A Series of Heterometallic Three-Dimensional Frameworks

Article pubs.acs.org/crystal

A Series of Heterometallic Three-Dimensional Frameworks Constructed from Imidazole−Dicarboxylate: Structures, Luminescence, and Magnetic Properties Xun Feng,† Lu-Fang Ma,† Lang Liu,§ Li-Ya Wang,*,†,‡ Hong-Liang Song,† and Shi-Yu Xie† †

College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang, 471022, P. R. China College of Chemistry and Pharmacy Engineering, Nanyang Normal University, Nanyang, 473601, P. R. China § College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou, 450001, China ‡

S Supporting Information *

ABSTRACT: The combination of lanthanide and zinc nitrates reaction with a rigid 1-H-2-methyl-4,5-imidazoledicarboxylic acid yields a new zinc complex of Zn(Hmimda)2·2H2O (1) and a family of heteronuclear polymers, namely, {[Ln 2 Zn 2 (μ 3 -Hmimda) 2 (μ 3 -mimda) 2 ·4H 2 O]· mH2O}n, Ln = Sm, m = 2 (2), Ln = Eu, m = 3 (3), Ln = Gd, m = 2 (4), Ln = Tb, m = 3 (5), Ln = Dy, m = 3 (6) (H3mimda = 1H-2-methyl-4,5-imidazole-dicarboxylic acid). Structural analysis shows that complexes 2−6 are isostructural and all crystallize in the monoclinic system, except for a slight disparity for 2. They have an extended nonporous structure constructed from tetraheteronuclear edifices with new framework topology, in which the Ln(III)−Zn square tetranuclear complexes act as the second building unit, which are further interconnected by the Hmimda to afford a two-dimensional corrugated layer. Both compounds 5 and 6 exhibit characteristic fluorescence in the visible region. The variable temperature magnetic investigations indicate that the magnetic interactions are mainly ascribed to depopulation of the Stark levels or possible antiferromagnetic couplings. The Dy(III)−Zn(II) compound exhibits possible ferromagnetic couplings and slow magnetic relaxation behavior of a single-molecule magnet nature.

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to the lanthanide ion.10 Usually, the presence of an aromatic organic system affords the fully allowed π−π* transitions, thus leading to a possible energy transfer.11 Meanwhile, the magnetic properties of large magnetic anisotropy of heavier lanthanides such as Tb(III) and Dy(III) continue to be an attractive research field because of their unique and intriguing properties and potential applications in high-density data storage technologies and molecular spintronics.12 The anisotropic barrier (U) of an Singe-molecule magnet (SMM) is derived from a combination of an appreciable spin ground state (S) and uniaxial Ising-like magneto-anisotropy (D).13 However, reports of the slow magnetic relaxation behavior existing in two- or three-dimensional (2D or 3D) lanthanide−organic frameworks, which also exhibit luminescence, are relatively rare, and luminescent SMMs would constitute a new class of bifunctional materials with an untapped potential in spintronics and MRI imaging. In a reported luminescent SMM, photoluminescent properties strictly arise from the luminescent large chelating ligands,14 while little attention has been devoted to the coordination chemistry of the simple rigid ligand such as

INTRODUCTION In recent years, the rational design and construction of extended frameworks containing both lanthanide and transitional metals bridged by nitrogen-heterocyclic ligands have attracted a great deal of interest in the chemistry and materials fields, not only because of their fascinating topology and intriguing architectures,1 but also due to their potential applications in magnetism, luminescence, electronic apparatus, and bimetallic catalysis.2−4 This is because introducing heterometallic ions makes the structures abundant and the energy levels more controllable.5 Meantime, coordination polymers with open frameworks that possess unprecedented pore sizes, shapes, and functions have attracted increasing interest for their potential applications due to long-lived lanthanide centered emissions,6−8 and many lanthanidemetal−organic frameworks (MOFs), especially the Eu(III)and Tb(III)-MOFs, have been applied in chemical sensors and light emitting devices and biomedicine. The vast majority of this research is focused on the enhancement or quenching of the intensity of Ln(III) ions emission, however.9 In the case of lanthanide-based fluorescence, one common strategy to overcome the problem of the Laporte forbidden f−f transitions (which leads to a low absorption coefficient) is to use organic ligands as chromophores in order to ensure an energy transfer © 2013 American Chemical Society

Received: June 26, 2013 Revised: July 26, 2013 Published: August 7, 2013 4469

dx.doi.org/10.1021/cg4009587 | Cryst. Growth Des. 2013, 13, 4469−4479

Crystal Growth & Design

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Table 1. Crystal Data and Structure Refinement for Complexes 1−6a compounds no.

