Syntheses and Structural Characterization of Lithium Carboxylate

Article pubs.acs.org/crystal

Syntheses and Structural Characterization of Lithium Carboxylate Frameworks and Guest-Dependent Photoluminescence Study Sokhrab B. Aliev,† Denis G. Samsonenko,†,‡ Mariana I. Rakhmanova,† Danil N. Dybtsev,†,‡ and Vladimir P. Fedin*,†,‡ †

Nikolaev Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of Sciences, Novosibirsk 630090, Russian Federation ‡ Natural Science Department, Novosibirsk State University, Novosibirsk 630090, Russian Federation S Supporting Information *

ABSTRACT: Five novel 3D lithium−organic frameworks, [Li2(H2pml)]·C4H8O2 (1, H4pml = 1,2,4,5-benzenetetracarboxylic or pyromellitic acid), [Li2(H2pml)] (2), [Li(H2tatab)]·5H2O (3, H3tatab = 4,4′,4″-s-triazine-1,3,5-triyltri-p-aminobenzoic acid), [Li11(H3O)(H2O)5(Html)6]·8C4H8O2 (4, H3tml = 1,2,4-benzenetricarboxylic or trimellitic acid), and [Li{Li(nmp)}{Li(H2O)(nmp)}{Li(nmp)2}2(H2tms)(Htms)2]·NMP·EtOH (5, H3tms = 1,3,5-benzenetricarboxylic or trimesic acid, nmp = 1-methyl-2pyrrolidinone), were synthesized by solvothermal methods and characterized by single crystal X-ray diffraction, powder X-ray diffraction (PXRD), IR spectroscopy, and thermogravimetric analyses (TGA). Compounds 1 and 2 feature the same framework composition but different guest-controlled structures and topology. Compound 4 consists of a unique undecanuclear Li carboxylate complex {Li11(H2O)5(RCOO)18} and contains pores with diameter 7 Å. Furthermore, compound 4 demonstrates highly pronounced guest-dependent photoluminescence behavior: nitrobenzene completely quenches the light emission, while toluene enhances it.

■

INTRODUCTION Porous metal−organic frameworks are one of the most rapidly expanding areas of modern chemistry due to very prominent application capabilities in adsorption, separation, catalysis, sensing, etc.1,2 In particular, recent demonstrations of high CH4 and H2 storage in porous metal−organic frameworks3 are very encouraging since these gases are recognized to be mobile fuels for today and tomorrow. The gravimetric gas uptake value is directly dependent on the density of the porous material; therefore, the assembly of light metal-based porous coordination frameworks is a straightforward strategy to enhance gas storage.4 In this context, lithium is unquestionably the metal of choice since it is the lightest metal, is relatively abundant, and has low toxicity. Among several dozen Li-based coordination polymers with structurally robust ligands known to date, only a few structures possess micropores sufficient for reversible encapsulation of small organic molecules and gas adsorption.5 On the other hand, coordination complexes based on diamagnetic metal cations and aromatic ligands often display strong ligand-centered luminescence. Extension of such complexes into regular coordination frameworks is a plain approach toward new materials combining both luminescence properties and uniform porosity. Indeed, many such compounds do demonstrate luminescence dependent on the nature or composition of the guest molecules.6,7 Therefore, the synthesis and characterization of new porous metal−organic © XXXX American Chemical Society

frameworks with luminescent properties, suited for potential sensing applications, is highly important both academically and practically. In the current work, we describe five new 3D lithium− organic frameworks with different robust carboxylate ligands: [Li2(H2pml)]·C4H8O2 (1), [Li2(H2pml)] (2), [Li(H2tatab)]· 5H2O (3), [Li11(H3O)(H2O)5(Html)6]·8C4H8O2 (4), and [Li{Li(nmp)}{Li(H2O)(nmp)}{Li(nmp)2}2(H2tms)(Htms)2]· NMP·EtOH (5), where H4pml = 1,2,4,5-benzenetetracarboxylic or pyromellitic acid, H3tatab = 4,4′,4″-s-triazine-1,3,5-triyltrip-aminobenzoic acid, H3tml = 1,2,4-benzenetricarboxylic or trimellitic acid, H3tms = 1,3,5-benzenetricarboxylic or trimesic acid, and nmp = 1-methyl-2-pyrrolidinone. Compounds 1 and 3 have open framework structures. Compound 4 consists of unique undecanuclear Li−carboxylate units and features relatively wide 7 Å channels allowing the facile exchange of solvent molecules. Most interestingly, the framework luminescence was found to be highly dependent on the nature of those guest molecules: toluene enhances the light emission intensity, while nitrobenzene completely quenches it. Such guest-dependent behavior of porous coordination polymers attracts increasingly more Received: April 9, 2014 Revised: July 11, 2014

