Synthesis, Crystal Structure, and Luminescent Properties of Novel Zinc

Sep 14, 2017 - Department of Natural Sciences, Novosibirsk State University, Pirogova Str. 2, 630090 Novosibirsk, Russia. ∥ Department of Chemical E...
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Synthesis, Crystal Structure, and Luminescent Properties of Novel Zinc Metal−Organic Frameworks Based on 1,3-Bis(1,2,4-triazol-1yl)propane Evgeny Yu. Semitut,†,‡ Taisiya S. Sukhikh,‡,§ Evgeny Yu. Filatov,‡,§ Galina A. Anosova,∥ Alexey A. Ryadun,‡ Konstantin A. Kovalenko,‡,§ and Andrei S. Potapov*,† †

Department of Biotechnology and Organic Chemistry, National Research Tomsk Polytechnic University, 30 Lenin Ave., 634050 Tomsk, Russia ‡ Nikolaev Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of Sciences, Lavrentieva Ave., 3, 630090 Novosibirsk, Russia § Department of Natural Sciences, Novosibirsk State University, Pirogova Str. 2, 630090 Novosibirsk, Russia ∥ Department of Chemical Engineering, Altai State Technical University, 46 Lenin Ave., 656038 Barnaul, Russia S Supporting Information *

ABSTRACT: Three new zinc three-dimensional coordination polymers with flexible 1,3-bis(1,2,4-triazol-1-yl)propane ligand were synthesized. The crystal structures of synthesized compounds were determined, and structural peculiarities are discussed. Coordination compounds with composition [Zn(btrp)(bdc)]·nDMF are interpenetrated frameworks, while metal−organic framework (MOF) [Zn3(btrp)(bdc)3]·nDMF is not. Thermal stability and luminescent properties of synthesized compounds have been investigated. The possibility of usage of such compounds as sensitive materials for some aromatic compounds are explored, and it was shown that luminescence of coordination polymers is completely quenched in the presence of nitrobenzene. Sorption properties of synthesized MOFs toward nitrogen, carbon dioxide, and hydrogen were evaluated.

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MOF applications are optical sensors,32−38 electronics,39−41 magnetic devices,42,43 biomedical imaging,31,44 and high-energy materials.45,46 Functional properties of MOFs can be tailored almost at will by a careful choice of organic ligands, which can be rigid or flexible.47 The use of flexible ligands often leads to increased disordering, which prevents the formation of crystalline MOF material. However, conformationally mobile molecules can meet the geometrical demands of different metal coordination spheres and form structures otherwise inaccessible with rigid ligands.48 Bis(azol-1-yl)alkanes are neutral nitrogen ligands that are often used to prepare coordination polymers.49−56 Bis(1,2,4triazol-1-yl)alkanes are probably the most widely used among other similar azole derivatives. The system of zinc−carboxylic acid−bis(1,2,4-triazol-1-yl)alkane coligand has been explored by several authors; however, in most cases only onedimensional (1D), 2D, or interpenetrated 3D structures have been prepared,57−60 which prevented the studies of their sorption properties. Therefore, the aim of our work was to explore the system zinc−terephthalic acid−1,3-bis(triazol-1-

INTRODUCTION Metal−organic frameworks (MOFs) attract great interest of many researchers because of the spectacular range of their potential applications.1−3 MOFs are constructed from metal ions and organic ligands, which are joined together by covalent bonds into an infinite porous three-dimensional structure.4 The first examples of MOFs were copper(II)−4,4′-bipyridine,5 cobalt(II)−1,3,5-benzenetricarboxylate,6 and zinc−terephthalate7 coordination polymers prepared by Omar Yaghi in 1990s. Since then, the area of MOF chemistry has been experiencing a tremendous growth both in the number of structures reported and areas of potential applications explored. The main area of MOF application−gas sorption and separation arises from their inherent porosity.8 Two of the most promising fields in this area are methane9,10 and hydrogen11−13 fuel storage and carbon capture and storage14,15 both of these fields are crucial for solving the global problem of climate change.16 Some MOFs with good gas storage capacity, such as HKUST-1,17 ZIF-8,18 or UiO-6619 are now produced on industrial scale.20 Other areas of MOF applications now include catalysis,21−23 including organocatalytic reactions inside the pores,24 biomimetic25,26 and enantioselective transformations,27 enantiomer resolution,28 immobilization of biomolecules,29 antimicrobial materials,30 and drug delivery.31 Among emerging © 2017 American Chemical Society