1

2

3

4

5

6

chemical formula formula weight temperature (K) crystal system space group θ range for collection (°) unit cell dimensions (Å, °)

C12H14N4O10Zn 439.64 293(2) orthorhombic Pbca 2.44−26.00

C12H13N4O11SmZn 604.98 296(2) monoclinic P2(1)/c 2.44−25.50

C12H16N4O12.5EuZn 633.62 293(2) monoclinic C2/c 2.36−27.50

C12H10N4O10GdZn 592.86 293(1) monoclinic C2/c 2.32−28.30

C12H16N4O11.5TbZn 624.56 293(2) monoclinic C2/c 2.39−25.50

C12H15N4O12DyZn 635.67 296(2) monoclinic C2/c 2.31−28.36

a = 6.8248(12)

a = 13.037(3)

a = 24.1908(19)

a = 24.399(5)

a = 24.368(17)

a = 24.293(5)

b = 13.871(3) c = 16.678(3) β = 90 1578.9(5), 4 1.624 1551/0/138 1.850 896 1.077 0.0268, 0.0714 0.0313, 0.0747 0.498, −0.269

b = 11.604(3) c = 13.350(3) β = 115.572(2) 1821.8(8), 4 4.576 3376/0/264 2.206 1172 1.076 0.0473, 0.1379 0.0488, 0.1390 4.711, −1.235

b = 9.1472(7) c = 18.9567(15) β = 114.2520(10) 3824.5(5), 8 4.579 4368/18/281 2.201 2472 1.034 0.0300, 0.0866 0.0337, 0.0893 2.516, −1.437

b = 9.2236(19) c = 19.186(4) β = 114.052(2) 3942.9(14), 8 4.624 4783/0/257 2.151 2476 1.090 0.0429, 0.1058 0.0621, 0.1132 2.017, −1.681

b = 9.214(6) c = 19.198(13) β = 114.289(7) 3929(5), 8 4.858 3644/0/269 2.105 2408 1.079 0.0233, 0.0592 0.0267, 0.0606 3.234, −0.732

b = 9.182 (17) c = 19.022(4) β = 114.220(3) 3869.3(15),8 5.143 4416/0/255 2.181 3776 0.976 0.0290, 0.0399 0.0644, 0.0684 1.627, −0.781

V(Å3), Z μ (mm−1) data/restr/param ρ (g cm−3) F(000) GOF R1, wR2[I > 2σ(I)] R1, wR2 (all data) residuals (e Å−3) a

R = ∑||F0| − |Fc||/∑|F0|, wR={∑[w(F02 − Fc2)2]/∑(F02)2}1/2. Powder Analysis software package. The simulated powder patterns were calculated using Powder Cell 2.4 program. Absorption spectra were obtained using a Cary 50 Bio UV/visible spectrophotometer (Varian, Inc., Palo Alto, CA) in quartz cuvettes. Solid state luminescence (emission and excitation) spectra in the visible ranges were measured at room temperature with an Edinburgh instrument FLS920 fluorescence spectrometer. Syntheses of the Compounds. The synthesis of Zn(II) complex 1 is described in the Supporting Information. Procedure Syntheses of the Series of Heterometallic Complexes {[Ln2Zn2(μ3-Hmimda)2 (μ3-mimda)2·4H2O]·mH2O}n (Ln = Sm(2), Eu(3), Gd(4), Tb(5), Dy(6)). The same procedure was employed in the preparation of all the heterometallic complexes; hence, only complex 2 will be described in detail. Zinc nitrate hexahydrate (0.0298 g, 0.1 mmol) and 0.1 mmol of lanthanide(III) nitrate hexahydrate, ((2) = Sm(NO3)3·6H2O, 0.045 g; (3) = Eu(NO3)3·6H2O, 0.046 g, (4) = Gd(NO3)3·5H2O, 0.044 g, (5) = Tb(NO3)3·6H2O, 0.047 g, (6) = Dy(NO3)3·6H2O, 0.047 g) were mixed in a ethanol−water solution (10 mL) of H3mimda (0.3 mmol, 0.079 g). After being stirred for 30 min in air, the aqueous mixture was placed into 25 mL Teflon-lined autoclave under autogenous pressure being heated at 155 °C for 96 h, and then the autoclave was cooled over a period of 24 h at a rate 5 °C/ h. Light yellow crystals of (2) were obtained suitable for X-ray diffraction analysis. For (2), yield: 0.0416 g (53%) based on lanthanide element. Elemental analysis (%): calcd for C12H13N4O11SmZn: C 23.82, H 2.17, N 9.26, found: C 23.59, H 2.16, N 9.63. IR (KBr pellet, cm−1): 3434br, 3046s, 2958s, 1592s, 1473vs, 1396s, 834s, 763s, 696m. For (3), yield: 0.0304 g (39%). Elemental analysis (%): calcd for C12H16N4O12.5EuZn: C 22.75, H 9.15, N 28.76, found: C 22.69, H 9.27, N 28.68. IR: 3504s, 3046br, 2358m, 1618s, 1446vs, 1379m, 1078s, 854m. For (4), yield: 0.0331 g (43%). Elemental analysis (%): calcd for C12H13N4O11GdZn: C 24.35, H 1.71, N 9.45, found: C 24.10, H 2.07, N 9.67. IR: 3351s, 3037br, 2918s, 1633s, 1572vs, 1463vs, 1367s, 1264m, 1054s. For (5), yield: 0.0341 g (43%). Elemental analysis (%): calcd for C12H14N4O11.5TbZn: C 23.15, H 14.11, N 8.99, found: C 23.03, H 14.06, N 8.61. IR: 3325s, 3036br, 2158m, 1756m, 1613s, 1572vs, 1430s, 1127m, 1009s, 832s, For (6), yield: 0.0298 g (38%). Elemental analysis (%): calcd for C12H15N4O12DyZn: C 22.69, H 2.38, N 8.82, found: C 22.48, H 2.21, N 8.85. IR: 3432s, 3327br, 1653s, 1562s, 1433m, 1374s, 1017s, 929s. Crystallographic Data Collection and Refinement. Single-crystal diffraction data of compounds were collected on a Bruker SMART

