to N-donor atoms. In addition, through in situ ligand synthesis, we can obtain unusual three-dimensional frameworks, which are difficult to obtain by routine synthetic 1153



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Figure 1. (a) Ball and stick plot showing the coordination environments of Dy and Cu atoms in compound 4. All H atoms are omitted for clarity. (a, −x, y, 0.5 − z; b, −x, −y, −z; c, x, −y, 0.5 + z; d, 0.5 − x, 0.5 − y, 2 − z). (b) Coordination mode of 3-tzba ligand in 4. (c) View of the 1D anionic chain [Dy(3-tzba)2(H2O)4]− containing a double-strand meso-helical chain (P + M) along the b-axis direction in compound 4.

vertex symbol of (312.424.59) when Dy(III) ions are only considered as nodes (Figure 3). To the best of our knowledge,

molecules. The Dy-based chain with Dy−O−C−O−Dy connectivity contains 1D double-stranded meso-helix built up along the b-axis direction, and the distance between neighboring Dy(III) ions in one chain is 4.728 Å (Figure 1c and Figure S1, Supporting Information). This analogous Ln− O−C−O−Ln arrangement of 1D infinite chains with mesohelical character can be found in homometallic lanthanide− carboxylate frameworks.18 The chains in the structure of 4 extend along the c-axis and are further assembled through adjacent Cu(I) ions into a 3D coordination framework (Figure

Figure 2. View of the 3D networks of 4 assembled by 1D anionic chains [Dy(3-tzba)2(H2O)4]− linking the adjacent Cu(I) ions. The ligands were simplified as lines for clarity.

2). A careful review of the literature suggests that this is the first example of Dy(III)−Cu(I) heterometallic compound through in situ tetrazole synthesis. In order to simplify the complicated framework, the network topology of 4 was analyzed. Herein, each Dy(III) ion connects to 10 adjacent metal ions through a pair of 3-tzba ligands and a Cu(I) ion affording a 3D 10-connected framework. The topology of the 3D framework can be rationalized in terms of a uninodal 10-connected topology with the short (Schläfli)

Figure 3. View of the 3D uninodal 10-connected topological networks.

so far, just one example of uninodal 10-connected framework considering the isolated metal ions as nodes has been reported.19 Thermal Analyses and Powder X-ray Diffraction (PXRD) Measurements. To examine the thermal stabilities of compounds 1−4, the thermogravimetric analyses of these 1154



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compounds were carried out from 35 to 800 °C at a heating rate of 10 °C·min−1 in dry air atmosphere (Figure S2, Supporting Information). Compounds 1−4 show similar thermal behaviors, maybe because they are isomorphic. Thus, only the thermal stability of 1 is discussed in detail. The curve of 1 shows two consecutive steps of weight loss below 200 °C corresponding to the gradual loss of four coordinated water molecules (found 10.93%; calcd 10.85%). After that, the rapid weight loss above 250 °C corresponds to the decomposition of the host network. At above 420 °C, the whole framework collapses completely, and the residue has a composition of 1 /2Eu2O3·CuO (found 37.88%; calcd 38.49%). Simulated and experimental powder X-ray diffraction (PXRD) patterns of 1−4 are shown in Figure 4. They are in

Figure 5. Solid state emission spectra of 1 (a) and 3 (b) at room temperature.

