Functionalization of Microporous Lanthanide ... - ACS Publications

May 11, 2016 - Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology, School of Chemistry and Chemical. Engineering,...
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Functionalization of Microporous Lanthanide-Based Metal−Organic Frameworks by Dicarboxylate Ligands with Methyl-Substituted Thieno[2,3‑b]thiophene Groups: Sensing Activities and Magnetic Properties Suna Wang,*,† Tingting Cao,† Hui Yan,†,§ Yunwu Li,† Jing Lu,† Ranran Ma,† Dacheng Li,† Jianmin Dou,† and Junfeng Bai*,‡ †

Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology, School of Chemistry and Chemical Engineering, and §School of Pharmacy, Liaocheng University, Liaocheng, 252059, People’s Republic of China ‡ State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing, 210093, People’s Republic of China S Supporting Information *

ABSTRACT: From a methyl-substituted thieno[2,3-b]thiophene dicarboxylate, three types of three-dimensional (3-D) microporous lanthanide-based metal−organic frameworks, {[Ln(DMTDC)1.5(H2O)2]·DEF}n (type I, Ln = Eu 1, Tb 2), {[Ln(DMTDC)1.5(H2O)2]·0.5DMF·0.5H2O}n (type II, Ln = Gd 3, Dy 4, Er 5), and {[Ln4(DMTDC)6(DMF)2]·0.5DMF·1.5H2O}n (type III, Ln = Er 6) (H2DMTDC = 3,4-dimethylthieno[2,3-b]thiophene2,5-dicarboxylic acid, DEF = N,N′-diethylformamide, DMF = N,N′dimethylformamide), have been solventhermally synthesized. Types I and II are isostructural, which exhibit 1-D triangular channels constructed by double-stranded rod-shaped {Ln(CO2)2}n chains. Type III demonstrates an intriguing framework with triple-stranded rod-shaped {Ln(CO2)3}n chains arranged along the (1,1,0) and (1,−1,0) axes and possesses two kinds of triangular channels along two axes, respectively. Immobilization of the Lewis basic sites of thiophene groups induced gas adsorption and sensing properties into these microporous frameworks. Complexes 5(Er) and 6(Er) display moderate adsorption properties toward N2 and CO2, and the Qst of CO2 are as high as 36.3 and 34.8 kJ mol−1, respectively. Complexes 1(Eu) and 2(Tb) exhibit sensing properties toward nitrobenzene, acetone, and the Cu2+ ion in both DMF and aqueous solution. Complex 3(Gd) shows a significant magnetocaloric effect with ΔSm = 24.3 J·kg−1· K−1 at 3.0 K and 7 T. Complex 4(Dy) exhibits slow magnetic relaxation with the energy barrier Δ/kB of 48.29 K.

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INTRODUCTION

rational design and synthesis, and variable coordination conformations of the organic groups. At the same time, immobilization of Lewis basic sites within porous MOFs has been more challenging considering the strong trend of such basic sites to bind metal centers and form condensed structures. Carboxylate ligands were at present the most popular ligands in the assembly of MOFs. Except for comparable stability, the diverse coordination and bridging modes of carboxylate groups can lead to interesting topological architectures, magnetic coupling, and fluorescence properties. In the construction, hydroxyl,5a,b carboxyl,5c,d fonate,6a amide,6b−d and pyridyl or other N-containing groups7 were also embedded into this kind of ligand. These functional groups in the obtained MOFs could act as Lewis basic sites, tailor the pore structure, and improve the properties, especially reorganization, selective adsorption, or sensing abilities. Comparably, S-containing functional groups

The design and synthesis of metal−organic frameworks (MOFs) has received tremendous attention over the past two decades, originally stemming from intriguing porous architectures and special properties including gas adsorption and separation. Functionalization of these porous frameworks with other properties, such as magnetism, fluorescence, sensing, electronic, and drug delivery, has now been of special interest for chemists.1−4 From the viewpoint of crystal engineering, the nature of frameworks and the pore surface play decisive roles in the functions of such materials. Thus, fabrication of the structures should be performed from two essential building blocks: a metal atom and an organic ligand. Consequently, framework and surface functionalization are extensively investigated though immobilization of open metal sites, free functional Lewis basic sites, or both.5−8 To the best of our knowledge, however, although some multifunctional MOFs have been reported using the former method, the choice of the latter one seems to be more favored due to the diversity, the © XXXX American Chemical Society

Received: December 7, 2015

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DOI: 10.1021/acs.inorgchem.5b02801 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

magnetocaloric effect with ΔSm = 24.3 J·kg−1·K−1 at 3.0 K and 7 T. Complex 4(Dy) exhibits slow magnetic relaxation.

were less employed. In fact, S possesses weaker binding abilities with metal atoms than O and thus easily forms “S-naked” structures, especially in lanthanide-based MOFs. When situated within a highly electron-rich conjugated system, the lone pair electrons of S atoms can be spread and thereby realize the effective transfer of electrons and further influence the optical properties. Therefore, introduction of sulfide or thiophene into carboxylic acid ligands may be a highly promising platform for the construction of a new class of materials. To the best of our knowledge, however, only a few coordination compounds have been obtained from simple thiophene dicarboxylic acid.9,10 Only a few complexes have been reported from bithiophendicarboxylic acids, such as 2,2′-bithiophene-5,5′-dicarboxylic acid, thieno[3,2-b]thiophene-2,5-dicarboxylic acid, [3,2-b;2′,3′-d]thiophene-2,6-dicarboxylate, 3,3′-dipheyl-2,2′- bithiophene5,5′-dicarboxylic acid, and dithieno[3,2-b:2′,3′-e]benzene-2,6dicarboxylic acid.11,12 Recently, the Su group have reported two S-containing MOFs based on 3,4-dimethylthieno[2,3-b]thiophene-2,5-dicarboxylic acid (H2DMTDC), which reveal high selective adsorption for Cu2+ ions of S atoms within the MOFs as active Lewis basic sites, and they have been applied as a chromatographic column for separating transition metal ions for the first time.12 We have previously reported transition metal compounds based on H2DMTDC that exhibit sensing properties for small organic molecules.13 In this work, we synthesized several microporous lanthanide-based MOFs from this ligand, namely, {[Ln(DMTDC)1.5(H2O)2]·DEF}n (type I, Ln = Eu 1, Tb 2), {[Ln(DMTDC)1.5(H2O)2]·0.5DMF·0.5H2O}n (type II, Ln = Gd 3, Dy 4, Er 5), and {[Ln4(DMTDC)6(DMF)2]·0.5DMF· 1.5H2O}n (type III, Ln = Er 6) (DEF = N,N′-diethylformamide, DMF = N,N′-dimethylformamide). A combination of organic groups of carboxyl and thiophene with characteristic lanthanide metals gave rise to the functionalization of these special MOFs (Scheme 1). All structure types demonstrate

