Nanoscale {LnIII24ZnII6} Triangular Metalloring with Magnetic

Nov 24, 2015 - “Three-in-one”: a novel family of high-nuclearity nanoscale triangular ringlike {LnIII24ZnII6} [Ln = Gd (1), Tb (2), and Dy (3)] co...
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Nanoscale {LnIII24ZnII6} Triangular Metalloring with Magnetic Refrigerant, Slow Magnetic Relaxation, and Fluorescent Properties Li Zhang,†,‡ Lang Zhao,† Peng Zhang,†,‡ Chao Wang,† Sen-Wen Yuan,† and Jinkui Tang*,† †

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *

ABSTRACT: The self-assembly of Ln(ClO4)3·6H2O and Zn(OAc)2· 2H2O with pyrazine-2-carboxylic acid (HL) results in the formation of three novel nanosized {Ln III 24 Zn II 6 } triangular metallorings, [Gd24 Zn6 L 24 (OAc) 22 (μ3 -OH)30 (H 2 O) 14 ](ClO 4 ) 7 (OAc)·2CH 3OH· 26H2O (1), [Tb 24Zn6L 24(OAc) 22(μ3-OH)30(CH3O) 2(CH3OH) 2(H2O)10](ClO4)5(OH)·6CH3OH·12H2O (2), and (H3O)[Dy24Zn6L24(OAc)22(μ3-OH)30(H2O)14](ClO4)7(OAc)2·4CH3OH·22H2O (3), having the largest nuclearity among any known Ln/Zn clusters. Magnetic and luminescent studies reveal the special prowess for each lanthanide complex. Magnetic studies reveal that 1 exhibits a significant cryogenic magnetocaloric effect with a maximum −ΔSm (isothermal magnetic entropy change) value of 30.0 J kg−1 K−1 at 2.5 K and 7 T and that a slow magnetization relaxation is observed for the dysprosium analogue. In addition, the solid-state photophysical properties of 2 display strong characteristic TbIII photoluminescent emission in the visible region, suggesting that TbIII-based luminescence is sensitized by the effective energy transfer from the ligand HL to the metal centers.

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good candidates because gadolinium (S = 7/2) has a high isotropic spin, and small organic ligands result in a large metalto-ligand mass ratio to guarantee a high magnetic density. However, it is worth noting that anisotropic ions and antiferromagnetic or negligible interactions may also be suitable for specific temperature ranges and/or applications.23 Within this scope, the development of 3d−4f heterometallic molecular coolants appears to be promising for the building of future energy-efficient and environmentally friendly cryogenic magnetic cooling materials as replacements for rare and expensive helium-3. Nowadays, high-nuclearity heterometallic coordination clusters having 30 metal ions or larger are still scarce because their design and controllable synthesis represent a formidable challenge. One possible strategy for constructing clusters of higher nuclearities is the judicious decision of suitable polydentate ligands with abundant coordination modes. Previous researches have shown that the incorporation of small ligands and anions is an effective method for achieving large coordination clusters and also naturally leads to a large metal-to-ligand mass ratio, such as carboxylate (i.e., aromatic carboxylic acid, pivalic acid, and α-amino acid),4,18,20,24,25 alcamine,17,26,27 phosphonate,9,16,28,29 and some anions (i.e., acetate, nitrate, and perchlorate).30−32 It is worth noting that

INTRODUCTION High-nuclearity heterometallic clusters have blossomed into a promising topic of modern coordination chemistry and attracted increasing attention of physicists, chemists, and material scientists over recent years because their dimensions and their potential applications as functional materials, including magnetism,1,2 electricity,3,4 and luminescence,5,6 play a crucial role in linking the classical macroscale and quantum microscale systems. Retrospectively, considerable effort has been dedicated to the preparation of high-nuclearity d−f clusters with interesting properties,1,7,8 such as singlemolecule magnets (SMMs), the magnetocaloric effect (MCE), and fluorescence. Consequently, a great variety of nanosized d− f clusters of varying nuclearities and topologies range from gridlike Ln4Co8 and Ln8Co8,9 wheel-shaped Dy10Co210 and Gd6Cu12,11 sandwich-type Ln3Mn612 and Ln3Cu8,13 drumlike Ln6Cd18 and Ln8Cd24,14 cagelike Gd7Cu1515 and Ln10Co4,16 ringlike Yb10Fe1017 and Ln24Cu36,18 bowl-like Ln42Co10,19 to shell-type Ln6Cu24,20 Gd54Ni54,21 etc. Several aforementioned clusters have been shown to display significant MCE, for instance, the bowl-like Gd42Co10 cluster with the largest −ΔSm (isothermal magnetic entropy change) of 41.3 J kg−1 K−1 among 3d−4f complexes. Generally speaking, a molecule featuring a large-spin ground state, negligible magnetic anisotropy, a large magnetic density, and dominant ferromagnetic exchange may favor a large MCE.8,22 Therefore, Gd3+ 3d high-nuclearity heterometallic clusters with small ligands are © 2015 American Chemical Society