been widely used as a measure of the coordination state and the site symmetry of the rare earth ions.21 For compound 1, the intensity of the 5D0 → 7F2 transition is much stronger than that of the 5D0 → 7F1 transition; the intensity ratio I(5D0 → 7F2)/ I(5D0 → 7F1) is ca. 9.02, which suggests that the Eu(III) ions in 1 have a noncentrosymmetric coordination environment. This is also consistent with the result of the single-crystal X-ray analysis. Compound 3 emits green light when excited at 369 nm (Figure S5b, Supporting Information), and it gives a typical Tb(III) emission spectrum (Figure 5b). The narrow and strong peaks at about 491, 544, 582, and 623 nm are ascribed to the characteristic emissions of Tb(III) corresponding to electronic transitions from the excited state 5D4 to the multiplets 7FJ (J = 6−3), respectively. In these compounds, direct transitions to the excited 4f levels of the Ln(III) ion are spin and parity forbidden, but efficient population of the emitting 4f excited states is achieved through the so-called “antenna effect”, that is, via intramolecular energy transfer from the ligand excited state to a Ln f-excited state.22 In other words, the organic ligands absorb light, energy is transferred from organic ligands to the lanthanide ions, and then luminescence is generated from the lanthanide ions. The emission of free 3-H2tzba ligand was observed at about 407 nm with a relatively intense broad band when excited at 362 nm.13c For compounds 1 and 3, there is no apparent residual ligandbased emission in the 400−480 nm region, indicating an efficient energy transfer from the ligand p-excited states to the Ln f-excited states. Because of the energy transfer of the organic ligands to the lanthanide ion, the fluorescence of the ligands is not observed. Though compounds 1 and 3 are isomorphic, it is to be noted that the excitation spectra for both samples are quite a difference. Compounds 1 and 3 exhibit the maximum excitation wavelength at 394 and 369 nm, respectively. Considering the ligand of the excitation peak at 362 nm, the luminescent mechanism of compound 3 can be proposed. The ligands first absorb light, and then the energy is delivered to the center ions

Figure 4. PXRD patterns (a) simulated based on the X-ray singlecrystal diffraction data of 1, (b) for as-synthesized 1, (c) for assynthesized 2, (d) for as-synthesized 3, and (e) for as-synthesized 4.

fairly good agreement with the experimental patterns, which clearly confirms the phase purity of the as-prepared products. Infrared Spectroscopy. The IR spectra of all products show similarities. There are the strong peaks around 3300 cm−1, which should be ascribed to the stretching vibrations of O−H, suggesting the presence of free or coordinated water molecules. The peaks 1400−1500 cm−1 are found clearly confirming the formation of tetrazole groups.12 The strong peaks of carboxyl groups in 1−4 appear in the region of 1682−1597 cm−1 (antisymmetric stretching vibrations) and 1411−1409 cm−1 (symmetric stretching vibrations). And no peaks around 1700 cm−1 are discovered illustrating complete deprotonation of carboxyl groups in all compounds.20 Photoluminescence Properties. Compounds 1 and 3 emit intense red and turquoise fluorescence under UV light, respectively. Their solid-state excitation and emission spectra were measured at room temperature (Figure 5 and Figure S5, Supporting Information). When excited at 394 nm, compound 1 yields intense red luminescence and exhibits characteristic peaks at 592 (5D0 → 7F1), 613 (5D0 → 7F2), 650 (5D0 → 7F3), and 699 nm (5D0 → 7F4) originating from the transition of 5D0 → 7FJ (J = 1−4) of the Eu(III) ion. It is to be noted that the 5 D0 → 7F1 transition is a magnetic dipole transition, and its intensity varies with the crystal field strength acting on Eu(III). On the other hand, the 5D0 → 7F2 transition is an electric dipole transition and is extremely sensitive to chemical bonds in the vicinity of Eu(III). Furthermore, the intensity of the 5D0 → 7 F2 transition increases as the site symmetry of Eu(III) decreases. Therefore, the intensity ratio of the 5D0 → 7F2 transition compared with that of the 5D0 → 7F1 transition has 1155



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Figure 6. (a) Plots of χMT vs T for 2 in the range of 1.8−300 K in 1000 Oe. (b) Plots of M vs H measured below 5 K in different fields.