imidazole/pyridine-dicarboxylic acid in luminescent SMMs. These coplanar rigid ligands always render a short separation between adjacent metal ions, facilitating energy transition to the Ln(III) centers, and leading to their potential applications in the areas of magnetic and electroluminescent devices.15 Our strategy to obtain magnetic and luminescent complexes is introducing new functionality of “metalloligands”, taking opportunities to transition metal complexes as building blocks.12 Such metalloligands have been verified to have the ability to introduce rich spectroscopic and magnetic features to MOFs,12,16 in addition to providing sites for introducing unsaturated metal centers.17−20 As a continuation of our previous investigation21 and in order to better understand the influence exerted by the second metal ion on structures and the properties in these systems, a new family of Zn-4f heterometallic frameworks has been isolated through the lanthanide and zinc salts reaction with the simple rigid Nhetero carboxylate ligand.

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EXPERIMENTAL SECTION

Materials and Physical Measurements. Lanthanide oxides were purchased from J & K Chemical Limited. Lanthanide nitrate was prepared by the reaction of lanthanide oxides and nitric acid (10 mol/ L−1). Elemental analyses (C, H, and N) were performed on a PerkinElmer 2400 element analyzer. IR spectra were recorded in the range 400−4000 cm−1 using a VECTOR-22 spectrometer using KBr discs. Magnetic data were obtained using a Quantum Design MPMS SQUID 7S magnetometer at an applied field of 2000 Gs using multicrystalline samples of 2, 4, 5, and 6 in the temperature range of 1.8−300 K. The magnetic susceptibilities were corrected using Pascal’s constant and the diamagnetism of the holder. Thermogravimetric and differential thermal analysis experiments were performed using a TGA/ NETZSCH STA449C instrument heated from 25−900 °C (heating rate of 10 °C/min, nitrogen stream). The powder X-ray diffraction (PXRD) patterns were measured have been performed at room temperature using a Bruker D8 advance powder diffractometer at 40 kV, 40 mA for Cu Kα radiation (λ = 1.5418 Å), with a scan speed of 0.2 s/step and a step size of 0.02 (2θ). The data were analyzed for dspacing measurements using the EVA program from the Bruker 4470



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APEX II CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at room temperature. The structures were solved using direct methods and successive Fourier difference synthesis (SHELXS-97) and refined using the full-matrix least-squares method on F2 with anisotropic thermal parameters for all nonhydrogen atoms (SHELXL-97). An empirical absorption correction was applied using the SADABS program,22 while conventional refinement of compounds converged reasonably well. The hydrogen atoms of organic ligands were placed in calculated positions and refined using a riding on attached atoms with isotropic thermal parameters 1.2 times those of their carrier atoms. For complexes 4 and 6, the disordered solvate molecules are difficult to identify; correspondingly, solvate molecules were accounted for by using the program PLATON/SQUEEZE (Spek, 2009) in order to remove the contributions of disordered solvent.23 For details about the Squeezed material, see cif data in Supporting Information. The disordered solvent has been included with the chemical formula, formula weight, density, F(000), etc. and now has been added to the cif check report. The summary crystallographic data, structure refinements, and selected bond lengths and angles are listed in Tables 1 and S1−S3, in Supporting Information, respectively.

addition to the slight difference of 2. They all crystallize in the monoclinic system, with C2/c space group, containing both Ln(III) and Zn(II) ions. Selected bond lengths and angles of the series of compounds are presented in Table S1, Supporting Information. Therefore, only the structure of 5 is described here in detail as an example. Figure 2a gives a perspective view

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RESULTS AND DISCUSSION PXRD Patterns Analysis. The X-ray diffraction powder pattern of the series of compounds 1−6 are given in Figures 1

Figure 2. (a) Coordination environments of the Tb(III) and Zn (II) cations with partial atomic labels in 5. (H atoms omitted for clarity, color code: Tb: green; Zn: white; cyan; O: red; N: blue; C: gray). (b) View of a 1-D heterometallic zigzag chain structure composed of tetranuclear complex units.

of the basic unit in 5 together with the atomic labeling system. The asymmetry unit contains one Tb(III) ion, one zinc(II) ion, four Hmimda ligands, and two coordinated water molecules, and two solvate water molecules. The octa-coordinated Tb(III) cation exhibits distorted dodecahedral prism geometry, being accomplished by an O8 donor set, among which two oxygen atoms are from water molecules, and six are from imidazolyl carboxylate. As far as the Zn(II) ion is concerned, it exhibits a octahedron geometry, being coordinated by four oxygen atoms from two carboxylic groups and two nitrogen of the imidazolyl ring from the next mimda ligand. It just demonstrates a slightly different coordination environment being compared with the simple Zn compound of 1 for the bond lengths and angles. Two dicarboxylic ligands sharing the common Zn(II) ion adopt two types of bridging coordination modes, but all in a chelating bridging bidentate fashion. The first category has been completed deprotonated for the carboxyl group, but an H atom was added to imidazole N bearing one positive charge, and the imidazole N in the second category has been deprotonated. They are denoted as mimda2− and Hmimda−, correspondingly. The former acts as a pentadentate ligand, and adopts a μ3-kN, O: kO, O’: kO, O modes connecting two Tb(III) and one Zn(II) centers in a bis-(bridging) and cheating mode. The 5-carboxylic group adopts bidentate, monodentate,

