Ligand Field Affected Single-Molecule Magnet Behavior of Lanthanide

Article pubs.acs.org/IC

Ligand Field Affected Single-Molecule Magnet Behavior of Lanthanide(III) Dinuclear Complexes with an 8‑Hydroxyquinoline Schiff Base Derivative as Bridging Ligand Wen-Min Wang,† Hong-Xia Zhang,† Shi-Yu Wang,† Hai-Yun Shen,† Hong-Ling Gao,† Jian-Zhong Cui,*,† and Bin Zhao*,‡ †

Department of Chemistry, Tianjin University, Tianjin 300072, People’s Republic of China College of Chemistry, Key Laboratory of Advanced Energy Material Chemistry, MOE, TKL of Metal and Molecule Based Material Chemistry, and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, People’s Republic of China

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S Supporting Information *

ABSTRACT: New dinuclear lanthanide(III) complexes based on an 8-hydroxyquinoline Schiff base derivative and β-diketonate ligands, [Ln2(hfac)4(L)2] (Ln(III) = Gd (1), Tb (2), Dy (3), Ho (4), Er (5)), [Ln2(tfac)4(L)2] (Ln(III) = Gd (6), Tb (7), Dy (8), Ho (9)), and [Dy(bfac)4(L)2·C7H16] (10) (L = 2-[[(4-fluorophenyl)imino] methyl]-8-hydroxyquinoline, hfac = hexafluoroacetylacetonate, tfac = trifluoroacetylacetonate, and bfac = benzoyltrifluoroacetone), have been synthesized. The single-crystal X-ray diffraction data show that complexes 1−10 are phenoxo-O-bridged dinuclear complexes; each eight-coordinated center Ln(III) ion is in a slightly distorted dodecahedral geometry with two bidentate β-diketonate coligands and two μ2-O bridging 8-hydroxyquinoline Schiff base derivative ligands. The magnetic study reveals that 1 and 6 display cryogenic magnetic refrigeration properties, whereas complexes 3, 8, and 10 show different SMM behaviors with energy barriers of 6.77 K for 3, 19.83 K for 8, and 25.65 K for 10. Meanwhile, slow magnetic relaxation was observed in 7, while no out-of-phase alternating-current signals were found for 2. The different dynamic magnetic behaviors of two Tb2 complexes and the three Dy2 complexes mainly derive from the tiny crystal structure changes around the Ln(III) ions. It is also proved that the β-diketonate coligands can play an important role in modulating magnetic dynamics of the lanthanide 8-hydroxyquinoline Schiff base derivative system.

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INTRODUCTION During the past two decades, the design and construction of single-molecule magnets (SMMs) have attracted increasing attention because of the prospect and potential applications of processing and storing magnetic information at a molecular level.1 Since the seminal discovery of SMM behavior in a Mn12 complex during the 1990s, many SMMs based on transitionmetal complexes have been reported.2 In recent years, with a deepening of the knowledge of the magneto-chemical properties, heavy lanthanide (Ln) ions (Tb(III), Dy(III), Ho(III), and Er(III)) have been much more used to design and construct new SMMs, which is mainly due to the significant magnetic anisotropy arising from their large, unquenched orbital angular momentum.3 In view of this, design and constructions along this line have resulted in the discovery of many SMMs, including Ln(III) complexes with polyoxometalate,4 phthalocyanine,5 macrocyclic Schiff base ligands,6 β-diketone,7 or nitronyl nitroxide radicals.8 Although there are a large number of SMMs based on polyoxometalate, phthalocyanine, macrocyclic Schiff base ligands, β-diketone, and nitronyl nitroxide radicals which have been reported, there have been fewer SMMs based on 8-hydroxyquinoline or 8-hydroxyquinoline Schiff base © XXXX American Chemical Society

Scheme 1. 2-[[(4-Fluorophenyl)imino]methyl]-8hydroxyquinoline (L)

ligand derivatives. Accordingly, we focused on the attractive 8-hydroxyquinoline Schiff base derivative ligand L (Scheme 1), which has excellent structural features: (i) it can easily coordinate with Ln(III) ions through three donor atoms; (ii) the phenoxy atom of L can act as a phenoxo-O bridge between Ln(III) ion centers, which can transmit magnetic exchange efficiently between neighboring Ln(III) ions. Taking advantage of 8-hydroxyquinoline Schiff base derivative ligand L, we have used it to research the relationship of the structure and the magnetic properties of lanthanide complexes. Received: June 22, 2015

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DOI: 10.1021/acs.inorgchem.5b01404 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