Figure 7. (a) Plots of χMT vs T for 4 in the range of 1.8−300 K in 1000 Oe. (b) Plots of M vs H measured below 5 K in different fields.

increases sharply, and the slope is very steep. Above this field, the magnetization of 2 saturates, reaching a maximum value of 7.15μB at 2 K and 70 kOe. Also, the magnetization curve illustrates the antiferromagnetic interactions existing in compound 2. For compound 4, as shown in Figure 7a, the value of χMT was 14.33 cm3 K mol−1, close to the 14.17 cm3 K mol−1 expected for one uncoupled Dy(III) (S = 5/2, L = 5, 6H15/2, g = 2) at 300 K. The value of χMT barely decreases until 100 K and then sharply drops to a minimum value of 12.08 cm3 K mol−1 at 2.0 K, which reveals an overall antiferromagnetic behavior in compound 4 as well. The χM−1 vs T also obeys the Curie− Weiss law with C = 14.42 cm3 mol−1 K and θ = −1.57 K (Figure S4, Supporting Information). The M vs H (Figure 7b) data were collected at different temperatures; below 22 kOe, M increases sharply. Above this field, the magnetization of 4 saturates, reaching a maximum value of 7.06μB at 2 K and 70 kOe. Both of them give the evidence of antiferromagnetic interactions existing in compound 4.

Tb(III); finally terbium ions emit characteristic luminescence. It needs to be mentioned that the starting chemical Tb(NO3)3 does not emit such luminescence when excited at 369 nm, indicating that the molecule of 3-H2tzba ligand does sensitize the fluorescence of lanthanide metal centers. On the other hand, the broad band overlap of excitation spectra of 3-H2tzba ligand and Eu(III) ion was observed for compound 1 (Figure S5, Supporting Information). The strongest excitation peak located at 394 nm caused by the f → f transitions from 7FJ of Eu(III) to excited levels, that is to say, the transition 7F0−5L6 of Eu(III) attribute to the 394 nm. The Eu(III) ion absorbs energy by itself, and the energy state moves to the lowest excited energy-level (5D0) through relaxation. Finally, a red emission occurs through the 5D0−7FJ transition.23 Recently similar results were reported, which also supported our present results.12c,24 In addition, the energy transfer from ligand-tocopper is inefficient, which is testified by no other emission peaks existing in the emission spectrum except the characteristic emission peaks of Eu(III) or Tb(III) ions. Magnetic Properties. Direct-current (dc) magnetic susceptibility studies were performed on microcrystalline samples of 2 and 4 at 1000 Oe in the temperature range of 1.8 and 300 K. At room temperature, the χMT value was ca. 8.12 cm3 K mol−1 for 2, close to the value (7.87 cm3 K mol−1) expected for one uncoupled Gd(III) (S = 7/2, L = 0, 8S7/2, g = 2). As shown in Figure 6a, the χMT value gradually decreases with the temperature decreasing in the range of 300−15 K and then further drops more sharply to reach a minimum value of 7.30 cm3 K mol−1 at 1.9 K, which reveals an overall antiferromagnetic behavior in compound 2. The χM−1 vs T obeys the Curie−Weiss law, χ = C/(T − θ), with Curie constant C = 8.13 cm3 mol−1 K and Weiss constant θ = −0.48 K (Figure S3, Supporting Information). The M vs H (Figure 6b) were measured at different temperatures; below 26 kOe, M

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CONCLUSION A series of intriguing 3d−4f heterometallic tetrazole-based coordination frameworks have been obtained through in situ hydrothermal synthesis, whereby these isostructural 3D coordination frameworks with 1D anionic chains [Ln(3tzba)2(H2O)4]− linking the adjacent Cu(I) ions possess a uninodal 10-connected topology. Their thermal stability and optical and magnetic properties were also investigated. Our successful synthetic route may be applicable to preparation of other novel 3d−4f heterometallic tetrazole-based coordination frameworks with interesting structures, topologies, and functional properties. Further systematic studies for the design and synthesis such crystalline materials with 3-cyanobenzoic acid are underway in our laboratory. 1156



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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic files in CIF format for structures 1−4, TGA curves, and additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

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ACKNOWLEDGMENTS We thank Professor M. J. Gerald Lesley, Chairman of Chemistry at the Southern Connecticut State University, for helpful discussions. This work was supported by National Natural Science Foundation of China, (Grant Nos. 20871048 and 21171060) and Supported by Natural Science Foundation of Guangdong Province (Grant No. 10351063101000001).

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A New Family of 3dâ4f Heterometallic Tetrazole-based Coordination

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