Rational Synthesis and Investigation of Porous MetalâOrganic

Article pubs.acs.org/IC

Rational Synthesis and Investigation of Porous Metal−Organic Framework Materials from a Preorganized Heterometallic Carboxylate Building Block Aleksandr A. Sapianik,† Ekaterina N. Zorina-Tikhonova,‡ Mikhail A. Kiskin,‡ Denis G. Samsonenko,†,§ Konstantin A. Kovalenko,†,§ Alexey A. Sidorov,‡ Igor L. Eremenko,‡ Danil N. Dybtsev,*,†,§ Alexander J. Blake,⊥ Stephen P. Argent,⊥ Martin Schröder,†,∥ and Vladimir P. Fedin†,§ †

Nikolaev Institute of Inorganic Chemistry, SB RAS, 3 Akad. Lavrentiev Avenue, 630090 Novosibirsk, Russia N. S. Kurnakov Institute of General and Inorganic Chemistry, RAS, 31 Leninsky Avenue, 119991 Moscow, Russia § Novosibirsk State University, 2 Pirogova Street, 630090 Novosibirsk, Russia ⊥ School of Chemistry, University of Nottingham, Nottingham NG7 2RD, U.K. ∥ School of Chemistry, University of Manchester, Manchester M13 9PL, U.K. ‡

S Supporting Information *

ABSTRACT: The tetranuclear heterometallic complex [Li2Zn2(piv)6(py)2] (1, where piv− = pivalate and py = pyridine) has been successfully employed as a presynthesized node for the construction of four porous metal−organic frameworks (MOFs) [Li2Zn2(R-bdc)3(bpy)]·solv (2-R, R-bdc2−; R = H, Br, NH2, NO2) by reaction with 4,4′-bipyridine (bpy) and terephthalate anionic linkers. The [Li2Zn2] node is retained in the products, representing a rare example of the rational step-by-step design of isoreticular MOFs based on complex heterometallic building units. The permanent porosity of the activated frameworks was confirmed by gas adsorption isotherm measurements (N2, CO2, CH4). Three compounds, 2-H, 2-Br, and 2-NH2 (but not 2-NO2), feature extensive hysteresis between the adsorption and desorption curves in the N2 isotherms at low pressures. The substituents R decorate the inner surface and also control the aperture of the channels, the volume of the micropores, and the overall surface area, thus affecting both the gas uptake and adsorption selectivity. The highest CO2 absorption at ambient conditions (105 cm3·g−1 or 21 wt % at 273 K and 1 bar for 2NO2) is above the average values for microporous MOFs. The photoluminescent properties of the prototypic 2-H as well as the corresponding host−guest compounds with various aromatic molecules (benzene, toluene, anisole, and nitrobenzene) were systematically investigated. We discovered a rather complex pattern in the emission response of this material depending on the wavelength of excitation as well as the nature of the guest molecules. On the basis of the crystal structure of 2-H, a mechanism for these luminescent properties is proposed and discussed.

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INTRODUCTION Metal−organic framework (MOF) materials are comprised of two components: a metal ion or cluster of metal centers and an organic linker. The chemistry of permanently porous MOFs has been developed overwhelmingly with carboxylate organic linkers,1,2 which facilitate the formation of polynuclear metal complexes as inorganic building blocks.3 Typically, such polynuclear metal units are self-assembled from the mononuclear cations during crystallization of the MOF under optimized reaction conditions. In certain cases, MOFs based on heterometallic nodes can be obtained from the corresponding multicomponent reaction mixtures,4−8 although the isolation of phase-pure mixed-metal products can be challenging. Despite synthetic complications, porous materials with heterogeneous compositions are of special interest because of their intrinsic features and unique physical properties.9,10 © XXXX American Chemical Society

However, the synthesis of mixed-metal materials can be rationalized if discrete presynthesized polynuclear complexes are used as starting materials. A number of examples of such step-by-step preparations of porous structures, e.g., IRMOFs,11,12 UiO-66,13,14 MIL-88,89,15 MIL-127,16 and other MOFs,17−24 have been reported. Such an approach optimizes the route to heterometallic MOFs as long as the heteronuclear node retains its structural integrity, and this can be a more effective route than less predictable self-assembly processes.18,22,23 Tetranuclear pivalates of the type [Li2M2(piv)6(py)2] (M = Co, Ni, Zn) are cluster complexes that can be obtained by a simple procedure and feature a number of functional properties, Received: November 9, 2016

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DOI: 10.1021/acs.inorgchem.6b02713 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry such as magnetism and luminescence.25−28 Because of the robust structure and fixed geometry of the carboxylate ligand, these complexes are excellent candidates as heterometallic building blocks for MOF synthesis. Only a handful of the corresponding coordination polymers have actually been synthesized,29,30 and these are restricted to 1D or 2D nonporous structures, which limits their application to gas adsorption and capture. We describe herein the rational stepby-step synthesis of 3D porous MOFs by substituting both pyridine and pivalate ligands in [Li2Zn2(piv)6(py)2] (1) with 4,4′-bipyridine (bpy) and terephthalate bridges, respectively. The systematic variation of the substituents in the terephthalate linkers allowed us to decorate the inner surface of the resultant microporous channels with bromo, amino, or nitro groups, which affect the gas adsorption properties of the resultant materials and afford routes to guest-dependent luminescence.