Received: August 13, 2017 Revised: September 13, 2017 Published: September 14, 2017 5559

DOI: 10.1021/acs.cgd.7b01133 Cryst. Growth Des. 2017, 17, 5559−5567

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Table 1. Crystallographic Data for Compounds 1−3 compound empirical formula formula weight temperature (K) crystal system space group a [Å] b [Å] c [Å] α [deg] β [deg] γ [deg] volume [Å3] Z density (calcd.) [g cm−3] Abs. coefficient [mm−1] F(000) crystal size [mm3] 2θmax [deg] index range

reflections collected independent reflections reflections, I ≥ 2σ(I) completness to θ [%] parameters final R indices [I > 2σ(I)] R indices (all data) GoF residual electron density (min/max, e/Å3)

1

2

3

C30H28N12O8Zn2·3(C3H7NO) 1034.67 296(2) orthorhombic Pbca 19.4772(14) 19.1029(12) 25.1966(17) 90 90 90 9374.9(11) 8 1.466 1.097 4288 0.4 × 0.4 × 0.25 51.54 −23 ≤ h ≤ 22 −23 ≤ k ≤ 18 −30 ≤ l ≤ 30 102130 8958 [R(int) = 0.0428] 7504 99.9 610 R1 = 0.0532 wR2 = 0.1397 R1 = 0.0636 wR2 = 0.1464 1.095 −0.564/0.701

C31H22N6O12Zn3·n(C3H7NO) 1010.83 150(2) triclinic P1̅ 9.5833(4) 18.4856(8) 19.0300(9) 118.0590(10) 92.909(2) 92.560(2) 2962.1(2) 2 1.133 1.258 1028 0.45 × 0.2 × 0.08 51.54 −11 ≤ h ≤ 10 −21 ≤ k ≤ 22 −23 ≤ l ≤ 23 35148 11344 [R(int) = 0.0342] 9348 99.1 529 R1 = 0.0512 wR2 = 0.1503 R1 = 0.0600 wR2 = 0.1550 1.048 −0.952/1.629

C15H14N6O4Zn·n(C3H7NO) 407.69 150(2) triclinic P1̅ 11.731(2) 15.182(2) 17.902(3) 90.989(7) 108.633(4) 90.057(5) 3020.7(9) 4 0.896 0.833 832 0.2 × 0.2 × 0.1 51.00 −14 ≤ h ≤ 14 −18 ≤ k ≤ 18 −21 ≤ l ≤ 21 23262 11402 [R(int) = 0.0375] 7285 98.8 399 R1 = 0.1006 wR2 = 0.3161 R1 = 0.1266 wR2 = 0.3315 1.196 −0.836/3.050

yl)propane (btrp) more deeply and find the conditions for the formation of porous coordination polymers.