Received: September 25, 2015 Published: November 24, 2015 11535

DOI: 10.1021/acs.inorgchem.5b02215 Inorg. Chem. 2015, 54, 11535−11541

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Inorganic Chemistry Table 1. Crystallographic Data and Structure Refinement Details of Compounds 1−3 formula Mr cryst syst space group T [K] a [Å] b [Å] c [Å] α [deg] β [deg] γ [deg] V [Å3] Z ρcalcd [g cm−3] μ(Mo Kα) [mm−1] F(000) reflns collected unique reflns Rint param/restraints GOF R1 [I > 2σ(I)] wR2 (all data)

1

2

3

C168H259Cl7Gd24N48O194Zn6 10469.60 monoclinic P21/c 186(2) 47.7859(10) 42.7862(10) 24.7719(5) 90 113.087(2) 90 46591.67 4 1.493 3.781 19960 117594 53770 0.0824 2042/76 1.052 0.0875 0.2807

C174H251Cl5N48O175Tb24Zn6 10198.78 monoclinic P21/c 186(2) 48.250(4) 42.573(3) 24.6952(18) 90 114.0970(10) 90 46308(6) 4 1.463 4.016 19424 190456 31094 0.0833 1962/140 1.189 0.0860 0.3114

C172H265Cl7Dy24N48O195Zn6 10665.69 monoclinic P21/c 186(2) 47.586(3) 42.449(3) 24.6782(17) 90 113.1730(10) 90 45828(5) 4 1.546 4.285 20304 115741 40321 0.0627 2065/78 1.093 0.0691 0.2445

diamagnetism estimated from Pascal’s tables40 and sample holder calibration. The photoluminescence (PL) excitation and emission spectra were recorded with a Hitachi F-7000 spectrophotometer with a 150 W xenon lamp as the excitation source. The luminescence decay lifetimes were measured using a Lecroy Wave Runner 6100 digital osilloscope (1 GHz) with a tunable laser (pulse width = 4 ns; gate = 50 ns) as the excitation source (Contimuum Sunlite OPO). Crystallographic Data Collection and Refinement. Crystallographic data of complexes 1−3 were collected on a Bruker Apex II charge-coupled-device diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 186(2) K. The structure was solved by direct methods and refined on F2 by full-matrix least squares by means of the SHELXS-97 and SHELXL-97 programs.41,42 The location of the Ln atoms was easily determined, and the O, N, C, and Cl atoms were subsequently determined from difference Fourier maps. The non-H atoms were refined anisotropically, except C85, C87, C90, O70, O71, O73, O74, O76, O91, O92, and O99 for 1, C11, C12, C13, C14, C21, C49, C59, C64, C65, C68, C69, C86, O44, O47, O80, O81, O82, O83, O84, and O85 for 2, and C82, O44, O45, O58, O59, O86, O87, O95, and O97 for 3 were refined isotropically because of the slight disorder of the monodentated acetate anions. The H atoms were introduced in calculated positions and refined with fixed geometry with respect to their carrier atoms. Details for the crystallographic data and refinement are summarized in Table 1, and selected bond distances and angles are listed in Table S1 of the SI. CCDC 1422854 (1), 1422855 (2), and 1422856 (3) contain the supplementary crystallographic data for this paper (see the SI). These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Synthesis of 1. Gd(ClO4)3·6H2O (0.2 mmol, 0.113 g) and Zn(OAc)2·2H2O (0.2 mmol, 0.044 g) were added to a solution of HL (0.2 mmol, 0.025 g) in CH3OH/CH3CN (10 mL/5 mL), followed by the addition of Et3N (0.3 mmol, 0.042 mL). The resultant colorless solution was stirred for 4 h and subsequently filtered. The filtrate was exposed to air to allow slow evaporation of the solvent. Colorless needles of 1 suitable for X-ray diffraction analysis were collected in a few weeks (yield: 25%, based on Gd). Elem anal. Calcd for C168H259Cl7Gd24N48O194Zn6: C, 19.27; H, 2.49; N, 6.42. Found: C, 18.99; H, 2.42; N, 6.37. IR (KBr, cm−1): 3528 (br), 3180 (br), 1659 (m), 1582 (s), 1522 (m), 1449 (w), 1409 (m), 1380 (m), 1338 (s),

the anions employed, particularly, perchlorate with a weakly coordinating ability can also balance the high cationic charge of the oxo-bridged metal core so as to stabilize the structure.19 Herein, we successfully synthesized three novel nanosized {LnIII24ZnII6} triangular metallorings by applying the elaborately chosen pyrazine-2-carboxylic acid (HL) ligand, which exhibits versatile coordination modes (Scheme S1 of the Supporting Information, SI) as chelating and also bridging agents, in the assembly of various 4f- or 3d-based metal clusters.33−36 Magnetic studies reveal that the {Gd4Zn6} complex exhibits significant cryogenic MCE and the dysprosium analogue displays slow relaxation of magnetization. In addition, given the strong luminescent emission of the ZnII cluster and the antenna effect of the ZnII segments in 3d−4f complexes to transfer energy to the Ln ions,37−39 we simultaneously investigated their luminescent properties, and therefore the solid-state luminescent spectra of complexes 2 and 3 display strong ligand-sensitized lanthanide-characteristic emission in the visible region and the complexes serve as the important candidates of multifunctional nanomaterials.