Figure 1. Comparing the simulated PXRD (all in black) and experimental patterns of compounds 2−6. Color code: pink, 2; cyan, 3; purple, 4; red, 5; blue, 6, respectively.

and S1 (Supporting Information). The phase purities of the bulk materials of the family complexes were confirmed by comparison of those calculated from the single-crystal studies. Compounds 1−6 are in good agreement with the calculated patterns obtained from single crystal structures, while the XPRD patterns of compounds 2−6 are almost identical, indicating that they are isomorphous. The stoichiometries of 2−6 have been confirmed by single X-ray crystallography, elements analysis, and thermogravimetric analysis to be {[Ln2Zn2(Hmimda)2 (mimda)2·4H2O]·mH2O}, while, Ln = Sm(2), Eu(3), Gd(4), Tb(5), Dy(6)), which are isostructural. Description of the Structures. The X-ray diffraction analysis reveals that 1 is just a mononuclear compound similar to that reported previously by Song,4d and we briefly describe the structure of [Zn(Hmimda)2·2H2O] in Figures S2 and S3, Supporting Information. Compounds 2−6 are isostructural, and they exhibit very similar molecular structures except for the metal ion and a different number of water molecules in the crystal lattice, in 4471



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and cheated modes. The latter also features a pentadentate ligand that adopts μ3-kN, O: kO, O’: kN′, O’ modes connecting two Zn(III) and one Tb(III) centers in bis-(bridging) and monodentate modes, as illustrated in Scheme 1. In this Scheme 1. (a, b) Diversity of Coordination Modes of H3mimda Ligand in Compound 5

structure, the shortest separation of Tb−Zn is 6.274 Å. The multidentate functionality of Hmimda− and the tendency of Ln(III) to have a high coordination number allowed water to act as a terminal ligand to Ln(III) in the structure. Two adjacent Tb(III) ions are doubly bridged by two 5carboxylic group of from Hmimda− ligand to form a dimer unit in an anti-anti chelating mode in a same plane, and the separation of Tb···Tb is 4.19 Å. At the same time, the 4,5carboxylate groups act as two armies to connect adjacent Zn ions into binuclear unit with the shortest distance Zn···Zn of 6.340 Å. An alternate description: four mimda2− ligand alternately linked two adjacent Tb(III) ions and Zn(II) ions into a heterometallic square [TbZn(mimda)4] aggregate as the secondary building unit (SBU) through the carboxylic oxygen and imidazolyl nitrogen as displayed in Figure S5, Supporting Information. The carboxylate further propagates these planar clusters into a 1D robust polymeric chain approximately along the crystallographic bc plane, as described in Figure 2b. The carboxylic oxygen and imidazolyl nitrogen atoms further cross-link these 1D chains into a 2D corrugated-shape layer approximately along the crystallographic bc plane, as illustrated in Figure 3a. Finally, the 3D pillar-layered coordination frameworks were constructed by the linkages of 2D heterometallic layers by the carboxylic oxygen and imidazolyl nitrogen atoms from two mimda ligands, as described in Figure 3b. Viewed along the b direction, the hexagonal windows are constructed by four methyl-imidazole-4,5-dicarboxylate ligands and six Tb(III) units with a size of ca. 7.56 × 6.05 Å. The relatively smaller rectangle windows (ca. 7.28 × 5.89 Å) are formed by four Hmimda ligand molecules and four Tb(III) units. All these windows sharing the corrugated walls are packed parallel along the b axis to form a 1D channel. The larger channel is accompanied by the both coordinated and solvate water molecules, and a series of extensive hydrogen bonding has been found (and is listed in Table S3, Supporting Information). This porous assemble is very different from the reported pure lanthanide or transition metal complexes based on an analogous ligand.24 As discussed above, H3mimda ligand acts as three-connected nodes connecting two Tb(III) and one Zn(II) ion, or connecting two Zn(II) ions and one Tb(III) ion. Both Tb(III) and Zn(III) are coordinated to three H3mimda ligands, assuming both the metal cation and H3mimda ligand act as three-connected nodes, as shown in Figure 4. The TOPOS analysis of this network results in a zeolite-like topology with the point number of (4.82)(82.12).25a Total

Figure 3. (a) Polyhedral view of corrugated-shaped 2D layer constructed from Tb2Zn2 SBUs. (b) Crystal packing diagram of 3D pillar-layer heterometallic framework of 5 along the a axis showing the existence of 1D channels along the crystallographic b axis in porous compound 5; guest water molecules are omitted for clarity.

Figure 4. Topology showing the connectivity between Tb2Zn2 nodes. Nonbridging atoms of ligands and hydrogen atoms have been omitted for clarity. Color codes, Zn: green; Tb: purple; Hmimda ligand: gray.

