Magnetic Properties and Photoluminescence of Lanthanide

Article pubs.acs.org/crystal

Magnetic Properties and Photoluminescence of Lanthanide Coordination Polymers Constructed with Conformation-Flexible Cyclohexane-Tetracarboxylate Ligands Yong-Cong Ou,*,†,‡ Xiang Gao,† Yue Zhou,† Yan-Cong Chen,‡ Long-Fei Wang,‡ Jian-Zhong Wu,† and Ming-Liang Tong*,‡ †

Guangzhou Key Laboratory of Materials for Energy Conversion and Storage, School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China ‡ Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China S Supporting Information *

ABSTRACT: To investigate the effects on the physical properties of complexes with different conformations of flexible ligands, four lanthanide (Ln) coordination polymers with flexible ligand cyclohexane-1e,2a,4a,5e-tetracarboxylic acid (H4LI), [Ln(HLII)(H2O)2]·1.5H2O (Ln = Dy, 1; Tb, 2) and [Ln2(H2LIV)(LIV)(H2O)4]·1.5H2O (Ln = Dy, 3; Tb, 4), have been synthesized and characterized. Complexes 1 and 2 are 2D layers composed of LII ligands and [Ln2O2] dinuclear units whereas complexes 3 and 4 are 3D coordination frameworks constructed with LIV ligands and two kinds of dinuclear units, [Ln2O2] and [Ln2(COO)2]. Notably, the conformations of ligands in two structures, which were different and changed from the original LI conformation to LII (e,a,e,e) and LIV (e,e,e,e) conformations, impact the construction of different structures and the magnetic properties. Complex 1 shows antiferromagnetic interactions whereas complex 3 exhibits weak ferromagnetic interactions and slow magnetic relaxation properties. Additionally, the photoluminescence of complexes 2 and 4 have been investigated.

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INTRODUCTION

Cyclohexane-1e,2a,4a,5e-tetracarboxylic acid (H4L) is a good candidate to construct novel frameworks with lanthanide ions which are oxyphilic elements. Additionally, H4L ligand could transform their conformations in the complexes with transition metal ions38−42 such as Mn2+, Co2+, Ni2+, and Zn2+ under different conditions, and it is intriguing that lanthanide coordination polymers have not been reported with other conformations except the LI conformation.43 Therefore, it is pregnant to construct novel LCPs with other conformations of H4L. Furthermore, the physical properties of LCPs could be affected strikingly while the conformations transformed. Herein, four lanthanide coordination polymers with H4L ligand in two different conformations have been synthesized and characterized, [Ln(HLII)(H2O)2]·1.5H2O (Ln = Dy, 1; Tb, 2) and [Ln2(H2LIV)(LIV)(H2O)4]·1.5H2O (Ln = Dy, 3; Tb, 4). Notably, complexes 1 and 3 show dramatically different magnetic signals, which is a surprising magnetostructural characterization.

Functional materials, such as metal organic frameworks (MOFs), whose properties can be tuned precisely at the molecular level, have giant application prospects1−4 and have attracted intensive attentions. Flexible MOFs, members of the third generation of functional MOFs, have great advantages in separation, sensing, and catalysis, with stimuli including thermal, photochemical, or high pressure.5−7 Compared to transition metal organic frameworks, the research in lanthanide coordination polymers (LCPs) has increased dramatically in recent years not only due to the high coordination number of lanthanide ions and structural diversities, but also because of the attractive physical properties arising from the 4f electrons of lanthanide and extensive applications in many fields,8−16 especially in magnetic17−24 and luminescent25−31 properties. The fabrication of LCPs with rigid multicarboxylate ligands to obtain promising compounds has been extensively studied. However, the complication of the coordination modes for lanthanide ions and flexible ligands usually led to difficulties in tuning the structures and/or properties. Therefore, reports of the fabrication of LCPs with flexible ligands are relatively rare.32−37 © XXXX American Chemical Society

Received: October 22, 2015 Revised: December 3, 2015

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DOI: 10.1021/acs.cgd.5b01500 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Table 1. Crystal Data and Structure Refinements for 1−4 empirical formula M wavelength (Å) crystal system space group a/Å b/Å c/Å α/deg β/deg γ/deg Vol/Å3 Z ρcalcd/g cm−3 μ/mm−1 reflns collected unique reflns S RED R1a, wR2b (I > 2σ(I)) R1a, wR2b (all data) a

1 (293 K)

2 (150 K)

3 (293 K)

4 (150 K)