range 2θ = 5−50° with a Rigaku Ultima IV instrument with Cu Kα radiation (λ = 1.54056 Å), with a scan speed of 5° min−1. TGA measurements were obtained in an air atmosphere on a Labsys NETZSCH TG 209 Setaram apparatus from 30 to 800 °C with a heating rate of 10 °C min−1. The magnetic measurements were carried out with Quantum Design MPMS-XL7 and PPMS-9 ACMS magnetometers. The diamagnetic corrections for the complexes were estimated using Pascal’s constants, and magnetic data were corrected for diamagnetic contributions of the sample holder.12 Synthesis of 2-[[(4-Fluorophenyl)imino]methyl]-8-hydroxyquinoline (L). The 8-hydroxyquinoline Schiff base derivative ligand L was synthesized in a simple aldimine condensation reaction of 8-hydroxyquinoline-2-carboxaldehyde (0.15 mmol) with 4-fluoroaniline (0.15 mmol) in methanol. The reaction mixture was stirred for 4 h at room temperature. The resulting yellow precipitate was then washed with methanol and dried in a vacuum chamber. After about 12 h, the L ligand as a yellow solid was obtained. Synthesis of Complexes 1−5. A solution of Ln(hfac)3·2H2O (0.025 mmol; Ln(III) = Gd (1), Tb (2), Dy (3), Ho (4), Er (5)) in 20 mL of n-heptane was heated to reflux temperature for 2 h. Then it was cooled to 70 °C, and a CH2Cl2 solution (5 mL) containing L (0.025 mmol) was added. The resulting mixture was stirred for 1 h at 70 °C and then cooled to room temperature. After filtration, the resulting solution was kept in the dark and concentrated slowly by evaporation at 4 °C. Block-shaped deep red crystals were obtained after 3 days. [Gd2(hfac)4(L)2] (1). Yield: 45% (based on Gd). Anal. Calcd for C52H24Gd2F26N4O10 (1673.25): C, 37.33; H, 1.45; N, 3.35. Found: C, 37.29; H, 1.47; N, 3.36. IR (KBr, cm−1): 3418 (s), 1657 (s), 1618 (m), 1465 (m), 13187 (w), 1257 (m), 1205 (m), 1141 (m), 1102 (m), 845 (w), 758 (w), 625 (m), 486 (w). [Tb2(hfac)4(L)2] (2). Yield: 56% (based on Tb). Anal. Calcd for C52H24Tb2F26N4O10 (1676.59): C, 37.25; H, 1.44; N, 3.34. Found: C, 37.24; H, 1.47; N, 3.31. IR (KBr, cm−1): 3417 (s), 1656 (s), 1618 (m), 1480 (m), 1463 (m), 1317 (w), 1257 (m), 1205 (m), 1141 (m), 1102 (m), 845 (w), 756 (w), 623 (m), 486 (w). [Dy2(hfac)4(L)2] (3). Yield: 63% (based on Dy). Anal. Calcd for C52H24Dy2F26N4O10 (1683.75): C, 37.10; H, 1.44; N, 3.33. Found: C, 37.21; H, 1.37; N, 3.24. IR (KBr, cm−1): 3415 (s), 1655 (s), 1618 (m), 1463 (m), 1318 (w), 1259 (m), 1203 (m), 1142 (m), 1102 (m), 845 (w), 756 (w), 623 (m), 487 (w). [Ho2(hfac)4(L)2] (4). Yield: 60% (based on Ho). Anal. Calcd for C52H24Ho2F26N4O10 (1688.61): C, 36.98; H, 1.43; N, 3.32. Found: C, 36.81; H, 1.35; N, 3.35. IR (KBr, cm−1): 3418 (s), 1657 (s), 1619 (m), 1462 (m), 1316 (w), 1259 (m), 1203 (m), 1145 (m), 1101 (m), 845 (w), 756 (w), 625 (m), 490 (w). [Er2(hfac)4(L)2] (5). Yield: 65% (based on Er). Anal. Calcd for C52H24Er2F26N4O10 (1693.27): C, 36.89; H, 1.43; N, 3.31. Found: C, 37.10; H, 1.40; N, 3.30. IR (KBr, cm−1): 3415 (s), 1658 (s), 1614 (m), 1467 (m), 1315 (w), 1255 (m), 1203 (m), 1142 (m), 1108 (m), 843 (w), 753 (w), 620 (m), 486 (w). Synthesis of Complexes 6−9. The syntheses of complexes 6−9 are very similar to those of 1−5, the only difference being the use of Ln(tfac)3·2H2O (0.025 mmol; Ln(III) = Gd (6), Tb (7), Dy (8), Ho (9)) instead of Ln(hfac)3·2H2O (0.025 mmol; Ln(III) = Gd (1), Tb (2), Dy (3), Ho (4), Er (5)). [Gd2(tfac)4(L)2] (6). Yield: 43% (based on Gd). Anal. Calcd for C52H36Gd2F14N4O10 (1457.35): C, 42.86; H, 2.49; N, 3.84. Found: C, 42.87; H, 2.45; N, 3.79. IR (KBr, cm−1): 3415 (s), 1657 (s), 1619 (m), 1463 (m), 1317 (w), 1257 (m), 1205 (m), 1141 (m), 1103 (m), 844 (w), 800 (w), 756 (w), 625 (m), 486(w). [Tb2(tfac)4L2] (7). Yield: 47% (based on Tb). Anal. Calcd for C52H36Tb2F14N4O10 (1460.69): C, 42.76; H, 2.48; N, 3.84. Found: C, 42.78; H, 2.45; N, 3.81. IR (KBr, cm−1): 3415 (s), 1656 (s), 1618 (m), 1463 (m), 1317 (w), 1257 (m), 1205 (m), 1141 (m), 1102 (m), 844 (w), 801 (w), 756 (w), 623 (m), 486 (w). [Dy2(tfac)4L2] (8). Yield: 56% (based on Dy). Anal. Calcd for C52H36Dy2F14N4O10 (1467.85): C, 42.55; H, 2.47; N, 3.82. Found: C, 42.50; H, 2.56; N, 3.81. IR (KBr, cm−1): 3416 (s), 1655 (s), 1618 (m),