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w, 1953 w, 1624 s, 1505 m, 1369 s, 1221 m, 1151 m, 1075 m, 1017 m, 877 w, 827 s, 748 s, 645 m, 590 w, 531 m, 418 w. Synthesis of [Li2Zn2(Br-bdc)3(bpy)]·2DMF·CH3CN·H2O (2-Br). The method of synthesis was similar to that for 2-H. The reagents and amounts used were bpy (7.8 mg, 0.06 mmol), 1 (17 mg, 0.02 mmol), and H2Br-bdc (29 mg, 0.12 mmol). Colorless crystals were collected by filtration and dried in air (yield: 12 mg, 0.01 mmol, 50% based on elemental analyses and TGA). Anal. Calcd for C42H36Br3Li2N5O15Zn2: C, 40.8; H, 2.9; N, 5.6. Found: C, 40.8; H, 3.2; N, 5.3. IR (KBr, cm−1): 3400 m, 3067 w, 1951 w, 1628 s, 1541 m, 1485 m, 1388 s, 1222 m, 1151 m, 1073 m, 1038 m, 889 w, 817 s, 769 s, 732 m, 644 w, 544 m, 492 w, 440 w. Synthesis of [Li2Zn2(NO2-bdc)3(bpy)]·2DMF·H2O (2-NO2). The method of synthesis was similar to that for 2-H. The reagents and amounts used were bpy (7.8 mg, 0.06 mmol), 1 (17 mg, 0.02 mmol), and H2NO2-bdc (24 mg, 0.12 mmol). Pale-yellow crystals were collected by filtration and dried in air (yield: 11.5 mg, 0.01 mmol, 52% based on elemental analyses and TGA). Anal. Calcd for C40H33Li2N7O21Zn2: C, 44.0; H, 3.1; N, 9.0. Found: C, 43.7; H, 3.3; N, 9.3. IR (KBr, cm−1): 3420 m, 3087 m, 1957 w, 1634 s, 1614 s, 1535 s, 1494 m, 1374 s, 1255 w, 1222 w, 1159 w, 1130 w, 1072 m, 1018 w, 924 w, 826 m, 782 m, 750 m, 729 m, 668 w, 646 w, 536 w. Synthesis of [Li2Zn2(NH2-bdc)3(bpy)]·DMF·CH3CN·3H2O (2-NH2). A solution of bpy (7.8 mg, 0.06 mmol) in DMF (0.1 mL) was added to 1 (17 mg, 0.02 mmol) dissolved in DMF (0.5 mL), and CH3CN (4 mL) was then added. H2NH2-bdc (21 mg, 0.12 mmol) in DMF (0.5 mL) was added, and the resultant mixture was sealed in a glass tube, heated to 130 °C (1 °C·min−1), and kept at 130 °C for 24 h. Brownyellow crystals were filtered off and dried in air (yield: 9 mg, 0.009 mmol, 45% based on elemental analyses and TGA). Anal. Calcd for C39H39Li2N7O16Zn2: C, 46.5; H, 3.9; N, 9.7. Found: C, 46.2; H, 4.0; N, 10.0. IR (KBr, cm−1): 3454 m, 3346 m, 3059 w, 2931 w, 2874 w, 1670 s, 1621 s, 1580 s, 1495 m, 1422 m, 1377 s, 1256 s, 1220 m, 1152 w, 1097 m, 1073 w, 1017 w, 962 w, 897 w, 831 m, 771 s, 662 w, 643 w, 598 m, 514 m, 450 w. Guest Exchange in 2-H. The general procedure for guest exchange in 2-H involved immersing crystals into benzene, toluene, anisole, or nitrobenzene. The solutions above the crystals were changed twice over 48 h, and the crystals were then collected by filtration and dried in air. The PXRD data support the stability of framework 2-H after guest exchange (Figure S18). Some deviations in the positions of the weaker reflections at 2θ = 10−12° and for peaks at a wider angle can be explained by slight distortions of the framework upon inclusion of the aromatic guest. Elemental analyses for 2-H with guest molecules are given. Anal. Calculated for the activated sample [Li 2 Zn 2 (bdc) 3 (bpy)]C34H20Li2N2O12Zn2: C, 51.5; H, 2.55; N, 3.5. Found: C, 51.7; H, 2.7; N, 3.5. Anal. Calcd for the sample with benzene [Li2Zn2(bdc)3(bpy)]·2.5C6H6-C49H35Li2N2O12Zn2: C, 59.5; H, 3.55; N, 2.8. Found: C, 59.2; H, 3.5; N, 2.9. Anal. Calcd for the sample with toluene [Li2Zn2(bdc)3(bpy)]·2C7H8-C48H36Li2N2O12Zn2: C, 59.0; H, 3.7; N, 2.9. Found: C, 59.3; H, 3.8; N, 2.8. Anal. Calcd for the sample with anisole [Li2Zn2(bdc)3(bpy)]·2C7H8O-C48H36Li2N2O14Zn2: C, 57.1; H, 3.6; N, 2.8. Found: C, 57.4; H, 3.6; N, 2.7. Anal. Calcd for the sample with nitrobenzene [Li2Zn2(bdc)3(bpy)]·2C6H5NO2C46H30Li2N4O16Zn2: C, 53.2; H, 2.9; N, 5.4. Found: C, 53.4; H, 3.0; N, 5.8. X-ray Crystallography. Diffraction data for complex 1 were collected at 296 K on a Bruker SMART APEX II diffractometer equipped with a CCD detector [graphite monochromator, λ(Mo Kα) = 0.71073 Å],31 and absorption correction was applied using the SADABS32 program. Diffraction data for a single crystal of 2-H were obtained at 120 K on an automated Agilent GV1000 diffractometer equipped with a 2D AtlasS2 detector [rotating anode, λ(Cu Kα) = 1.54184 Å]. Diffraction data for single crystals of 2-Br, 2-NO2, and 2NH2 were obtained at 130 K on an automated Agilent Xcalibur diffractometer equipped with a 2D AtlasS2 detector [graphite monochromator, λ(Mo Kα) = 0.71073 Å]. Integration, absorption correction, and determination of the unit cell parameters were performed using the CrysAlisPro program package.33 The structures of