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EXPERIMENTAL SECTION

General Methods. H2DMTDC was synthesized according to previous documents.14 Other reagents were commercially available and used as purchased without further purification. The elemental analysis was carried out with a PerkinElmer 240C elemental analyzer. The FTIR spectra were recorded from KBr pellets in the range 4000−400 cm−1 on a VECTOR 22 spectrometer. Powder X-ray diffraction (PXRD) data were collected over the 2θ range 5−50° on a Philips X’pert diffractometer using Cu Kα radiation (λ = 1.5418 Å) at room temperature. Thermal analyses were performed on a TGA V5.1A Dupont 2100 instrument from room temperature to 700 °C with a heating rate of 10 °C/min under flowing nitrogen. Gas sorption isotherms were measured using an Autosorb-IQ-C analyzer of Quantachrome. The N2 and CO2 adsorption isotherms for desolvated compounds were collected in a relative pressure range from 10 to 1.0 × 105 Pa. The cryogenic temperatures of 77 K required for N2 sorption measurements were controlled by liquid nitrogen, and the 273 K required for CO2 was controlled using an ice−water bath. The cryogenic temperature of 293 K for CO2 was controlled using a water bath. The initial outgassing process for the sample was carried out under a high vacuum (less than 10−4 Pa) at 150 °C for 10 h. Magnetic measurements were carried out on SQUID-VSM. The emission/ excitation spectra were recorded on a Hitachi 850 fluorescence spectrophotometer. UV−vis spectra were recorded on a Shimadzu-260 UV−vis spectrophotometer at room temperature. Synthesis of the Complexes. All complexes were solventhermally synthesized. {[Eu(DMTDC)1.5(H2O)2]·DEF}n (1). A mixture of H2DMTDC (0.1 mmol) and Eu(NO3)3·6H2O (0.1 mmol) in H2O/DEF (1:1, 10 mL) was placed in a Parr Teflon-lined stainless steel vessel and heated to 120 °C for 72 h. Then the reaction system was cooled to room temperature slowly. Single crystals were filtrated, washed with water, and dried in air. Yield: 52.3% based on H2DMTDC. C20H24NO9S3Eu (670.56): calcd C 35.82, H 3.61, N 2.09; found C 35.91, H 3.67, N 2.11. IR (KBr pellet: cm−1): 3451(vs,br), 1646(s), 1567(s), 1499(s), 1384(m), 1310(m), 1112(w), 1025(w), 919(w), 822(w), 808(w), 786(w), 602(m). {[Tb(DMTDC)1.5(H2O)2]·DEF}n (2). The procedure was the same as that for complex 1 except that Eu(NO3)3·6H2O was replaced by Tb(NO 3 ) 3 ·6H 2 O. Yield: 44.3% based on H 2 DM TDC. C20H24NO9S3Tb (677.53): calcd C 35.45, H 3.57, N 2.07; found C 35.61, H 3.52, N 2.13. IR (KBr pellet: cm−1): 3451(vs,br), 1646(s), 1569(s), 1531(s), 1499(s), 1394(s), 1283(m), 1209(w), 1113(m), 1025(w), 824(w), 786(w), 604(m), 492(m). {[Gd(DMTDC)1.5(H 2 O)2 ]·0.5DMF·0.5H 2O}n (3). A mixture of H2DMTDC (0.1 mmol) and Gd(NO3)3·6H2O (0.1 mmol) in H2O/ DMF (1:1, 10 mL) was placed in a Parr Teflon-lined stainless steel vessel and heated to 120 °C for 72 h. Then the reaction system was cooled to room temperature slowly. Single crystals were filtrated, washed with water, and dried in air. Yield: 34.6% based on H2DMTDC. C16.5H17.5N0.5O9S3Gd (620.26): calcd C 31.95, H 2.84, N 1.13; found C 31.86, H 2.86, N 1.14. IR (KBr pellet: cm−1): 3455(vs,br), 1668(vs), 1561(s), 1499(m), 1436(w), 1386(s), 1143(w), 1023(w), 832(w), 811(m), 784(m), 665(w), 601(w), 494(w). {[Dy(DMTDC)1.5(H2O)2]·0.5DMF·0.5H2O}n (4). The procedure was the same as that for complex 3 except that Gd(NO3)3·6H2O was replaced by Dy(NO3)3·6H2O. Yield: 47.3% based on H2DMTDC. C16.5H17.5N0.5O9S3Dy (625.51): calcd C 31.68, H 2.82, N 1.12; found C 31.51, H 2.80, N 1.10. IR (KBr pellet: cm−1): 3381(vs,br), 1654(m), 1557(vs), 1499(m), 1383(vs), 1155(s), 1101(w), 1023(w), 833(w), 784(m), 665(w), 604(w), 495(w). {[Er(DMTDC) 1.5 (H 2 O) 2 ]·0.5DMF·0.5H 2 O} n (5). A mixture of H2DMTDC (0.1 mmol), Er(NO3)3·6H2O (0.1 mmol), and 4,4-bpy (0.1 mmol) in H2O/DMF (1:1, 10 mL) was placed in a Parr Teflonlined stainless steel vessel and heated to 120 °C for 72 h. Then the