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EXPERIMENTAL SECTION

Materials and Physical Measurements. All reagents and solvents were commercially available and were used without further purification. Elemental microanalyses for C, H, and N were performed with a PerkinElmer 2400 analyzer. Fourier transform infrared (FT-IR) spectra were obtained with a PerkinElmer FT-IR spectrophotometer using the reflectance technique (4000−300 cm−1) from KBr pellets. Powder X-ray diffraction (PXRD) patterns for polycrystalline samples were measured on a Bruker D8 Advance diffractometer (Cu Kα, λ = 1.54056 Å) by scanning over the range of 5−40° with steps of 5° min−1. Magnetic susceptibilities were measured on a Quantum Design MPMS-XL7 SQUID magnetometer equipped with a 7 T magnet. The direct-current (dc) measurements were collected with an external magnetic field of 1000 Oe in the temperature range 1.9−300 K, and the alternating-current (ac) measurements were carried out in a 3.0 Oe ac field oscillating at different frequencies from 1 to 1500 Hz. The experimental magnetic susceptibility data are corrected for the 11536

DOI: 10.1021/acs.inorgchem.5b02215 Inorg. Chem. 2015, 54, 11535−11541

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Inorganic Chemistry 1253 (w), 1167 (w), 1149 (w), 1090 (m), 1066 (m), 1038 (s), 925 (w), 860 (w), 789 (m), 729 (w), 621 (w). Synthesis of 2. A procedure similar to that used for 1 but with Tb(ClO4)3·6H2O (0.2 mmol, 0.113 g) produced colorless needles of 2 suitable for X-ray diffraction analysis in a few weeks (yield: 40%, based on Tb). Elem anal. Calcd for C174H251Cl5N48O175Tb24Zn6: C, 20.49; H, 2.48; N, 6.59. Found: C, 19.94; H, 2.43; N, 6.48. IR (KBr, cm−1): 3524 (br), 3361 (br), 1626 (m), 1577 (m), 1526 (m), 1478 (w), 1428 (m), 1380 (s), 1195 (w), 1166 (w), 1086 (m), 1047 (m), 855 (w), 792 (w), 723 (w), 650 (w), 618 (w). Synthesis of 3. A procedure similar to that used for 1 but with Dy(ClO4)3·6H2O (0.2 mmol, 0.114 g) produced colorless needles of 3 suitable for X-ray diffraction analysis in a few weeks (yield: 30%, based on Dy). Elem anal. Calcd for C172H265Cl7Dy24N48O195Zn6: C, 19.37; H, 2.50; N, 6.30. Found: C, 18.75; H, 2.48; N, 6.27. IR (KBr, cm−1): 3528 (br), 3364 (br), 1627 (m), 1558 (m), 1481 (w), 1428 (m), 1378 (s), 1195 (w), 1166 (w), 1091 (m), 1049 (m), 1033 (m), 858 (w), 792 (w), 723 (w), 648 (w), 618 (w).

formulated as [Gd 24 Zn 6 L 24 (OAc) 22 (μ 3 -OH) 30 (H 2 O) 14 ](ClO4)7(OAc)·2CH3OH·26H2O (1), [Tb24Zn6L24(OAc)22(μ3OH)30(CH3O)2(CH3OH)2(H2O)10](ClO4)5(OH)·6CH3OH· 12H 2 O (2), and (H 3 O)[Dy 24 Zn 6 L 24 (OAc) 22 (μ 3 -OH) 30 (H2O)14](ClO4)7(OAc)2·4CH3OH·22H2O (3), as determined by X-ray crystallographic studies and elemental analysis. Complexes 1−3 were found to be isomorphous, as revealed by single-crystal diffraction and PXRD as well as IR spectroscopy (Figures S1 and S2 of the SI), differing only in the coordinated terminal solvents (Figure S3 of the SI) and the number of solvent molecules in the lattice and counterions; hence, only the detailed description of 3 is given here. Complex 3 crystallized in the monoclinic space group C2/c with Z = 4. As represented in Figure 1, six cubanelike [Dy4(μ3-OH)4]8+ subunits and six ZnII ions are linked together by 24 deprotonated L− ligands and six μ3-OH− groups to form a triangular metalloring (Figure 2a), with the {Dy2Zn2O6}

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RESULTS AND DISCUSSION Recently, Zhao et al.34 have reported a 1D lanthanide chain based on a distorted cubic {Ln4(μ3-OH)4}8+ motif encapsulated by four 1-naphtholate (na) ligands and repeatedly extended by 4-fold chelating and bridging L− linkers (Figure 1a).