EXPERIMENTAL SECTION

General Procedures. Unless specified, all reagents were obtained from commercial sources and used as received. Dimethylformamide (DMF) was treated over activated 3 Å molecular sieves. Powder X-ray diffraction (PXRD) for 1 was performed on a Bruker D8 instrument (Cu Kα radiation), and all other PXRD data were obtained on a Shimadzu XRD 7000S diffractometer (Cu Kα radiation) with the following parameters: 2θ step = 0.03, counting time = 1.0−2.5 s, and 2θ scan range = 3−30°. For compounds 1 and [Li2Zn2(R-bdc)3(bpy)]· solv (2-R, R-bdc2−; R = H, Br, NH2, NO2), the PXRD patterns are presented in Figures S13−S18. Fourier transform infrared (FT-IR) spectra in the range of 4000−300 cm−1 were measured on a Vertex 80 spectrometer and are presented in Figure S21. Thermogravimetric analyses (TGA) were obtained on a NETZSCH TG 209 F1 analyzer. The sample quantity ranged from 2 to 10 mg, and all samples were heated under helium from room temperature to 600 °C at a rate of 10 °C·min−1. Elemental analytical data of carbon, hydrogen, and nitrogen were obtained on a Eurovector 600 analyzer, and excitation and emission spectra of solid samples were recorded on a Cary Eclipse (Varian) fluorescence spectrophotometer. The emission spectra of the complexes were recorded at room temperature under the following experimental conditions: λEx = 320 and 380 nm; V = 500 V; spectral slit width = 1 nm. Photoluminescence spectra were run on powdered samples between two quartz sheets. The thin layer of powder was placed at 45° to the excitation light beam, and a xenon flash lamp was used as a light source to excite the solid-state photoluminescent spectra. Synthesis of Li2Zn2(piv)6(py)2 (1). CH3CN (25 mL) was added to a mixture of [Zn(piv)2]n (0.205 g, 0.77 mmol) and Lipiv (0.085 g, 0.79 mmol). The reaction mixture after the addition of pyridine (0.07 mL, ∼ 0.8 mmol) was heated at 80 °C for 40 min, and the resultant colorless solution was cooled to room temperature. Slow evaporation of CH3CN led to the formation of colorless crystals suitable for X-ray diffraction. The crystalline precipitate that formed after 24 h (Vsolution = 10 mL) was separated by decantation, washed with CH3CN, and dried in air. The yield of compound 1 was 0.195 g (56%). Anal. Calcd for C40H64Li2N2O12Zn2: C, 52.82; H, 7.09; N, 3.08. Found: C, 52.72; H, 6.95; N, 3.03. IR (KBr, cm−1): 3079 vw, 2961 m, 2925 m, 2092 m, 2870 w, 1606 s, 1577 s, 1567 s, 1481 s, 1449 s, 1422 s, 1406 s, 1369 s, 1358 s, 1218 s, 1153 w, 1070 m, 1044 m, 1030 w, 1016 w, 937 w, 895 m, 793 m, 754 m, 696 s, 678 v.w, 639 m, 613 s, 571 m, 555 w, 438 s. Synthesis of [Li2Zn2(bdc)3(bpy)]·3DMF·CH3CN·H2O (2-H). A solution of bpy (7.8 mg, 0.06 mmol) in DMF (0.1 mL) was added to 1 (17 mg, 0.02 mmol) dissolved in DMF (0.5 mL), followed by the addition of CH3CN (1.5 mL). H2bdc (20 mg, 0.12 mmol) in DMF (0.5 mL) was added, and the resultant mixture was sealed in a glass tube, heated to 100 °C (1 °C·min−1), and retained at 100 °C for 24 h. Colorless crystals were collected by filtration and dried in air (yield: 14 mg, 0.013 mmol, 67% based on elemental analyses and TGA). Anal. Calcd for C45H46Li2N6O16Zn2: C, 50.4; H, 4.3; N, 7.8. Found: C, 50.6; H, 4.4; N, 7.8. IR (KBr, cm−1): 3420 m, 3067 w, 2955 w, 2926 w, 2856 B


