A Series of Lanthanide-Based MetalâOrganic Frameworks Derived

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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A Series of Lanthanide-Based Metal−Organic Frameworks Derived from Furan-2,5-dicarboxylate and Glutarate: Structure-Corroborated Density Functional Theory Study, Magnetocaloric Effect, Slow Relaxation of Magnetization, and Luminescent Properties Manesh Kumar,† Lin-Hui Wu,‡ Mukaddus Kariem,† Antonio Franconetti,§ Haq Nawaz Sheikh,*,† Sui-Jun Liu,*,‡ Subash Chandra Sahoo,⊥ and Antonio Frontera§ †

Department of Chemistry, University of Jammu, Baba Sahib Ambedkar Road, Jammu 180006, India School of Chemistry and Chemical Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, Jiangxi Province, P. R. China § Department of Chemistry, Universitat de les Illes Balears (UIB), Palma (Baleares) 07122, Spain ⊥ Department of Chemistry & Centre of Advanced Studies in Chemistry, Panjab University, Chandigarh 160014, India

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‡

S Supporting Information *

ABSTRACT: Herein, through a dual-ligand strategy, we report eight isorecticular lanthanide(III) furan-2,5-dicarboxylic acid metal−organic frameworks (Ln-MOFs) with the general formula {[Ln(2,5-FDA)0.5(Glu)(H2O)2]·xH2O}n [Ln = Sm (1), Eu (2), Gd (3), Tb (4), Dy (5), Ho (6), Er (7), and Yb (8); 2,5-FDA2− = furan-2,5-dicarboxylate and Glu2− = glutarate; x = 0.5 for 1, 2, and 4 and x = 0 for 3 and 5−8], synthesized under solvothermal conditions by using an N,N′-dimethylformamide/H2O mixed solvent system. Crystallographic data reveal that all eight Ln-MOFs 1−8 crystallize in the orthorhombic Pnma space group. All of the MOFs are isostructural as well as isomorphous with distorted monocapped squareantiprismatic geometry around the Ln1 metal center. In Ln-MOFs 1−8, the 2,5-FDA2− and Glu2− ligands exhibit μ2-κ4,η1:η1:η1:η1 and μ3-κ5,η2:η1:η1:η1 coordination modes, respectively. Topologically, assembled Ln-MOFs 1−8 consist of the 2D cem topological type. The designed Ln-MOFs 1−8 are further explored for structure-corroborated density functional theory study. Meanwhile, room temperature photoluminescence properties of Ln-MOFs 2 and 4 and magnetic properties of Ln-MOFs 3 and 5 have been explored in detail. A highly intense, ligand-sensitized, Ln3+ f−f photoluminescence emission is exhibited by Ln-MOFs 2 [Eu3+ (red emission)] and 4 [Tb3+ (green emission)]. Magnetic studies suggest weak antiferro- and ferromagnetic interactions between adjacent GdIII ions in Ln-MOF 3, thereby displaying a large magnetocaloric effect. The magnetic data measured at T = 2 K and ΔH = 30 kOe depict that the −ΔSm value per unit mass reaches 32.1 J kg−1 K−1, which is larger than most of the GdIII-based complexes reported. The alternating-current susceptibility measurements on Ln-MOF 5 revealed that out-of-phase signals are frequency- and temperature-dependent under both 0 and 2 kOe direct-current fields, thereby suggesting a typical slow magnetic relaxation behavior with two relaxation processes. This is further supported by the Cole−Cole plots at 2.4−6 K.

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rescent materials,16−19 hydrogen storage,20 adsorption/storage of gases,21 optoelectronic devices,22 magnetic devices,23−25 materials science,26−28 proton conductivity,29 and pH/ humidity sensors.30 It is quite difficult to design Ln-MOFs with predetermined structural properties compared with their transition-metal counterparts because of the high coordination numbers and flexible geometries exhibited by trivalent Ln ions. Various factors, viz., choice of organic linkers, reaction conditions, solvent(s) ratio (H2O/volatile organic solvent), molar ratio of the reactants, and temperature and pH of the

INTRODUCTION Crystal engineering of functional inorganic−organic materials has become a leading research field over the last 3 decades as a result of rapid growth in the assembly of novel functional metal−organic frameworks (MOFs).1 The dimensionality of metal−organic assemblies ranges from discrete metal−organic polyhedron to porous three-dimensional (3D) MOFs.2−7 Restricted to the lanthanide-based MOFs (Ln-MOFs), an exponential growth has been observed in the crystal engineering of Ln-MOFs because of intriguing precedent topologies, fascinating properties, and potential applications. The applications include organic heterogeneous catalysis,8−10 microporous materials,11−14 nonlinear-optical devices,15 fluo© XXXX American Chemical Society

Received: January 23, 2019

A

DOI: 10.1021/acs.inorgchem.9b00219 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

linker and Glu2− as the flexible coligand, it seems to be a more useful avenue to introduce a rigid linker with a flexible coligand to construct Ln-MOFs 1−8 with fascinating structural and functional peculiarities. To date, various Ln-MOFs/CPs with 2,5-FDA2− linkers have been reported.46−49 It is pertinent to mention here that 2,5-FDA2− is a poorly explored rigid Vshaped linker in combination with flexible tethers.46−49 To the best of our knowledge, no related experiment has ever been carried out by taking 2,5-H2FDA as a rigid ligand and glutaric acid as a coligand. In this contribution, eight new homonuclear isorecticular lanthanide furan-2,5-dicarboxylate series of LnMOFs with the formula {[Ln(2,5-FDA)0.5(Glu)(H2O)2]· xH2O}n [Ln = Sm (1), Eu (2), Gd (3), Tb (4), Dy (5), Ho (6), Er (7), and Yb (8); 2,5-FDA2− = furan-2,5-dicarboxylate and Glu2− = glutarate; x = 0.5 for 1, 2, and 4 and x = 0 for 3 and 5−8] have been synthesized by a solvothermal method. Their structures, elemental analysis, thermal stabilities, Brunauer−Emmett−Teller (BET) analysis, and photoluminescent and magnetic properties have been explored. Furthermore, structure-corroborated density functional theory (DFT) studies have been investigated to study the structure−property relationship of synthesized Ln-MOFs systematically (see the Supporting Information data for a detailed description of the DFT study). Upon a comparison of the magnetoluminescent properties of reported Ln-MOFs of rigid 2,5-FDA2− 46−49 to those of Ln-MOFs 1−8, it can be interpreted that Glu2− shows cooperativity to enhance the “antenna effect” of rigid 2,5FDA2− efficiently, which results in an increase in the intensities and a sharpness in the photoluminescent spectral bands. Glu2− used in combination with 2,5-H2FDA as a coligand via a mixed-ligand strategy also modulates interlanthanide distances in assembled MOFs 1−8, which, in turn, affects the magnetic properties of the designed LnIII-MOFs.