It is proven that the SMM behaviors are closely related to the coordination geometry, ligand field effects, and the strength of the magnetic interactions between the lanthanide sites.9 Among these influencing factors, it is worth noting that the ligand field plays an important role in modulating the significant magnetic anisotropy of Ln(III)-based SMMs.7a The magnetic relaxation of lanthanide SMMs is very sensitive to the coordination environment of the Ln(III) ion, and the SMMS behaviors can be modulated by a slight change in the coordination environment around the Ln(III) ion center. In order to explore how a slight change of coordination environment around the center lanthanide ions affects the SMMS behavior of the complexes, we used three slightly different β-diketonates (Scheme 2), Scheme 2. Structures of Three Different β-Diketonates

hexafluoroacetylacetonate (hfac), trifluoroacetylacetonate (tfac), and benzoyltrifluoroacetone (bfac), to construct Ln(III) complex SMMs. Herein, the crystal structures and magnetic properties of 10 dinuclear lanthanide(III) complexes based on the 8-hydroxyquinoline Schiff base derivative ligand L, [Ln2(hfac)4(L)2] (Ln(III) = Gd (1), Tb (2), Dy (3), Ho (4), Er (5)), [Ln2(tfac)4(L)2] (Ln(III) = Gd (6), Tb (7), Dy (8), Ho (9)), and [Dy(bfac)4(L)2·C7H16] (10) (L = 2-[[(4fluorophenyl)imino]methyl]-8-hydroxyquinoline, hfac = hexafluoroacetylacetonate, tfac = trifluoroacetylacetonate, and bfac = benzoyltrifluoroacetone), have been reported. Among them, complexes 1 and 6 display cryogenic magnetic refrigeration properties, while 3, 8, and 10 show SMM behavior. However, the effective anisotropy barrier (ΔE/kB) of 10 is 25.65 K, and it is larger than that of 19.83 K for 8 and 6.77 K for 3. In addition, complexes 2 and 7 show distinct magnetic behaviors, in which 7 shows slow magnetic relaxation, while no frequency-dependent out-of-phase signals were noticed for complex 2. The different dynamic magnetic behaviors of the three Dy2 complexes 3, 8, and 10 and the two Tb2 complexes 2 and 7 mostly originated from the different coordination environments of the Ln3+ ions, which are directly related to the strength and symmetry of the three β-diketonate coligands. Therefore, the synthetic and β-diketonate coligand tuned approach illustrated in the lanthanide(III) complexes based on an 8-hydroxyquinoline Schiff base derivative ligand may open up new opportunities to develop Ln-based SMMs.

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EXPERIMENTAL SECTION

General Procedures. All starting solvents and chemicals were reagent grade without further purification. The materials Ln(hfac)3·2H2O (Ln(III) = Gd, Tb, Dy, Ho, Er), Ln(tfac)3·2H2O (Ln(III) = Gd, Tb, Dy, Ho), and Dy(bfac)3·2H2O were prepared according to the reported literature.10 The 8-hydroxyquinoline Schiff base derivative ligand L was synthesized by the methods already reported.11 Elemental analyses for C, H, and N were measured by a PerkinElmer 240 CHN elemental analyzer. IR spectra were performed on a Bruker TENOR 27 spectrophotometer using a KBr pellet in the range of 4000−400 cm−1. UV−vis spectra were measured with a JASCO V-570 spectrophotometer at room temperature. Luminescence properties were recorded on an F-4500 FL spectrophotometer with a xenon arc lamp as the light source. PXRD data were examined in the B


Article

Inorganic Chemistry

β-diketonate coligands, and the center Dy(III) ion is in a slightly distorted dodecahedral geometry. Different from the case for 3, the Dy−O bond lengths of 8 and 10 are in the ranges 2.326(7)−2.430(7) and 2.303(3)−2.399(3) Å, respectively. Moreover, the Dy- - -Dy distance is 3.8800(10) for 8 and 3.8326(10) Å for 10 and the bond angles Dy−O−Dy of 8 and 10 are 110.1(3) and 108.88(10)°, respectively. According to the description and Table S1 in the Supporting Information, we can find that the Dy−O−Dy bond angles and Dy- - -Dy distances of complexes 8 and 10 are larger than those of 3. These slight differences in bond lengths and angles of the three Dy2 complexes are due to the different strengths and symmetries of the three β-diketonate coligands. To our knowledge, the introduction of an electron-donating CH3 or a phenyl ring group to replace one CF3 electron-withdrawing group in hfac, to a great extent, may result in the decrease of the ligand field strength of the lanthanide ion and it can also change the symmetry of the complex molecule. TG Analyses and Powder X-ray Diffraction. The thermal stabilities of 1−10 were studied on crystalline samples in an air atmosphere with a heating rate of 10 °C min−1 in the temperature range 30−800 °C (Figure S1 in the Supporting Information). For 1−5, the TGA curves are very similar since they are isomorphic. Herein, complex 3 will used as a representative illustration. From 30 °C to about 280 °C, no weight loss occurs in 3. Between 280 and 350 °C, a weight loss of 32.4% occurs, which is related to the loss of two 8-hydroxyquinoline Schiff base derivative ligands (L) (calcd 31.6%), and after that the complex decomposes gradually. For 6−9, all of the TGA curves are very similar and the three complexes are thermally stable up to 240 °C; after that, complexes 6−9 decompose gradually. For 10, from 30 to 310 °C, a weight loss of 5.6% is related to the loss of one free n-heptane solvent molecule (calcd 5.5%) and then there is a weight loss of 30.5% between 310 and 380 °C, which is related to the release of two ligands (L) (calcd 29.5%); after that, 10 decomposes gradually. The purity of crystalline powders of 1−10 was confirmed by powder X-ray diffraction (PXRD), and the results are shown in Figure S2 in the Supporting Information. The experimental PXRD patterns of 1−9 are in accord with the corresponding simulated patterns obtained from the single-crystal data, indicating the high purity of the crystal samples. For 10, the PXRD pattern data show that it might be impure on comparison with the simulated values. This is very probably due to the loss of the n-heptane solvent molecule in the unit cell during the sample preparation, which is unavoidable. UV−Vis Spectra. The UV−vis spectra of ligand L, Dy(hfac)3·2H2O, Dy(tfac)3·2H2O, and complexes 1−10 are measured at room temperature in MeOH solution and are displayed in Figure S3 in the Supporting Information. For the L ligand, two broad absorption bands were clearly observed at ca. 210 and 246 nm. The observed strong peak at ca. 246 nm may be attributed to a π → π* transition of the aromatic rings. The UV−vis spectra of 1−5 (Figure S3a) display similar absorption profiles centered at 212 and 300 nm. The high-energy band at 213 nm results from the ligand L, and the absorption bands at 300 nm are ascribed to the π → π* transition of hfac−. For 6−10 (Figure S3b), the absorption bands at 213 and 298 nm result from the ligand L and β-diketonate coligands, respectively. The maximum absorption peaks at about 246 nm disappear in complexes 1−10 in comparison with the L ligand, which may be due to the coordination effects of different β-diketonate coligands (hfac−, tfac−, and bfac−) with the Ln(III) ion.