Article

Inorganic Chemistry Table 1. Crystal Data and Structure Refinement for 1 and 2-R empirical formula M, g·mol−1 cryst syst space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z D(calcd), g·cm−3 μ, mm−1 F(000) cryst size, mm θ range for data collection, deg index ranges reflns collected/indep Rint reflns with I > 2σ(I) GOF on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff peak/hole, e·Å−3

1

2-H

2-Br

2-NO2

2-NH2

C40H64Li2N2O12Zn2 909.55 triclinic P1̅ 9.1017(19) 11.773(3) 12.241(3) 116.876(3) 97.357(3) 90.416(3) 1157.2(4) 1 1.305 1.093 480 0.22 × 0.12 × 0.08 1.89−28.34 −12 ≤ h ≤ 11, −15 ≤ k ≤ 15, −16 ≤ l ≤ 16 11682/5662 0.0411 4255 1.078 R1 = 0.0452, wR2 = 0.1132 R1 = 0.0678, wR2 = 0.1226 0.601/−1.322

C47H47Li2N7O15Zn2 1094.53 monoclinic C2/c 17.0602(9) 18.8603(9) 16.9468(11) 90 113.193(7) 90 5012.1(5) 4 1.450 1.805 2256 0.19 × 0.04 × 0.03 5.27−73.39 −21 ≤ h ≤ 15, −20 ≤ k ≤ 23, −20 ≤ l ≤ 19 10728/4943 0.0316 4444 1.041 R1 = 0.0488, wR2 = 0.1326 R1 = 0.0523, wR2 = 0.1355 1.029/−0.962

C40H31Br3Li2N4O14Zn2 1176.04 monoclinic C2/c 17.8034(9) 18.4763(7) 17.1568(9) 90 115.411(6) 90 5097.6(5) 4 1.532 3.354 2328 0.49 × 0.06 × 0.04 3.36−25.68 −16 ≤ h ≤ 21, −22 ≤ k ≤ 20, −20 ≤ l ≤ 20 12067/4828 0.0301 3790 1.030 R1 = 0.0606, wR2 = 0.1707 R1 = 0.0775, wR2 = 0.1805 1.769/−0.713



all compounds were solved by direct methods and refined by a fullmatrix least-squares technique in the anisotropic approximation (except H atoms) using the SHELX-2014 software.34 The positions of the H atoms of the organic ligands were calculated geometrically and refined in the riding model. The crystallographic data and details of the structure refinements are summarized in Table 1 with selected interatomic distances, and the valence angles are given in Tables S1− S5. CCDC 1486669−1486673 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Center at http://www. ccdc.cam.ac.uk/data_request/cif. Surface Area and Porous Structure. Analysis of the porous structure was performed on a Quantochrome Autosorb iQ analyzer at 77 K. Samples of 2-H, 2-Br, and 2-NO2 were first degassed under vacuum at 313 K for 12 h, while 2-NH2 was degassed under vacuum at 423 K for 12 h. N2 adsoprtion−desorption isotherms were measured within the range of relative pressures of 10−6−0.99 bar. The specific surface area was calculated from data based on the Brunauer− Emmett−Teller (BET) and Langmuir models. The Gourvich and density functional theory (DFT) approaches, the most appropriate for the studied materials, were employed to estimate the total pore volume and the pore-size distribution, respectively. CO2 and CH4 Sorption Experiments. The CO2 and CH4 adsorption isotherms were measured volumetrically on a Quantochrome Autosorb iQ analyzer at 273 K using an ice−water bath. The samples (ca. 50 mg) were activated before measurements using the standard “outgas” option of the equipment at 323 K. Adsorption− desorption isotherms were measured within the range of pressures of 10−3−1 bar. The database of the National Institute of Standards and Technology35 was used as a source of p−V−T relations at experimental pressures and temperatures.

pyridine in a CH3CN solution following the synthetic procedure reported previously for [Li2Co2(piv)6(2,4-lut)2] (2,4-lut = 2,4-lutidine).28 1 crystallizes in the triclinic space group P1.̅ The center of symmetry of the tetranuclear molecule is at the intersection of the diagonals of the almost square [Li2(μ2-O)2] moiety (Figure 1a), and the Li2Zn2 core has a planar structure [Zn···Li 3.098(5) Å, Li···Li 2.665(9) Å, and Zn−Li−Li 121.8(3)°]. Each ZnII cation is bound to the N donor of a pyridine ligand and to three O donors of three carboxylate groups, thus adopting a distorted tetrahedral coordination [Zn−N 2.067(2) and Zn−O 1.930(2)−1.952(2) Å]. The coordination environment of the Li+ ions is also

RESULTS AND DISCUSSION Synthesis and Structures. Compound 1 was synthesized by the reaction of lithium pivalate, zinc(II) pivalate, and

Figure 1. Crystal structure of complex 1: ellipsoid presentations at the 50% probability level (left); possible extension of the coordination structure through the bridging ligands (right). H atoms are omitted. Color code: Zn, green; Li, cyan; N, blue; C, gray.