C10H16O11.5Dy 482.22 0.71073 Monoclinic C2/c 23.801(3) 6.4594(7) 19.401(2) 90 110.485(3) 90 2794.1(5) 8 2.293 5.411 6123 2964 (0.0717) 1.063 2.49 0.0555, 0.1283 0.0778, 0.1403

C10H16O11.5Tb 479.15 1.54178 Monoclinic C2/c 23.816(2) 6.4464(5) 19.4193(13) 90 110.344(9) 90 2795.4(4) 8 2.277 25.500 4409 2060 (0.0713) 0.919 1.68 0.0471, 0.1075 0.0697, 0.1136

C20H29O21.5Dy2 938.43 0.71073 Triclinic P1̅ 6.4343(13) 13.116(3) 17.546(4) 71.43(3) 82.65(3) 88.76(3) 1391.9(5) 2 2.239 5.424 10714 5267 (0.0360) 1.039 1.01 0.0286, 0.0499 0.0444, 0.0552

C20H29O21.5Tb2 931.27 1.54178 Triclinic P1̅ 6.4415(2) 13.1031(8) 17.5383(10) 71.445(5) 82.599(4) 88.731(4) 1391.38(12) 2 2.223 25.551 7487 4104 (0.0368) 0.952 0.95 0.0303, 0.0704 0.0378, 0.0721

R1 = ∑||F0| − |Fc||/∑|F0|. bwR2 = [∑w(F02 − Fc2)2/∑w(F02)2]1/2, RED is residual electron densities.

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IR (4000−400 cm−1) spectra for 1−4 are similar (Figure S2): the absorption bands of COO− grouped are 1562, 1410, and 1708 cm−1. The adsorptions for 1 and 2 at 1708 cm−1 are much stronger, probably due to interlayer hydrogen bonds.44 TGA (Figure S3): 13.2%, 14.0%, 11.3%, and 11.7% weight loss before long plateaus corresponds to the loss of water molecules in the four complexes, respectively. X-ray Crystallography. Diffraction data for complexes 1 and 3 were collected on a Rigaku R-AXIS SPIDER Image Plate diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 293(2) K. Diffraction data for complexes 2 and 4 were recorded on an Oxford Diffraction Gemini R CCD diffractometer with Cu Kα radiation (λ = 1.54178 Å) at 150 K. The measurement temperature was controlled using an Oxford Cryosystems Cryostream cooling apparatus. Absorption corrections were applied by using the multiscan program SADABS.45 The structures were solved by a direct method, and all non-hydrogen atoms were refined anisotropically by full-matrix least-squares techniques using the SHELXTL program.46 Further details for structural analysis for complexes 1−4 are summarized in Table 1.

EXPERIMENTAL SECTION

Materials and Physical Measurements. The reagents and solvents used were commercially available and used as received without further purification. Cyclohexane-1,2,4,5-tetracarboxylic acid (H4L) used in all the reactions was in the LI (e,a,a,e) conformation. C, H, and N microanalyses were carried out with an Elementar Vario-EL CHN elemental analyzer. FT-IR spectra were recorded in KBr tablets in the range 4000−400 cm −1 on a Nicolet FT-IR-170SX spectrophotometer. X-ray powder diffraction (XPRD) intensities for polycrystalline samples of 1−4 (Figure S1) were measured at 293 K on a Bruker D8 Advance Diffratometer (Cu Kα, λ = 1.54056 Å) by scanning over the range of 5−50° with steps of 0.12°/s. Simulated XPRD patterns of 1−4 were generated with Mecury software. Thermogravimetric (TG) analysis was carried out on a NETZSCH TG209F3 thermogravimetric analyzer. Variable-temperature magnetic susceptibility measurements were carried out using a SQUID magnetometer MPMS XL-7 (Quantum Design) at 1.0 kOe for 1 and 3. Diamagnetic correction was applied from Pascal’s constants. The emission/excitation spectra of solid samples of 1−4 were measured on a Hitachi F-2500 instrument at room temperature with a xenon arc lamp as the light source. Synthesis Methods. [Ln(HLII)(H2O)2]·1.5H2O (Ln = Dy, 1; Tb, 2). A mixture of Ln(NO3)3·6H2O (0.2 mmol, Ln: Dy, 90 mg, Tb, 92 mg), H4LI (0.2 mmol, 53 mg), and NaOH (0.6 mmol, 39 mg) in deionized water (10 mL) was heated in a stainless steel reactor with a Teflon liner (23 mL) at 160 °C for 120 h and cooled to ambient temperature at a rate of ca. 5 °C·h−1 to give colorless needle crystals of 1 and 2, respectively (yield, based on H4LI: 1, 38 mg, 53%; 2, 25 mg, 35%). Elemental analysis calcd. (%) for C10H16O11.5Dy (1): C 24.88, H 3.34; Found: C 25.14, H 3.46; Elemental analysis calcd. (%) for C10H16O11.5Tb (2): C 25.07, H 3.37; Found: C 25.15, H 3.43. [Ln2(H2LIV)(LIV)(H2O)4]·1.5H2O (Ln = Dy, 3; Tb, 4). A mixture of Ln2O3 (0.1 mmol, Ln: Dy, 38 mg, Tb, 35 mg) and H4LI (0.2 mmol, 53 mg) in deionized water (10 mL) was heated in a stainless steel reactor with a Teflon liner (23 mL) at 180 °C for 120 h and cooled to ambient temperature at a rate of ca. 5 °C·h−1 to give colorless sheet crystals of 3 and 4, respectively (yield, based on H4LI: 3, 58 mg, 62%; 4, 40 mg, 43%). Elemental analysis calcd. (%) for C20H29O21.5Dy2 (3): C 25.60, H 3.11; Found: C 26.01, H 3.24; Elemental analysis calcd. (%) for C20H29O21.5Tb2 (4): C 25.79, H 3.14; Found: C 25.95, H 3.33.