Figure 2. Central core structure (a) and metallic skeleton (b) of the {Dy24Zn6} molecule in compound 3. (c) Core structure of one side (Dy8O14Zn2) of the triangular metalloring. (d and e) Binding modes of the L− ligand represented with a black line and a circle in part b, respectively.

butterfly motif being the midpoint of its three sides (Figure 2c). The adjacent Dy4 cubic subunits are connected by 4-fold chelating−bridging L− connectors in a μ2-η1:η1:η1 fashion (Figure 2d), while a heterometallic ZnII ion and two DyIII ions are bridged in another μ3-η1:η1:η1 mode (Figure 2e). The metallic skeleton of the {Dy24Zn6} ring (Figures 2b and S4 of the SI) demonstrates that six Dy4 cubic subunits alternately locate on opposite sides of the metallic plane defined by three coplanar {Dy2Zn2O6} butterfly motifs. The asymmetric unit is half the molecule consisting of three types of cubanelike [Dy4(μ3-OH)4]8+ subunits (Figure S5 of the SI), whose structures are almost similar with slight difference of the terminal ligands of the Dy10 and Dy4 (or Dy5) ions. The Dy4 subunit is made up of four DyIII ions, four μ3-OH− groups, four L− ligands, three syn-syn-η1:η1:μ2-acetate bridges, and three H2O molecules [or two H2O molecules along with a monodentated acetate coordinated to Dy4 (or Dy5)]. All eight-coordinate DyIII ions have an O7N or O8 coordination sphere and each ZnII ion is six-coordinate, adopting a distorted trigonal-prismatic geometry with an O4N2 environment (Figure S6 of the SI). The Dy−O bond length ranges from 2.277(10) to 2.488(10) Å, and the Zn−O distances are in the range of 2.056(9)−2.228(9) Å, which are comparable to those

Figure 1. Structure of (a) the 1D dysprosium chain based on the cubic {Dy4(μ3-OH)4}8+ motif with na represented by acetate for clarity and (b) the {Dy24Zn6} triangular metalloring of complex 3, with solvents, H atoms, and external counteranions omitted for clarity. Color code: Dy, sky blue; Zn, pink; O, red; N, blue; C, gray.

Interestingly, the further introduction of ZnII ions interrupts the 1D chain into Ln8 units coupling two Ln4 cubes, and the alternative connection between the Ln8 and Zn2 units leads to the formation of a 3d−4f cyclic structure. The reaction of Ln(ClO4)3·6H2O (Ln = Gd, Tb, or Dy) and Zn(OAc)2·2H2O with HL and triethylamine in MeOH/MeCN in a mole ratio of 2:2:2:3 led to a colorless solution. The resulting solution was left undisturbed to allow slow evaporation of the solvent, producing colorless needles 11537

DOI: 10.1021/acs.inorgchem.5b02215 Inorg. Chem. 2015, 54, 11535−11541

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Inorganic Chemistry reported.38,43 The average Dy−N and Zn−N distances are 2.627 and 2.130 Å, respectively. The Dy···Dy separations within the Dy4 subunits are 3.6073(8)−3.8947(9) Å, and the closest intercubic Dy···Dy distance equals 5.4032(10) Å. As depicted in Figure 3, the space-filling representations of the metallic ring facilitate the better appreciation of the

DyIII (S = 5/2, L = 5, 6H15/2, g = 4/3, and C = 14.17 cm3 K mol−1). Upon cooling of the samples, the χMT value of 1 is almost constant from 300 to 20 K and then sharply decreases to a minimum value of 85.0 cm3 K mol−1 at 2 K, indicating the presence of weak antiferromagnetic interactions between the GdIII spin carriers within the [Gd4]2 subunits. A fit of the data to the Curie−Weiss law in the range of 2−300 K (Figure S7 of the SI) leads to Curie and Weiss constants of 179.53 cm3 K mol−1 and −1.91 K for 1, further supporting the antiferromagnetic interactions. For 2 and 3, the χMT values drop gradually to 230.0 cm3 K mol−1 at 28 K and 289.8 cm3 K mol−1 at 30 K, respectively, followed by rapid drops to 2 K with minima of 146.9 and 196.1 cm3 K mol−1, which possibly result from a combination of thermal depopulation of the Stark sublevels and weak antiferromagnetic coupling interactions.19,21 Magnetization measurements at low temperature (2.0−10 K) were performed on compounds 1−3. The magnetization data of 1 (Figure 5a) show a steady increase in the 0−70 kOe field

Figure 3. Space-filling representations from the viewpoints perpendicular (left) and parallel (right) to the plane of the triangular metalloring, showing the dimensions and thickness of compound 3. Color code: Dy, sky blue; Zn, pink; O, golden; N, blue; C, gray; H, light gray.

structure, which shows a diagonal dimension, height of the triangle, and thickness of approximately 2.9, 2.6, and 1.5 nm, respectively, with a size-negligible central hole for 3. The cationic core structures of 1 and 3 are essentially identical, and the only difference is the number of counterions and lattice solvents, as observed from their formula. Besides, 2 has a core structure similar to those of the other analogues (Figure S3 of the SI) except that two methanol and two deprotonated CH3O− anions in 2 replace four coordinated aqua ligands in 1 or 3 for the charge balance. For the three complexes, the closest intermolecular Ln···Ln separations are 9.558, 9.543, and 9.532 Å, respectively, indicating negligible intermolecular magnetic interactions in 1−3. Magnetic Properties. The dc magnetic susceptibility data of compounds 1−3 were collected under an applied magnetic field of 1000 Oe between 2 and 300 K, as shown in Figure 4. The χMT products at room temperature for 1−3 are 177.6, 250.3, and 319.4 cm3 K mol−1, respectively, which are lower than the theoretical value (1, 189.1 cm3 K mol−1; 2, 283.7 cm3 K mol−1; 3, 340.1 cm3 K mol−1) for 24 uncoupled LnIII ions: GdIII (S = 7/2, L = 0, 8S7/2, g = 2, and C = 7.88 cm3 K mol−1), TbIII (S = 3, L = 3, 7F6, g = 3/2, and C = 11.82 cm3 K mol−1), or

