Inorganic Chemistry Communications 35 (2013) 195–199

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Inorganic Chemistry Communications

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2D lanthanide-based pyridine-substituted triazole benzoate coordinationpolymers: Structure, optical and magnetic properties

Linyan Yang a, Wen Gu a,b,⁎, Jinlei Tian a,b, Shengyun Liao a, Liangliang Xin c, Ming Zhang a, Xiaohua Wei a,Peiyao Du a, Lili Shen d, Xin Liu a,b,⁎a Department of Chemistry, Nankai University, Tianjin 300071, Chinab Tianjin Key Laboratory of Metal and Molecule Based Material Chemistry, Tianjin 300071, Chinac School of Science, Tianjin University, Tianjin 300072, Chinad Tianjin Sungene Biotech Co., Ltd., Tianjin 300457, China

⁎ Corresponding authors at: Department of Chemistry, NChina. Tel.: +86 13512865360; fax: +86 22 23502779.

E-mail address: liuxin64@nankai.edu.cn (X. Liu).

1387-7003/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.inoche.2013.06.010

a b s t r a c t

a r t i c l e i n f o

Article history:Received 18 April 2013Accepted 6 June 2013Available online 14 June 2013

Keywords:Lanthanide-basedCoordination polymersLuminescentMagnetism

A series of novel lanthanide-based coordination polymers, [Ln(L)(atpa)(H2O)2]n·1.5nH2O (HL = 4-[3-methyl-5-(pyridin-4-yl)-1,2,4-triazol-4-yl]benzoic acid, H2atpa = 2-aminoterephthalic acid and Ln = Tb(1), Dy(2),Ho(3), Er(4), Tm(5), Yb(6)) have beenhydrothermally synthesized and characterized by single-crystal X-ray dif-fraction, optical andmagnetic measurements. Compounds 1–6 are isomorphous, which can be described as a 2Dnetwork based on Ln2(CO2)2 units. Themagnetic properties of compound 1 (Tb) show typical antiferromagneticinteraction, while compound 2 (Dy) has ferromagnetic behavior. The room-temperature photoluminescencespectra of 1 exhibit strong characteristic emissions in the visible region, whereas compounds of Dy(2), Er(4)and Yb(6) display NIR luminescence upon irradiation at the ligand band.

© 2013 Elsevier B.V. All rights reserved.

Lanthanide-based polymers continue to attract considerable atten-tion because of their fascinating architectures and rich optical andmagnetic properties [1–4]. The advantages of lanthanide luminescenceare characteristic narrow emission bands mostly in the visible andnear-infrared spectral regions, long lifetimes (μs toms), and high quan-tum yields [5,6]. TbIII compounds are used generally as optical materialsfor their strong, visible, and easily detected emissions, while NIR emit-ting lanthanide ions YbIII and ErIII are ever-growing studied in biologicalanalysis, because of their almost-transparent emissions in biologicalissues [7]. The large anisotropic magnetic moment for some of the LnIII

ions makes them very appealing for the preparation of magnetic mate-rials (for instance, TbIII and DyIII) [8–26].

The association of modulable functionalized (carboxylates,phosphonates…) organic moieties with rare-earth metals allows the cre-ation of a large number of compounds with dimensions and sometimesunique physical properties [27,28]. While pure carboxylate ligands havebeen extensively studied, less investigated nitrogen-containing heterocy-cles such as pyrazoles, 1,2,4-triazoles, and tetrazoles were found topossess promising coordination abilities [29–31]. Given the important po-tential applications of lanthanide-based polymers and the fascinatingproperties of the HL ligand, we have investigated a series of lanthanide-based polymers, [Ln(L)(atpa)(H2O)2]n·1.5nH2O (Ln = Tb(1), Dy(2),Ho(3), Er(4), Tm(5), Yb(6)) based on 4-[3-methyl-5-(pyridin-4-yl)-

ankai University, Tianjin 300071,

rights reserved.