1462 (m), 1317 (w), 1256 (m), 1205 (m), 1141 (m), 1102(m), 843 (w), 800 (w), 756 (w), 624 (m), 485 (w). [Ho2(tfac)4L2] (9). Yield: 62% (based on Ho). Anal. Calcd for C52H36Ho2F14N4O10 (1472.71): C, 42.41; H, 2.46; N, 3.80. Found: C, 42.44; H, 2.38; N, 3.76. IR (KBr, cm−1): 3414 (s), 1655 (s), 1617 (m), 1463 (m), 1319 (w), 1256 (m), 1205 (m), 1145 (m), 1101 (m), 843 (w), 805 (w), 756 (w), 625 (m), 489 (w). Synthesis of [Dy2(bfac)4(L)2]·C7H16 (10). The complex 10 was prepared by a procedure similar to that for 1, the only difference being the use of Dy(bfac)3·2H2O (0.025 mmol) instead of Gd(hfac)3·2H2O (0.025 mmol). In the end, needle-shaped red crystals were obtained. Yield: 35% based on Dy. Anal. Calcd for C79H46Dy2F14N4O10: C, 52.65; H, 2.57; N, 3.11. Found: C, 52.53; H, 2.68; N, 3.32. IR (KBr, cm−1): 3420 (m), 1597 (s), 1519 (s), 1460 (s), 1397 (m), 1368 (s), 1291 (m), 1263 (m), 1172 (m), 1012 (m), 920 (m), 752 (w), 486 (w). X-ray Crystallography. Single-crystal X-ray diffraction data of 1−10 were collected at a temperature of 113 K on a computercontrolled Rigaku Saturn CCD area detector diffractometer equipped with confocal monochromatized Mo Kα radiation with a radiation wavelength of 0.71073 Å using the ω−φ scan technique. The structures were solved by direct methods and refined by a full-matrix leastsquares technique based on F2 using the SHELXS-97 and SHELXL-97 programs.13 Anisotropic thermal parameters were assigned to all non-hydrogen atoms. Crystallographic data parameters for 1−10 are given in Tables 1 and 2. The important bond angles and lengths for 1−10 are given in Table S1 in the Supporting Information. CCDC (1061556, 1; 1061078, 2; 1061079, 3; 1061080, 4; 1061081, 5; 1405046, 6; 1405047, 7; 1405048, 8; 1405049, 9; 1061100, 10) contain supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/data_request/cif.

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RESULTS AND DISCUSSION Descriptions of Crystal Structures of 1−5. Complexes 1−5 are isomorphous and crystallize in the monoclinic space group P21/n. Therefore, the crystal structure of 3 will be selected and described in detail. For 3, as shown in Figure 1, each molecule contains two Dy3+ ions and two L and four hfac− ligands. The center Dy3+ ion is eight-coordinated with two phenoxide oxygen atoms (O5 and O5a), one nitrogen atom (N1) in the pyridyl ring and one imine nitrogen atom (N2) of the two L ligands, and four oxygen atoms of two hfac− ligands. The two Dy3+ ions are bridged by two μ2-O atoms (O5 and O5a) of two L ligands, leading to a approximate rhombic Dy2O2 core with a Dy−O−Dy angle of 107.572(90)° and a Dy---Dy distance of 3.7633(8) Å. The Dy−O bond lengths are in the range 2.289(2)−2.375(2) Å, and the Dy−N bond distances are 2.438(3) and 2.606(3) Å for 3, which are comparable to those of the already reported phenoxo-bridged dysprosium complexes.14 On comparison of the bond lengths and bond angles of 1−5 (Table S1 in the Supporting Information), from 1 to 5, with a decrease in the ionic radius, the Ln−O bond lengths gradually decrease, which originates from the effect of the lanthanide contraction.15 In addition, with a decrease in the ionic radius of the Ln3+ ions decrease, the Ln---Ln distances gradually decrease, while the Ln−O−Ln bond angles almost increase as the ionic radius of the Ln3+ ions decreases. Descriptions of Crystal Structures of 6−10. Complexes 6−9 have similar crystal structures, Therefore, the molecular structures of 8 and 10 will be described in detail. In comparison with 3, complex 8 is composed of two [Dy(tfac)2L] units bridged by two phenoxo groups (Figure 2a), while 10 consists of two [Dy(bfac)2L] units and one free n-heptane molecule (Figure 2b). For 3, 8, and 10, each center Dy3+ ion is eightcoordinated with two μ2-O bridging L ligands and two C


no. of rflns collected no. of params GOF on F2 R1, wR2 (I > 2σ(I)) R1, wR2 (all data)

formula Mr T (K) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z cryst size (mm3) Dc (g cm−3) μ (mm−1) Rint limiting indices