medium, play crucial roles in the rational design of LnMOFs.31 Reports on MOFs or coordination polymers (CPs) with Ln ions are limited compared with transition metals.32−37 Ln ions have unique electronic properties by virtue of which they show high and versatile coordination numbers. The strong spin−orbit coupling and large magnetic moments endow their potential applications in magnetic resonance imaging and luminescent sensing in particular.38−41 Ln-MOFs show special optical features with characteristic sharp lines, optical color purity, and luminescent properties usually unaffected by the ligand field.42,43 Also, trivalent LnIII-MOFs can be considered to be promising molecular magnetic materials because they have large magnetic anisotropy and strong spin−orbit coupling except for gadolinium(III)-based MOFs.44,45 Among the organic linkers used for the construction of Ln-MOFs, rigid polycarboxylates such as furandicarboxylic,46−49 thiophenedicarboxylic,50−52 pyridinedicarboxylic,53−57 isophthalic,58,59 terephthalic,60−63 2,6-naphthalenedicarboxylic,64−67 and trimesic68−71 acids are of special interest. They have been widely exploited for the synthesis of Ln-MOFs. These dicarboxylates and polycarboxylates exhibit versatile coordination modes, thereby generating intriguingly robust topological structures, imparting rigidity, and enforcing a porous network. Additionally, these aromatic carboxylate linkers assembled in Ln-MOFs can sensitize photoluminescent emission, thereby enhancing the quantum efficiency of the designed framework.72−75 The Ln-MOFs designed from various isomers of furandicarboxylic acids have received attention not only because of the intriguing topologies of Ln-MOFs but also because of their enhanced photoluminescent and magnetic properties.46−49 In particular, rigid multidentate V-shaped furan-2,5-dicarboxylate (2,5FDA2−) has been exploited for assembling Ln-MOFs because it shows an “antennae” effect, which may efficiently sensitize photoluminescent trivalent TbIII and EuIII ions in their LnMOFs.46,47,49 Various combinations of rigid and flexible dicarboxylate/ polycarboxylate linkers foster a structural diversity in LnMOFs. As part of our ongoing research work, herein we have chosen a H2FDA/H2Glu/N,N′-dimethylformamide (DMF)/ H2O to assemble Ln-MOFs by considering the following: (1) H2FDA can act as a rigid multidentate V-shaped linker with special C2 symmetry. Such a V-shaped coordination mode leads to unforeseen structural patterns and frameworks. The carboxylate groups of the 2,5-FDA2− linker serve as donor/ acceptor hydrogen-bonding sites, thereby generating 3D intriguing supramolecular frameworks fostered by inter- or intramolecular hydrogen bonds. (2) Glutaric acid as a flexible polymethylene [−(CH2)3−] dicarboxylic tether can act as a bridging linker, thereby generating a spacious MOF. (3) These carboxylate ligands can act as mediators in magnetic exchange interactions transmitted between paramagnetic Ln centers (LnMOFs 3 and 5) because of their versatile coordination modes and can also act as sensitizers in photophysical luminescent studies on Ln-MOFs 2 and 4. (4) The flexible tethers (Glu2−) can easily bend and rotate freely when coordinated Ln ions produce useful functional properties and peculiar structural properties. (5) The DMF/H2O mixed-solvent system used can take part in the coordination of LnIII ions, thereby satisfying its geometry, while lattice H2O molecules may induce hydrogen bonding to produce supramolecular frameworks. To this end, we are inspired to synthesize and explore Ln-MOFs decorated with an entangled 2,5-FDA2− linker and Glu2− as the flexible coligand. Considering the merits of both a rigid 2,5-FDA2−

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EXPERIMENTAL SECTION AND SYNTHESES

Materials and Physical Measurements. Chemicals such as lanthanide nitrates, furan-2,5-dicarboxylic acid, glutaric acid, and N,N′-dimethylformamide (DMF) are of analytical grade and were employed as received without further purification. The designed LnMOFs 1−8 were microanalytically analyzed on a CHNS-932 Leco elemental analyzer. IR spectra for all structures 1−8 were recorded (4000−400 cm−1) on a Shimadzu Fourier transform infrared (FT-IR) model no. Prestige-21 spectrophotometer by employing KBr pellets as a reference. The room temperature photoluminescent spectra of LnMOFs 2 and 4 were investigated by using an Agilent Cary Eclipse fluorescence spectrophotometer with a xenon lamp as the excitation source. Thermogravimetric analysis (TGA) was recorded on a PerkinElmer (SGSA 6000) thermal analyzer at a heating rate of 10 °C min−1. To confirm the phase purity of synthesized Ln-MOFs 1−8, powder X-ray diffraction (PXRD) patterns in a 2θ range of 5−50° were recorded on an Agilent Supernova X-ray diffractometer (Nifiltered Cu Kα irradiation; λ = 0.1542 nm) at 45 kV and 40 mA. The simulated PXRD patterns were plotted by using single-crystal X-ray data on the Mercury 3.8 program.76 The N2 adsorption−desorption, porosity, and specific surface area of Ln-MOF 1 were calculated using the BET method using a Quantachrome Nova 2000e BET analyzer. Before sorption measurements, degassing of the sample was performed at 403 K for 16 h. The magnetic data for Ln-MOFs 3 and 5 were collected by using polycrystalline phases on a Quantum Design MPMS-XL-7 SQUID magnetometer. Diamagnetic corrections were estimated by employing Pascal constants to approximate the diamagnetic susceptibility, and background corrections were done by experimental measurement on sample holders. Synthetic Procedures for the Syntheses of Ln-MOFs 1−8. Single crystals suitable for single-crystal X-ray diffraction analysis were B


Article

Inorganic Chemistry Table 1. Crystal Data and Structure Refinements for Ln-MOFs 1−8 Ln-MOF-1 empirical formula CCDC fw cryst syst space group temperature (K) a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z μ (mm−1) cryst size (mm3) Tmin, Tmax no. of reflns

Rint (sin θ/λ)max (Å−1) R [F2 > 2σ(F2)], wR(F2), S

no. of param no. of restraints Δρmax, Δρmin (e Å−3) GOF on F2

Ln-MOF-2

Ln-MOF-3

Ln-MOF-4

Ln-MOF-5

Ln-MOF-6

Ln-MOF-7

Ln-MOF-8

C8H11O9Sm 1868174 401.52 orthorhombic Pnma 293(2) 8.8073(3) 24.9439(9) 14.1692(6) 90 90 90 3112.8(2) 8 3.80 0.18 × 0.15 × 0.12 0.482, 1.000 15967 3421 3055 0.045 0.648

C8H11O9Eu 1868175 403.14 orthorhombic Pnma 293(2) 8.7917(2) 24.8459(7) 14.1229(3) 90 90 90 3084.97(13) 8 4.10 0.3 × 0.25 × 0.2 0.212, 1.000 14098 3355 2869 0.078 0.640