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RESULTS AND DISCUSSION Synthesis Conditions. The nature of chemistry is focused on the construction of new compounds, and the most important part in that is to control the synthesis conditions. As reported in our previous works, there are many factors that could affect the conformation transformation of the flexible ligand H4LI, which contains four possible conformations in free phase (Table S1), such as temperature, pH values, reaction time, and the addition of auxiliary ligands. In addition, we have reported a series of LCPs, [Ln2LI3(H2O)10]·9H2O, which were constructed via a hydrothermal method with triethylamine to adjust the pH values.43 Fortunately, while just replacing triethylamine with NaOH under the same conditions, complexes 1 and 2 appeared as byproducts, respectively. If increasing the temperature from 120 to 160 °C, complexes 1 and 2 appeared as pure crystals in the reactions with NaOH, but no crystals appeared under the same conditions with triethylamine. This shows that the kinds of base might play an important role to control the conformation transformation of B



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Chart 1. Coordination Modes of LI in Previous Reported LCPs: the HLII in 1 and 2, the LIV, H2LIV in 3 and 4

Figure 1. (a) ORTEP drawing of the coordination environment of the Dy atom and the bridging mode of the HLII ligand with ellipsoids at the 30% probability level, (b) side-view, (c) top-view and simplified (4,6) layer network (d) of the 2D coordination layer constructed via dinuclear units (purple balls in (d)) and HLII (gray balls in (d)) (H atoms are omitted for clear), and (e) 3D supramolecular structure (blue-white multiband bond represents hydrogen bonding interactions) in complex 1.

flexible ligands and that raising the reacting temperatures could create thermodynamically more stable conformations, such as the stable conformation LII in complexes 1 and 2, and LIV in complexes 3 and 4 constructed from Ln2O3 at 180 °C. From the theoretical calculations on free H4L ligand in different

conformations in gas phase or as a solvate in water,41 conformations LIV, LIII, and LII are sequentially more stable. The possible reason that conformation LIII has not been captured yet might be its closer energy to conformation LII, which would make it hard to precisely control the synthesis C



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Figure 2. (a) ORTEP drawing of the coordination environment of the Dy atoms and the bridging modes of the LIV and H2LIV ligands with ellipsoids at the 30% probability level, (b) top-views of the 2D layer unit constructed by the LIV ligands and dinuclear units, (c) top- and (d) side-view of the 3D MOF structure, (e) the novel (6,4) topological network in complex 3. Green and purple polyhedrons correspond to Dy1 and Dy2 ions.

conditions. Although conformation LIV is not as stable as conformation LII, coordination bonds (Chart 1) and strong hydrogen environments of the ligands in the lanthanide complexes could also stabilize it. Crystal Structures of 1−4. Complexes 1 and 2 are isomorphic, the same as complexes 3 and 4; hence, only the structures of 1 and 3 are discussed in detail here. Single crystal X-ray crystallography reveals that complex 1 is a twodimensional coordination layer structure and contains one independent Dy3+ atom, one HLII ligand, two coordinated water molecules, and one and a half lattice water molecules in the asymmetric unit (Figure 1a). Dy1 adopts a muffin geometry (Figure S4a), calculated by SHAPE,47 surrounded by six oxygen atoms from three equatorial carboxylate groups of ligand, two water molecules, and one bridging atom O1 which connects two Dy atoms into a dinuclear unit Dy1(O)2Dy1 (Dy1−O1 =