C52H24Gd2F26N4O10 1673.25 113(2) monoclinic P21/n 11.207(2) 19.142(4) 13.093(3) 90 94.83(3) 90 2798.7(10) 2 0.20 × 0.18 × 0.12 1.986 2.498 0.0497 −13 ≤ h ≤ 13, −22 ≤ k ≤ 22, −15 ≤ l ≤ 13 23237 452 1.005 0.0255, 0.0544 0.0317, 0.0568

1 C52H24Tb2F26N4O10 1676.59 113(2) monoclinic P21/n 11.229(2) 19.093(4) 13.064(3) 90 94.85 90 2790.8(10) 2 0.20 × 0.18 × 0.12 1.995 2.663 0.0400 −13 ≤ h ≤ 13, −22 ≤ k ≤ 22, −15 ≤ l ≤ 15 22586 452 1.093 0.0254, 0.0575 0.0290, 0.0593

2

Table 1. Crystallographic Data and Structure Refinement Details for 1−5 3 C52H24Dy2F26N4O10 1683.76 113(2) monoclinic P21/n 11.245(2) 19.040(4) 13.046(3) 90 94.68(3) 90 2783.9(1) 2 0.20 × 0.18 × 0.12 2.009 2.813 0.0400 −12 ≤ h ≤ 13, −22 ≤ k ≤ 22, −14 ≤ l ≤ 15 23052 452 1.066 0.0263, 0.0566 0.0319, 0.0590

4 C52H24Ho2F26N4O10 1688.62 113(2) monoclinic P21/n 11.266(2) 19.005(4) 13.043(3) 90 94.47 90 2784.1(10) 2 0.20 × 0.18 × 0.12 2.014 2.971 0.0424 −13 ≤ h ≤ 13, −22 ≤ k ≤ 22, −15 ≤ l ≤ 15 25714 452 1.101 0.0225, 0.0526 0.0253, 0.0540

5 C52H24Er2F26N4O10 1693.19 113(2) monoclinic P21/n 11.277(2) 18.986(4) 13.026(3) 90 94.42 90 2780.7(10) 2 0.20 × 0.18 × 0.12 2.022 3.147 0.0319 −13 ≤ h ≤ 13, −21 ≤ k ≤ 22, −15 ≤ l ≤ 15 22771 452 1.148 0.0236, 0.0623 0.0270, 0.0641

Inorganic Chemistry Article

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no. of rflns collected no. of params GOF on F2 R1, wR2 (I > 2σ(I)) R1, wR2 (all data)

formula Mr T (K) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z cryst size (mm3) Dc (g cm−3) μ (mm−1) Rint limiting indices

C52H36Gd2F14N4O10 1457.35 113(2) monoclinic P21 10.863(2) 19.489(4) 13.226(3) 90 108.25(3) 90 2659.1(9) 2 0.20 × 0.18 × 0.12 1.820 2.583 0.0379 −12 ≤ h ≤ 12, −23 ≤ k ≤ 23, −11 ≤ l ≤ 15 21785 743 0.985 0.0437, 0.1049 0.0472, 0.1079

6 C52H36Tb2F14N4O10 1460.69 113(2) monoclinic P21 10.860(2) 19.431(4) 13.198(3) 90 107.99(3) 90 2648.8(9) 2 0.20 × 0.18 × 0.12 1.831 2.759 0.0442 −12 ≤ h ≤ 12, −23 ≤ k ≤ 23, −15 ≤ l ≤ 15 27341 743 0.983 0.0460, 0.1028 0.0510, 0.1069

7

Table 2. Crystallographic Data and Structure Refinement Details for 6−10 8 C52H36Dy2F14N4O10 1467.85 113(2) monoclinic P21 10.959(2) 19.531(4) 13.299(3) 90 108.30(3) 90 2648.8(9) 2 0.20 × 0.18 × 0.12 1.831 2.759 0.0442 −12 ≤ h ≤ 12, −23 ≤ k ≤ 23, −15 ≤ l ≤ 15 27341 743 0.983 0.0460, 0.1028 0.0510, 0.1069

9 C52H36Ho2F14N4O10 1472.71 113(2) monoclinic P21 10.877(2) 19.428(4) 13.149(3) 90 107.78 90 2646.0(9) 2 0.20 × 0.18 × 0.12 1.848 3.080 0.0818 −12 ≤ h ≤ 12, −23 ≤ k ≤ 23, −15 ≤ l ≤ 15 27449 741 1.045 0.0644, 0.1568 0.0698, 0.1624

10

−14 ≤ h ≤ 14, −17 ≤ k ≤ 17, −19 ≤ l ≤ 19 22175 524 1.109 0.0467, 0.1522 0.0493, 0.1574

C79H46Dy2F14N4O10 1802.20 113(2) triclinic P1̅ 11.034(2) 13.209(3) 14.584(3) 68.82(3) 86.43(3) 76.58(3) 1927.4(7) 1 0.20 × 0.18 × 0.12 1.553 2.016