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temperature. Large prismatic colorless crystals formed on the bottom of vial, and they were filtered and washed twice with 10 mL of DMF and dried in vacuum. The yield was 720 mg (47%). IR bands, cm−1: 3123, 2928, 1958, 1669, 1603, 1530, 1499, 1441, 1387, 1348, 1279, 1210, 1132, 1090, 997, 966, 889, 828, 750, 677, 657. Elemental analysis: found, % C 45.3, H 4.6, N 20.3; calculated ([Zn(btrp)(bdc)]· 1.5DMF), % C 45.3, H 4.6, N 20.5. Synthesis of Compound [Zn3(bdc)3(btrp)]·nDMF (2). To the mixture of 148.8 mg (0.5 mmol) of Zn(NO3)2·6H2O, 83.0 mg (0.5 mmol) of H2bdc, and 89.1 mg (0.5 mmol) of btrp in a glass vial, 20.0 mL of DMF was added. The mixture was stirred for several minutes at room temperature until complete dissolution of all reagents. The vial was then placed in the oven at 95 °C for 44 h. After this time, the vial was removed from the oven and cooled to room temperature. Colorless crystals formed on the bottom and the walls of the vial, and they were washed twice with 20 mL of DMF and dried in vacuum. The yield was 72.0 mg (37%). IR bands, cm−1: 3121, 2928, 1663,1601, 1505, 1385 (broad), 1281, 1130, 1100, 1019, 999, 887, 826, 749, 675 (broad). Elemental analysis: found, % C 44.3, H 4.7, N 12.1; calculated ([Zn3(btrp)(bdc)3]·nDMF, n = 1.5), % C 44.6, H 4.3, N 12.1. Synthesis of Compound [Zn(btrp)(bdc)]·nDMF (3). To the mixture of 30.0 mg (0.1 mmol) of Zn(NO3)2·6H2O, 17.0 mg (0.1 mmol) of H2bdc, and 19.6 mg (0.11 mmol) of btrp in a glass vial, 4.0 mL of DMF were added. The mixture was stirred for several minutes at room temperature until complete dissolution of all reagents. Then the vial was placed in the oven at 105 °C for 42 h. After this time, colorless crystals of compound 1 (according to XRD analysis) formed on the bottom and walls of the vial, and they were filtered and washed with DMF. The filtered mother liquor was left for one month at room temperature. During this period, small crystals formed on the bottom

EXPERIMENTAL SECTION

Starting Materials and Experimental Procedures. The starting reagents used for synthesis of coordination compounds, Zn(NO3)2· 6H2O (chemical grade), dimethylformamide (analytical grade), and terephthalic acid (analytical grade) were used as received. Synthesis of 1,3-Bis(1,2,4-triazol-1-yl)propane (btrp). A suspension of 3.45 g (50 mmol) of 1,2,4-triazole and 5.6 g (100 mmol) of powdered KOH in 20 mL of DMSO was vigorously stirred at 80 °C during 30 min. The reaction flask was then immersed into a cold water bath, and after cooling to room temperature, 5.05 g (25 mmol) of 1,3-dibromopropane in 10 mL of DMSO was added dropwise during 30 min. After the addition was completed, the reaction mixture was stirred overnight at 80 °C, then it was quenched with 200 mL of water and extracted with 1-butanol (5 × 20 mL), and the extract was washed with water (2 × 10 mL). Evaporation of solvents from the extract on a rotary evaporator and recrystallization from isopropyl alcohol gave 4.05 g (91%) of the product as colorless crystals, mp 122−123 °C. NMR 1H (CDCI3) δ ppm: 2.37 (q, 2H, CH2CH2CH2, J 6.3 Hz), 4.12 (t, 4H, TrCH2, J 6.3 Hz), 7.87 (s, 2H, H3-Tr), 8.06 (s, 2H, H5-Tr). NMR 13C (CDCI3) δ ppm: 29.7 (CH2CH2CH2), 45.7 (TrCH2), 143.2 (Tr-C3), 152.1 (Tr-C5). Synthesis of Compound [Zn(btrp)(bdc)]·1.5DMF (1). Solution of 534.6 mg (3 mmol) of btrp and 498.4 mg (3 mmol) of H2bdc in 7.5 mL of DMF was placed in a glass vial, and 3.0 mL of Zn(NO3)2·6H2O (1.0M) solution in DMF was added. The mixture was stirred for several minutes at room temperature and then placed into an oven at 95 °C. After 20 h of heating, the vial was cooled to the room 5560