C8H11O8.5Gd 1868176 400.42 orthorhombic Pnma 293(2) 8.7374(3) 24.8728(8) 14.0643(4) 90 90 90 3056.51(17) 8 4.36 0.3 × 0.25 × 0.2 0.685, 1.000 13685 3332 2990 0.072 0.639

C8H11O9Tb 1868177 410.09 orthorhombic Pnma 293(2) 8.7271(3) 24.7841(7) 14.0331(4) 90 90 90 3035.28(16) 8 4.69 0.3 × 0.25 × 0.2 0.621, 1.000 14636 3320 2859 0.056 0.648

C8H11O8.5Dy 1868178 811.33 orthorhombic Pnma 293(2) 8.7553(3) 24.7475(6) 13.9800(4) 90 90 90 3029.07(15) 4 4.96 0.25 × 0.2 × 0.18 0.287, 1.000 15138 3306 2900 0.050 0.647

C8H11O8.5Ho 1868179 799.15 orthorhombic Pnma 293(2) 8.6746(3) 24.9337(9) 13.8842(4) 90 90 90 3003.01(17) 4 5.29 0.25× 0.2 × 0.18 0.276, 1.000 14876 3282 2715 0.042 0.639

C8H11O8.5Er 1868180 410.43 orthorhombic Pnma 293(2) 8.5999(1) 24.9662(5) 13.8506(2) 90 90 90 2973.82(8) 8 5.67 0.20× 0.18 × 0.15 0.697, 1.000 35164 3369 2934 0.056 0.641

C8H11O8.5Yb 1868181 416.21 orthorhombic Pnma 293(2) 8.7405(3) 24.7017(10) 14.0740(6) 90 90 90 3038.7(2) 8 6.18 0.23× 0.2 × 0.18 0.718, 1.000 13881 3240 2887 0.040 0.637

0.035

0.048

0.051

0.033

0.038

0.096

0.031

0.055

0.116 1.09 168 0 1.43, −0.69

0.157 1.02 168 0 1.53, −1.82

0.192 1.02 162 0 1.65, −1.45

0.089 1.09 168 0 1.58, −0.98

0.123 1.05 162 1 2.20, −1.01

0.247 1.02 147 1 5.76, −4.84

0.090 1.05 184 1 1.01, −1.03

0.158 1.04 162 1 2.99, −1.19

1.088

1.124

1.013

1.089

1.052

1.021

1.113

1.045

block-shaped crystals for Ln-MOF 3 with a yield of 54% based on GdIII ions. FT-IR (KBr pellets, cm−1): 3238(m), 2980(w), 2393(w), 1667(s), 1546(s), 1452(m), 1417(s), 1315(s), 1249(m), 1059(s), 947(m), 811(m), 670(m), 615(w), 446(w). Anal. Calc. (%) for C8H11O8.5Gd: C, 23.99; H, 2.76. Found (%): C, 23.88; H, 2.75. Synthesis of {[Tb(2,5-FDA)0.5(Glu)(H2O)2]·0.5H2O}n (4). The procedure employed for the synthesis of 4 was similar to that used for Ln-MOF 1 with a stoichiometric equivalent of Tb(NO3)3·6H2O (0.0906 g, 0.2 mmol) used instead of Sm(NO3)3·6H2O. We obtained colorless needlelike crystals for Ln-MOF 4 with a yield of 54% based on TbIII ions. FT-IR (KBr pellets, cm−1): 3407(m), 2895(m), 2376(w), 1655(s), 1540(s), 1422(s), 1308(m), 1155(m), 1059(s), 654(m), 546(w), 497(w). Anal. Calc. (%) for C8H11O9Tb: C, 23.41; H, 2.68. Found (%): C, 23.64; H, 2.65. Synthesis of [Dy(2,5-FDA)0.5(Glu)(H2O)2]n (5). The procedure employed for the synthesis of 5 was similar to that used for LnMOF 1 with a stoichiometric equivalent of Dy(NO3)3·6H2O (0.0912 g, 0.2 mmol) used instead of Sm(NO3)3·6H2O. We obtained colorless hexagonal crystals for Ln-MOF 5 with a yield of 57% based on DyIII ions. FT-IR (KBr pellets, cm−1): 3242(m), 2980(w), 2391(w), 1660(s), 1549(s), 1455(s), 1418(s), 1250(s), 1199(m), 1059(s), 1026(s), 972(m), 837(s), 786(w), 673(m), 620(w), 511(s), 442(w). Anal. Calc. (%) for C8H11O8.5Dy: C, 23.67; H, 2.71. Found (%): C, 23.56; H, 2.69. Synthesis of [Ho(2,5-FDA)0.5(Glu)(H2O)2]n (6). The procedure employed for the synthesis of 6 was similar to that used for LnMOF 1 with a stoichiometric equivalent of Ho(NO3)3·5H2O (0.0882 g, 0.2 mmol) used instead of Sm(NO3)3·6H2O. We obtained darkbrown block-shaped crystals for Ln-MOF 6 with a yield of 62% based on HoIII ions. FT-IR (KBr pellets, cm−1): 3336(m), 2930(m),

obtained by similar procedures for Ln-MOFs 1−8. So, only the detailed synthesis of Ln-MOF 1 is given as follows: Synthesis of {[Sm(2,5-FDA)0.5(Glu)(H2O)2]·0.5H2O}n(1). To a suspension of furan-2,5-dicarboxylic acid (2,5-H2FDA; 0.0312 g, 0.2 mmol) and glutaric acid (H2Glu; 0.0264 g, 0.2 mmol) in 3 mL of DMF was added dropwise 9 mL of a Sm(NO3)3·6H2O (0.088 g, 0.2 mmol) aqueous solution over a magnetic stirrer with continuous stirring, and the reaction mixture was further stirred for 30 min. Subsequently, the reaction mixture was sealed in a 23 mL Teflon-lined stainless steel autoclave and heated under autogenous pressure at 125 °C for 3 days. After completion of the reaction, the mixture was gradually cooled to 25 °C at a rate of 5 °C h−1. We obtained lightgreen prismatic crystals with a yield of 58% based on SmIII ions. FTIR (KBr pellets, cm−1): 3586(m), 3300(m), 2989(w), 1668(m), 1564(m), 1487(s), 1114(s), 1275(s), 1197(s), 1058(s), 967(s), 754(m), 651(w), 515(s), 497(w). Anal. Calc. (%) for C8H11O9Sm: C, 23.94; H, 2.74. Found (%): C, 23.82; H, 2.69. Synthesis of {[Eu(2,5-FDA)0.5(Glu)(H2O)2]·0.5H2O}n (2). The procedure employed for the synthesis of 2 was similar to that used for Ln-MOF 1 with a stoichiometric equivalent of Eu(NO3)3·5H2O (0.0856 g, 0.2 mmol) used instead of Sm(NO3)3·6H2O. We obtained colorless block-shaped crystals for Ln-MOF 2 with a yield of 54% based on EuIII ions. FT-IR (KBr pellets, cm−1): 3216(m), 2976(w), 2388(w), 1668(s), 1546(s), 1454(m), 1417(s), 1254(w), 1060(s), 1026(m), 833(s), 787(w), 689(w), 622(m), 507(w). Anal. Calc. (%) for C8H11O9Eu: C, 23.83; H, 2.75. Found (%): C, 23.65; H, 2.68. Synthesis of [Gd(2,5-FDA)0.5(Glu)(H2O)2]n (3). The procedure employed for the synthesis of 3 was similar to that used for LnMOF 1 with a stoichiometric equivalent of Gd(NO3)3·6H2O (0.0902 g, 0.2 mmol) used instead of Sm(NO3)3·6H2O. We obtained colorless C