Scheme 1. Presence of Lewis Basic Sites for Sensing and Bridges for Magnetic Coupling within the Porous Frameworks

noninterpenetrating porous structures constructed by rodshaped lanthanide−carboxylate chains, and the pores are all tailed by thiophene groups. Gas adsorption measurements indicated that complexes 5(Er) and 6(Er) display moderate adsorption properties toward N2 and CO2. Owing to the presence of the Lewis basic S sites, complexes 1(Eu) and 2(Tb) exhibit characteristic lanthanide luminescent properties in the solid state and sensing properties of acetone, nitrobenzene (NB), and the Cu2+ ion. Connectivity of the metal−carboxylate chains leads to antiferromagnetic behaviors between adjacent lanthanide centers. Complex 3(Gd) shows a significant B

DOI: 10.1021/acs.inorgchem.5b02801 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Refinement of the Crystal Structures of Complexes 1−6 formula fw T [K] cryst syst space group a [Å] b [Å] c [Å] α [deg] β [deg] γ [deg] V [Å3] Z Dcalcd [g cm−3] μ [mm−1] θ range index ranges

R1; wR2a [I > 2σ(I)] GOF formula fw T [K] cryst syst space group a [Å] b [Å] c [Å] α [deg] β [deg] γ [deg] V [Å3] Z Dcalcd [g cm−3] μ [mm−1] θ range index ranges

R1; wR2a [I > 2σ(I)] GOF

1

2

3

C20H24NO9S3Eu 670.56 121(2) monoclinic C2/c 26.0190(13) 12.3698(4) 19.5603(11) 90 121.672(7) 90 5357.9(4) 8 1.663 19.356 3.99−66.20 −31 ≤ h ≤ 14 −11 ≤ k ≤ 14 −19 ≤ l ≤ 23 0.0618; 0.1510 1.019 4

C20H24NO9S3Tb 677.53 121(2) monoclinic C2/c 25.935(3) 12.3434(4) 22.869(2) 90 133.605(18) 90 5301.2(8) 8 1.698 15.745 4.00−25.02 −30 ≤ h ≤ 30 −14 ≤ k ≤ 14 −27 ≤ l ≤ 27 0.0542; 0.1320 1.056 5

C16.5H17.5N0.5O9S3Gd 620.26 293(2) monoclinic C2/c 26.578(3) 12.4498(9) 19.9618(18) 90 122.923(2) 90 5544.4(8) 8 1.377 2.646 2.49−25.02 −27 ≤ h ≤ 31 −14 ≤ k ≤ 14 −23 ≤ l ≤ 16 0.0862; 0.2413 1.101 6

C16.5H17.5N0.5O9S3Dy 625.51 293(2) monoclinic C2/c 26.333(3) 12.2760(9) 19.5387(19) 90 122.809(9) 90 5308.7(8) 8 1.451 3.080 2.52−25.02 −31 ≤ h ≤ 31 −13 ≤ k ≤ 14 −23 ≤ l ≤ 23 0.1066; 0.2831 1.137

C16.5H17.5N01.5O9S3Er 620.27 293(2) monoclinic C2/c 26.3205(9) 12.2748(3) 19.6085(7) 90 122.972(3) 90 5314.7(3) 8 1.461 3.422 2.53−25.02 −31 ≤ h ≤ 29 −10 ≤ k ≤ 14 −19 ≤ l ≤ 23 0.0385; 0.0945 1.050

reaction system was cooled to room temperature slowly. Single crystals were filtrated, washed with water, and dried in air. Yield: 48.3% based on H2DMTDC. C16.5H17.5N0.5 O9S3Er (630.27): calcd C 31.44, H 2.80, N 1.11; found C 31.51, H 2.70, N 1.07. IR (KBr pellet: cm−1): 3395(vs,br), 2901(m), 1668(m), 1563(s), 1500(s), 1447(m), 1389(vs), 1101(w), 1022(w), 834(m), 812(w), 801(m), 665(w), 612 (m), 486(m). {[Er4(DMTDC)6(DMF)2]·0.5DMF·1.5H2O}n (6). The procedure was the same as that for complex 5 except that 4,4′-bpy was replaced by 1,4-bis(tetrazol-5-yl)benzene. Yield: 35.6% based on H2DMTDC. C67.5H56.5N2.5O28S12Er4 (2404.49): calcd C 33.72, H 2.37, N 1.46; found C 33.41, H 2.42, N 1.56. IR (KBr pellet: cm−1): 3369(s,br), 2924(m), 1668(s), 1564(vs), 1499(s), 1436(m), 1389(vs), 1159(w), 1101(w), 1022(w), 834(m), 811(m), 784(m), 666(w), 601(w), 497(w). X-ray Crystallography. Suitable single crystals of complexes 3−6 were selected for indexing, and intensity data were measured on a Siemens Smart CCD diffractometer with graphite-monochromated

C67.5H56N2.5O28S12Er4 2404.49 293(2) monoclinic C2/c 13.1603(2) 33.4072(7) 42.2530(7) 90 98.111(2) 90 18390.6(6) 8 1.679 3.952 2.41−25.02 −15 ≤ h ≤ 15 −39 ≤ k ≤ 36 −50 ≤ l ≤ 49 0.0448; 0.1039 1.039

Mo Kα radiation (λ = 0.710 73 Å) at 298 K. Complexes 1 and 2 were processed using the CrysAlisPro software package (Agilent Technologies 2013, CrysAlisPro Software system, version 1.171.35.19, Agilent Technologies UK Ltd., Oxford, UK) at 121 K. The raw data frames were intergrated into SHELX-format reflection files and corrected using SAINT program.15 Absorption corrections based on multiscans were obtained by the SADABS program.16 The structures were solved with direct methods and refined with full-matrix leastsquares technique using the SHELXS-97 and SHELXL-97 programs, respectively.17 Displacement parameters were refined anisotropically, and the positions of the H atoms were generated geometrically, assigned isotropic thermal parameters, and allowed to ride on their parent carbon atoms before the final cycle of refinement. Basic information pertaining to crystal parameters and structure refinement is summarized in Table 1, and selected bond lengths and angles are listed in Table S1 in the Supporting Information. CCDC 1424435 (for 1), 1424436 (for 2), 1424432 (for 3), 1424433 (for 4), 1424434 (for 5), and 1424437 (for 6) contain the supplementary crystallographic C

DOI: 10.1021/acs.inorgchem.5b02801 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif. There are some solvent-accessible void volumes in complexes 3−6, which are occupied by highly disordered DMF and H2O molecules. No satisfactory disorder model could be achieved, and therefore the SQUEEZE program implemented in PLATON was used to remove these electron densities.18 The solvents are tentatively assigned based on TGA, elemental analysis, and the SQUEEZE results. All structures were examined using the Addsym subroutine of PLATON to ensure that no additional symmetry could be applied to the models.