Schlafli symbol is {3;4;52;6;7;82;112;124;13}{43;8;102;123;143;15;16;17}, which is similar to other reported heterometallic coordination polymers.25b After hypothetical removal of the guest and coordinated waters, the incipient void space for the 3D noninterpentrating framework was found to be 615.7 Å3 per cell volume (accounting for 15.8% of the total unit cell system, as volume for 3929 Å3), calculated using the Platon PLATON/ SQUEEZE programs.26 It should be pointed that in complex 2 (in chiral space group of P21/c), the zinc ion is just five coordinated, possessing a 4472



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Table 2. Comparing of the Metal Ion Separations for Series of Complexes compound

shortest separation of Ln···Ln (Å)

shortest separation of Ln···Zn (Å)

shortest separation of Zn···Zn (Å)

4 (Gd) 5 (Tb) 6 (Dy)

4.201 4.190 4.170

6.380 6.362 6.331

6.327 6.340 6.319

tetragonal pyramid coordination environment, bonding to two Hmimda ligands and one terminal water molecule. It is different from the other complexes in this family, in which the zinc ion is coordinated to three Hmimda ligands, with octahedron geometry, as displayed in Figure S5a, Supporting Information. Sm(III) ion adopts seven-coordinated mode with an O6N1 donor set, without water molecules’ participation in coordination. Two Sm(III) ions and four Zn(II) ions are combined with two μ3-Hmimda ligands to produce a heterometallic hexanuclear [Sm2Zn4 (Hmimda)4] as second building unit, rather than tetraunclear ringlike aggregate. These hexanuclear clusters are grafted onto an infinite 1D doublestranded structure, as shown in Figure S5b, Supporting Information. Two adjacent Sm(III) ions are just monobridging linked by a carboxylic group rather than double connected, resulting in the nearest distance between Sm−Sm of 6.604 Å, which is larger than those of complexes 2−6. This array can be comparable with those of another series of 3D d−f heterometallic coordination polymers based on 2-(pyridine-3yl)-1H-4,5-imidazole dicarboxylic acid.27 To the best of our knowledge, such a diverse construction of heterometallic nuclear SBUs is still rare explored.28 However, similarities of building blocks in these family lanthanide complexes allow us to compare the metal−ligand distances in the same array, as reported in Table S1, Supporting Information. Comparing the average Ln−N and Ln−O bond lengths among the polymers finds that the average lengths display a slightly decreasing trend with increasing order of elements, as the ionic radius of the lanthanide center becomes smaller and smaller in the order of Gd(III) > Tb(III) > Dy (III). Furthermore, except for complexes 2 and 3 as mentioned above, in the series of complexes that exhibit the same array, separation of Ln···Ln double bridged by carboxylic bridging (along the a axis) and separation of Ln···Zn connected by carboxylate from the mimda ligand also display their slightly decreasing trend with increasing order of elements, which is also in accordance with the “lanthanide contraction” effect,29 as listed in Table 2 in detail. Thermal Gravimetric Analysis. To characterize the thermal stability of this series complexes, thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out, and the results are reported in Figures 5 and S6, Supporting Information, respectively. Complexes 2−6 showed almost similar TG curves, and 4 is used to describe as a representative. As indicated in Figure S6, Supporting Information, it shows an endothermic peak in the range of 120−150 °C with a weight loss of 4.44%. This was caused by the release of three lattic water molecules performula, and the result is close to the theoretical value 4.50% for {[Gd2Zn2(Hmimda)2(mimda)2·4H2O]·3H2O}n. The second weight loss of ca. 5.48% in the range of 220−280 °C is attentively assigened to the release of coordination water. After the loss of water molecules, the 3D framework was stable up to 400 °C and then began to decompose upon further heating. It also exhibits two exdothermic peaks, with a total weight loss of 27.1%, and this weight loss is close to the theoretical value

Figure 5. The TGA curves for compounds 2, 3, 5, and 6.

(26.3%) calculated for {[Gd2Zn2(Hmimda)2·4H2O]·2H2O}n. The larger exothermic peak located at about 450 °C with a weight loss of 13.9% was caused by the decomposition of the [Gd(Hmimda)] moiety. The smaller exothermic peak located at 550 °C with a weight loss of 23.2% was due to the decomposition of the [Zn(Hmimda)] unit into ZnO and release of CO2 and H2O. The complex collapses beyond the temperature of 400 °C. The residue substance is 39.48% (the calculated ZnO and Gd2O3 weight is 39.17%). Photoluminescence Properties. Considering the excellent luminescent properties of Eu(III), Tb(III), and Dy(III), the solid state photo luminescent spectra of 2−6 in multicrystal samples, as well as the free Hmimda ligand, were measured at room temperature. As displayed in Figure 6, the solid Hmimda ligand shows an emission band at 351 nm upon the excitation at 290 nm, which may be assigned to the π* → n electron transition (ILCT) character.30 The emission spectrum of Sm(III) complex 2 bears a large similarity with the one for

Figure 6. The photoemission spectra of the polymer 2 (excited at 281 nm) and free H3eimda ligand in the solid state. 4473



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in other reported terbium(III) complexes.36 whereas the emission band from the free ligand is not found, indicating effective energy transfer from the ligand to the Tb(III) center during the photoluminescence process.37 It is interesting to find the larger separation between the excited and emission lengths of 5. Sensitized emission gives rise to large Stokes’ shifts, facilitating separation of the observed signal from scattered light arising from excitation.38 Systems containing lanthanide ions and sensitizing chromophores have been widely applied to the development of ultrasensitive bioassays and have considerable potential for use in imaging, both through the development of time-gated protocols for luminescence microscopy and through the use of lanthanide probes with conventional fluorescence microscopy.39−41 The photoluminescent spectrum of 6 excited at 284 nm is illustrated in Figure 8. The emission spectrum consists of three