1,2,4-triazol-4-yl]benzoic acid (HL) and 2-aminoterephthalic acid(H2atpa).

Compounds 1–6 are isomorphous, so compound 6 is employed asa representative to be discussed in detail. In compound 6, ligands L−

and atpa2− both present only one coordination mode (see Scheme 1:configuration μ2 for L−, and each carboxylate group coordinates totwo YbIII ions; configuration μ2 for atpa2−, and each carboxylategroup coordinates to one YbIII ion). Compound 6, crystallizing in thetriclinic system with the Pī space group. As shown in Fig. 1A, theasymmetric unit of 6 consists of one crystallographically independentYbIII ion, one L− ligand, one atpa2− ligand, two coordinated watermolecules and one and a half lattice water molecules. Each YbIII issurrounded by an O8 donor set, formed by two oxygen atoms fromtwo different L− ligands, four oxygen atoms from two atpa2− ligands,and two terminal water molecules. The coordination geometry aroundthe metal center can be described as a distorted triangular dodecahe-dron. This type of configuration of eight-coordinated YbIII ion is rarelyreported, compared to the square antiprism, which is confirmed bycomparing chosen ideal and observed pertinent dihedral angles basedon calculations of the shape factor S to evaluate the degree of distortionfrom an ideal geometry [7,32]. The Yb\O bond lengths vary from2.199(3) Å to 2.435(3) Å. Yb1 and its corresponding centrosymmetricgenerated atom Yb1A are joined by two μ2-O atoms of carboxylategroups to form a binuclear unit, Yb2(CO2)2 units (Fig. 1B), in whichthe distance of Yb1-Yb1A is 4.822 Å. Atpa2− ligands and YbIII ionsconnected to 1D chain (Fig. 1C). And the 1D chains were connected toa 2D sheet by the connection of the binuclear unit formed by two L−

ligands and two YbIII ions (Fig. S1). Each Yb2(CO2)2 unit is connected

Scheme 1. Coordination modes of L− and atpa2− in 1–6.

196 L. Yang et al. / Inorganic Chemistry Communications 35 (2013) 195–199

with four adjacent units through four atpa2− ligands sustaining a 2Dsheet, as illustrated in Figs. 1D and S2. The Yb⋯Yb distance across thebridging atpa2− ligand is 11.178 Å. The atpa2− ligands connect theunits, which play a vital role in the extension of the dimensionality for6. However, L− ligands coordinate to the metal centers with terminalbridging-bidentate binding mode. As shown in Fig. 1E, hydrogen bonds

Fig. 1. (A) Coordination environment of the YbIII ion (all hydrogen atoms and solvent molec1D chain for 6. (All L− ligands, hydrogen atoms and solvent molecules are omitted for clariO\H⋯O interaction in the bc-plane.

form among the oxygen atoms from carboxylate groups, the nitrogenatoms from amino groups and coordinated water molecules. Thecrystallization O9w and O10w water molecules are placed in theintermellar region, establishing strong hydrogen bonding interactionswith the layer network. The structures of 1–6 show the effect of lantha-nide contraction: Ln\O bond length and Ln⋯Ln separations decreasealong with the decrease of ionic radii from Tb(1) to Yb(6).

Based on the crystal data, we carried out the calculations of theshape factor S to estimate the degree of distortion of the coordinationstructure in first coordination sphere. The S value is given by Eq. (1):

S ¼ min

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1m

� �Xmi¼1

δi−θið Þ2vuut ð1Þ

in which m, δi and θi are the number of possible edges (m = 18 in thisstudy), the observed dihedral angle between planes along the ith edgeand the dihedral angle for the ideal structure, respectively. The estimat-ed S values of YbIII ions are summarized (see Table S8 in the Supportinginformation). For [Yb(L)(atpa)(H2O)2]n·1.5nH2O, the S value for theeight-coordinated square antiprism structure (8-SAP, point group:D4d, S = 15°) is bigger than that for the eight-coordinated trigonaldodecahedron structure (8-TDH, point group: D2d, S = 11°), thereby

ules are omitted for clarity). (B) Structure of a paddle-wheel unit for 6. (C) Structure ofty.) (D) 2D layer of 6 running along the bc-plane. (E) 3D framework of 6 connected by

Fig. 3. Lifetime curve of compound 1-Tb.