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RESULTS AND DISCUSSION Effect of the Solvents and N-Containing Ligands. It is well known that the synthesis conditions play an important role. Change of raw material, temperature, or solvents may lead to different products. Solventhermal methods were employed in our syntheses. A systematic investigation on the influence of solvents and different N-coligands was carried out. As shown in Table S2 in the Supporting Information, cases were interesting for distinctive lanthanides. For Eu and Tb complexes, only the reactions in DEF/H2O mixed solvents without N-ligands resulted in the formation of crystals with type I structure. Dy and Gd complexes, however, were obtained from the reactions in DMF/H2O mixed solvents without N-ligands. When Nligand 1,4-bis(tetrazol-5-yl)benzene was added, a Gd-tetrazole complex without carboxylate ligands was found. Er complexes could not be obtained without N-ligands, and the structures show diversity from type II to III with the presence of different N-ligands. Significantly, the polarity and solubility of the mixed solvents, as well as the size, electron configuration, and conjugation of pyridine and tetrazol groups within the Nligands, all have influences on the synthesis. Structures of {[Ln(DMTDC)1.5(H2O)2]·DEF}n (Type I) (Ln = Eu 1, Tb 2) and {[Ln(DMTDC)1.5(H2O)2]·0.5DMF· 1.5H2O}n (Type II) (Ln = Gd 3, Dy 4, Er 5). Single-crystal X-ray diffraction studies reveal that types I and II are isostructural, and only complex 5 is described here representatively. Complex 5 crystallizes in the monoclinic space group C2/c. As shown in Figure 1a, each asymmetric unit consists of one Er(III) ion, one and a half DMTDC ligands, two coordinated aqua molecules, and a half-lattice of DMF and H2O molecules. The Er1 atom is eight-coordinated with four dimonodentate carboxyl groups (O1, O2B, O5, O6D), one chelating bisdentate carboxyl group (O3C and O4C), and two terminal H2O molecules (O7 and O8). The whole coordination geometry could be viewed as a distorted bicapped trigonal prism with two H2O molecules situated in cis positions. The Er−O bonds and O−Er−O angles are in the ranges 2.253(4)− 2.449(3) Å and 72.81(12)−155.85(14)°, respectively. Two crystallographically independent DMTDC ligands adopt (κ0κ2)-(κ1-κ1)-μ3 tridentate (DMTDC1) and (κ1-κ1)-(κ1-κ1)-μ4 bis-bidentate (DMTDC2) coordination modes, respectively. Er1 atoms are linked together through two syn−syn carboxyl groups of discrete DMTDC2 ligands, resulting in an infinite rod-like inorganic chain {Er(μ2-CO2)2}n with Er1···Er1A and Er1···Er1#1 (symmetry codes: #1, −x, 1−y, −z; B, −x, y, 1/2− z) distances of 4.8449(7) and 4.9961(7) Å, respectively (Figure 1b). Each chain contains equal amounts of left- and righthanded double-stranded helixes along the c axis. These chains are connected through DMTDC1 ligands to construct a 2-D meso-helical layer along the bc plane with adjacent chains showing opposite helicity. DMTDC1 ligands link these layers further, generating a three-dimensional (3-D) extended frame-

Figure 1. (a) Local coordination environment of complex 5. All hydrogen atoms are omitted for clarity. Symmetry codes for the generated atoms: A, −x, y, 1/2−z; B, 1/2−x, 1/2+y, 1/2−z; C, −x, 1− y, −z; D, x, −1+y, z. Hydrogen atoms are omitted for clarity. (b) Perspective view of the 2-D sheet, showing the 1-D double-stranded inorganic rods. (c) Perspective view of the 3-D framework of complex 5 along the c axis. (d) Connolly surface in complex 5 along the a and c axes, showing the solvent accessibly void volume.

work (Figure 1c). Interesting 1-D triangular channels with dimensions of 12.867 × 12.867 × 12.275 Å3 (measured by Er1 centers across DMTDC ligands) were formed along the c axis. The channels are occupied by the lattice DMF and H2O molecules. PLATON program calculation indicated that this 3D porous architecture has a void volume of 1663.3 Å3 (31.3% of the unit cell of 5314.7 Å3) after removing these lattice molecules (Figure 1d). Structures of {[Ln4(DMTDC)6(DMF)2]·0.5DMF·1.5H2O}n (Type III) (Ln = Er 6). Reaction of Er(NO3)3·6H2O with H2DMTDC in DMF/H2O results in the formation of type III structure, which is much more complicated than those of types I and II. Complex 6 crystallizes in the monoclinic space group C2/c. There are four crystallographically independent Er atoms, D