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C


Article

Inorganic Chemistry tetrahedral [Li−O 1.889(5)−1.985(5) Å]. Three carboxylate bridges link the ZnII and Li+ cations, with two carboxylates acting as bridges between two heterometallic centers and one acting as a μ3-bidentate linker bridging two Li+ cations through one O atom. The six carboxylate anions are directed perpendicularly to the Zn−Zn axis of the node (Figure 1b), while the Zn−N bonds are directed along this axis, and thus the node overall acts as a potential 8-connected building unit for the design of extended MOFs by replacing the terminal piv− and py ligands with bridging ones. We used complex 1 as the starting material for the synthesis of coordination polymers by linking these units through terephthalate anions and bpy. The syntheses of coordination polymers 2-R (R = H, Br, NH2, NO2) were carried out under solvothermal conditions. The order of the addition of the reagents has a marked impact on the product(s) isolated, and in particular the bpy solution should be added to the polynuclear complex first, followed by the addition of terephthalic acid. An inverse order of the addition results in the formation of unknown amorphous products. It should also be noted that the presynthesized tetranuclear complex 1 is necessary to form the target materials, and up to now the use of mixtures of LiI and ZnII salts instead of 1 under a variety of conditions results either in the formation of amorphous precipitates (by PXRD) or in the crystallization of known zinc(II) terephthalate products. Single-crystal X-ray structures of compounds 2-R (R = H, Br, NO2, NH2) confirmed the successful realization of the above step-by-step strategy. All products are based on the 8connected tetranuclear heterometallic [Li2Zn2(OOCR)6L2] node linked by both terephthalates and bpy. The compounds are isostructural aside from the substituent on the terephthalate moiety. The coordination polymers 2-R crystallize in the monoclinic space group C2/c. Both Li+ and ZnII cations show tetrahedral coordination environments, with Li+ bound to four O donors of four R-bdc2− (R = H, Br, NO2, NH2) ligands. The Li−O bond lengths lie in the ranges of 1.888(5)−2.007(5), 1.884(8)−2.024(8), 1.880(7)−2.012(7), and 1.875(5)− 2.048(5) Å for compounds 2-H, 2-Br, 2-NO2, and 2-NH2, respectively. ZnII cations are coordinated by three O atoms of three different R-bdc2− ligands and one N atom of the 4,4′-bpy molecule. The Zn−O bond distances are in the ranges of 1.900(2)−1.961(2), 1.904(3)−1.944(3), 1.909(3)−1.942(3), and 1.910(2)−1.945(2) Å for 2-H, 2-Br, 2-NO2, and 2-NH2, respectively, with Zn−N bonds of 2.034(2), 2.026(3), 2.026(3), and 2.041(2) Å for 2-H, 2-Br, 2-NO2, and 2-NH2, respectively. The [Li2Zn2(OOCR)6N2] nodes are situated on a 2-fold rotation axis, and there are two crystallographically independent R-bdc2− ligands. Each [Li2Zn2(OOCR)6N2] node is connected to eight others by six terephthalate and two bpy linear linkers, forming an open 3D MOF with complicated selfpenetrated topology (Figure 2). Most interestingly, porous channels of varying diameters running along the [101] direction could be identified within these structures (Figure 3). For the prototypic compound 2-H, the widest diameter is 7 Å, and these are connected by wider windows of 5 × 7 Å along the main direction and by smaller lateral windows of 3 × 4 Å in the perpendicular direction. The functional groups of the terephthalate linkers in 2-R are directed toward the inner space of the channels, thus reducing the diameter of those windows (Figures S9−S11) or even blocking some of the lateral passages in the case of the bulkiest group R = NO2. For R = Br and NH2, the diameters of the windows vary from 2 × 4 to 4 × 4 Å and from 2 × 3 to 5 × 6 Å,

Figure 2. Connectivity of the [Li2Zn2(RCOO)6(bpy)] complex as an 8-connected node in compound 2-H. Bridging terephthalate and bpy linkers are highlighted in blue and magenta, respectively, zinc is in green, and lithium is in turquoise (left). Topological presentation of MOF in 2-H. Each black ball represents a [Li2Zn2(RCOO)6(bpy)] node (right).