197L. Yang et al. / Inorganic Chemistry Communications 35 (2013) 195–199

suggesting that the 8-TDH structure is less distorted than the 8-SAPstructure. We thus determined that the coordination geometry of[Yb(L)(atpa)(H2O)2]n·1.5nH2O is 8-TDH.

The TGA was carried out on crystalline samples of 1–6 in theflowing air with a heating rate of 10 °C/min in the temperaturerange of 25–800 °C. TG curves of isomorphous compounds 1–6 are,respectively, similar, exhibiting two main weight loss steps until thedecomposition of the framework. Here, compound 3 is used as onerepresentative as illustrated in Fig. S3 (Supporting information). TheTG curve of compound 3 shows the first stage of weight loss from25 °C to 160 °C (9.6% contrast to theoretical 9.2%), which is attributedto the release of the coordinated and guest water molecules andpartial decomposition of the organic ligands. The second stage ofweight loss occurs from 380 °C to 670 °C (72.43%). The remnants ofcompound 3 are close to the calculated values (27.83% base on theHo2O3).

The purity of the bulky crystalline samples was confirmed byPXRD. The PXRD patterns of compounds 1–6 are illustrated in Fig. S4(Supporting information), which are in good agreementwith simulatedones, confirming the phase purity of the as-synthesized products.

The solid-state photoluminescence spectra of 1 were recorded atroom temperature and depicted in Fig. 2. Under excitation of300 nm, compound 1 exhibits characteristic TbIII emission spectraat about 490 (5D4 → 7F6), 545 (5D4 → 7F5), 585 (5D4 → 7F4), and621 nm (5D4 → 7F3). The dominant peak corresponding to the5D4 → 7F5 transition at 545 nm is hypersensitive, which impliesgreen luminescence of TbIII compound. The emission decay profilesof compound 1 in the solid state were obtained at room temperature.Fig. 3 shows the linear fit curve of compound 1. The fluorescence life-time value of compound 1 was calculated by the single-exponentialmode. From the result, it was found that the determined lifetimesfor compound 1 are 0.57 μs (19.30%); 4.67 μs (51.79%) and 13.30 μs(28.91%), respectively, determined by monitoring the 5D4 → 7F5transition. And that is close to the literature [7].

Luminescence spectra of DyIII, ErIII and YbIII compounds in solid stateupon excitation at 333 nm, 375 nm, and 397 nm, display typical nar-row band emissions in the NIR region at room temperature. The emis-sion spectrum of DyIII compound peaked at 1308 nm, and 888 nm isattributed to the transition from the excited states 6F11/2(6H9/2) and6F7/2 to the ground state 6H15/2, respectively (Fig. 4A) [33]. The ErIII com-pound exhibits the emission band in the near-infrared region with themaximum at 1500 nm attributed to the transition from the excitedstate (4I13/2) to the ground state (4I15/2) of the ErIII ion (Fig. 4B) [34].The YbIII compound displays a strong emission at 1003 nm (Fig. 4C),which is the characteristic transition 2F5/2 → 2F7/2 of the YbIII ion innear-infrared region [35]. However, the luminescence of HoIII andTmIII in compounds 3 and 5 is too weak to be measured with our

Fig. 2. Solid state emission spectra for compound 1-Tb at room temperature upon ex-citation at 300 nm.

instrument setup. The relative intensities of these emissions are sensi-tive to the detailed nature of the ligand environment. Furthermore,the intensity of the compounds based on the HL ligand is completelyweak, possibly due to the coordinated water molecules, which reducesthe luminescent intensity of the rare earth ions, and the energy transferfrom the HL ligand to LnIII ions is inefficient in the theory of energytransfer [38].