2.686(7) Å, Dy1−O1A = 2.345(7) Å, Dy1−O1−Dy1A = 110.7(2)°). These units are linked together by two LII ligands into 2D coordination layers (Figure 1b and 1c) in which axial protonated carboxylate groups point outward. While the dinuclear units and ligands are considered as nodes, 2D layers can be simplified as (4,6) layers (Figure 1d). Therefore, the 3D supramolecular structure is constructed via hydrogen bonding interactions between hydrogen atoms from axial carboxylate groups and O4 atoms (Figure 1e). Complex 3 is a 3D coordination framework and crystallizes in P1̅ space group. The asymmetric unit consists of two unique Dy3+ atoms, one total deprotonated LIV ligand, one partial deprotonated H2LIV ligand, four coordination water molecules, and one and a half lattice water molecules (Figure 2a). The Dy1 atom is in a snub diphenoid geometry48 (Figure S4b) fulfilled with four oxygen atoms from two chelated carboxylate groups, D



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Figure 3. χmT versus T plots for 1 (a) and 3 (b) measured at 1000 Oe in the temperature range of 1.8−300 K. Inset: M versus H plots at 2, 3, and 5 K. Temperature dependence of the in-phase (χ′) and out-of-phase (χ″) susceptibilities for 1 (c) and 3 (d) under a 1000 Oe dc field. The solid lines are guidelines only.

units for HLII in complexes 1 and 2 are similar to that of three equatorial carboxylate groups for LIV in complexes 3 and 4, and the axial carboxylate groups for HLII are stabilized with hydrogen bonds whereas the remaining equatorial groups for LIV connect dinuclear units; therefore, it is dramatically different in forming the 2D layer frameworks, bilayer structure for HLII, and single ligand layer structure for LIV. The carboxylate groups for LI in the reported structure,43 [Ln2LI3(H2O)10]·9H2O, are coordinated with lanthanide atoms totally, and the fact that the two axial carboxylate groups chelate with the same metal atom makes greatly different structural modes (Chart 1). Magnetic Properties. As carboxylate groups can commendably process magnetic exchange between metal ions,49−52 magnetic susceptibility measurements were carried out on bulk samples of 1 and 3 in a direct current (dc) field in the temperature range of 1.8−300 K. As shown in Figure 3a, the χmT value is 27.3 cm3 K mol−1 for complex 1 at 300 K, which is in good agreement with the expected value of 28.3 cm3 K mol−1 for two uncoupled Dy3+ ions (S = 5/2, L = 5, J = 15/2, g = 4/ 3).53−55 The χmT value monotonically decreases with temperature to 20.6 cm3 K mol−1 at approximately 10 K and then drops steeply to a minimum of 14.4 cm3 K mol−1 at 1.8 K, indicating the depopulation of the MJ states of Dy ion (6H15/2) from the splitting of crystal field and/or antiferromagnetic interactions within the dinuclear units.56 For complex 3, the room temperature χmT value is 27.0 cm3 K mol−1, which is close to the value expected for two Dy3+ ions. Upon cooling, the value decreases gradually to reach a minimum of 21.9 cm3 K mol−1 at 4.1 K, and then increases to 22.0 cm3 K mol−1 at 1.8 K

two oxygen (O3, O4) atoms from a bridging carboxylate group, and two water molecules, whereas Dy2 is surrounded with nine oxygen atoms in the coordination mode, which is similar to that of lanthanide metal atom in complex 1. In the structure, there are two kinds of dinuclear units, Dy1(COO)2Dy1 (Dy1−O3 = 2.332(4) Å, Dy1−O4 = 2.232(5) Å) and Dy2(O)2Dy2 (Dy2− O7 = 2.848(4) Å, Dy2−O7C = 2.320(4) Å, Dy2−O7−Dy2C = 109.80(14)°), of which the latter one is similar to the dinuclear unit in complex 1. The total deprotonated LIV ligands link four dinuclear units into 2D layers (Figure 2b) that are connected further by partial deprotonated H2LIV ligands into a 3D coordination framework (Figure 2c, 2d). As the two kinds of dinuclear units can be considered as six-connected nodes and the LIV and H2LIV ligands as four-connected nodes and connectors, respectively, the whole framework can be simplified as a novel (6,4) net with the short Schläfl i symbol (4 4 .6 1 0 .8)(4 4 .6 2 ) and the long vertex symbols of (4.4.4.4.65.6 5.65.6 5.65.6 5.65 .6 5)(4.4.4.4.65.6 5.65.6 5.65 .6 5.65 .6 4) (4.4.4.4.62.62) (Figure 2e). Comparing the structures of complexes 1−4 with the reported structures which are only comprised of H4L and metal ions, the conformation of the flexible ligand plays a vital role in construction. For conformation LII, we have reported a structure,41 Mn2(LII)(H2O)3, where the four carboxylate groups coordinated with metal ions, is a three-dimensional framework. Interestingly, when the manganese atoms connecting with the axial carboxylate group are omitted, the structure turns into a 2D layer which is similar to the structure of complexes 1 and 2. Furthermore, the coordination modes of all equatorial carboxylate groups which combined with dinuclear E