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of the vial, and they were filtered and washed twice with 2 mL of DMF. The yield was 7 mg (10%). Elemental analysis: found, % C 43.8, H 3.8, N 19.9; calculated ([Zn(btrp)(bdc)]·nDMF, n = 0), % C 44.2, H 3.5, N 20.6. Analytical Methods. NMR spectra were recorded on a Bruker AV300 instrument operating at 300 MHz for 1H and 75 MHz for 13C. Solvent residual peaks were used as an internal standard. Elemental analyses were carried out on Eurovector EuroEA 3000 analyzer. Infrared (IR) spectra of solid samples as KBr pellets were recorded on a FT-801 spectrometer (4000−550 cm−1). Polycrystalline samples were studied in the 2θ range og 5−60° on a DRON RM4 powder diffractometer equipped with a CuKα source (λ = 1.5418 Å) and graphite monochromator for the diffracted beam. Indexing of the diffraction patterns was done using data for compounds reported in the JCPDS-ICDD database.61 Thermal stability of coordination polymers was studied in inert (He) atmosphere. Thermogravimetric measurements were carried out on a NETZSCH thermobalance TG 209 F1 Iris. Open Al2O3 crucibles were used (loads 10−20 mg, heating rate 10 K·min−1). Room-temperature excitation and emission spectra were recorded with a Horiba Jobin Yvon Fluorolog 3 photoluminescence spectrometer equipped with a 450 W ozone-free Xe-lamp, cooled PC177CE-010 photon detection module with a PMT R2658, and double grating excitation and emission monochromators. Quantum yields were determined using a Quanta-φ integrating sphere. Excitation and emission spectra were corrected for source intensity (lamp and grating) and emission spectral response (detector and grating) by standard correction curves. For measurements, powdered samples were placed between two nonfluorescent quartz plates. X-ray Structure Determination. Single-crystal XRD data for the complexes were collected by a Bruker Apex DUO diffractometer equipped with a 4K CCD area detector at 150(2) K for [Zn3(bdc)3(btrp)]·nDMF (2) and [Zn(bdc)(btrp)]·nDMF (3) and 298(2) K for [Zn(bdc)(btrp)]·1.5DMF (1) using the graphitemonochromated MoKα radiation (λ = 0.71073 Å) (Table 1). The φand ω-scan techniques were employed to measure intensities. Absorption corrections were applied with the use of the SADABS program.62 The crystal structures were solved by direct methods and refined by full-matrix least-squares techniques with the use of the SHELXTL package.63 Atomic thermal displacement parameters for non-hydrogen atoms were refined anisotropically. The positions of hydrogen atoms were calculated corresponding to their geometrical conditions and refined using the riding model. DFIX, DANG, and FLAT restraints and EADP constraints were applied to atoms of disordered units where needed. In compounds [Zn3(bdc)3(btrp)]· nDMF (2) and [Zn(bdc)(btrp)]·nDMF (3), solvent molecules displayed an unresolvable disorder. They were removed from the refinement using the “SQUEEZE” option available in the PLATON program suite.64 Solvent accessible volume was estimated for the disordered structure ignoring noninteger occupancies of the atoms of the ligands. Thus, the real free volume of [Zn3(bdc)3(btrp)]·nDMF (2) and [Zn(bdc)(btrp)]·nDMF (3) should be higher than the reported one. For 2 and 3, n was estimated to be 6 and 4, respectively. Sorption Studies. Analysis of textural properties was performed by the nitrogen adsorption technique at 77 K using a Quantochrome’s Autosorb iQ instrument. Initially the compound was activated in a dynamic vacuum using the standard “outgas” option of the equipment in the most benign conditions: using very slow heating (1 K/min) up to only 50 °C, which is wittingly below the decomposition temperature. Nitrogen adsorption−desorption isotherms at 77 K were measured within the range of relative pressures of 10−3 to 0.998. Carbon dioxide adsorption−desorption isotherms at 195 K were measured within the range of pressures of 1 to 800 Torr. The specific surface area was calculated from the data obtained based on the conventional BET and Langmuir models. External surfaces were valued using the t-plot approach. DFT calculations were tried for pore size distribution analysis. Liquid nitrogen bath, ice bath, or cryostat CryCooler was used to adjust temperatures at 77, 273, or 195 K.