The spin-orbital coupling in general plays an important role in themagnetism of lanthanide compounds due to the internal nature of thevalence f orbitals. This large spin–orbit coupling partly removes the de-generacy of the 2S + 1L group term of lanthanide ions, giving 2S + 1LJstates, which further split into Stark levels under the crystalfield pertur-bation [36]. The variable-temperature magnetic susceptibilities weremeasured at an applied magnetic field of 1000 Oe in the temperaturerange of 2.0–300 K for 1–2, and the χMT vs T plots are shown in Figs. 5and 6. For compound 1, the χMT value at 300 K of 12.557 cm3 K mol−1

is slightly larger than the expected value of 11.82 cm3 K mol−1 of oneisolated Tb3+ ion in the 7F6 state, and it gradually decreases with de-creasing temperature to reach a minimum of 8.29 cm3 K mol−1 at 2 K.The decrease of the χMT product suggests the presence of weak an-tiferromagnetic interaction between TbIII ions. The low tempera-ture decrease is likely due to a combination of the depopulationof Stark levels of TbIII ions and the presence of interlayer antiferro-magnetic interactions. For compound 2, the χMT value at roomtemperature is 14.598 cm3 K mol−1, slightly higher than the expected14.17 cm3 K mol−1 with the ground state 6H15/2 and g = 4/3. The χMTvalue decreases slowly with a minimum in temperature to ca. 12 K.When the temperature is lowered below 12 K, the χMT value increasesrapidly and reaches the maximum of 14.37 cm3 K mol−1 at 2 K. Thefact that the χMT value increases after reaching the minimum value at12 K shows that a ferromagnetic interaction is expected to exist be-tween the DyIII ions in 2 [37]. Because no out-of-phase (χM″) signalsare observed in the ac magnetic susceptibility between 2 and 20 K,there is no evidence of slow-relaxation for 2 (see Fig. S5 in theSupporting information).

In conclusion, a series of novel lanthanide-based coordination poly-mers, [Ln(L)(atpa)(H2O)2]n·1.5nH2O (HL = 4-[3-methyl-5-(pyridin-4-yl)-1,2,4-triazol-4-yl]benzoic acid, H2atpa = 2-aminoterephthalicacid and Ln = Tb(1), Dy(2), Ho(3), Er(4), Tm(5), Yb(6)) have been hy-drothermally synthesized. Compounds 1–6, crystallizing in the triclinicsystem with the Pī space group, feature a 2D network based onLn2(CO2)2 units. Atpa2− ligands and LnIII ions connected to 1D chain,and the 1D chains were connected to a 2D sheet by the connection oftwo L− ligands and two LnIII ions. Compound 1 exhibits the characteris-tic emission of TbIII ions, showing that the ligands are luminescencesensitizers. Compounds 2, 4, and 6 exhibit the characteristic transitionof DyIII, ErIII, and YbIII ions in near-infrared region. Themagnetic proper-ties of compound 1 show typical antiferromagnetic interaction, whilecompound 2 has ferromagnetic behavior.

Fig. 4. (A) NIR emission spectra of compound 2-Dy in the solid state. (B) NIR emissionspectra of compound 4-Er in the solid state. (C) NIR emission spectra of compound6-Yb in the solid state.

Fig. 5. Temperature dependence of xM and xMT for 1-Tb.

Fig. 6. Temperature dependence of xM and xMT for 2-Dy.

198 L. Yang et al. / Inorganic Chemistry Communications 35 (2013) 195–199

Acknowledgment

This work was supported by the National Natural Science Founda-tion of China (No. 21071083 and No. 20771062).

Appendix A. Supplementary material

CCDC 916887, 916883, 916886, 916884, 916888, 916889 for com-pounds 1–6 contain the supplementary crystallographic data for this

paper. These data can be obtained free of charge from the CambridgeCrystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

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