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between 4f orbitals, which are excited by an antenna effect.9,67,68 Herein the reason for no emission peaks for 1 and 3 may be that the energy transmissions of organic ligands, LII and LIV, are suitable for the excited energy of Tb3+ ions but not Dy3+ ions.

(Figure 3b). The decrease in the low temperature region could be ascribed to the crystal field effect on Dy3+ ions, and the increase in the lowest temperature region is due to the weak ferromagnetic interactions of dysprosium dinuclear units.57,58 The field dependences of the magnetization for complexes 1 and 3 at 2, 3, and 5 K have shown that a steep increase occurs, which, without true saturation and the nonsuperimposable nature of a large magnetic field, suggests the presence of strong magnetic anisotropy and/or a population of low lying excited states (insets Figure 3a and 3b). To investigate the dynamic properties of complexes 1 and 3, the temperature dependent ac susceptibilities were measured. In the absence of dc field, both of the complexes show no outof-phase signals with the frequencies v = 1 and 1488 Hz (Figure S5) which is possibly due to the quantum tunneling of magnetization (QTM) effect that prevents isolation of effective energy barriers at zero-field.59,60 In order to distinguish the difference between the complexes, we performed ac magnetic measurement under a 1000 Oe dc field.61,62 The obvious temperature dependence of the in-phase (χ′) and out-of-phase (χ″) for 3 (Figure 3d) indicates that slow magnetic relaxation is improved while the magnetic signals show that 1 is frequencyindependent (Figure 3c). According to the structural analysis, complex 3 comprises two magnetic coupling units, one of which is an oxygenbridging coupling unit, Dy2−O7−Dy2C, and similar to that unit, Dy1−O1−Dy1A, in complex 1. The tricapped trigonal prismic geometry of metal ions, bond lengths, and angles for the two units are similar (Table S2). The distance between the carboxylate-bridging dinuclear units and the oxygen-bridging dinuclear unit is 8.72 Å at least, which indicates the magnetic interactions could be ignored. Herein the overall magnetic phenomenon of 3 is comprised of antiferromagnetic Dy(O)2Dy units and ferromagnetic Dy(COO)2Dy units, which are in accord with the reported carboxylate-bridging dinuclear dysprosium complexes.63−66 Luminescent Properties. The solid state photoluminescence spectra of complexes 1−4 have been studied at room temperature and show that the corresponding characteristic emission bands of the Tb3+ ions of complexes 2 and 4 are displayed but no emission bands of Dy3+ are captured in complexes 1 and 3 upon excitation at 352 nm (Figures 4 and S6). Four emission peaks at 490, 544, 585, and 622 nm for 2 and 489, 544, 585, and 622 nm for 4, respectively, are assigned to be the transitions from 5D4 to 7FJ (J = 6, 5, 4, and 3) for Tb3+ ions. The 5D4 → 7F5 transition with a peak at 544 nm is dominant in the spectra. It is well-known that the luminescent properties of lanthanide ions come from an electronic transition

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CONCLUSION Two series of novel lanthanide complexes have been constructed with H4L in this paper and fully characterized. First, ligand conformation transformations in lanthanide complexes are successfully captured from LI to LII and LIV. Furthermore, the magnetic properties show obvious differences from antiferromagnetic to ferromagnetic interaction along with the different coordination modes of the e/a position carboxylate group. This work shows a useful strategy for tuning the complexes structures and properties, and may give a new way to combine novel functional materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01500. Lanthanide atoms polyhedral diagrams, PXRD data, IR spectra, TG data, temperature dependence of ac magnetic susceptibilities, excitation spectra, theoretical calculation of free conformations, and selected bond lengths, angles, and hydrogen-bond parameters. (PDF) Accession Codes

CCDC 1430325−1430328 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (Grant nos. 21401058 and 91422302), Yong Innovative Talents Program of Guangdong (2014KQNCX058) and the SCNU Foundation for Fostering Young Teachers (13KJ15).

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Figure 4. Solid state emission spectra of complexes 1−4. F



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