Figure 5. (a) Field-dependent magnetization plots for 1 in the temperature range of 2−10 K. (b) −ΔSm calculated from the magnetization data of 1 at various fields and temperatures.

range and reach 154.8 μB at 70 kOe and 2.0 K, which is close to the expected saturation value of 168 μB for 24 GdIII ions (S = 7 /2 and g = 2.00). For 2 and 3, nonsuperposition of the M versus H/T data (Figure S8 of the SI) on a single master curve below 5 K and the lack of saturation of M versus H data (Figure S9 of the SI) in the 0−70 kOe field range confirm the presence of significant magnetoanisotropy and/or low-lying excited states.44 The ac susceptibility measurements were carried out for 2 and 3 under zero dc field in a 3 Oe oscillating ac field to gain insight into the dynamics of magnetization. As shown in Figure S11 (SI), complex 2 did not display any out-of-phase ac signal. In contrast, complex 3 exhibited frequency-dependent out-of-phase signals below 5 K down to the lowest measured temperature of 1.9 K, indicating the onset of slow magnetization relaxation (Figure 6), which is consistent with the magnetic properties of the Kramers ion.45

Figure 4. Temperature dependence of the χMT products at 1000 Oe for 1−3. 11538

DOI: 10.1021/acs.inorgchem.5b02215 Inorg. Chem. 2015, 54, 11535−11541

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Inorganic Chemistry

Figure 6. Temperature dependence of the out-of-phase ac susceptibility under zero dc field for 3. Inset: Corresponding temperature dependence of the in-phase ac susceptibility.

Figure 7. Solid-state PL spectrum of 2 excited at 342 nm. Inset: (left) photograph of green emissive 2 excited at 365 nm and (right) emission decay curve of 2 monitored at 544 nm. The solid line represents the best fit to the data using a single-exponential function.

Because compounds 1−3 are possible candidates for magnetic refrigeration materials, the MCEs were evaluated by making use of the Maxwell relation ΔSm(T) = ∫ [∂M(T,H)/ ∂T]H dH.46 The magnetic entropy change ΔSm can be estimated from the experimentally obtained M(H, T) data (Figure 5a). As shown in Figure 5b, the maximum −ΔSm value of 1 at 2.5 K for the largest ΔH (from 7 to 0 T) is 30.0 J kg−1 K−1, which is larger than the value previously reported for larger 3d−4f ringlike compounds, such as {Gd24Cu36} (21.0 J kg−1 K−1)18 and {Gd24Co16} (26.0 J kg−1 K−1),47 owing to the use of the small HL ligand with abundant coordination sites, which is helpful to achieve high spin density in high-nuclearity clusters. Furthermore, the value is comparable to that (30.0 J kg−1 K−1 at 2.5 K) of the previously reported 1D Gd4 molecular magnetic coolers.34 Nevertheless, the entropy change of 1 is lower than the corresponding calculated values per mole of 39.63 J kg−1 K−1 expected for 24 uncorrelated GdIII (s = 7/2) ions, based on the equation −ΔSm = nR ln(2s + 1) = 24R ln 8, where R is the gas constant, which is attributable to the presence of intrametalloring antiferromagnetic interactions. For 2 and 3, the −ΔSm values (Figure S12 of the SI) of 10.2 J kg−1 K−1 (T = 8.0 K) and 9.4 J kg−1 K−1 (T = 5.0 K) at ΔH = 7 T are much smaller than that of complex 1 and the expected values of 38.07 and 33.42 J kg−1 K−1 calculated for the spins of 24 uncorrelated TbIII (s = 3) or DyIII ions (s = 5/2) because of the antiferromagnetic interactions between LnIII ions and strong magnetic anisotropy of TbIII and DyIII ions in 2 and 3.46 In addition, the presence of diamagnetic ZnII ions decreases the magnetic density to a certain extent and interrupts the magnetic interactions between Ln8 units, which appears to be unfavorable to the enhancement of −ΔSm. The following work should focus on the replacement of ZnII ions by paramagnetic ions such as CuII, CoII, and NiII. PL. The solid-state PL spectra of compounds 2 and 3 and ligand HL were investigated at room temperature to explore the photophysical behavior in the visible range under ultraviolet (UV) irradiation and are shown in Figures 7 and S13 and S14 of the SI. The purity of the polycrystalline powder samples of 2 and 3 was determined by the PXRD data (Figure S1 of the SI), and the results indicated that the experimental data are in agreement with the simulated data from their Crystallographic Information File. The excitation spectra of 2 and 3 were obtained by monitoring the 544 nm line of the 5D4 → 7F5 emission and the 575 nm line of the 4F9/2 → 6H13/2 emission, respectively. As shown in Figure S14 of the SI, the wide excitation band at 342 nm for complex 2 (or 320 nm for