respectively. Assuming the presence of fully open channels, the solvent-accessible volume derived from PLATON/SOLV analysis36 was found to be 45%, 40%, 37%, and 42% for the structures 2-H, 2-Br, 2-NO2, and 2-NH2, respectively. These voids are occupied by highly disordered solvent molecules that could not be modeled as a set of discrete atomic sites. The solvent compositions of the crystalline porous compounds 2-R were established by TGA and elemental analytical data and FTIR spectroscopy (see the Supporting Information). The TGA data show a steady weight decrease upon heating to 200 °C due to a continuous loss of loosely bound guest solvent molecules (see the Supporting Information). The observed weight losses were 25.5% for 2-H, 16.5% for 2-Br, 16.1% for 2-NO2, and 18.0% for 2-NH2. It should be noted that the weight losses of the crystalline samples during TGA, and hence the amounts of solvent guest molecules present in the pores, are in line with the calculated free volume of the porous coordination polymers as well as with the unassigned electron density within the channels obtained by the SQUEEZE routine (see the Supporting Information). The structural integrity and phase purity of the crystalline compounds 2-R were confirmed by PXRD. Minor discrepancies between the experimental PXRD patterns and theoretical simulations are anticipated because the latter are based on guest-free structures (see the Supporting Information). Also, some breathing of the porous frameworks upon the loss of solvent molecules during data collection can take place, resulting in slight shifts of certain peaks in the PXRD patterns. The stability of solvent-free porous MOFs was established by X-ray diffraction analyses of the activated compounds (Figures S14−S17). Such results prompted us to investigate the porosity of the coordination frameworks 2 by gas adsorption isotherm measurements. Gas Adsorption. The permanent porosity of 2-R was established by N2 adsorption isotherms at 77 K (Figure 4). D


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Inorganic Chemistry

pressure or from the sieving effect by the kinetically hindered adsorption−desorption in narrow channels. Considering the relatively narrow windows in the porous frameworks, we suggest that a sieving effect is the basis of the observed hysteresis. Also, as discussed above, the nitro groups in 2-NO2 block some of the lateral passages, thus isolating the 1D channels of the porous structure. This structural feature may result in the distinct hysteresis-free behavior of the N2 isotherm of 2-NO2, compared to the other compounds 2-H, 2-Br, and 2NH2, which have 3D porous structures with intersecting channels. The calculated values of the specific surface area and the parameters of the porous structure are shown in Table 2. Because of significant hysteresis, the specific surface areas for compounds 2-H, 2-Br, and 2-NH2 were calculated from the desorption curves. Table 2. Parameters of the Porous Structures of Frameworks 2-R specific surface area (m2·g−1)

compound 2-H 2-Br 2-NO2 2-NH2 a

Langmuir b

1200 535, 735b 915 90, 982b

BET

DFT

Vpore (cm3·g−1)

Vads(N2)a [cm3(STP)· g−1]

b

881 803

0.466 0.310

301 200

1124 651

0.354 0.392

229 253

1052 431, 655b 742 85, 876b

Measured or calculated at P/P0 = 0.95. desorption curve.

b

Calculated from the

2-R also shows reversible adsorption of CH4 and CO2, and the isotherms, measured at 273 K, are presented in Figures 5 and 6. CH4 uptake at 1 bar reached 0.62, 1.02, 1.62, and 0.36 wt %, while CO2 uptake at 1 bar and 273 K reached 6.38, 11.54, 20.76, and 5.69 wt % for 2-H, 2-Br, 2-NO2, and 2-NH2, respectively. Unlike the N2 adsorption−desorption data, no significant hysteresis was observed for the CO2 and CH4 isotherms in the low-pressure range. CO2 uptake for 2-NO2 at ambient temperature (105 mL·g−1 or 20.76 wt %) and low pressure is among the better values for the reported MOFs.37−42 Interestingly, we found an inverse correlation in this pressure domain between the CO2 and CH4 sorption capacities with respect to the pore size/volume, with materials having larger pore volumes and surface areas (2-H and 2-NH2) showing the lowest CO2 and CH4 uptakes. In contrast, the highest CO2 and CH4 uptakes are observed for 2-NO2, with the lowest pore volume and surface area. In the absence of relatively strong interactions between the gas molecules and surface, for example, to Lewis acidic (open metal site) or Lewis basic (amine groups) centers, weak van der Waals forces must play a key role in the substrate binding at ambient conditions. The highly polar NO2 groups in 2-NO2 may well form stronger dipole−dipole interactions with the CO2 quadrupole or the induced dipole in CH4. The narrower aperture of channels in 2Br and 2-NO2 is likely another reason here because narrow channels give better potential overlap between walls, and so uptake at low pressures is higher in narrower-pored materials.43 In contrast, 2-H with larger channels and the absence of the polar groups on the surface show relatively low uptake of CO2 and CH4 at ambient conditions and at low pressures. CO2/CH4 Adsorption Selectivity. The selective adsorption of CO2 from natural gas is a highly important technology to

Figure 3. (top) Projection of the MOF 2-H along the channels (van der Waals presentations). Visualization of the void structure in different projections: red, inner surface (channels); gray, outer surface. (bottom) Framework shown by blue ware.