Article

Inorganic Chemistry 2395(w), 1659(s), 1549(s), 1421(s), 1250(s), 1059(s), 1027(m), 972(m), 949(m), 839(m), 673(m), 623(w), 510(m), 465(w). Anal. Calc. (%) for C8H11O8.5Ho: C, 23.53; H, 2.69. Found (%): C, 23.52; H, 2.68. Synthesis of [Er(2,5-FDA)0.5(Glu)(H2O)2]n (7). The procedure employed for the synthesis of 7 was similar to that used for LnMOF 1 with a stoichiometric equivalent of Er(NO3)3·5H2O (0.0886 g, 0.2 mmol) used instead of Sm(NO3)3·6H2O. We obtained lightpink block-shaped crystals for Ln-MOF 7 with a yield of 56% based on ErIII ions. FT-IR (KBr pellets, cm−1): 3234(m), 2982(w), 2387(w), 1665(s), 1547(s), 1462(s), 1422(s), 1279(s), 1199(m), 1156(m), 1059(s), 838(m), 815(m), 753(w), 653(w), 514(w), 464(w). Anal. Calc. (%) for C8H11O8.5Er: C, 23.39; H, 2.68. Found (%): C, 23. 37; H, 2.63. Synthesis of [Yb(2,5-FDA)0.5(Glu)(H2O)2]n (8). The procedure employed for the synthesis of 8 was similar to that used for LnMOF 1 with a stoichiometric equivalent of Yb(NO3)3·5H2O (0.0898 g, 0.2 mmol) used instead of Sm(NO3)3·6H2O. We obtained colorless needle-shaped crystals for Ln-MOF 8 with a yield of 60% based on YbIII ions. FT-IR (KBr pellets, cm−1): 3246(m), 2893(m), 2392(w), 1657(m), 1551(s), 1416(s), 1250(s), 1199(m), 1059(m), 1027(s), 972(m), 836(m), 785(w), 676(m), 506(w), 446(w). Anal. Calc. (%) for C8H11O8.5Yb: C, 23.05; H, 2.64. Found (%): C, 22.95; H, 2.32. To optimize the reaction conditions, a series of reactions were tried by changing conditions such as variable solvent ratios (DMF/H2O), different temperatures, and autogenous heating duration. We also attempted to design these Ln-MOFs in neat DMF as well as H2O only as a solvent with different heating intervals. However, no fruitful product was procured. A series of attempts led to the successful assembly of designed Ln-MOFs by employing a DMF/H2O mixedsolvent system in the ratio of 1:3 (for details, see Table S1). It was established that the synthesized Ln-MOFs 1−8 are generally insoluble in common solvents such as H2O, methyl alcohol, ethyl alcohol, acetone, ethyl acetate, acetonitrile, etc. Crystallographic Data Collection and Structure Determination. Single-crystal X-ray diffraction data of Ln-MOFs 1−8 were recorded at 293(2) K on a SuperNova, single source at offset/far, HyPix3000 diffractometer by employing graphite-monochromatic Mo Kα radiation (λ = 0.71073 Å). The program Olex277 was used to solve the structures of Ln-MOFs 1−8, with ShelXT78 using intrinsic phasing and refined with least-squares minimization on F2 by using the ShelXL79 refinement package. In the crystal data of Ln-MOFs 1−8, the non-H atoms were refined anisotropically.80−82 The DIAMOND83 and Mercury 3.876 software programs were employed for final pictorial drawing of the crystal structures. The MOF topological type of the isorecticular series was explored from the crystallographic data by using TOPOS Pro software.84 We have deposited crystallographic data for the reported Ln-MOFs 1−8 to the Cambridge Crystallographic Data Centre. CCDC 1868174 (1; Sm), 1868175 (2; Eu), 1868176 (3; Gd), 1868177 (4; Tb), 1868178 (5; Dy), 1868179 (6; Ho), 1868180 (7; Er), and 1868181 (8; Yb) consist of the supplementary crystallographic data for the reported Ln-MOFs 1−8. The crystal data and structural refinement parameters for the assembled Ln-MOFs 1− 8 are tabulated in Table 1. For Ln-MOFs 1−8, the selected bond lengths, bond angles, and hydrogen-bonding geometries are summarized in Tables S2−S25. In the reported isorecticular LnMOFs 1−8, the rigid 2,5-FDA2− and flexible Glu2− exhibit μ2κ4,η1:η1:η1:η1 and μ3-κ5,η2:η1:η1:η1 bonding modes, respectively, as depicted in Scheme 1.

Scheme 1. Identified Coordination Modes of Dicarboxylates in Ln-MOFs 1−8

as isomorphous compounds. So, here we will only discuss the structure of Ln-MOF 2 in detail as a representative example of the isorecticular series. The asymmetric unit of Ln-MOF 2 has only one crystallographically characterized EuIII ion, half of a fully deprotonated 2,5-FDA2− unit, one fully deprotonated Glu2−, and two H2O molecules, besides having 1/2H2O as a guest in the interstitial sites. In Ln-MOF 2, the rigid 2,5-FDA2− features μ2-κ4,η1:η1:η1:η1 and the flexible Glu2− exhibits μ3κ5,η2:η1:η1:η1 coordination modes (see Scheme 1). The ORTEP view of 2 is depicted in Figure 1a. In the ORTEP view, H atoms and guest H2O molecules are omitted for structural clarity. In the asymmetric unit of Ln-MOF 2, a single-crystal diffraction study reveals that there is only one EuIII ion surrounded by the [O9] donor set, thereby generating a distorted monocapped square-antiprismatic polyhedron (Figure 1b, inset). As shown in Figure 1b, the coordination environment depicts that in Ln-MOF 2 each Eu1 is bonded to one FDA2− linker, three Glu2− units, and two H2O molecules. In the coordination environment of Eu1, there are five O atoms belonging to the Glu2− units, out of which three O atoms (viz., O8, O9, and O9#) originate from two tridentate chelating carboxylates of two symmetry-equivalent Glu2− ligands and O2 and O10 belong to the bidentate chelating carboxylate group of the third Glu2− unit. The two O atoms O14 and O16 stem from the bidentate chelate carboxylate group of FDA2−. The O atoms O11 and O12 belonging to coordinated H2O molecules complete the nine-coordinated monocapped square-antiprismatic distorted polyhedral geometry around Eu1 (Figure 1b, inset). The coexistence of the μ2-κ4,η1:η1:η1:η1 and μ3-κ5,η2:η1:η1:η1 bonding modes of 2,5-FDA2− and Glu2−, respectively, leads to the formation of a rectangular array of LnIII ions characteristic of a {33.44.53} topological net, where Ln ions act as a 5connected uninodal net connected by rigid 2,5-FDA2− and flexible Glu2− ligands (see Figure 2; view along the c axis). In Ln-MOF 2, the Eu−O(CO2−) bond lengths range from 2.381(4) to 2.598(5) Å (Table S5), whereas the O−Eu−O bond angles range from 50.47(14)° to 151.37(14)° (Table