free ligand, albeit with much larger intensity, also ascribed to the ligand-centered (LC) fluorescence, based on the emission spectrum of the free Hmimda ligand. The maxima of compound 3 is slightly red-shifted with respect to the free Hmimda ligand, which is mainly due to an intraligand π−π* transition.31 Interestingly, being comparable with zinc complex 1, the final Zn−Eu complex 3 just exhibits one broad peak maximum at 398 nm; it is not the characteristic emission of the Eu(III) ion, but it is originated from the Hmimda ligand, as described in Figure S7, Supporting Information. Maybe the [Zn3mimda] chromophore rather than Eu-centered red emission with the residual broad Zn emission was observed in the complex 3. Consequently, the Eu(III) ions are optically active, but their emission is fully quenched, at least at room temperature, which is not uncommon due to the many crossrelaxation decay paths.25b,32 A similar evolution is observed in the case of gadolinium compound 4, it also exhibits a broad and strong luminescent emission maximum at 397 nm (see Figure S9, Supporting Information), which corresponds to the emission of ligand-to-ligand charge transfer, based on the solution excitation spectrum of 4, as presented in Figure S10, Supporting Information. There is no typical 4f−4f emission lines observed, but this is not surprising. The metal-centered (MC) electronic levels of Gd(III) ion are known to be located at 31 000 cm −1, typically well above the ligand-centered electronic levels. Therefore, ligand-to metal energy transfer and the consequent MC luminescence cannot be observed.33 Among these heterometallic d−f compounds, Zn[Hmimda]2 antenna chromophores were adopted to populate the excited state of the Tb(III) ion, and the lanthanide luminescence is indeed “lit up” by excitation of the Zn-based chromopores, which emits at a sufficiently high energy to act as a donor for sensitizing Tb(III) in the complex 5,34 as described in Figure 7.

Figure 8. Fluorescence spectrum of 6 excited at 284 nm.

main bands at 352, 483, and 576 nm. The similarity of emission of free Hmimda ligand 1 and 4 at about 355 nm indicates that Hmimda plays an important role in the emission spectrum. The emissions at 483 and 576 nm are attributed to Dy(III) ions, corresponding to 4 F 9/2 → 6 H 13/2 and 4 F 9/2 → 6 H 15/2 , respectively.42 Considering complex 6 shows excitation luminescence maximum at 351 nm, as illustrated in Figure S8, Supporting Information, the emission maximum at 352 should be attributed to the ligand-to-ligand transition (π−π* transition) of the Hmimda or Zn complex chromophore. The existence of the antenna complex to Dy(III) energy transfer is also evidenced in the excitation spectra monitored within the intra-4f transitions.43 These Zn-Ln complexes exhibit the socalled “dual emission” originating from both Zn-based luminophores and lanthanide emitters. Therefore, two simultaneously emissive excited states would be operative in this system at room temperature,44 being particularly suitable for generating yellow light emission.15a Magnetic Properties. The magnetic susceptibilities for complexes 2, 4, 5, and 6 were measured using the polycrystalline samples in an applied direct current (dc) field of 2000 Oe. For all complexes, the observed paramagnetism arises uniquely from the 4f of Ln(III) ions. All the χMT values display a sharp decrease below 50 K, because the observed decrease of χMT at low temperature may be due to the large anisotropy effects observed for lanthanides.46

Figure 7. Emission spectra of 5 (excitation at 309 nm) in the solid state at room temperature.

The emission spectrum presents four sharp line bands at 488, 545, 585, and 622 nm, which are the 5D4 → 7Fn transitions for terbium emission, where n = 6, 5, 4, and 3, respectively, among which, the 5D4 → 7F5 is the most intense transition showing strong green light and consisting of an intense band without a shoulder at higher frequency. This transition has the largest probability for both electric-dipole and magnetic-dipole induced transitions,35 and this phenomenon was also observed 4474



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As displayed in Figure 9, the χMT value is 0.452 cm3 K mol−1 for 2 at room temperature (300 K). Because the 6H ground

χ=

term of the free Sm(III) ion is split into six states by spin−orbit coupling (with the parameter λ on the order of 200 cm−1), and both the crystal field effect and the possible thermal population of the higher states have inferences on magnetic properties of the Sm(III) complex. The thermal variation of χMT is almost linear over the whole temperature range. As temperature is lowered, the χMT value decreases rapidly to a value of 0.274 cm3 K mol−1 at 2 K, which is much larger than the value of 0.178 cm3 K mol−1 expected for two uncoupled Sm(III) ions, indicating an antiferromagnetic interaction possibly exists between Sm(III) ions at lower temperature. Because of the presence of thermally populated excited state, the magnetic properties of the samarium complex 2 remain difficult to interpret even at room temperature. However, it is possible to extract the value of the spin−orbit coupling constant λ in the complex by using the analytical approach. Considering magnetic properties just originate from Ln(III) ions connected by carboxylic bridging, while the Zn(II) ion in these compounds does not contribute to magnetic property, and it only provides diamagnetism, and to model the magnetic properties of 2 in a particular low-temperature region, the intramolecular Zn···Sm and Zn···Zn magnetic interactions are going to be negligible due to the extremely weak paramagnetism center.47 The molar magnetic susceptibility for a mononuclear species is then given by M. Andruh, and the approximate treatment equation previously deduced from the monomeric Sm(III) system with free-ion approximation was applied to estimate the magnetic interactions.48