A

dx.doi.org/10.1021/cg500477s | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

Synthesis of [Li(H2tatab)]·5H2O (3). A solution of LiOH·H2O (4.0 mg, 0.10 mmol) in 1.0 mL of CH3OH was added to H3tatab (7.0 mg, 0.014 mmol) dissolved in 1 mL of 1,4-dioxane by a thorough sonication within 30 min. The resultant solution was sealed in a glass tube and heated at 100 °C during 3 days. Colorless needle-shaped crystals were filtered and washed with 0.5 mL of acetone (yield 6.2 mg, 0.011 mmol, 76% based on H3tatab). Anal. Calcd for C24H27LiN6O11 (%): C, 49.50; H, 4.67; N, 14.40. Found: C, 49.40; H, 4.50; N, 14.20. IR (KBr, cm−1): 1700 (s), 1610 (s), 1570 (m), 1515 (m), 1490 (s), 1408 (s), 1400 (s), 1330 (w), 1280 (s), 1240 (s), 1220 (m), 1170 (s), 1120 (w), 1080 (w), 1050 (w), 1010 (w), 990 (s), 890 (m), 880 (m), 855 (s), 790 (s), 770 (s), 730 (w), 690 (w), 680 (w), 660 (w), 640 (w), 615 (m), 580 (w), 540 (m), 520 (w), 490 (m), 390 (w). Synthesis of (H3O)[Li11(H2O)5(Html)6]·8C4H8O2 (4). A solution of LiOH·H2O (20 mg, 0.48 mmol) in 1.0 mL of cellosolve was added to a 1,2,4-benzenetricarboxylic acid (80 mg, 0.38 mmol) dissolved in 1.0 mL of 1,4-dioxane. The resultant solution was sealed in a glass tube and heated at 80 °C during 24 h. Colorless octahedral crystals were filtered off and washed with 0.5 mL of acetone (yield 83.3 mg, 0.040 mmol, 90% based on lithium). Anal. Calcd for C86H101Li11O58 (%): C, 48.29; H, 4.76. Found: C, 48.00; H, 4.51. IR (KBr, cm−1): 1700 (s), 1614 (s), 1490 (m), 1440 (s), 1400 (s), 1377 (s), 1297 (m), 1253 (m), 1170 (w), 1120 (s), 1078 (m), 915 (s), 840 (m), 820 (m), 770 (s), 719 (m), 670 (m), 612 (m), 580 (m), 540 (m), 480 (w), 416 (m). Synthesis of [Li{Li(nmp)}{Li(H2O)(nmp)}{Li(nmp)2}2(H2tms)(Htms)2]·NMP·EtOH (5). A solution of LiOH·H2O (10 mg, 0.24 mmol) in 1.0 mL of C2H5OH was added to a 1,3,5benzenetricarboxylic acid (80 mg, 0.38 mmol) dissolved in 3.0 mL of NMP by sonication within 30 min. The resultant solution was sealed in a glass tube and heated at 80 °C during 3 days. Colorless octahedral crystals were filtered and washed with 0.5 mL of acetone (yield 8.0 mg, 0.0056 mmol, 12% based on lithium). Anal. Calcd for C64H84Li5N7O18 (%): C, 60.33; H, 6.64; N, 7.69. Found: C, 59.90; H, 6.80; N, 7.51. IR (KBr, cm−1): 1858 (w), 1700 (s), 1590 (s), 1513 (m), 