DOI: 10.1021/acs.inorgchem.5b02801 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry five and two half DMTDC ligands, two coordinated DMF molecules, and one H2O and two lattice DMF molecules in the asymmetric unit. As shown in Figure 2a, four Er centers exhibit two kinds of coordination spheres. Er1 and Er4 atoms are both in six-coordinated distorted octahedral geometries with six carboxyl oxygen atoms from six DMTDC ligands: O2, O3A, O6, O12B, O20, and O24 for Er1 and O4E, O8C, O16D, O19G, O22, and O23F for Er4, respectively. Er2 and Er3 atoms both adopt seven-coordinated pentagonal bipyramidal geometries. Each is surrounded by four carboxyl oxygen atoms from four DMTDC ligands and one DMF oxygen atom in the equatorial plane (O1, O9, O11B, O17, and O25 for Er2; O7C, O14, O15D, O18, and O26 for Er3) and another two carboxyl oxygen atoms at the apical positions (O5 and O13 for Er2; O10 and O21 for Er3). The Er−O bonds and O−Er−O angles are in the ranges 2.181(5)−2.390(8) Å and 72.3(2)−174.4(2)°, respectively. All DMTDC ligands adopt (κ1-κ1)-(κ1-κ1)-μ4 bisdentate coordination modes to connect Er centers, also generating 1-D infinite rod-shaped inorganic chains. These chains are further connected across DMTDC ligands to give rise to the overall 3-D framework (Figure 2b). Further investigations on the chain structures indicated that both the composition and arrangement of the chains are more complicated than those in types I and II. First, adjacent Er atoms are triply bridged by three syn−syn carboxyl groups of different DMTDC ligands, generating the {Er(μ2-CO2)3}n unit. The metal···metal distances for Er1···Er2, Er2···Er3, Er3···Er4, and Er4···Er1#2 (symmetry code: #2, 1/2+x, −1/2+y, z) are 4.4935(5), 4.7338(5), 4.6859(5), and 4.5119(5) Å, respectively, significantly shorter than those distances of doubly bridged centers in type I. Second, the triply stranded chains are interconnected across the thiophene ring of DMTDC ligands to form a double-layer structure. The helicity of the chains in adjacent layers is opposite: left−left−right (MMP)-handed in one layer and right−right−left (PPM)-handed in the other layer (Figure 2c, Supporting Information, Figure S1). As a consequence, the 2-D double layer is mesomeric. Third, these layers are linked through DMTDC ligands to generate the overall 3-D framework. It is more interesting that the rod-like chains in different layers are not parallel but extended along the [110] and [1−10] plane, respectively. To the best of our knowledge, such a MOF network is unusual, which may represent the rare example of a lanthanide network containing meso three-stranded helical chains.19 The special connectivity leads to an appealing 3-D architecture and further attractive porous structure (Figure 3). The 1-D trigonal channels in this case had different sizes within and between the double layers. Within the double layer, the dimensions are about 13.00 × 11.76 × 12.23, 12.26 × 13.16 × 12.67, and 12.67 × 12.59 × 12.18 Å3 (measured by Er centers across the DMTDC ligands, Supporting Information, Figure S2), respectively. The terminal coordination of DMF molecules is alternately situated on Er2 and Er3 along the inorganic chains, and the channels are partly blocked. Between the double layers, however, the channels with the size of 12.67 × 12.59 × 12.18 Å3 were almost fully blocked by condensed packing of the DMTDC ligands. Moreover, the special unparallel arrangement of the inorganic chains resulted in the formation of unparallel channels. The lattice DMF and H2O molecules are situated within the channels, and the solvent-accessible volume calculated by PLATON is 4295.0 Å3 (23.4% of a unit cell of 18 390.7 Å3) after removing the lattice molecules.

Figure 2. (a) Local coordination environment of complex 6. All hydrogen atoms are omitted for clarity. Symmetry codes for the generated atoms: A, 1−x, y, 1/2−z; B, −1+x, y, z; C, 2−x, 1−y, −z; D, 2−x, y, 1/2−z; E, 3/2−x, −1/2+y, 1/2−z; F, 1+x, y, z; G, 1/2+x, −1/ 2+y, z; H, 1−x, 1−y, −z; I, 1/2−x, 3/2−y, −z. Hydrogen atoms are omitted for clarity. (b) View of the 3-D network of 6, showing the arrangement of different double layers and triangular channels along the [110] (topo) and [1−10] (bottom) plane, respectively. (c) View of the 2-D double layer, showing the different extending directions of adjacent triply stranded chains with opposite helicity (bottom).

Powder XRD Diffractions and Thermogravimetric Analysis. Powder XRD diffractions of complexes 1−6 are shown in Figure S3 in the Supporting Information. All peaks E

DOI: 10.1021/acs.inorgchem.5b02801 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

steady weight loss, corresponding to the evacuation of the coordinated and lattice DMF and H2O molecules (found about 13.0%). The decomposition of the overall structures was observed from 450 °C. For type III, the removal of the coordinated and dissociative DMF and H2O molecules occurs from room temperature to 240 °C (found 8.9%). No weight loss occurred until approximately 400 °C, indicating the decomposition of the whole structure from then on. Gas Adsorption Properties. To investigate the porous structures, the sorption isotherms of the complexes were measured. Also, considering the isostructural structures, only complexes 5 and 6 were selected representatively. Fresh samples of these two complexes were first solvent-exchanged with dry methanol and chloroform and then outgassed at 150 °C for 10 h under vacuum to obtain the activated samples. The PXRD pattern of activated samples is almost consistent with that of the pristine samples, indicating that the structures are stable after the removal of the guest molecules (Supporting Information, Figure S5). As shown in Figure 4a and b, N2 adsorption isotherms at 77 K of both complexes indicate that they exhibited type I sorption behavior with a saturated uptake of 116.3 and 84.8 cm3 g−1 at 1.0 bar, respectively, typical of microporous materials. The Brunauer−Emmett−Teller (BET) surfaces calculated from the adsorption isotherm are 300.2 and 215.7 m2 g−1 for complexes 5 and 6, respectively. Almost no N2 adsorption was observed at 273 K for both complexes. The CO2 sorption measurements for the complexes at 273 and 298 K are also reversible and show the steady rise with uptake of 50.4 and 36.9 cm3 g−1 (corresponding to 2.25 and 1.65 mmol/g) for complex 5 at 1 bar, respectively. The CO2 sorption uptakes of complex 6 at these two temperatures are similar, and they display uptakes of 52.6 and 33.9 cm3 g−1 (corresponding to 2.35 and 1.52 mmol/ g) at 1 bar. To quantitatively evaluate the interactions between

Figure 3. Connolly surface along different crystallographically axes in complex 6 showing the solvent-accessible void volume.

measured were similar to those in the simulated patterns, indicating the purity of all the complexes. Thermogravimetric analysis (TGA) was performed on the complexes to investigate the thermal stability of these polymers (Supporting Information, Figure S4). Each type of isomorphous structures showed very similar TGA curves. Type I displays steady weight loss from room temperature to 230 °C, corresponding to the evacuation of the coordinated and lattice DEF molecules (found about 20.5%). The decomposition of the overall structures corresponds to the rapid weight loss from 450 °C. From room temperature to about 200 °C, type II exhibits