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Inorganic Chemistry

is 315 nm. The emission peak at 491 nm could be attributed to the 5D4 → 7F6 transition, and the other three peaks at 545, 585, and 626 nm could be assigned to the 5D4 → 7F5, 5D4 → 7F4, and 5 D4→ 7F3 transitions, respectively.16a Among them, the 5D4 → 7 F5 transition is the strongest. For 7 (Figure 3b), the excitation wavelength for emission spectra is 292 nm. The emission spectra are composed of four main bands at 492 nm (5D4 → 7F6), 545 nm (5D4 → 7F5), 586 nm (5D4 → 7F4), and 623 nm (5D4 → 7 F3), and the emission peak at 545 nm (5D4 → 7F5) is clearly stronger than the other peaks. The fluorescence spectra show the characteristic emission peaks of terbium ions.16b Magnetic Properties. The solid-state magnetic susceptibility measurements of 1−10 were studied in the 2−300 K range under an applied magnetic field of 1000 Oe (the magnetic property measurements of 1−10 were carried out with crystalline samples). For complexes 1−5 (Figure 4a), the χMT values at 300 K are 15.63 (1), 24.02 (2), 27.53 (3), 28.08 (4), and 22.92 (5) cm3 K mol−1, respectively. These are in good agreement with the expected values for two isolated Ln3+ ions as follows: two free Gd3+ ions (8S7/2, g = 2) give 15.76 cm3 K mol−1 for 1; two Tb3+ ions (7F6, g = 3/2) give 23.64 cm3 K mol−1 for 2; two

Figure 1. Molecular structure for 3. Hydrogen atoms are omitted for clarity.

Luminescence Properties. The photoluminescence of 2 and 7 was measured at room temperature in a methanol solution. For 2 (Figure 3a), the excitation wavelength for emission spectra

Figure 2. Molecular structures for 8 (a) and 10 (b). The n-heptane molecule of 10 and all hydrogen atoms of 8 and 10 are omitted for clarity.

Figure 3. Room-temperature luminescence spectra of 2 (λex 315 nm) (a) and 7 (λex 292 nm) (b) in methanol solution. F


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Inorganic Chemistry

for complexes 6−9 (Figure 4b), the χMT values are 15.86, 23.91, 28.02, and 28.25 cm3 K mol−1, respectively. These are also very close to the expected values for two isolated Ln3+ ions: two Gd3+ ions (8S7/2, g = 2) give 15.76 cm3 K mol−1 for 6; two Tb3+ ions (7F6, g = 3/2) give 23.64 cm3 K mol−1 for 7; two Dy3+ ions (6H15/2, g = 4/3) give 28.34 cm3 K mol−1 for 8; two Ho3+ ions (5I8, g = 5/4) give 28.14 cm3 K mol−1 for 9. For 6, the χMT vs T curve is similar to that of 1; upon cooling, the χMT value is almost constant over the temperature range 300−20 K and then it drops sharply to a minimum value of 7.95 cm3 K mol−1 at 2 K. The decreasing trend indicates the presence of a weak antiferromagnetic interaction between the Gd(III) ions.16 For complexes 7−9, on cooling, the χMT values decrease gradually in the temperature range 300−50 K, and then the χMT values rapidly reach minima of 6.41, 8.20, and 4.88 cm3 K mol−1 at 2 K, respectively. This can be ascribed to the weak magnetic exchange between the Ln(III) ions in the system and/or the thermal depopulation of the Ln(III) Stark sublevels.18a−c For complex 10 (Figure 4c), the χMT value is 28.44 cm3 K mol−1 at room temperature, which is slightly larger than the expected value of 28.34 cm3 K mol−1 for two free Dy(III) ions (6H15/2, g = 4/3). Upon a decrease in the temperature, the χMT value gradually decreases and falls to the minimum of 8.81 cm3 K mol−1 at 2.0 K, which also can be ascribed to weak magnetic exchange between the Dy(III) ions in the complex and/or the thermal depopulation of the Dy(III) Stark sublevels.18d,e In addition, the magnetic susceptibilities of complexes 1 and 6 can be fitted to the Curie−Weiss law, and the parameters Θ = −0.90 K, C = 15.69 cm3 K mol−1 for 1 and Θ = −3.23 K, C = 15.82 cm3 K mol−1 for 6 were obtained. The negative parameters of Θ further prove that a weak antiferromagnetic coupling between Gd(III) ions existed in 1 and 6. For complexes 2−5 and 7−10, the plots of χM−1 versus T between 2 and 300 K follow the Curie−Weiss law with negative Θ values, resulting from the depopulation of Stark sublevels (Figure S4 in the Supporting Information). The magnetization data of 1 and 6 are carried out at a field of 0−8 T between 2 and 10 K (Figure 5a,b). The M versus H curves display a gradual increase with the increasing field and saturation values of 13.98 Nβ for 1 and 14.02 Nβ for 6 at 8 T and 2 K, being extremely approximate with the theoretical value of 14 Nβ for two individual Gd(III) (S = 7/2, g = 2) ions. Magnetic entropy changes ΔSm of 1 and 6 are calculated from the M versus H data to evaluate the MCE. ΔSm can be calculated by using the Maxwell equation:

Figure 4. χMT versus T for 1−5 (a), 6−9 (b), and 10 (c) under an applied dc field of 1000 Oe.