Figure 4. View of the adsorption (filled symbols) and desorption (open symbols) isotherms at 77 K for N2 in 2-H (black squares), 2-Br (red circles), 2-NO2 (blue triangles), 2-NH2 (green diamonds).

Only 2-NO2 exhibits a clear type I isotherm typical for the microporous materials. The other materials show more complex behavior with pronounced hysteresis between the adsorption and desorption curves in the low-pressure range. Such a hysteresis could result either from reversible structural rearrangements (breathing) of the frameworks upon the gas E


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Inorganic Chemistry

Figure 5. View of the adsorption isotherms at 273 K for CH4 in 2-H (black squares), 2-Br (red circles), 2-NO2 (blue triangles), and 2-NH2 (green diamonds) as the absorbed volume per sample mass (top) and weight % (bottom).

Figure 6. View of the adsorption isotherms at 273 K for CO2 in 2-H (black squares), 2-Br (red circles), 2-NO2 (blue triangles), and 2-NH2 (green diamonds) as the absorbed volume per sample mass (top) and weight % (bottom).

prevent the deterioration of pipes over time.44 We estimated the selectivity factors α = CO2/CH4 by three commonly used methods: (i) as a ratio of adsorbed volumes; (ii) as a ratio of Henry constants derived from the slope of the linear approximation of the isotherms at lower pressures, and (iii) by ideal adsorbed solution theory (IAST). Table 3 shows the results of the selectivity factor calculations for 2-R by the first two methods, with the highest values of α observed for 2-NH2 consistent with the potential interactions of CO2 molecules with amino groups. The IAST calculations (see the Supporting Information) allowed us to obtain the selectivity factors as a function of the gas mixture compositions as well as the absolute pressure at 273 K (Figure 7). As could be seen from these data, the selectivity (the slope of the function) for 2-NH2 is rather different from the other porous compounds in the series. The values for the selectivity are consistently higher for 2-NH2, while the selectivity of the nonfunctionalized 2-H is the lowest among the studied porous compounds, following the wellknown compromise of uptake versus selectivity observed in porous materials.43 The less porous 2-Br and 2-NO2 show intermediate selectivity values independent of the method of analysis used. Guest-Exchange and Photoluminescence Studies. Complex 2-H has the highest accessible pore volume of the series and was selected therefore for guest-dependent luminescence experiments. The solvent molecules in the as-

Table 3. Carbon Dioxide and Methane Adsorption Volumes, Calculated Henry Constants for Compounds 2-R, and selectivity factors of CO2/CH4 Adsorption Vadsa [cm3(STP)· g−1]

Henry constants KH (mmol·g−1·bar−1)

selectivity factors α

compound

CO2

CH4

CO2

CH4

V(CO2)/ V(CH4)

2-H 2-Br 2-NO2 2-NH2

33 59 106 29

9 14 23 5

47.44 106.96 224.8 54.62

6.93 17.90 29.48 5.836

3.7 4.2 4.6 5.8

a

KH(CO2)/ KH(CH4) 6.8 6.0 7.6 9.3

Measured at P = 1 bar.

synthesized crystals were substituted by aromatic molecules with different substituents by immersing the crystals into benzene, toluene, anisole, or nitrobenzene, and excitation and emission spectra of the compounds are displayed in Figure 8. The excitation spectrum of 2-H features two broad bands with maxima at 320 and 380 nm, and on the basis of literature data, the observed excitation maxima could be assigned to the absorption of terephthalate (320 nm)45−47 and bipyridyl (380 nm) moieties.48−50 Accordingly, the emission spectra were recorded at two different excitation wavelengths. In the case of λEx = 380 nm, the activated guest-free sample displays a blue F