complex 3) assigned to the S0 → S1 transition of the organic ligand demonstrates an efficient energy transfer from the ligand to the Ln centers in both compounds.46,47 Upon UV excitation at 342 nm, complex 2 exhibits a strong TbIII characteristic emission with four typical narrow peaks at 489, 544, 586, and 622 nm, which corresponds to the 5D4 → 7FJ (J = 6, 5, 4, and 3) transitions of the TbIII centers.47−49 The emission spectrum is dominated by the most intense band at 544 nm assigned to 5 D4 → 7F5, giving the complex strong bright-green emission (inset of Figure 7, left). It is noted that no sign of the ligand emission suggests almost complete energy transfer from the ligand π excited states to the Tb f excited states.46 For 3, photoexcitation at 320 nm results in the appearance of two relatively weak characteristic bands at 482 and 576 nm attributed to the 4F9/2 → 6HJ (J = 15/2 and 13/2) transitions of the DyIII ions.49 However, an obvious broad band in the purple spectral region ascribed to ligand-centered emission displaying some red shift in comparison to the corresponding free ligands (Figure S13 of the SI) is still observed because of a partial energy transfer from the ligand HL to the DyIII ions, which is recently observed in some heterometallic Dy/Zn complexes based on the Schiff base ligand.38,50 The emission decay curves were monitored within the 5D4 → 7F5 transition and well fitted with the single-exponential function [I = I0 exp(−t/τ)],51 affording a luminescence lifetime value of 1082 μs for 2 (inset of Figure 7, right), which is relatively high in comparison with those reported in the literature for other Tb or Tb/Zn complexes.38,47,52,53 Actually, the radiationless deactivation coming from the interaction of OH and CH oscillators of the solvent molecules (coordinating or noncoordinating with TbIII ions), which can efficiently depopulate the excited states of the metal ions, may decrease the lifetime value of 2.54−57

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CONCLUSION In conclusion, a novel family of high-nuclearity nanoscale triangular ringlike {LnIII24ZnII6} [Ln = Gd (1), Tb (2), and Dy (3)] complexes have been successfully synthesized by employing a versatile HL ligand. Importantly, each lanthanide complex exhibits their individual functionality based on the different Ln ions. The {Dy24Zn6} complex displays slow magnetic relaxation behavior, as evidenced by the frequency-dependent out-ofphase signals, while a significant MCE is present in the gadolinium analogue with a maximum −ΔSm (isothermal magnetic entropy change) value of 30.0 J kg−1 K−1 at 2.5 K and 11539