Article

RESULTS AND DISCUSSION

Synthesis of btrp Ligand and Coordination Polymers. The ligand was prepared using our newly developed procedure based on the reaction between 1,2,4-triazole and 1,3dibromopropane in a superbasic medium DMSO−KOH (Scheme 1). Scheme 1. Synthesis of btrp Ligand

Syntheses of coordination polymers by the reaction of zinc nitrate, btrp, and terephthalic acid (H2bdc) were carried out under solvothermal conditions in dimethylformamide (DMF). The conditions (metal−ligand−acid ratio, temperature in the range of 90−105 °C, duration of heating) were optimized to achieve the formation of crystalline product. As a result, three new coordination polymers were synthesized and characterized by single-crystal structure analysis, thermal analysis, luminescence, and sorption measurements. When equimolar amounts of zinc nitrate, btrp ,and H2bdc were heated in DMF solution (Zn2+ concentration 0.3 M) at 95 °C for 20 h, crystals of coordination polymer 1 of composition [Zn(btrp)(bdc)]·1.5DMF were obtained. After filtration of the crystals and prolonged standing of the mother liquor at room temperature, crystals of another coordination polymer [Zn(btrp)(bdc)]·nDMF (3) were isolated. At lower Zn 2+ concentrations (0.025 M), crystals of yet another coordination polymer [Zn3(bdc)3(btrp)]·4DMF (2) deposited at the same temperature. Varying the concentration of the solution did not allow us to find the conditions of selective formation of compound 3. It should be noted that the elemental composition and IR spectral data for compounds 1 and 3 are identical and that only XRD analysis allows to distinguish them reliably. The purity of all experimental samples was thus confirmed by comparison of experimental diffraction patterns with theoretical patterns derived from single-crystal diffraction analysis data (Figures S1 and S2). Crystal Structures of Coordination Polymers. All three coordination polymers formed single crystals suitable for X-ray structural analysis. Crystallographic parameters and details of the diffraction experiment are given in Table 1. Crystal Structure of Polymer [Zn3(bdc)3(btrp)]·nDMF (2). Complex [Zn3(bdc)3(btrp)]·nDMF (2) has a 3D network. The structure reveals two crystallographically independent trinuclear linear {Zn3} units of similar topology. This unit is a relatively common building block of carboxylate MOFs, particularly those containing bdc2− (see, for example, refs 65 and 66). In [Zn3(bdc)3(btrp)]·nDMF, central Zn atoms lie on an inversion center and adopt the octahedral environment of O atoms of bdc2− (Figure 1a). The coordination geometry of outer Zn atoms can be considered as tetrahedral consisting of three oxygen and one nitrogen atoms (with average Zn−O/N distance of 1.97 Å) distorted by inclusion of the fifth oxygen atom with significantly elongated Zn−O distance (2.63 and 2.54 Å). Central and outer Zn atoms are connected via two bridged bidentate μ2-RCOO-O,O′ groups and one bridged monodentate μ2-RCOO-O group with Zn···Zn distance of 3.23 Å. Each {Zn3} unit is linked to six neighboring ones via bdc 5561

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Figure 1. (a) Thermal ellipsoid plot of {Zn3} unit of [Zn3(bdc)3(btrp)]·nDMF (2) showing 50% probability ellipsoids. Weak Zn···O interactions are shown as dashed lines. (b) Representation of {Zn3(bdc)3} layer of [Zn3(bdc)3(btrp)]·nDMF. Hydrogen atoms are omitted for clarity.