Figure 4. (a and b) Gas sorption isotherms of complexes 5 and 6 of N2 (77 and 273 K) and CO2 (273 and 298 K), respectively: filled shape, adsorption; open shape, desorption. (c and d) CO2 adsorption isotherms for complexes 5 and 6 with fitting by the Virial 2 model, respectively. F

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Figure 5. (a) PL spectra and (b) the intensities of complex 1 (the 5D0 → 7F2 transition) introduced into various organic solvents. (c) PL spectra and (d) intensities of complex 2 (the 5D4 → 7F5 transition) introduced into various organic solvents.

including methanol (MeOH), ethanol (EtOH), acetonitrile (CH3CN), N,N′-dimethylformamide (DMF), dichloromethane (CH2Cl2), tetrahydrofuran (THF), acetone, and nitrobenzene (NB), respectively. Before measurements, the suspension was treated with ultrasonication for 30 min and immersed in these solvents for several days. The PXRD of both complexes after being dispersed in these organic solvents indicated the frameworks were almost unchanged (Supporting Information, Figure S8). As shown in Figure 5, the suspensions of the complexes in different solvents still exhibited the characteristic peaks of corresponding lanthanide ions, while the luminescence intensities of the emulsions are largely dependent upon the solvent molecules. For complex 1, the order of the fluorescence intensities is DMF > THF > CH3CN > CH2Cl2 > MeOH > EtOH ≫ acetone > NB. For complex 2, however, the order is slightly different: DMF > CH2Cl2> CH3CN > MeOH > EtOH > THF ≫ acetone > NB. Such solvent-dependent fluorescence quenching of microporous MOFs has also been found in other compounds and reported by the Chen, Qian, Su, Sun, Bu, Zhang, and Mukherjee groups.24−27 The luminescence of lanthanide compounds mainly depends upon the sensitization of organic ligands. The physical interactions between the ligands and the solvents could affect the energy absorbed by the ligand, further resulting in the changes in the emissions of the compounds. It is notable that the emission bands for most solvents used were visible, while for NB and acetone the luminescence signals almost disappeared. That is, these two solvent molecules exhibit the most significant quenching effect. These sensing properties are of much interest since the former is an important explosive and the latter is very harmful to human. The luminescent intensities were further monitored by increasing the amount of NB or acetone in the dispersed suspension of both complexes

CO2 molecules and the framework structure, the adsorption enthalpies (Qst) were calculated by the Clausius−Clapeyron equation and the virial-type fitting method from adsorption isotherms at 273 and 298 K (Figure 4c,d and Supporting Information Figure S6). The Qst values for these complexes at zero-coverage are ca. 36.3 and 34.8 kJ mol−1, respectively, which are comparable to NaX zeolites (31−37 kJ mol−1)20a,b and hydroxyl-functionalized porous MOFs (39−47 kJ mol−1).20c These values are slightly larger than those MOFs functionalized with some open metal sites and some Ncontaining Lewis basic sites (pyrazolyl, pyridyl, and polyamine) (27−29 kJ mol−1),21 but lower than those in some trizazolateand polyamine-tethered MOFs (45−110 kJ mol−1).22 Sensing Properties of Organic Molecules and Metal Ions of Complexes 1 and 2. Due to the intriguing luminescent properties of Eu3+ and Tb3+ ions, the solid-state luminescence of complexes 1 and 2 was investigated in the visible region. Both complexes exhibited characteristic transitions of Eu3+ and Tb3+ ions (Supporting Information, Figure S7). The sharp peaks at 589, 610, 648, and 698 nm of complex 1 are characteristic of Eu-centered transitions: 5D0 → 7F1, 5D0 → 7F2, 5D0 → 7F3, and 5D0 → 7F4, respectively. The strongest transition, at 610 nm, was ascribed to 5D4 → 7F5. For complex 2, the sharp peaks at 486, 545, 584, and 619 nm are characteristic of Tb-centered transitions: 5D4 → 7F6, 5D4 → 7F5, 5 D4 → 7F4, and 5D4 → 7F3, respectively. The strongest transition, 5D4 → 7F5 , showing green-light emission, is attributed to a magnetic-dipole-induced transition.23 Considering the stable pore structures of complexes 1 and 2, the sensing properties of both complexes in diverse organic solvent suspensions and metal ions were also explored. The suspensions were prepared by introducing 2 mg of finely ground samples of the complexes into 4 mL of solvents, G

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Figure 6. (a and b) Effect on the emission spectra of complex 1 dispersed in DMF upon incremental addition of NB and acetone in DMF solution (1 mM), respectively.

Figure 7. (a) PL spectra and (b) intensities of the dispersed complex 1 (the 5D0 → 7F2 transition) in DMF with an M ion concentration of 10−3 M (excited at 276 nm). (c) PL spectra and (d) intensities of the dispersed complex 2 (the 5D4 → 7F5 transition) in DMF with an M2+ ion concentration of 10−3 M (excited at 330 nm).

ppm. The calculated quenching constants, Ksv, are 1.74 × 104 (complex 1 for NB), 7.89 × 103 (complex 1 for acetone), 1.69 × 104 (complex 2 for NB), and 1.38 × 104 (complex 2 for acetone) M−1, respectively, indicating complex 1 exhibited the most efficient quenching effect for NB (see Supporting Information Figure S10). The calculated detection limits, D, are in the range 4.2−10.1 ppm (see Supporting Information Figure S11). It is interesting that similar quenching phenomena were observed in aqueous solutions, while the quenching effects were slightly different from those in DMF, which may be also related to the solvent effects (see Supporting Information Figures S12, S13 and Table S3).

in DMF solvent to investigate the quenching effects. As one can see from Figure 6, the fluorescent intensity of complex 1 in DMF decreased with the addition of NB or acetone solvents (for other complexes see Supporting Information Figure S9). To quantitatively investigate the quenching effect, the quenching constant (Ksv) was calculated by the Stern−Volmer equation: I0/I = 1 + Ksv[M], in which the values of I0 and I are the luminescent intensity of the suspensions before and after the addition of analyte and [M] is the molar concentration of the analyte. Furthermore, the detection limit (D) was calculated based on the equation D = 3σ/k, in which σ and k are the standard deviation and slope, respectively.25e,f The linear range of both complexes for the detection of NB and acetone is 5−30 H

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Figure 8. Luminescence spectra of complexes 1 (a) and 2 (b) in DMF solutions with Cu(NO3)2 at different concentrations (excited at 267 and 330 nm, respectively).