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Inorganic Chemistry

inclusion of nitrobenzene results in luminescence quenching likely due to charge transfer to the electron-deficient aromatic system, as a result of the electron-withdrawal effect of the nitro group.52−54 Interestingly, the luminescence spectra of the compounds irradiated by λEx = 320 nm (the absorption of terephthalate anions) show very different shapes, depending on the nature of the guest molecules (Figure 8). The luminescence of the guestfree framework 2-H results in a broad peak at λ = 430 nm. The inclusion of nitrobenzene expectedly quenches the luminescence due to a similar charge-transfer mechanism with subsequent nonemissive energy dissipation. The inclusion of the other aromatic guest molecules enhances the luminescence intensity due, at least partly, to the same reasons as those described above. More importantly, the wavelength of the luminescence peak is shifted depending on the nature of the guest: λ = 380 nm for benzene, 480 nm for toluene, and 500 nm for anisole. Such remarkable differences in the luminescence behavior of the solvent-loaded compounds, depending on the excitation wavelength (λEx = 320 or 380 nm), can be explained by the structural features of the framework 2-H. According to the single-crystal X-ray diffraction data, the 4,4′bpy moieties are only exposed to porous channels by the edges of the aromatic rings (Figure S12). No short interaction between the aromatic π-electron system of bpy and a potential guest molecule is possible. In contrast, the terephthalate groups line the surface of the channels with their aromatic π electrons in potential close contact with solvent molecules through a π−π stacking or similar specific van der Waals interactions. Consequently, photoexcitation of the bpy linker (λEx = 380 nm) may only result in a ligand-centered π* → π relaxation through the luminescence at 450 nm, and this wavelength does not vary with the nature of the solvent because the π-electronic structure of bpy can only interact poorly with guest molecules in the channels. The photoexcitation of terephthalate moieties (λEx = 320 nm) also results in a radiative relaxation, but in this case, the π-electronic structure of bdc2− is available for the interaction with aromatic guest molecules to form excited complexes or exciplexes.55 Such exciplexes exist for a very short time as charge-transfer dimers of the photoexcited ligand bdc2−* and an aromatic guest molecule. The exciplex-centered nature of the luminescence is indirectly supported by a notable red shift of the luminescence when electron-donor groups −CH3 or −OCH3 are present in the solvent molecules (480 and 500 nm, respectively), compared to the unmodified aromatic system of benzene (380 nm). Note also that the luminescence of the guest-free compound 2-H (λEx = 320 nm) is centered at λ = 430 nm. This is typical for terephthalatebased MOFs51 because no electronic structure altering exciplex dimers could obviously be formed in the absence of guest molecules. The luminescent properties of these porous coordination frameworks thus show distinct responses depending on both the wavelength of excitation and the nature of the guest molecules. This could, in principle, be implemented in sensing devices for the detection of multiple types of aromatic molecules, including potentially explosive nitroaromatics.

Figure 7. CO2/CH4 adsorption selectivity calculations determined by IAST for 2-R. Top: dependences of the adsorption selectivities on the total pressure for the equimolar CO2/CH4 mixture at 273 K. Bottom: dependences of the adsorption selectivities on the mole fraction of CO2 in CO2/CH4 mixtures at 273 K and 1 bar. Color code: 2-H, black; 2-Br, red; 2-NO2, blue; 2-NH2, green.

emission with a broad band at λ = 450 nm. The guestexchanged samples with benzene, toluene, and anisole show similar photoluminescence spectra with broad maxima in the same region. Both ZnII and Li+ are redox-inactive and not known to interfere with the ligand-centered luminescence.51 Therefore, the observed peak at λ = 450 nm could be assigned to an intraligand π* → π transition in the bpy linker.48−50 A noticeable increase of the luminescence intensity of the host− guest compounds with benzene, toluene, and anisole, compared to the guest-free host framework 2-H (Figure 8), may result from a higher mechanical stability (stiffness) of the framework upon saturation by guest molecules. Indeed, the complete filling of channels hinders possible oscillations of the organic linkers and framework as a whole, thus reducing the probability of nonemissive relaxation of the photoexcited electronic state through the consecutive vibrational states. In contrast, the

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CONCLUSIONS Substitution of both pyridine and pivalate ligands in 1 with bpy and terephthalate bridges (R-bdc2−, R = H, Br, NH2, NO2) results in a series of four isoreticular porous frameworks [Li2Zn2(R-bdc)3(bpy)]·solv (2-R), in which the 8-connected tetranuclear heterometallic {Li2Zn2(OOCR)6L2} node is linked G


Article

Inorganic Chemistry

Figure 8. Luminescence data of host−guest compounds of 2-H. Excitation (dashed line) and emission (solid line) spectra of 2-H·guest for λEx = 380 nm (left). Photographic images of 2-H·nitrobenzene, activated 2-H, and 2-H·benzene (from left to right) upon irradiation at λEx = 380 nm. Excitation (dashed line) and emission (solid line) spectra of 2-H·guest for λEx = 320 nm (right). Photographic images of 2-H·nitrobenzene, 2-H· benzene, activated 2-H, 2-H·toluene, and 2-H·anisole (from left to right) upon irradiation at λEx = 320 nm.

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by dicarboxylate and bpy ligands. The introduction of different substituents R to the terephthalate linkers affects the free volume of the porous compounds and the N2 adsorption behavior. The CH4 and CO2 adsorption and relative selectivities have been investigated in detail, and interestingly a fascinating interplay of the luminescence properties with the wavelength of excitation and the nature of the host aromatic guest molecules has been observed.

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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.6b02713. PXRD patterns, additional crystal structures and structural descriptions, TGA diagrams, FT-IR spectra, IAST calculations, and crystal data (PDF) Crystallographic data in CIF format for 1 and 2-R (CIF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ekaterina N. Zorina-Tikhonova: 0000-0002-6456-9584 Danil N. Dybtsev: 0000-0001-8779-0612 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by a grant of the Government of the Russian Federation (Grant 14.Z50.31.0006). E.N.Z-T., M.A.K., A.A.S., and I.L.E. (synthesis and investigation of the structure of the molecular Li2Zn2 pivalate complex) acknowledge the Russian Science Foundation (Project 14-23-00176) for financial support. M.S. acknowledges support from EPSRC and ERC (Advanced Grant). H


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