Crystal Structure of Polymer [Zn(bdc)(btrp)]·nDMF (3). The complex [Zn(bdc)(btrp)]·nDMF (3) is a 3D polymer with two crystallographically independent metal centers in the structure (Figure S5). Both Zn atoms lie in a general position and have distorted tetrahedral environment of two nitrogen and two oxygen atoms of btrp and bdc2− ligands. Average Zn−N and Zn−O bond lengths are ca. 2.01 and 1.95 Å, correspondingly. Two additional interactions of Zn with remaining oxygen atoms of bdc2− ligands are observed with significantly elongated Zn−O distances of ca. 2.83 Å. Overall, both btrp and bdc2− ligands are monodentate bridging. Btrp ligands connect Zn atoms to form chains that are linked in pairs by bdc2− ligands into ladder-bended ribbons (Figure 3a). The latter are linked via additional bdc2− ligands forming 3D framework (Figure 3b). The whole structure comprises three interpenetrated frameworks arranged to fill octagonal voids (Figure 4a); each rectangle of the ribbon passes through two others (Figure 4b). The structure of [Zn(bdc)(btrp)]·nDMF (3) is very flexible, resulting in disordering of all atoms of both btrp ligands (with the occupancies of 65/35% and 62/38%) and one bdc2− (with the occupancy of 52/48%) (Figure S6). Despite the interpenetration, the structure contains channels of large volume of ca. 50%, filled by DMF molecules. Crystal Structure of Polymer [Zn(bdc)(btrp)]·1.5DMF (1). The complex [Zn(bdc)(btrp)]·1.5DMF (1) is also 3D polymeric (Figure S7). Two crystallographically independent Zn atoms lying in general positions have the same distorted tetrahedral environment as that in [Zn(bdc)(btrp)]·nDMF (3) with average Zn−N and Zn−O bond lengths of ca. 2.02 and

ligands to form layers (Figure 1b). The neighboring layers are shifted with independent {Zn3} units stacked one over the other (Figure S3). Bidentate btrp ligands are located along the Zn···Zn line connecting the layers into 3D net (Figure 2). It

Figure 2. Representation of {Zn3(bdc)3} layers of [Zn3(bdc)3(btrp)]· nDMF (2) linked by btrp ligands. Hydrogen atoms are omitted for clarity.

should be noted that due to the high flexibility of the btrp ligand one of the triazole rings is disordered by two positions with the occupancy of 59 and 41% (Figure S4). Two crystallographically independent solvent DMF molecules can be located in the structure, while the rest of the free volume of the MOF channels of ca. 35% is filled by highly disordered DMF molecules. The total free volume in the absence of solvent molecules is estimated to be ca. 53%.

Figure 3. Fragment of [Zn(bdc)(btrp)]·nDMF (3) showing structure of 3D framework. Zn atoms connected via bridging ligands are linked by bold blue lines. Rectangles of the ladder-bended ribbons are filled yellow and blue. 5562

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Figure 6. Relative arrangement of four interpenetrated frameworks of [Zn(bdc)(btrp)]·1.5DMF (1) colored blue, green, red, and brown. Figure 4. Relative arrangement of three interpenetrated frameworks of [Zn(bdc)(btrp)]·nDMF (3) colored with blue, green, and red.

1.96 Å. Two additional interactions of Zn with remaining oxygen atoms of bdc2− ligands are also observed with Zn−O distances of ca. 2.75 Å. However, topology of the framework [Zn(bdc)(btrp)]·1.5DMF (1) is different. btrp ligands connect Zn atoms to form chains as those in [Zn(bdc)(btrp)]·nDMF (3), but these chains are linked by bdc2− ligands into a diamond-like net with hexagon rings (Figure 5). On the whole, four-fold interpenetration of the frameworks is observed (Figure 6); each ring passes through four others. Contrary to [Zn3(bdc)3(btrp)]·nDMF (2) and [Zn(bdc)(btrp)]·nDMF (3), the structure of [Zn(bdc)(btrp)]·1.5DMF (1) does not contain disordered ligands. Three crystallographically independent DMF molecules are located, which completely fill narrow channels in the structure; free volume in the absence of solvent molecules is estimated to be ca. 37%. Despite the fact that the structure of MOF 1 is interpenetrated and have narrow channels, it is potentially capable of demonstrating sorption properties and has a stable network without guest molecules at ambient conditions. Thermal and XRD Analyses. The analysis of thermal properties of synthesized compounds revealed that the process of removing guest molecules from coordination polymers (1) and (2) has some significant differences. The main feature for compound (1) is that it has well-defined steps with plateau at 95 and 165 °C on TG curves assigned for compositions [Zn(btrp)[bdc)]·nDMF (where n = 1.0 and 0) (Figure 7). It indicates the possibility to obtain stable intermediate products for 1. Thermolysis of 2 proceeds in more complex way