The quenching mechanism of NB has been well investigated by both the experiments and theoretical calculation. A preliminary consensus has been formed that the quenching effect can be ascribed to the electron nature of the frameworks and highly electron deficient nature of NB.25,26 With both the rich electron-conjugated bithiophene group and electrondonating methyl groups, the organic ligand H2DMTDC could enrich the electrons of the frameworks of complexes 1 and 2. Thus, the electron could easily transfer from these electrondonating frameworks to an electron-withdrawing nitro group of NB. To explain the possible quenching mechanism of acetone, the UV−vis absorption spectroscopy of the ligand H2DMTDC and other organic molecules in n-hexane was investigated (Supporting Information, Figure S14). The solvents used, except for acetone, with absorption between 200 and 350 nm, have no absorption in such a wavelength region. Furthermore, the absorption of the organic ligand H2DMTDC could be overlaid by the absorbing band of acetone. That is to say, upon excitation, there was competition of the absorption of the light source energy between acetone molecules and the ligand between 225 and 320 nm, and the energy absorbed by the organic ligands could be transferred to acetone molecules. As a consequence, the luminescence intensity of the complex largely decreased and was even fully quenched. A similar mechanism has been reported for some other MOFs.24,27 Due to the solubility and the photoluminescence enhancement of the complexes in DMF, the effects of a variety of biologically relevant cations, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Ag+, were determined. The compound was dispersed in 0.001 M DMF solutions containing M(NO3)2 (M = Co2+, Ni2+, Cu2+, Zn2+, Cd2+) and AgNO3 for luminescent-sensing studies. As shown in Figure 7, the introduction of these metal ions caused the intensity to weaken. The order of the fluorescent intensities for both complexes was also slightly different. For complex 1, the order is Cd2+ > Zn2+ > Ag+ > Ni2+ > Co2+> Cu2+. For complex 2, however, the order is Ag+ > Cd2+ > Zn2+ > Co2+> Ni2+ > Cu2+. This phenomenon is related to the electron configuration of the metal ions. Ag+, Cd2+, and Zn2+ ions, with a closed-shell electron configuration of d10, have a trivial effect on the luminescence intensity. Co2+, Ni2+, and Cu2+ ions are paramagnetic metal ions with an unsaturated d orbital, which can induce the electron transfer, affect the energy, and increase the luminescence intensity. Significantly, both complexes show the most quenching effect on the Cu2+ ion. Measurements of the fluorescence intensities of the emulsions of complexes 1 and 2 in DMF solutions with varying concentrations of Cu2+

ions were preformed. As presented in Figure 8, the luminescence intensity of both complexes decreases obviously as the concentration of Cu2+ ions increases from 10−8 M to 10−3 M, and the fluorescence almost disappears when the concentration of Cu2+ ions is as low as 10−3 M, comparable to those of MOFs.24b,28 An attempt to fit the data with the Stern− Volmer equation did not give a linear result, indicating the coexistence of the dynamic and static quenching processes.29 Moreover, on keeping the concentration of Cu2+ ions at 10−3 M in DMF and adding other metal ions to the system, the quenching effect of the Cu2+ ions was not influenced (see Supporting Information Figures S15 and S16), further suggesting that complexes 1 and 2 could act as promising luminescence sensors for Cu2+ ions. According to previous reports, examples of porous MOFs with Lewis basic sites, such as pyridyl sites, amide sites, and anionic sulfonate sites, could affect sensing of metal ions.6−8 The possible mechanism of quenching by Cu2+ ions might be related to the interaction between the Cu2+ ions and the electron-donating organic ligand H2DMTDC within the MOFs. Recently, the Su group has reported a Zn-MOF assembled from this ligand and its selective adsorption on Cu2+ ions.12 From the experimental and calculation results, they speculated that sulfur atoms with active coordination sites on the S-containing thiophene spacers are responsible for Cu2+ ion adsorption, and the selective adsorption ability for different metal ions could be attributed to different binding energies between sulfur atoms and metal ions. DFT calculations were also performed on our complexes (Supporting Information, Figure S18). Similar to the above report, based on the consideration of rational length and orientation of a covalent bond, two possible adsorption sites were also put forward, namely, S···M···S and S···M···O chelated coordination sites. The results showed that the energy is slightly lower when the Cu2+ ion is bonded to the S···O chelated coordination site, consistent with the reported example. The color change of both complexes from white to blue after immersing these complexes in Cu(NO3)2 solutions of DMF also indicated Cu2+ ions may have been absorbed by the MOFs (Supporting Information, Figure S19). Thus, the quenching of the luminescent intensity in our complexes may result from the fact that Cu2+ ions increase the antenna efficiency of the organic linkers, consequently diminishing the f−f transitions of Eu3+ or Tb3+ ions and further leading to the decrease of intramolecular energy transfer from the ligands to Ln3+ ions.30 I

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Figure 9. (a) Temperature dependence of the χMT values for complexes 3 and 4 with an applied field of 100 Oe. (b) M−H/T curve of complex 4.

Figure 10. Temperature dependence of the in-phase (χ′) and out-of-phase (χ″) ac susceptibilities at different frequencies for complex 4 with an oscillation of zero (a) and 2 kOe (b) dc field, respectively.