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RESULTS AND DISCUSSION Descriptions of the Structures. Single-crystal X-ray crystallographic studies on {[Ln(2,5-FDA)0.5(Glu)(H2O)2]· xH2O}n [Ln = Sm (1), Eu (2), Gd (3), Tb (4), Dy (5), Ho (6), Er (7), and Yb (8); x = 0.5 for 1, 2, and 4 and x = 0 for 3 and 5−8] show that isorecticular two-dimensional (2D) LnMOFs 1−8 crystallize in the orthorhombic Pnma space group (for details, see Table 1). Furthermore, X-ray crystallographic analysis suggested that Ln-MOFs 1−8 are isostructural as well D


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Inorganic Chemistry

Figure 1. Pictorial representations of the crystal structure of Ln-MOF 2: (a) ORTEP view of the structural unit (the thermal displacement ellipsoids are drawn at 40% probability; lattice H2O molecules and H atoms are omitted for clarity). (b) Coordination environment of Eu1 (the inset shows the distorted monocapped square-antiprismatic polyhedral geometry around the Eu1 center).

(4,4) rectangular grid array fostered by connecting LnIII nodes along the c axis (see Figure 5a). Figure 5b shows a space-filled ball representation of Ln-MOF 2 along the a axis. Furthermore, Ln-MOF 2 shows intriguing supramolecular structural frameworks and different packing arrangements along variable axes, as shown in Figures S1−S6. The hydrogen bonding (D−H···A) reinforces and stabilizes the covalent interactions to foster a 3D supramolecular framework. In all assembled MOFs, the hydrogen bonds are generated by carboxylates groups of 2,5-FDA2− as well as Glu2− and coordinated/lattice H2O molecules (see Figure 5c and Table S7; Ln-MOF 2). The additional insights on the hydrogen-bonding geometries are further explored theoretically by DFT studies. TOPOS Pro was used to explore the topology of 2 using the crystallographic data.84 Figure 6 (in the ab plane) depicts topological net of Ln-MOF 2. Topologically, the structure consists of 5-connected uninodal net. The point symbol for Eu1 is {33.44.53} and topological type is cem with Shubnikov plane net (33.42). Each Eu metal center is connected to five other Eu centers via 2,5-FDA2− and Glu2− moieties, which explains the 5-connected uninodal 2D coordination net (Figure 6; ab plane view). In a comparative study of Ln-MOFs 1−8, the values of the interlanthanide(III) distances in [M2O16] SBUs and volumes (Å3) follow the trend of the LnIII ionic radii of the ions involved.85 However, Ln···Ln separation trends are consistent only for Ln-MOFs 1−5. Thereafter, there is no regular trend. The Ln···Ln separations for Ln-MOFs 1−8, bridged through tridentate chelating-bridged carboxylates, are 4.169 Å (1) for SmIII, 4.150 Å (2) for EuIII, 4.138 Å (3) for GdIII, 4.124 Å (4) for TbIII, 4.118 Å (5) for DyIII, 4.128 Å (6) for HoIII, 4.134 Å (7) for ErIII, and 4.131 Å (8) for YbIII (for details, see the SBUs of Ln-MOFs 1−8 in Figures 3a and S7−S13 and Table 2). Also, the volume decreases from 3112.8(2) to 3038.5 Å3 for Ln-MOFs 1−8 (for details, see Table 1). Important structural parameters of the Ln ions in Ln-MOFs 1−8 are tabulated in Table 2. IR Spectroscopy. The IR spectra of Ln-MOFs 1−4 and 5−8 are depicted in Figures S14 and S15, respectively. All LnMOFs 1−8 have similar patterns of nearly overlapping

Figure 2. 2D view of a structural fragment of Ln-MOF 2 along the c axis.

S6). These observations are in close agreement with earlier reports.53 In a dimeric Eu2O16 secondary building unit (SBU), the Eu1···Eu1 (distance = 4.150 Å) metal centers are held together by two bidentate chelating-bridged carboxylates stemming from two symmetrically equivalent Glu2− units via O8 and O9 in an antiparallel fashion (see Figure 3a). The dinuclear [Eu2O16] SBUs, when extended, generate one-dimensional (1D) zigzag infinite rod-shaped chains along the c axis, as shown in Figure 3b. Figure 3c shows 2D extension of linearly extended 1D zigzag chains linked via 2,5-FDA2−. It can be seen that two 1D linearly extended chains are bridged together by two Glu2− units per Eu2O16 SBU (see Figures 3c and 4). In Figure 4, a tetramer consisting of Eu2O16 SBUs is shown, which further highlights the structural insight of isostructural Ln-MOFs 1−8. The Eu1···Eu1 distance is 9.589 Å between two adjacent Eu2O16 SBUs linked via 2,5-FDA2− linkers when extended linearly along the c axis. The Eu1···Eu1 distance between two Eu2O16 units bridged together by two Glu2− units is 8.792 Å. The Ln-MOFs 1−8 are isomorphous and show a unique 2D E


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Inorganic Chemistry

Figure 3. Representations of (a) a dinuclear Eu2O16 SBU of Ln-MOF 2, (b) a 1D linear extension of a SBU of Ln-MOF 2 viewed along the c axis, and (c) a perspective 2D extension of 1D chains of Ln-MOF 2.

vibrational bands because they have similar functionalities in their structural motifs and are also isostructural, as suggested by the crystal data. In IR spectra of assembled Ln-MOFs, a broad band in the 3686−3229 cm−1 region is assigned to asymmetric stretching vibrations of coordinated H2O molecules. The characteristic asymmetric carbonyl stretching bands are in the region 1668−1546 cm−1, and bands in the region 1457−1419 cm−1 are assigned to symmetric stretching vibrational modes of carboxylate groups [LnIII-coordinated carboxylate groups belonging to the 2,5-FDA2− and Glu2− moieties]. The highly intense bands in the 1725−1680 cm−1

region are absent in the FT-IR spectra of designed Ln-MOFs, thereby suggesting complete deprotonation of all coordinated ligands. The vibrational bands in the 1275−1058 cm−1 region are assigned to ring vibrations of the rigid 2,5-FDA2− ligand. The LnIII−O bonds in assembled Ln-MOFs can be ascertained by IR spectral bands ranging from 920 to 450 cm−1.53−57 The weaker-intensity IR spectral bands in the 2989−2830 cm−1 region are assigned to the asymmetric stretching vibrational modes of the furan ring and characteristic νC−H vibrational modes of −CH2− groups within the carbon chain of glutaric acid. F


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Inorganic Chemistry

Figure 4. Representation of a Eu2O16 SBU tetramer, highlighting the structural insights of Ln-MOF 2. The Eu1···Eu1 distances are 9.589 Å between two adjacent Eu2O16 SBUs linked via 2,5-FDA2− linkers, and the Eu1···Eu1 distance between two Eu2O16 units bridged together by two Glu2− units is 8.792 Å.