Figure 7. Curves of thermal analysis in helium atmosphere at 10 K/ min heating rate: 1, TG for MOF 1; 2, TG for MOF 2; 3, DTG for MOF 1; 4, DTG for MOF 2.

presumably having simultaneous degradation of the framework and loss of guest molecules. Considering this issue in more detail, we can describe thermal properties of synthesized compounds as follows. Thermal decomposition of MOF 1 can be divided into three stages (Figure 7). The first stage from 20 to 170 °C corresponds to the loss of 1.5 DMF molecules, which is in good agreement with elemental analysis and accompanies with two steps on the TG curve. The thermolysis of 1 up to 170 °C results in compound [Zn(btrp)[bdc)]·nDMF (n = 0) (Figure

Figure 5. Fragment of [Zn(bdc)(btrp)]·1.5DMF (1) showing the structure of the 3D framework. Zinc atoms connected via bridging ligands are linked by bold blue lines. Two octahedral rings are filled blue. 5563

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Figure 8. Emission (λex = 330 nm) (a) and excitation (b) spectra: 1, btrp ligand; 2, MOF 1; 3, MOF 2; 4, activated MOF 1.

Figure 9. Emission (λex = 330 nm) (a) and excitation (b) spectra of activated MOF 1 with different guests: 1 , DMF; 2, o-xylene; 3, p-xylene; 4, toluene; 5, aniline; 6, benzene; 7, nitrobenzene (λex = 330 nm, room temperature).

(∼10−3 mbar) for 5−10 min at room temperature and filled with argon (method 1). Depending on evacuation time, a different number of DMF molecules in MOF 1 can be located: from 0 to 0.7 per formula unit. In addition, compound [Zn(btrp)[bdc)]·nDMF (n = 0.7) can be obtained by thermal activation under isothermal conditions at 70 °C, 7 h (method 2), which is in good agreement with TG data. Activation for a few hours at ∼10−5 mbar (method 3) gave the product with different crystal structure compared to initial compounds. This is supposed to be a result of total removing of guest molecules and rearrangement of crystal structure under high vacuum. Thus, it indicates the essential role of guest molecules on the stability of the crystal structure. Method 2 is the fastest and the easiest preparation of the activated complex of definite composition. In order to avoid the collapsing of framework, we have conducted activation by method 1 with total loss of DMF. Then activated MOF 1 was treated with a number of organic solvents (o-xylene, p-xylene, benzene, aniline, toluene, and nitrobenzene), and a series of guest-impregnated MOFs was prepared. The changes in structure for impregnated samples are not significant (Figure S10). The samples prepared by this method were used for luminescence measurements. The experiments with direct exchanging of DMF molecules (method 4) to the o- and p-xylenes without preliminary treatment with dichloromethane and vacuum gave us opportunity to change guest molecules without significant structure changes. However, the degree of substitution of DMF to the xylenes was low (only 10−20% of DMF exchange to the o- and p-xylenes, confirmed by CHN and TG measurements).