ΔSm(T ) =

Dy3+ ions (6H15/2, g = 4/3) give 28.34 cm3 K mol−1 for 3; two Ho3+ ions (5I8, g = 5/4) give 28.14 cm3 K mol−1 for 4; two Er3+ ions (4I15/2, g = 6/5) give 22.96 cm3 K mol−1 for 5. For complex 1, the χMT value stays almost constant in the temperature range 300−20 K and then drops rapidly to a minimum value of 8.96 cm3 K mol−1 at 2 K, which indicates the presence of a weak antiferromagnetic interaction between the Gd3+ ions.17 In the cases of 2−5, χMT values decrease gradually in the temperature range 300−50 K and then decrease distinctly to reach minima of 5.10, 7.25, 13.88, and 12.77 cm3 K mol−1 at 2 K, respectively. Such behavior is generally due to the thermal depopulation of the Ln(III) Stark sublevels and/or the weak antiferromagnetic interactions between the Ln(III) ions in complexes 2−5.18 The χMT vs T plots of complexes 6−9 are very similar to those of complexes 1−4. At room temperature,

∫ [∂M(T , H)/∂T ]H dH

(1)

According to eq 1,19 the −ΔSm values of 1 and 6 can be obtained; the plots of −ΔSm versus T are shown in Figure 5c,d. For 1, the maximum value of −ΔSm is 17.05 J K−1 kg−1 (calculated as 2Rln(2S+1), expected maximum −ΔSm is 20.67 J K−1 kg−1) for a field change ΔH = 8 T at 3.0 K. The difference of − ΔSm between the experimental and theoretical values for 1 are mainly due to the MW /NGd ratio of 837 (where molecular mass is MW = 1673.25 g mol−1 and NGd is the number of Gd (III) ions present per mole of 1) and the antiferromagnetic interaction in 1. The expected maximum −ΔSm of 6 is 23.73 J K−1 kg−1, calculated as 2R ln(2S + 1) for two uncoupled Gd(III) ions, which is larger than the maximum value of −ΔSm (20.69 J K−1 kg−1) for the field change ΔH = 8 T at 2.0 K. The observed −ΔSm values of 1 and 6 are smaller than those of reported Gd(III)-based G


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Inorganic Chemistry

Figure 5. M versus H plots for 1 (a) and 6 (b) at T = 2.0−10.0 K and H = 0−8 T. Temperature dependence of magnetic entropy change (−ΔSm) as calculated from the magnetization data of 1 (c) and 6 (d) at T = 2−10 K and 0−8 T.

molecular systems;20 however, they are significantly larger than those for the antiferromagnetic {Gd2} complex (17.25 J kg−1 K−1, ΔH = 7 T at 3 K).21 The maximum −ΔSm value of complex 6 (20.69 J K−1 kg−1, ΔH = 8 T at 2 K) is larger than that of 1, which can be ascribed to the smaller Mw/NGd ratio (729) in 6. In order to study the magnetic dynamics of 2−5 and 7−10, the temperature dependences of the ac susceptibility were measured with a 3 Oe ac magnetic field and under 0 dc field (Figure 6 and Figure S5 in the Supporting Information). For complexes 3, 8, and 10, both the in-phase (χ′) and out-of-phase (χ″) signals of ac susceptibilities exhibit strong frequency dependence and good peaks are observed, implying that the three Dy2 complexes possess slow magnetic relaxation, typical of SMM behavior. For 7, the out-of-phase (χ″) magnetic susceptibility shows frequencydependent behavior and remarkable peak shapes are observed, indicating that complex 7 possesses SMM behavior. The relaxation time τ data derived from the out-of-phase (χ″) peaks can be fitted by the Arrhenius law: τ = τ0 exp(ΔE/kBT). The best fitting results yield the pre-exponential factor τ0 = 2.2 × 10−6 s and the effective energy barrier ΔE/kB = 10.39 K (Figure S6 in the Supporting Information). The obtained values of ΔE/kB and τ0 fall into the normal range of already reported Tb-based SMMs.22 For complexes 2, 4, 5, and 9, no frequency-dependent out-of-phase signals (χ″) were noticed.

In order to check the QTM effect above 2 K in 3, 8, and 10, the variable-temperature ac susceptibilities were determined again under a dc field of 2000 Oe (Figure S7 in the Supporting Information). For 3, remarkable peak shapes were observed in the low-frequency region (111−1111 Hz), which were not clearly observed in a 0 dc field. In addition, with an external 2000 Oe dc field, the peaks are shifted to a higher temperature range in comparison with the peaks in a 0 dc field. For 10, remarkable peak shapes were observed in a 2000 Oe dc field and the peaks are also shifted to higher temperature in comparison with those in a 0 dc field. These phenomena show that the quantum tunneling effect in the two Dy2 complexes is pronounced and the QTM effect is basically suppressed when they were under an external 2000 Oe dc field. However, in comparison, it is interesting that two peaks are observed in 8 under a 0 dc field but only one peak shape is observed under an external 2000 Oe dc field. This shows that an external dc field has a great influence on the relaxation of magnetization of 8. The frequency dependences of the ac susceptibility were measured under 0 dc field to further explore the SMM behavior of 3, 8, and 10. As show in Figure 7, both the in-phase (χ′) and out-of-phase (χ″) signals of the ac susceptibility of 3, 8, and 10 show temperature dependences, which further indicate the presence of slow relaxation of the magnetization in complexes H


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Inorganic Chemistry

Figure 6. Temperature dependence of χ′ and χ″ for 3 (a and b), 8 (c and d) and 10 (e and f) in a 0 dc field with an oscillation of 3.0 Oe.