DOI: 10.1021/acs.inorgchem.5b02215 Inorg. Chem. 2015, 54, 11535−11541

Article

Inorganic Chemistry

(13) Iasco, O.; Novitchi, G.; Jeanneau, E.; Luneau, D. Inorg. Chem. 2013, 52, 8723−8731. (14) Yang, X.; Schipper, D.; Jones, R. A.; Lytwak, L. A.; Holliday, B. J.; Huang, S. J. Am. Chem. Soc. 2013, 135, 8468−8471. (15) Dermitzaki, D.; Lorusso, G.; Raptopoulou, C. P.; Psycharis, V.; Escuer, A.; Evangelisti, M.; Perlepes, S. P.; Stamatatos, T. C. Inorg. Chem. 2013, 52, 10235−10237. (16) Moreno Pineda, E.; Tuna, F.; Pritchard, R. G.; Regan, A. C.; Winpenny, R. E. P.; McInnes, E. J. L. Chem. Commun. 2013, 49, 3522− 3524. (17) Baniodeh, A.; Anson, C. E.; Powell, A. K. Chem. Sci. 2013, 4, 4354−4361. (18) Leng, J.-D.; Liu, J.-L.; Tong, M.-L. Chem. Commun. 2012, 48, 5286−5288. (19) Peng, J.-B.; Zhang, Q.-C.; Kong, X.-J.; Zheng, Y.-Z.; Ren, Y.-P.; Long, L.-S.; Huang, R.-B.; Zheng, L.-S.; Zheng, Z. J. Am. Chem. Soc. 2012, 134, 3314−3317. (20) Xiong, G.; Xu, H.; Cui, J.-Z.; Wang, Q.-L.; Zhao, B. Dalton Trans. 2014, 43, 5639−5642. (21) Kong, X.-J.; Ren, Y.-P.; Chen, W.-X.; Long, L.-S.; Zheng, Z.; Huang, R.-B.; Zheng, L.-S. Angew. Chem., Int. Ed. 2008, 47, 2398− 2401. (22) Liu, J.-L.; Chen, Y.-C.; Guo, F.-S.; Tong, M.-L. Coord. Chem. Rev. 2014, 281, 26−49. (23) Sharples, J. W.; Collison, D. Lanthanides and the Magnetocaloric Effect. In Lanthanides and Actinides in Molecular Magnetism; Layfield, R. A., Murugesu, M., Eds.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2015; pp 293−314. (24) Mereacre, V. M.; Ako, A. M.; Clérac, R.; Wernsdorfer, W.; Filoti, G.; Bartolomé, J.; Anson, C. E.; Powell, A. K. J. Am. Chem. Soc. 2007, 129, 9248−9249. (25) Majeed, Z.; Mondal, K. C.; Kostakis, G. E.; Lan, Y.; Anson, C. E.; Powell, A. K. Dalton Trans. 2010, 39, 4740−4743. (26) Abbas, G.; Lan, Y.; Mereacre, V.; Buth, G.; Sougrati, M. T.; Grandjean, F.; Long, G. J.; Anson, C. E.; Powell, A. K. Inorg. Chem. 2013, 52, 11767−11777. (27) Langley, S. K.; Moubaraki, B.; Tomasi, C.; Evangelisti, M.; Brechin, E. K.; Murray, K. S. Inorg. Chem. 2014, 53, 13154−13161. (28) Pineda, E. M.; Tuna, F.; Zheng, Y.-Z.; Teat, S. J.; Winpenny, R. E. P.; Schnack, J.; McInnes, E. J. L. Inorg. Chem. 2014, 53, 3032−3038. (29) Zheng, Y.-Z.; Evangelisti, M.; Winpenny, R. E. P. Angew. Chem., Int. Ed. 2011, 50, 3692−3695. (30) Peng, J.-B.; Kong, X.-J.; Zhang, Q.-C.; Orendác,̌ M.; Prokleška, J.; Ren, Y.-P.; Long, L.-S.; Zheng, Z.; Zheng, L.-S. J. Am. Chem. Soc. 2014, 136, 17938−17941. (31) Peng, J.-B.; Zhang, Q.-C.; Kong, X.-J.; Ren, Y.-P.; Long, L.-S.; Huang, R.-B.; Zheng, L.-S.; Zheng, Z. Angew. Chem., Int. Ed. 2011, 50, 10649−10652. (32) Guo, F.-S.; Chen, Y.-C.; Mao, L.-L.; Lin, W.-Q.; Leng, J.-D.; Tarasenko, R.; Orendác,̌ M.; Prokleška, J.; Sechovský, V.; Tong, M.-L. Chem. - Eur. J. 2013, 19, 14876−14885. (33) Devereux, M.; McCann, M.; Leon, V.; McKee, V.; Ball, R. J. Polyhedron 2002, 21, 1063−1071. (34) Li, Y.; Yu, J.-W.; Liu, Z.-Y.; Yang, E.-C.; Zhao, X.-J. Inorg. Chem. 2015, 54, 153−160. (35) Zheng, L.-M.; Wang, Y.; Wang, X.; Korp, J. D.; Jacobson, A. J. Inorg. Chem. 2001, 40, 1380−1385. (36) Leciejewicz, J.; Ptasiewicz-Bąk, H.; Premkumar, T.; Govindarajan, S. J. Coord. Chem. 2004, 57, 97−103. (37) Cao, J.; Gao, Y.; Wang, Y.; Du, C.; Liu, Z. Chem. Commun. 2013, 49, 6897−6899. (38) Long, J.; Vallat, R.; Ferreira, R. A. S.; Carlos, L. D.; Almeida Paz, F. A.; Guari, Y.; Larionova, J. Chem. Commun. 2012, 48, 9974−9976. (39) Sun, W.; Qin, X.-T.; Zhang, G.-N.; Ding, S.; Wang, Y.-Q.; Liu, Z.-L. Inorg. Chem. Commun. 2014, 40, 190−193. (40) Boudreaux, E. A.; Mulay, L. N. Theory and Applications of Molecular Paramagnetism; John Wiley & Sons: New York, 1976. (41) Sheldrick, G. M. SHELXS-97, Program for Crystal Structure Solution; University of Göttingen: Göttingen, Germany, 1997.

7 T. Further work focusing on the replacement of Zn ions by paramagnetic Cu, Co, and Ni ions is in progress to explore the critical influence of magnetic interactions on magnetocaloric and SMM properties in such high-nuclearity clusters. Furthermore, the solid-state Tb- and Dy-based clusters display strong characteristic Ln-centered emission in the visible region, which suggests that TbIII- and DyIII-based luminescence is sensitized by the effective energy transfer from the ligand to the metals (antenna effect). These results have provided new perspectives to nanosized 3d−4f clusters as possible candidates of multifunctional nanomaterials that could be detected on a surface by the emission spectrum.

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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.inorgchem.5b02215. Coordination modes (Scheme S1), PXRD patterns (Figure S1), IR spectra (Figure S2), figures of crystal structures (Figures S3−S6), magnetic measurements (Figures S7−S12), PL spectra (Figures S13 and S14), and selected bond lengths and angles (Table S1) (PDF) X-ray crystallographic data in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (Grants 21371166, 21331003, and 21221061) for financial support.