S8a) , and thermolysis at isothermal conditions at 70 °C during 1 h (Figure S8b) gives us compound [Zn(btrp)[bdc)]·nDMF (n = 1.0). The XRD analysis of these samples have shown that the crystal structures of [Zn(btrp)[bdc)]·nDMF (n = 1.0 and 0) did not undergo significant changes relative to MOF 1 (Figure S9). However, the complex with n = 0 transformed quickly to another compound with different structure. The second stage from 225 to 520 °C corresponds to polymer degradation with the formation of zinc- and amorphous carboncontaining products. Further thermolysis up to 800 °C leads to almost complete destruction of these products. The final products at 800 °C are nanoparticles of zinc oxide (with crystallite size 22 ± 10 nm) and traces of amorphous carbon, which is confirmed by elemental analysis. The steps of thermal decomposition of MOF 2 (Figure 7) are not as well-defined as those for MOF 1, and no stable intermediate products were observed. The powder XRD analysis of MOF 1 kept under DMF in a closed vial for a prolonged period of time (several months) showed that the coordination polymer does not undergo any noticeable changes compared to freshly synthesized compound. Comparison of XRD patterns of fresh samples and samples stored in air (Figure S1) has shown that the structure of MOF 1 is stable for at least 10 days. For further experiments on luminescent and gas sorption properties, we performed activation of synthesized frameworks by removing guest molecules. Activation of MOF 1 was achieved by the exchange of DMF molecules for easily removable CH2Cl2 (for 3 days) and drying under vacuum 5564

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Article

Luminescence Properties. The luminescence and excitation spectra of MOFs 1 and 2 are shown in Figure 8. Upon excitation at 330 nm, the photoluminescence spectra of these MOFs demonstrate wide bands with maxima at λem = 440 nm and shoulders near λem = 390 nm. The ligand (btrp) reveals a single maximum near 410 nm. The excitation bands of coordination polymers are relatively narrow, while for btrp ligand this band is very wide. Activated by method 1 (see above), MOF 1 and MOF 1 impregnated with different guests have similar emission band wavelengths (Figure 9). The highest quantum yield (up to 46%) was observed for the activated MOF 1. Inclusion of guest molecules led to significant decrease of quantum yield, especially in the case of nitrobenzene, which quenched the luminescence completely (Table 2). A bathochromic shift of excitation bands was observed for aniline and nitrobenzene guests (Figure 9).

does not have significant porosity. Nitrogen sorption isotherm (Figure S11a) belongs to type III according to the official IUPAC classification,69 which is common for macroporous compounds with weak adsorbate−adsorbent interactions. Preparation of MOF 1 under other conditions leads to materials and porosity of which was established by carbon dioxide adsorption at 195 K. The calculated values of the specific surface area based on Langmuir, BET, and DFT approaches, and the parameters of the porous structure are given in Table 3. The full guest removal was achieved by deep activation (method 3, soaking several times in CH2Cl2 then prolonged evacuation), while half of guest molecules could be removed using a softer activation procedure in helium flow at moderate temperature (70 °C) by method 2. Thus, analysis of sorption properties confirmed that the frameworks are not stable at prolonged evacuation and that collapsing of the structures occurs after complete removal of guest molecules, while the structure remains intact after removing at least up to half of guest molecules. It is possible to control the amount of removed guest molecules from pores of MOF 1 by changing the annealing time in method 2 and therefore perform the comparison of pore volume of MOF 1 with different amounts of guest molecules. Two samples, [Zn(bdc)(btrp)]·nDMF (n = 1, 0.7), have been prepared using annealing time of 1 and 7 h, correspondingly. Carbon dioxide sorption isotherms at 195 K confirm that the higher amount of removed guest molecules leads to the higher amount of gas adsorbed (Figure 10). Nevertheless, the pores remain inaccessible for nitrogen. Thus, despite of both low surface area and pore volume, some CO2/ N2 adsorption selectivity can be expected for MOF 1.

Table 2. Quantum Yields of Samples for Native and Activated MOFs (Method 1) with Different Guest Molecules

a

sample

quantum yield (%)

activated MOF 1 MOF 1a p-xylene@1 ligand (btrp)a o-xylene@1 toluene@1 benzene@1 aniline@1 MOF 2a nitrobenzene@1

22−46 10−15 2−6 3−5 1−3 0.5−1 0.5−1