3, 8, and 10. The relaxation time τ data of 3, 8, and 10 derived from the out-of-phase (χ″) peaks are in the form of ln τ plotted as a function of 1/Tp (Figure 8). Above 2.8 K, the relaxation time τ data obey the Arrhenius law: τ = τ0 exp (ΔE/kBT). The best fitting results give the energy barriers ΔE/kB = 6.77 K for 3, 19.83 K for 8, and 25.65 K for 10 and the pre-exponential factors τ0 = 9.12 × 10−6 s for 3, 7.62 × 10−8 s for 8, and 1.64 × 10−6 s for 10. The τ0 values of all three Dy2 complexes are consistent with the reported values of 10−6−10−12 s for SMMs.23,24

The Cole−Cole plots of χ″ vs χ′ for 3, 8, and 10 show nearly semicircle shapes and were fitted to the generalized Debye model to obtain α values (Figure S7 in the Supporting Information).25,26 For 3, α = 0.25−0.29 was obtained in the temperature range of 2−4 K, while α values of 0.34−0.40 for 8 and 0.27−0.32 for 10 were obtained. The relatively large distribution coefficients α of the three Dy2 complexes indicate that there are a wide distribution of relaxation time in complexes 3, 8, and 10. It is clear that the introduction of one electron-donating group I


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Inorganic Chemistry

Figure 7. Frequency dependence of χ′ and χ″ for 3 (a and b) at 2.0−4.0 K, 8 (c and d) at 2.0−4.0 K, and 10 (e and f) at 2.0−6.5 K under a 0 dc field.

−CH3 or a phenyl ring to replace the electron-withdrawing group (−CF3) has a great influence on the magnetic relaxation of complexes 3, 8, and 10. Although the crystal structures of complexes 3, 8, and 10 are very similar, they display different SMM behaviors. Indeed, complex 10 shows slow magnetic relaxation and the energy barrier (ΔE/kB) is 25.65 K, which is higher than that of both 3 (ΔE/kB = 6.77 K) and 8 (ΔE/kB = 19.83 K). The different magnetic behaviors can be caused by the slightly different

structures of the three Dy2 complexes. According to the singlecrystal X-ray diffraction data together with the semiquantitative method of polytopal analysis, for complexes 3, 8, and 10 the most reasonable geometry of the Dy(III) ion is dodecahedral (DD). Hence, the slightly different coordination environments of the Dy3+ ions for the three complexes can result in different magnetic relaxation behaviors. Moreover, different Ln- - -Ln distances are found for complexes 3 (Dy- - -Dy = 3.7633(8) Å), 8 (Dy- - -Dy = 3.8800(10) Å), and 10 (Dy- - -Dy = 3.8326(10) Å), J


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Inorganic Chemistry

Figure 8. ln τ versus T−1 plots for complexes 3 (a), 8 (b), and 10 (c). The red solid lines represent the least-squares fits of the experimental data to the Arrhenius law.

The differences in the energy barriers for the magnetic relaxation (ΔE/kB) of 3, 8, and 10 are mostly due to the slightly different coordination environments around the Dy(III) ions. Moreover, complexes 2 and 7 show distinct magnetic behaviors, in which 7 shows slow magnetic relaxation, while no out-of-phase (χ″) frequency-dependent signals were observed for 2. It was proven that the β-diketonate ligands play a significant role in modulating SMM behaviors by comparing the magnetic behaviors of the three Dy2 complexes 3, 8, and 10 and the magnetic behaviors of the two Tb2 complexes 2 and 7. This work provides an important approach for fine-tuning SMM behaviors by using different β-diketonate coligands. Further magnetic property studies on other polynuclear lanthanide(III) complexes based on an 8-hydroxyquinoline Schiff base ligand are in progress in our laboratory.

which may have a significant influence on the magnetic interaction between the Dy3+ ions. In addition, the three Dy2 molecules are of three different space groups: complex 3 crystallizes in P21/n and the two Dy3+ ions are related by the inversion center, while the rare-earth ions in 8 in the P21 space group are of different type, or symmetrically independent. Both the crystal system and space group of 10 are different fromthose of 3 and 8. These differences may directly result in the different SMM behaviors of the three Dy2 complexes. In conclusion, the different magnetic relaxation behaviors of complexes 3, 8, and 10 mostly originate from the different strengths and symmetries of the three β-diketonate coligands, and the distinct magnetic behaviors of 2 and 7 also proved this point. Our work testified that the SMM behavior of the lanthanide 8-hydroxyquinoline Schiff base derivative system can be finely tuned by changing the substituent group of the β-diketonate ligands.

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ASSOCIATED CONTENT

S Supporting Information *

CONCLUSION In summary, we reported the structures, luminescence properties, and magnetic properties of 10 dinuclear Ln(III) complexes based on an 8-hydroxyquinoline Schiff base derivative ligand. Complexes 1−10 are all phenoxo-O-bridged binuclear complexes; each eight-coordinated center Ln(III) ion is in a slightly distorted dodecahedral geometry with two bidentate β-diketonate coligands and two μ2-O bridging 8-hydroxyquinoline Schiff base derivative ligands. The magnetic study reveals that complexes 1 and 6 display cryogenic magnetic refrigeration properties, while 3, 8, and 10 show SMM behaviors with energy barriers (ΔE/kB) of 6.77, 19.83, and 25.65 K, respectively.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01404. Important bond lengths and angles, PXRD patterns, TGA data, UV−vis absorption spectra, and magnetic and crystallographic data of complexes 1−10 (PDF) Crystallographic data of complexes 1−10 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for J.-Z.C.: [email protected]. *E-mail for B.Z.: [email protected]. K


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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (No. 21473121, 21271137, and 21571138) for its financial support.

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