2

χM T is 8.26 cm3 mol−1 K, which is somewhat larger than the value of 7.88 cm3 mol−1 K, expected for a free Gd(III) ion with S = 7/2, g = 2.0.53 Upon cooling, this value gradually decreases up to the maximum of a value of 7.13 cm3 mol−1 K at 2 K. This behavior is indicative of the existence of weak antiferromagnetic interactions between adjacent Gd(III) ions54 and may also partially arise from the very small splitting of the 8S7/2 multiplet at zero field. The absence of spin−orbit coupling at the firstorder for the Gd(III) ion offers an opportunity to quantitatively study the interaction between the two lanthanides centers in this dinuclear complex. Considering the short distance between two adjacent Gd(III) is 4.201 Å in 4, and the large value of the

+ (a5x + b5)e−20x

+ (a6x + b6)e−55x /2⎤⎦/[3 + 4e−7x /2 + 5e−8x + 6e−27x /2 + 7e−20x + 8e−55x /2] x = λ /kT

(3)

Figure 10. Plots of the temperature dependence of molar magnetic susceptibility (χM) and χMT product for 4. The solid lines represent the theoretical values based on the equation.

Nβ 2 ⎣⎡a1x + b1 + (a 2x + b2)e−7x /2 3kTx

+ (a3x + b3)e−8x + (a4x + b4)e−27x

1 − (2zJ ′/Ng 2β 2)χM

In this expression, N, β, k, and g have their usual meanings, and λ is a spin−orbit coupling parameter. An additional coupling parameter zJ′ was added in eq 3 to take into account the molecular field approximation introduced to simulate the magnetic interaction between the Sm(III) ions49 (see Supporting Information for the fitting details). The best fit to the magnetic susceptibilities of 2 gave the parameters λ = 221.7 cm−1, g = 0.731, zJ′ = −0.32 cm−1, and R = 2.66 × 10−5 (R is the agreement factor defined as R = Σ[(χM)obs − (χM)calc]2/ Σ[(χM)obs]2). The value of λ is comparable with that of the Sm(III) dinuclear complex [Sm2(4-cba)6(phen)2(H2O)2] (4Hcba = 4-cyanobenzoic acid),50 and another recently reported 3D Sm(III) complex, Sm(2,5-PyDC) (3-PyC)(H2O) (2,5H2PyDC = 2,5-pyridinedicarboxylic acid, 3-HPyC = 3pyridinecarboxylic acid) (λ = 214.5 cm−1).51 It is interesting to evaluate the value extracted from the magnetic data compared to the spectroscopic one. Indeed, the separation between the (6H5/2) and the (6H7/2) state is directly equal to λ. The value is nearly in agreement with that obtained from the luminescence data (the value of λ = 398 nm). The small negative zJ′ value implied the presence of a very weak antiferromagnetic interaction between the two adjacent Sm(III) ions or the depopulation of Stark sublevels together with crystal affection.52 The temperature dependence of the χMT product and χM for 4 is illustrated in Figure 10. At room temperature, the value of

Figure 9. Temperature dependence of the χMT products at 2000 Oe for compound 2. The color lines correspond to the calculated behavior of 2 (see the text for details).

χM =

χM

(1) (2) 4475



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mol −1. The χMT product remains roughly constant at a higher temperature; then below 50 K the χMT value decreases when lowering temperature and displays a sharp decrease to the lowest value of 9.12 cm3 K mol −1 at 2.0 K. But this phenomenon can be ascribed to the gradual depopulation of the highest level of the 7F6 multiplets of 5, because of crystal effects rather than the magnetic exchange. This behavior can be attributed to several causes such as (i) the thermal depopulation of the Stark sublevels, (ii) the presence of significant anisotropy, and (iii) an antiferromagnetic interaction between the adjacent Tb(III) ions. For compound 6, the χMT value at 300 K is 13.98 cm3 K mol−1, which is somewhat smaller than the μeff value expected for an isolated Dy(III) ion (14.16 cm3 K mol−1): S = 5/2, L = 5, 6 H15/2, g = 4/3, as displayed in Figure 11. As the temperature is lowered, the χMT product increases steadily and reaches a value of 14.17 cm3 K mol−1 at 50 K. This behavior indicates that the very weak ferromagnetic coupling between Dy(III) ions is strong enough to overcome the effect of depopulation of the Stark components of Dy (III) due to the splitting of free ion ground state by the crystal field.53a,61 The ferromagnetic interaction is also confirmed by magnetization measurements in the 0−70 kOe at 2 K, as shown in Figure 12, which increases

local interacting spin [S = 7/2], the classical spin expression derived by Fisher is taken to describe the magnetic behavior of a uniform chain with large spins (eq 4) for 4.55 The eq 4 is induced from the simple dimer model with Hamiltonian Ĥ = − JŜ1Ŝ2 and Ŝ1 = Ŝ2 =7/2, using the Van Vleck equation evaluated for an exchange-coupled high-spin dinuclear Gd(III) complex.56 χM =

Ng 2β 2 ⎡ A ⎤ ⎢ ⎥ 4KT ⎣ B ⎦

A = 8(e x + 5e3x + 14e6x + 30e10x +55e15x + 91e 21x + 140e 28x)