Figure 5. Representations of (a) a 2D (4,4) rectangular grid array fostered by connecting EuIII nodes along the c axis, (b) a space-filled ball structure along the a axis, and (c) hydrogen-bonding positions in Ln-MOF 2.

G


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Inorganic Chemistry

Figure 6. Topological diagram of Ln-MOF 2 in the ab plane.

Table 2. Important Structural Parameters of the Ln Ions in Ln-MOFs 1−8 Ln-MOF

Ln−O (Å)

Ln---Ln (Å)

coordination no.

1 2 3 4 5 6 7 8

2.615(3)−2.386(3) 2.598(5)−2.381(4) 2.601(3)−2.363(5) 2.603(3)−2.341(3) 2.590(4)−2.334(4) 2.619(16)−2.308(14) 2.660(4)−2.296(3) 2.585(6)−2.364(6)

4.169 4.150 4.138 4.124 4.118 4.128 4.134 4.131

9 9 9 9 9 9 9 9

ligands 2,5-FDA2−, 2,5-FDA2−, 2,5-FDA2−, 2,5-FDA2−, 2,5-FDA2−, 2,5-FDA2−, 2,5-FDA2−, 2,5-FDA2−,

TGA. TGA suggested that Ln-MOFs exhibit high and flexible coordination numbers. This unique structural property may impart profound flexibility, sturdiness, robustness, and thermodynamic stability to the designed Ln-MOFs.52,86 The isorecticular Ln-MOFs 1−8 being isostructural follow similar thermal decomposition patterns except for differences in the dehydration and decomposition temperatures. Thermal analyses of Ln-MOFs 1−8 have been carried out in the temperature range of 30−1000 °C under an inert atmosphere (maintained by a N2 gas flow) at a heating rate of 10 °C min−1, as shown in Figure S16. We have chosen Ln-MOF 7 as a representative example for explaining the thermal stability and decomposition behavior of assembled MOFs. The TGA curve of Ln-MOF 7 suggested a typical two-step thermal decomposition pattern. The thermogravimetric curve shows that Ln-MOFs are thermally stable up to 140 °C. The robust Ln-MOF 7 undergoes thermal degradation in two consecutive steps in the temperature range of 150−950 °C. In the first step, the ∼14% weight loss that occurred from 150 to 240 °C can be attributed to coordinated H2O molecule loss. In TGA curve, the next significant weight loss starts at 450 °C and has an end set around 950 °C. In the second step, the weight loss involves a gradual loss of ∼35% (calcd 34.87%), which is possibly due to rapid collapse of the structural framework as a whole with decomposition of coordinated 2,5-FDA2− and Glu2− moieties. In the thermal analysis of Ln-MOF 7, the remaining residue at the end is Er2O3.53 The thermal results of Ln-MOFs 1−8 are fundamentally in close agreement with the observed structural

3Glu2−, 3Glu2−, 3Glu2−, 3Glu2−, 3Glu2−, 3Glu2−, 3Glu2−, 3Glu2−,

coordination geometry 2H2O 2H2O 2H2O 2H2O 2H2O 2H2O 2H2O 2H2O

monocapped monocapped monocapped monocapped monocapped monocapped monocapped monocapped

square square square square square square square square

antiprism antiprism antiprism antiprism antiprism antiprism antiprism antiprism

(distorted) (distorted) (distorted) (distorted) (distorted) (distorted) (distorted) (distorted)

features of assembled Ln-MOFs, and such thermal behavior depicts the robustness of the designed functional frameworks. PXRD. To check the phase purity of the polycrystalline materials, the PXRD patterns of Ln-MOFs 1−8 were recorded (see Figures S17a,b and S18a,b). The experimental PXRD patterns of the designed Ln-MOFs 1−8 are in close agreement with the simulated ones obtained from the crystal data, with slight differences in their intensities. The differences in the experimental and simulated PXRD patterns of synthesized phases arise as a consequence of the preferred orientation of the powder samples used for recording the PXRD data.87 Porosity and Surface Area Measurement. N2 adsorption−desorption isotherm was studied to determine the surface areas, pore radii, and pore volumes of Ln-MOFs. The sorption experiment on the activated phase of Ln-MOF 1 revealed a type I isotherm (Figure S19), indicating a microporous structure. On the basis of the BET multipoint N2 isotherm at 77 K, the Langmuir surface area of Ln-MOF 1 was calculated as 2.101 m2 g−1. The average pore diameter was found 2.101 nm. The MOF 1 shows low surface area and poor N2 uptake, thereby suggesting that Ln-MOFs do not possess permanent porosity. These observations can be attributed to the rapid collapse of the structural framework during N2 adsorption−desorption measurements.88 Photoluminescent Properties of Ln-MOFs 2 and 4. LnIII ions such as Sm3+ (4f5), Eu3+ (4f6), Gd3+ (4f7), Tb3+ (4f8), Dy3+ (4f9), Ho3+ (4f10), and Er3+ (4f11) show promising photoluminescent emission due to partially filled 4f subshells. However, the optical properties of trivalent Eu and Tb ions are H


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Inorganic Chemistry

Figure 7. Solid-state photophysical properties of Ln-MOF 2 recorded at room temperature: red color, emission spectrum; black color, excitation spectrum. The CIE plot is shown in the inset.

Figure 8. Photophysical properties of Ln-MOF 4 recorded at room temperature: green color, emission spectrum; black color, excitation spectrum. The CIE plot is shown in the inset.