1480 (w), 1448 (m), 1404 (m), 1380 (s), 1368 (s), 1306 (s), 1262 (m), 1205 (w), 1175 (w), 1122 (m), 1066 (w), 1024 (w), 989 (m), 941 (w), 805 (m), 763 (s), 746 (w), 695 (s), 610 (m), 570 (w), 470 (w), 387 (s). Guest Exchange in Porous Coordination Framework 4. In a typical procedure, the as-synthesized colorless crystals of compound 4 were immersed in organic solvent during 2 days at 50 °C. After that, the crystalline materials were filtered off, washed with CH2Cl2, and dried in a vacuum of a membrane pump during 5 min. Elemental Anal. Calcd for 4·6C6H5NO2 (C90H67Li11N6O54) (%): C, 49.75; H, 3.10; N, 3.87. Found: C, 50.00; H, 3.35; N, 4.00. Anal. Calcd for 4·8CH3NO2 (C62H61Li11N8O58) (%): C, 38.73; H, 3.20. Found: C, 38.60; H, 3.10. Anal. Calcd for 4·5C6H6 (C90H73Li11O42) (%): C, 56.80; H, 3.87. Found: C, 56.60; H, 3.75. Anal. Calcd for 4·5C 6 H 5 CH 3 (C89H77Li11O42) (%): C, 56.41; H, 4.10. Found: C, 56.50; H, 4.00. X-ray Crystal Structure Determination. Single crystal X-ray diffraction data of 1−5 were collected at 150 K on a Bruker Apex Duo automatic four-circle diffractometer equipped with an area detector (Mo Kα, λ = 0.71073 Å, graphite monochromator, φ and ω scans). Data collection, frame integration, and data processing were performed with the use of the APEX2 and SAINT program packages.11 The absorption correction was applied based on the intensities of equivalent reflections with the use of the SADABS program.11 The structures were solved by direct methods and refined on F2 by fullmatrix least-squares method in the anisotropic approximation (for non-hydrogen atoms) using the SHELX-97 program package.12 Positions of hydrogen atoms of organic ligands were calculated geometrically and refined by the riding model. The hydrogen atoms of water molecules were not located. X-ray data collections have been performed for several crystals of 5. The data given for structure 5 in the paper are the best we could achieve. Apparently, lower quality of X-ray data of 5 is due to nonideal crystal quality and disordering in the guest subsystem. Free solvent accessible volume was calculated by PLATON13 standard technique of filling the cavities with probe spheres of radius 1.2 Å. A summary of the crystallographic data and structural determination for 1−5 is provided in Table 1. Selected bond