B = (1 + 3e x + 5e3x + 7e6x + 9e10x + 11e15x + 13e 21x + 15e 28x)

x = |J | /KT

(4) −1

The best fit parameters obtained are J = −0.048 cm and g = 1.87. The weak antiferromagnetic coupling constant observed agrees well with the values reported for other phenoxo-bridged Gd(III) systems,57 especially for centrosymmetric complexes.58 We neglect all exchange interactions between nearest-neighbor Gd(III) ions, including that of being connected by hydrogen bonding. The best least-squares fitting parameters are J = −0.038 cm−1, g(Cu) = 2.02, and R = 2.83 × 10−4 (R is the agreement factor defined as R = ∑[(χM)obs − (χM)calc]2/ ∑[(χM)obs]2). The magnetic behavior in this case is similar to the reported other compounds with carboxylate as the bridging ligand.56a,59 The reason for the small J value results from the fact that the 4f electrons are influenced very little by the surrounding environment. The negative J value indicates the presence of the overall antiferromagnetic interactions between the adjacent Gd(III) ions in 4. Compare this conclusion with previous findings that unanimously document the existence of an antiferromagnetic coupling in homopolynuclear gadolinium complexes,60 in which the coupling constants vary from −0.0057 cm−1 to −0.053 cm−1. As exhibited in Figure 11, for the analogous Tb(III) complex 5, the χMT value at room temperature of 11.76 cm3 K mol−1 is in reasonable agreement with that expected for the J = 6 with gJ =3/2, which characterize the 7F6 state of Tb(III), 11.82 cm3 K

Figure 12. Field dependence of the magnetization of 6 at the indicated temperatures.

rapidly with the increasing magnetic field and reaches a saturation value of ca. 5.86 NμB. The χMT product then drops to a minimum value of 12.64 cm3 K mol−1 at 2 K. This behavior is mainly due to the depopulation of the Stark sublevels of the dysprosium ion, which arise from the splitting of the 6H15/2 ground term by the ligand field. This behavior is likely due to crystal-field effects leading to significant magnetic anisotropy. As indicated in Figure 12, for 6 the low temperature magnetization measurements as a function of the field H reveal a relatively rapid increase in the magnetization at low field and then a very slow nonlinear increase to reach a value of 5.86 NμB at the maximum applied field of 6 T, where the curves are not all superimposed on a single master-curve as expected for an isotropic system with a well-defined ground state. Similar magnetization saturation values were observed for other Dy(III) complexes with approximate axial symmetry.62 The slow increase of the magnetization at low field and a very slow saturation of the magnetization even at higher field suggests the

Figure 11. Plots of the temperature dependence of the χMT product for compounds 5 (black squares) and 6 (blue circles). 4476



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Stark levels or possible antiferromagnetic couplings in 2, 4, and 5. Interesting slow relaxation of magnetization of the SMM behavior has been observed for the Dy(III)-containing complex 6 and could be selected as a potential building block for the synthesis of fluorescent SMM.

possible presence of a magnetic anisotropy and/or more likely the presence of low-lying excited states expected with the weak Dy···Dy magnetic interactions discussed above. To investigate the dynamic behavior of the magnetization of these extended structures at low temperatures, the alternative current (ac) susceptibility was performed on a microcrystalline powder sample of 6. The strength of this signal increases by an order of magnitude and becomes frequency-dependent when a weak external dc field (1200 Oe) is applied, probably because quantum tunneling of magnetization is suppressed. However, hysteresis of the magnetization is not observable even at 1.9 K, so the blocking temperature is lower than the temperature limit of the magnetometer. Recent work63 suggests that dysprosium(III) ions may hold the key to obtaining high blockingtemperature lanthanide SMMs. The large intrinsic magnetic anisotropy and the large number of unpaired f-electrons of lanthanide metal ions can contribute to the height of the energy barrier for reversal of magnetization. Currently, the origin of different magnetic behaviors in these dysprosium(III) coordination compounds is still not clear.64

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ASSOCIATED CONTENT

S Supporting Information *

The additional structures and characterizations of polymers (Figures S1−S11), the selected bonds lengths and angles for series of polymers, etc. This information is available free of charge via the Internet at http://pubs.acs.org. Crystallographic files in CIF format have been deposited with the Cambridge Crystallographic Data Center with deposition numbers CCDC 932154, 932156, 932157, 932158, 932159, and 932162.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86 037965523593. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support of this work by the National Natural Science Foundation of China (Nos. 21071074, 21271098, and 21273101), the Foundation of the Program for Backbone Teachers in Universities of Henan Province, China (No. 2012GGJS158), and the Foundation of education committee of Henan province, China (No. 2011B150022).

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REFERENCES

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Figure 13. The temperature dependence of the out-of-phase (χ″) ac susceptibility of 6 at indicated frequencies (for 3−1200 Hz data) of ac field modulation and in zero dc field.

The series of complexes display different magnetic behaviors, although they are isostructural. The divergence mainly originates from the intrinsic natures of different lanthanide cores (heterometallic cluster) in these complexes, because the carboxyl groups bridging adjacent lanthanide ions communicate rather weak magnetic interactions. The large and different magnetic anisotropy and complicated Stark energy levels of lanthanide ions in 5 and 6 from the splitting of individual 2S+1LJ states should be responsible for the significant differences of magnetic behaviors for this family of compounds.48a,55a,56b,60a

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CONCLUSIONS In summary, we report a family of five novel mixed-metal 3d− 4f 3D zeolite-like MOFs constructed from robust heterotetrametallic-nuclear units. The heterometallic frameworks exhibit high thermal stability. The heterometallic cluster plays an important role of in luminescence and magnetic properties. Compound 5 displays strong green luminescence resulting from the sensitization of H3mimd ligand. The variable temperature magnetic investigations indicate that the magnetic interactions are mainly ascribed to the depopulation of the 4477



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