π-conjugated di/polycarboxylates ligands such as thiophenedicarboxylic,50−52 pyridinedicarboxylic,53−57 furan-2,5-dicarboxylic,46−49 isophthalic,58,59 terephthalic,60−63 and naphthalenedicarboxylic64−67 acids. These organic linkers display an “antenna effect”, thereby harvesting quanta of photoenergy, followed by transfer of the harvested light to the LnIII ion centers in assembled Ln-MOFs. These functional materials show characteristic photoluminescent emission at a specific wavelength.91

of special interest because they show characteristic strong emissions, longer excited lifetimes, and, importantly, high color purity.89 The intraconfigurational 4f → 4f transitions are responsible for narrow emission bands (linelike appearance) of trivalent Ln ions. In general, these intraconfigurational 4f → 4f transition bands are not profoundly influenced by the surroundings of the LnIII centers. The 4f → 4f transitions are Laporte-forbidden transitions and, hence, display weaker molar absorptivities, so it is hard to excite them.90 To overcome this difficulty, Ln-MOFs are designed by using photoluminescent I


Article

Inorganic Chemistry In this contribution, we have explored room temperature solid-state photophysical properties of Ln-MOF 2 (Eu) and Ln-MOF 4 (Tb), as shown in Figures 7 (Eu) and 8 (Tb). A highly intense red emission is exhibited by Ln-MOF 2 upon excitation at 363 nm (CIE plot; Figure 7, inset), and Ln-MOF 4 shows green emission when excited at 361 nm (CIE plot; Figure 8, inset). These observations suggest an effective transfer of photoenergy from 2,5-FDA2− to LnIII ions by means of an “antenna effect”.91 The various characteristic emission peaks at 548 nm (5D0 → 7 F0), 591 nm (5D0 → 7F1), 617 nm (5D0 → 7F2), 690 nm (5D0 → 7F3), and 697 nm (5D0 → 7F4) are obtained in the visible region for Ln-MOF 2 upon excitation at 363 nm. The CPs/ MOFs of Eu 3+ exhibit the 5 D0 → 7 F2 transition as hypersensitive and consist of a single intense peak. The relatively weak transition at 5D0 → 7F1 positioned at 591 nm (magnetic-dipole transitions) is independent of the coordination environment around the Eu3+ centers. It is pertinent to mention here that the hypersensitive 5D0 → 7F2 (positioned at 617 nm) transition is responsible for red emission of Ln-MOF 2 with CIE coordinates (0.610, 0.312; see Figure 7, inset). Moreover, the relative intensity of the 5D0 → 7F2 (617 nm) transition depends on the coordination environment and symmetry around the Eu3+ centers.92 This indicates that in LnMOF 2 Eu3+ ions have low symmetry and the inversion center is absent in accordance with the Judd−Ofelt theory.93 In LnMOF 2, 5D0 → 7F0 (548 nm) and 5D0→ 7F4 (697 nm) are the shortest- and longest-wavelength transitions, are electric as well as magnetic-dipole forbidden transitions, and possibly arise because of the crystal-field-induced J-mixing effect.94 Ln-MOF 4 exhibits typical solid-state photoluminescent emission of a ligand-sensitized TbIII ion upon excitation at 361 nm. In the photoluminescence spectrum of Ln-MOF 4, the various characteristic emission bands originate from the 5D4 excited state of Tb3+, which is efficiently populated by photoenergy transfer from the FDA2− linker excited state to the 5D4 excited state of Tb3+ via the well-known “antenna” mechanism. As shown in Figure 8, the transition bands at 5D4 → 7F6 (490 nm), 5D4 → 7F5 (544 nm), 5D4 → 7F4 (583 nm), and 5D4 →7F3 (621 nm) are due to the relaxation of excited state 5D4 to the 7FJ (J = 6, 5, 4, 3) state(s). The solid-state emission spectrum of Ln-MOF 4 displays a high-intensity 5D4 → 7F5 transition at 544 nm, being accountable for the intense green emission of a Tb3+ ion with CIE coordinates (0.293, 0.619; Figure 8, inset). Thus, photoluminescent studies on assembled Ln-MOFs 2 and 4 suggest some important structural insights of the assembled Ln-MOFs that are in agreement with the singlecrystal X-ray results.95 Magnetic Properties of Ln-MOFs 3 and 5. The phase purities of Ln-MOFs 3 and 5 have been confirmed by their PXRD patterns [Figures S17b (3) and S18a (5)]. As shown in Figure 9, under a direct-current (dc) field of 1 kOe, variabletemperature magnetic susceptibilities for MOFs 3 and 5 were recorded on their polycrystalline phases. The observed χmT product at 300 K of 17.83 cm3 mol−1 K for Ln-MOF 3 is somewhat larger than the theoretical value for two noninteracting GdIII (15.75 cm3 mol−1 K, 8S7/2, and g = 2), while the experimental χmT value at 300 K for Ln-MOF 5 is 24.72 cm3 mol−1 K, which is relatively less compared to the expected value for two noncoupled DyIII centers (28.34 cm3 mol−1 K, 6 H15/2, and g = 4/3). The χmT values of Ln-MOFs 3 and 5 stay

Figure 9. Plots of χmT versus T for Ln-MOFs 3 and 5.

essentially constant at high temperatures upon cooling. However, upon further cooling, the χmT value of Ln-MOF 3 decreases gradually and reaches 16.60 cm3 mol−1 K at 2 K and the χmT value of Ln-MOF 5 decreases from 100 to 2 K rapidly and reaches 16.03 cm3 mol−1 K at 2 K. There exists antiferromagnetic coupling between adjacent GdIII ions, indicated by a decline of the χmT curve, and this fact is further supported by a negative Weiss constant (θ = −1.03 K) from the Curie−Weiss fitting (see Figure S20). As shown in Figure S21, the magnetization measurements on Ln-MOFs 3 and 5 were investigated in the range of 0−7 T at 2 K. The M versus H plots depict a steady increase with increasing magnetic field. For Ln-MOF 3, the M value reaches 14.98 Nβ at 2 K and 7 T, which is in agreement with the theoretical value of 14 Nβ for two GdIII ions. The M value for Ln-MOF 5 is 10.62 Nβ measured at 2 K and 7 T. This value is far from the theoretical saturated value of 20 Nβ for two adjacent DyIII ions (g = 4/3 and J = 15/2). In Ln-MOF 5, the magnetic unsaturation even at 7 T may be assigned to the magnetic anisotropy and/or low-lying excited states of trivalent Dy3+ ion. In Ln-MOF 3, the weak magnetic couplings and large metal/ligand ratio by mass forge it as a promising candidate for low-temperature magnetic cooling. The magnetic entropy change (ΔSm) is a key parameter in evaluating the magnetocaloric effect (MCE), which can be derived from experimentally obtained magnetization data by applying the Maxwell equation ΔSm(T)ΔH = ∫ [∂M(T,H)/∂T]H dH.96−102 The ΔSm values at variable temperatures and applied magnetic fields are obtained from the experimental magnetization data, as displayed in Figure S22, with an impressive −ΔSmmax = 40.6 J kg−1 K−1 for T = 2 K and ΔH = 7 T (see Figure 10). The −ΔSmmax value is relatively small compared to 43.2 J kg−1 K−1 obtained for two uncoupled GdIII ions [as judged by 2R ln(2S + 1), where R is the gas constant and S is the spin state]. The difference in the experimental data and expected value mainly originated from the intrachain antiferromagnetic couplings in Ln-MOF 3. The magnetic entropy change −ΔSmmax above 40.0 J kg−1 K−1 is limited among reported molecule-based magnetic cryogen, as listed in Table S26. Considering the volumetric aspect with −ΔSmmax = 70.59 mJ cm−3 K−1, Ln-MOF has the highest −ΔSmmax value throughout the GdIII-containing complexes (Table S26). For practical applications, the volumetric aspect is more meaningful for evaluating the molecule-based magnetocaloric materials.96−102 In Ln-MOF 3, the large MCE per unit mass and/or per unit volume is primarily attributed to the small J


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Inorganic Chemistry

Figure 12. Cole−Cole plots for Ln-MOF 5 measured at 2.4−6 K with a 2 kOe dc field (the red solid line represents the least-squares fitting by using CC-FIT software).