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REFERENCES

(1) Liu, K.; Shi, W.; Cheng, P. Coord. Chem. Rev. 2015, 289−290, 74−122. (2) Liu, J.-L.; Guo, F.-S.; Meng, Z.-S.; Zheng, Y.-Z.; Leng, J.-D.; Tong, M.-L.; Ungur, L.; Chibotaru, L. F.; Heroux, K. J.; Hendrickson, D. N. Chem. Sci. 2011, 2, 1268−1272. (3) Zhuang, G.-L.; Jin, Y.-C.; Zhao, H.-X.; Kong, X.-J.; Long, L.-S.; Huang, R.-B.; Zheng, L.-S. Dalton Trans. 2010, 39, 5077−5079. (4) Zhang, J.-J.; Hu, S.-M.; Xiang, S.-C.; Sheng, T.; Wu, X.-T.; Li, Y.M. Inorg. Chem. 2006, 45, 7173−7181. (5) Wang, P.; Ma, J.-P.; Dong, Y.-B.; Huang, R.-Q. J. Am. Chem. Soc. 2007, 129, 10620−10621. (6) Kumar, G. A.; Riman, R. E.; Diaz Torres, L. A.; Banerjee, S.; Romanelli, M. D.; Emge, T. J.; Brennan, J. G. Chem. Mater. 2007, 19, 2937−2946. (7) Rosado Piquer, L.; Sanudo, E. C. Dalton Trans. 2015, 44, 8771− 8780. (8) Zheng, Y.-Z.; Zhou, G.-J.; Zheng, Z.; Winpenny, R. E. P. Chem. Soc. Rev. 2014, 43, 1462−1475. (9) Zheng, Y.-Z.; Evangelisti, M.; Tuna, F.; Winpenny, R. E. P. J. Am. Chem. Soc. 2012, 134, 1057−1065. (10) Zou, L.-F.; Zhao, L.; Guo, Y.-N.; Yu, G.-M.; Guo, Y.; Tang, J.; Li, Y.-H. Chem. Commun. 2011, 47, 8659−8661. (11) Liu, J.-L.; Chen, Y.-C.; Li, Q.-W.; Gomez-Coca, S.; Aravena, D.; Ruiz, E.; Lin, W.-Q.; Leng, J.-D.; Tong, M.-L. Chem. Commun. 2013, 49, 6549−6551. (12) Zheng, Y.; Kong, X.-J.; Long, L.-S.; Huang, R.-B.; Zheng, L.-S. Dalton Trans. 2011, 40, 4035−4037. 11540

DOI: 10.1021/acs.inorgchem.5b02215 Inorg. Chem. 2015, 54, 11535−11541

Article

Inorganic Chemistry (42) Sheldrick, G. M. SHELXL-97, Program for Crystal Structure Refinement; University of Göttingen: Göttingen, Germany, 1997. (43) Zheng, X.-Y.; Wang, S.-Q.; Tang, W.; Zhuang, G.-L.; Kong, X.J.; Ren, Y.-P.; Long, L.-S.; Zheng, L.-S. Chem. Commun. 2015, 51, 10687−10690. (44) Lin, P.-H.; Sun, W.-B.; Yu, M.-F.; Li, G.-M.; Yan, P.-F.; Murugesu, M. Chem. Commun. 2011, 47, 10993−10995. (45) Zheng, Z. Cluster Compounds of Rare-Earth Elements. In Handbook on the Physics and Chemistry of Rare Earths; Gschneidner, K. A., Jr., Bünzli, J.-C. G., Pecharsky, V. K., Eds.; Elsevier: Amsterdam, The Netherlands, 2010; Vol. 40, pp 109−239. (46) Ilmi, R.; Iftikhar, K. Inorg. Chem. Commun. 2012, 20, 7−12. (47) Bag, P.; Rastogi, C. K.; Biswas, S.; Sivakumar, S.; Mereacre, V.; Chandrasekhar, V. Dalton Trans. 2015, 44, 4328−4340. (48) Armelao, L.; Quici, S.; Barigelletti, F.; Accorsi, G.; Bottaro, G.; Cavazzini, M.; Tondello, E. Coord. Chem. Rev. 2010, 254, 487−505. (49) Yi, X.; Bernot, K.; Pointillart, F.; Poneti, G.; Calvez, G.; Daiguebonne, C.; Guillou, O.; Sessoli, R. Chem. - Eur. J. 2012, 18, 11379−11387. (50) Palacios, M. A.; Titos-Padilla, S.; Ruiz, J.; Herrera, J. M.; Pope, S. J. A.; Brechin, E. K.; Colacio, E. Inorg. Chem. 2014, 53, 1465−1474. (51) Li, Y.; Zhao, Y. J. Fluoresc. 2009, 19, 641−647. (52) Zhang, L.; Ji, Y.; Xu, X.; Liu, Z.; Tang, J. J. Lumin. 2012, 132, 1906−1909. (53) Ramya, A. R.; Sharma, D.; Natarajan, S.; Reddy, M. L. P. Inorg. Chem. 2012, 51, 8818−8826. (54) Ahmed, Z.; Iftikhar, K. Inorg. Chim. Acta 2012, 392, 165−176. (55) Quici, S.; Marzanni, G.; Forni, A.; Accorsi, G.; Barigelletti, F. Inorg. Chem. 2004, 43, 1294−1301. (56) Comby, S.; Imbert, D.; Chauvin, A.-S.; Bünzli, J.-C. G.; Charbonnière, L. J.; Ziessel, R. F. Inorg. Chem. 2004, 43, 7369−7379. (57) de Sá, G. F.; Malta, O. L.; de Mello Donegá, C.; Simas, A. M.; Longo, R. L.; Santa-Cruz, P. A.; da Silva, E. F., Jr Coord. Chem. Rev. 2000, 196, 165−195.

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DOI: 10.1021/acs.inorgchem.5b02215 Inorg. Chem. 2015, 54, 11535−11541