Magnetic Properties of Complexes 3 and 4. It is well documented that many lanthanide complexes may exhibit interesting single-molecule-magnet (SMM) and single-chainmagnet (SCMs) character, especially those containing Gd3+ and Dy3+ ions. The magnetic interactions of complexes 3 and 4 were investigated. Direct-current (dc) SQUID magnetometry was conducted at an applied field of 2000 Oe and in the temperature range 1.8−300 K. The temperature-dependent χMT values of these complexes are shown in Figure 9a. The χMT value of complex 3 at 300 K is 8.06 cm3·K·mol−1, slightly larger than the theoretical value of 7.73 cm3·K·mol−1 for one Gd3+ ion with the ground state S = 7/2 and g = 2. The value remains roughly constant until about 3.0 K, and then a steady decrease is clearly visible, reaching 7.50 cm3·K·mol−1 at 1.8 K. For complex 4, the χMT product at room temperature of 14.10 cm3·K·mol−1 is in good agreement with the expected value of 14.17 cm3·K·mol−1 for one Dy3+ ion (J = 15/2 and g = 4/3). As the temperature decreased, the χMT value decreases smoothly to 12.39 cm3·mol−1·K at 10.0 K, and then the value increases and reaches 13.33 cm3·K·mol−1 at 1.8 K. The decrease of the χMT value may be related with the following factors: the thermal depopulation of the Stark sublevels; the significant magnetic anisotropy of Ln3+ ions; and possible weak antiferromagnetic interactions between the Ln3+ ions across the organic ligands.31 For complex 4, the increase of the χMT value below 10 K may be caused by weak ferromagnetic interactions between the Ln3+ ions.

The dc magnetization data of the complexes were obtained in the ranges of magnetic field from 1 to 7 T and temperature from 10 to 1.8 K, respectively. The field dependence of the magnetization of complexes 3 and 4 demonstrates a gradual increase of the magnetization at low fields and reaching saturation at 7 T (Supporting Information, Figure S20). The reduced magnetization (M/NμB−H/T) curves of complex 4 exhibited significant nonsuperimposition, suggesting the presence of magnetic anisotropy and the lack of a well-defined ground state (Figure 9b). The ac susceptibility measurements of these complexes were performed in a zero-field and 2000 Oe dc field with a 2.0 Oe ac field oscillating at frequencies in the range 1−900 Hz and in the temperature range 1.8−10 K, respectively. Almost no out-of-phase susceptibility was observed for complex 3 in a zero field (Supporting Information, Figure S21a). Even when a static dc field of 2000 Oe was applied, only slight frequency-dependent out-of-phase signals were observed (Supporting Information, Figure S21b). For complex 4, however, obvious frequency-dependent out-ofphase signals indicated the onset of slow magnetization relaxation (Figure 10a). When in a zero dc field, the peaks in the out-of-phase susceptibility signals are not present at the operating limits of our SQUID instrument. Such a phenomenon is likely due to the fast quantum tunneling of the magnetization that is too fast to observe, which was also observed in the reported Gd3+- and Dy3+-containing system.32 When a static dc field of 2 kOe was applied, both the in-phase J

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Figure 11. (a) Plots of M versus H/T for complex 3 in the field range 0−70 kOe at 2−14 K. (b) −ΔSm calculated by using the magnetization data of complex 1 at different fields and temperatures.

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χM′ and out-of-phase χM″ signals of complex 2 display good peak shapes, which are strongly dependent upon the frequency (Figure 10b). This difference indicates the quantum tunneling was quenched by a dc field. These behaviors are the essential characteristic of slow magnetization relaxation. The relaxation data for a 2 kOe field exhibits a linear correlation complying with the Arrhenius law [ln(2πf) = ln τ0 − Δ/kBT] at frequencies above 10 Hz (Supporting Information, Figure S22). The energy barrier Δ/kB = 48.29 K and pre-exponential factor τ0 = 4.38 × 10−7 s can be obtained by fitting the plot of ln(2πf) versus 1/Tp. These two parameters are consistent with a superparamagnetic-like character of the relaxation dynamics.33 Considering the weaker antiferromagnetic interaction, the large-spin ground state, and relatively low molecular mass, the magnetocaloric effect of complex 3 was investigated. The isothermal magnetic entropy changes −ΔSm were obtained by applying the Maxwell equation (ΔSm(T)ΔH = ∫ [∂M(T,H)/ ∂T]HdH) from the experimental magnetization data. As shown in Figure 11, with an increasing magnetic field and a decreasing temperature, the observed −ΔSm values increased, reaching a maximum value of 24.3 J·kg−1·K−1 at 3.0 K with an applied field change of 70 kOe. The expected maximum −ΔSm value can be calculated from the equation −ΔSm = NGdR ln(2SGd + 1), where NGd is the GdIII percent per mole. Obviously, the experimental maximum of −ΔSm is smaller than the theoretical one, which can be ascribed to the presence of an antiferromagnetic exchange among the GdIII ions.34,35

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02801. PXRD, TGA, UV−vis, luminescent, and magnetic data as well as theoretical calculations (PDF) X-ray crystallographic data for CCDC 1424432 (CIF) X-ray crystallographic data for CCDC 1424433 (CIF) X-ray crystallographic data for CCDC 1424434 (CIF) X-ray crystallographic data for CCDC 1424435 (CIF) X-ray crystallographic data for CCDC 1424436 (CIF) X-ray crystallographic data for CCDC 1424437 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel (S. Wang): +86-0635-8230669. E-mail: wangsuna@lcu. edu.cn. *Tel (J. Bai): +86-025-83593384. E-mail: [email protected]. cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful to the National Natural Science Foundation of China (Nos. 21571092, 21403102, and 21203084), the Natural Science Foundation of Shandong Province (No. ZR2012BQ023), and the University Scientific Research Development Plan of the Education Department of Shandong Province (No. J14LC10).

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CONCLUSION Several microporous lanthanide-based MOFs from an Scontaining carboxylate ligand were obtained. The 3-D noninterpenetrating structures possess triangular channels constructed by rod-shaped lanthanide−carboxylate chains. The channels are all tailed by thiophene groups with S atoms pointing into the interior. These pores display moderate adsorption properties toward N2 and CO2. Within Eu and Tb complexes, the free S atoms act as Lewis basic sites to sense organic molecules of nitrobenzene, acetone, and especially Cu2+ ions in both DMF and aqueous solutions. The Dy complex exhibits slow magnetic relaxation. The Gd complex shows a significant magnetocaloric effect. The results demonstrated that immobilization of thiophene groups as Lewis basic sites in MOFs, together with characteristic lanthanide metals, may realize the functionalization of these special porous materials. Further investigations are ongoing.

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DOI: 10.1021/acs.inorgchem.5b02801 Inorg. Chem. XXXX, XXX, XXX−XXX