Figure 10. −ΔSm for Ln-MOF 3 calculated by using the magnetization data at variable fields and temperatures.

linker and high magnetic density among the cluster compounds. Furthermore, the −ΔSm value per unit mass approaches 32.1 J kg−1 K−1 for T = 2 K and ΔH = 30 kOe, which is larger than those of most of complexes based on the Gd3+ ion reported in the literature.96−102 To further explore the dynamic magnetic properties of LnMOF 5, under a zero dc field and a 3 Oe alternating-current (ac) magnetic field, we have collected frequency- and temperature-dependent ac susceptibilities (see Figure S23). The frequency-dependent signals below 9 K were obtained for the out-of-phase constituent of ac susceptibilities, suggesting the slow magnetic relaxation behavior of 5, which might be an indication of single-molecule-magnet (SMM) behavior. However, because of the fast quantum tunneling, the out-of-phase signal peaks were not obtained in the technically available temperature range similar to some reported Dy-based complexes.99 Therefore, to minimize the quantum-tunneling effect, a 2 kOe dc field was exerted. As depicted in Figure 11, the peaks and tails at superlow temperature can be observed obviously in both the χ′m and χ″m curves, which clearly imply the existence of slow magnetic relaxation behavior and two relaxation processes in Ln-MOF 5. In addition, the Cole−Cole plots at 2.4−6 K (Figure 12) further prove the existence of two relaxation processes. By using CC-FIT software,103 the leastsquares fitting parameters are listed in Table S27. Notably,

there is an overlap section between these two relaxation processes. Compared with some DyIII complexes based on carboxylate ligands,104,105 complex 5 exhibits weaker frequency-dependent χ″m signals and two obvious relaxation processes, which suggest that DyIII complexes assembled from two carboxylate linkers are not the ideal candidates for the SMMs.

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CONCLUSIONS In conclusion, employing 2,5-FDA2− and Glu2− via a mixedligand strategy, single crystals of eight similar Ln-MOFs were isolated and characterized by single-crystal X-ray diffraction. Crystallographic data reveal that Ln-MOFs 1−8 crystallize in the orthorhombic Pnma space group with the general formula [Ln(2,5-FDA)0.5(Glu)(H2O)2]·xH2O}n [Ln = Sm (1), Eu (2), Gd (3), Tb (4), Dy (5), Ho (6), Er (7), and Yb (8); 2,5FDA2−= furan-2,5-dicarboxylate and Glu2− = glutarate; x = 0.5 for 1, 2, and 4 and x = 0 for 3 and 5−8]. The assembled LnMOFs 1−8 are isostructural and isomorphous with distorted monocapped square-antiprismatic geometry around Ln1 metal centers. Topologically, all MOFs consist of a 5-connected uninodal net with the point symbol {33.44.53} and 2D cem topological type. We have analyzed the energies associated with the hydrogen-bonding interactions using DFT calcu-

Figure 11. Representation of in-phase (χ′m, a) and out-of-phase (χ″m, b) frequency dependence of the ac susceptibility components for Ln-MOF 5 with a 2 kOe dc field. K


Article

Inorganic Chemistry lations and NCIplot. Ln-MOFs 2 [Eu3+ (red emission)] and 4 [Tb3+ (green emission)] display a highly intense characteristic ligand-sensitized Ln3+ f−f photoluminescence. Magnetic studies suggested that MCE and SMM behaviors are shown by Ln-MOFs 3 and 5, respectively. The −ΔSm value for LnMOF 3 reaches 32.1 J kg−1 K−1 for T = 2 K and ΔH = 30 kOe, which is higher than most of reported complexes based on the Gd3+ ion. The ac susceptibilities (out-of-phase) below 9 K for Ln-MOF 5 show signals dependent on the frequency, which suggests the slow magnetic relaxation behavior of 5 and might be an indication of SMM behavior. At superlow temperature, the peaks and tails in both the χ′m and χ″m curves of Ln-MOF 5 can be observed, which further suggested the existence of slow magnetic relaxation behavior and two relaxation processes. These observations are further supported by the Cole−Cole plots at 2.4−6 K.

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Delhi) for helping in topological analysis. M.K. acknowledges University Grant Commission, New Delhi, India, for a SRF Fellowship (reference no. 20/12/2015(ii) Eu-V). A.F. acknowledges MINECO/AEI from Spain for a “Juan de la Cierva” contract. We are thankful to MINECO/AEI from Spain for financial support (Project CTQ2017-85821-R and FEDER funds). We gratefully acknowledge CTI (UIB) for free allocation of computer time. S.-J.L. thanks the National Natural Science Foundation of China (Grant 21501077) and Natural Science Foundation of Jiangxi Province (Grants 20161ACB21013 and 20171BCB23066). S.C.S. acknowledges DST-FIST for the single-crystal X-ray facility at Panjab University Chandigarh.

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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.9b00219. Single-crystal X-ray structural information (Figures S1− S13), FT-IR spectra (Figures S14 and S15), TGA curves (Figure S16), PXRD studies (Figures S17 and S18), N2 adsorption−desorption study (Figure S19), magnetic studies (Figures S20−S23), DFT studies (Figures S24− S27), reaction conditions in optimization attempts for the synthesis of Ln-MOFs 1−8 (Table S1), selected bond lengths, bond angles, and hydrogen-bonding geometries for Ln-MOFs 1−8 (Tables S2−S25), and a comparison of −ΔSmmax (ΔH = 70 kOe) of Ln-MOF 3 with select molecule-based magnetic coolers from the literature and fitting parameters α and τ using CC-FIT software (Tables S26 and S27, respectively) (PDF) Accession Codes

CCDC 1868174−1868181 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

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

Manesh Kumar: 0000-0001-7507-5144 Antonio Franconetti: 0000-0002-7972-8795 Haq Nawaz Sheikh: 0000-0002-2851-3831 Sui-Jun Liu: 0000-0002-7705-0634 Subash Chandra Sahoo: 0000-0003-2557-1937 Antonio Frontera: 0000-0001-7840-2139 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Department of Physics, SMVDU Katra, for PL studies and SAIF Chandigarh for PXRD and BET analysis. We thank Dr. Amanpreet K. Jassal (Indian Institute of Technology L


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