attention because new methods of sensing of nitroaromatics are demanded in various aspects of social life.6,8

■

EXPERIMENTAL SECTION

Materials and Physical Measurements. All compounds were synthesized under solvothermal conditions. Starting materials include lithium hydroxide (LiOH·H2O, 99%, Aldrich), 1,2,4,5-benzenetetracarboxylic acid (C10H6O8, pyromellitic acid, 96%, Acros Organics), 1,2,4-benzenetricarboxylic acid (C9H6O6, trimellitic acid, 99%, Aldrich), 1,3,5-benzenetricarboxylic acid (C9H6O6, trimesic acid, 95%, Aldrich), 1,4-dioxane (C4H8O2, reagent grade), 1-methyl-2pyrrolidinone (C5H9NO, N-methylpyrrolidone, NMP, 99.5%, SigmaAldrich), cellosolve (C4H10O2, 2-ethoxyethanol, 99%, Sigma-Aldrich), 4-aminobenzoic acid (C7H7NO2, 99%, Acros Organics), sodium hydroxide (NaOH, 98%, Sigma-Aldrich), sodium bicarbonate (NaHCO3, 99%, Sigma-Aldrich), cyanuric chloride (99%, Aldrich) and were used without any further purification. Ethanol and methanol were dried according to the literature method by distillation over Na and Mg, respectively.9 H3tatab was synthesized according to the literature method.10 1H NMR (DMSO-d6): δ 7.87 (d, 6H), 7.97 (d, 6H), 9.84 (s, 3H), 12.55 (s, br, 3H). The excitation and emission spectra of the solid samples were recorded on a fluorescence spectrophotometer Cary Eclipse (Varian). The emission spectra of the complexes were recorded at room temperature under the following experimental conditions: λexc = 340 nm, V = 500 V, spectral slit width = 5 nm. The PL spectra were registered for the samples prepared by grinding compounds to powder between two quartz glasses. The thin layer of powder between glasses was placed at 45° to the excitation light beam. The xenon flash lamp was used as a light source to excite the steady-state PL spectra. Powder X-ray diffraction (PXRD) data were obtained on Shimadzu XRD 7000S diffractometer (Cu Kα radiation, λ = 1.54178 Å): 2θ step = 0.03, counting time = 1.0−2.5 s, 2θ scan range = 3−25°. For compounds 1−5, the powder X-ray diffraction patterns are present in Figures S5−10, Supporting Information. Fourier-transform infrared (FT-IR) spectra in the range from 4000−300 cm−1 were measured on a Vertex 80 spectrometer. The IR spectra are present in Figures S16− S20, Supporting Information. Thermogravimetric analyses (TGA) were obtained on NETZSCH TG 209 F1. The sample quantity ranged from 2 to 10 mg. All samples were heated under a He atmosphere from room temperature up to 250 °C at a 2 °C·min−1 heating rate. Elemental C, H, N analysis data were obtained on Eurovector 600 analyzer. Synthesis of [Li2(H2pml)]·C4H8O2 (1). A solution of LiOH·H2O (10 mg, 0.24 mmol) in 1.1 mL of CH3OH was added to 1,2,4,5benzenetetracarboxylic acid (40 mg, 0.16 mmol) in 1.1 mL of 1,4-dioxane prepared by sonication within 20 min to slurry formation. The resultant solution was sealed in a glass tube and heated at 80 °C during 5 h. Colorless octahedral crystals were filtered and washed with 0.5 mL of acetone (yield 40 mg, 0.11 mmol, 94% based on lithium). Anal. Calcd for C14H12Li2O10 (%): C, 47.48; H, 3.42. Found: C, 47.50; H, 3.60. IR (KBr, cm−1): 1720 (s), 1580 (s), 1500 (s), 1490 (s), 1350 (s), 1330 (s), 1290 (m), 1260 (m), 1140 (s), 1120 (s), 1080 (w), 1050 (m), 1010 (s), 960 (s), 870 (s), 820 (w), 800 (w), 760 (s), 640 (s), 610 (m), 510 (s), 420 (s). Synthesis of [Li2(H2pml)] (2). A solution of LiOH·H2O (10 mg, 0.24 mmol) in 1.0 mL of CH 3OH was added to 1,2,4,5benzenetetracarboxylic acid (50 mg, 0.20 mmol) dissolved in 1.0 mL of THF by a sonication within 20 min. The resultant solution was sealed in a glass tube and heated at 80 °C during 12 h. Colorless octahedral crystals were filtered and washed with 0.5 mL of acetone (yield 28.7 mg, 0.11 mmol, 90% based on lithium). Anal. Calcd for C10H4Li2O8 (%): C, 45.15; H, 1.51. Found: C, 45.00; H, 1.80. IR (KBr, cm−1): 1700 (s), 1580 (s), 1500 (s), 1470 (s), 1400 (m), 1360 (s), 1290 (m), 1260 (m), 1160 (w), 1120 (w), 1010 (s), 960 (s), 870 (m), 820 (m), 780 (s), 750 (s), 590 (s), 520 (s), 490 (m), 450 (w), 400 (s). B



Article

Table 1. Crystal Data and Structure Refinement for 1−5 1 formula M, g/mol cryst syst space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z Dcalcd, g/cm3 μ, mm−1 F(000) cryst size, mm3 θ range, deg index ranges

reflns collected/indep Rint reflns with I > 2σ(I) Tmax/Tmin GOF final R indices [I > 2σ(I)] R indices (all data) largest diff. peak/hole, e/Å3

C14H12Li2O10 354.12 tetragonal I41/amd 17.301(3) 17.301(3) 10.1195(17) 90 90 90 3029.1(9) 8 1.553 0.132 1456 0.33 × 0.15 × 0.14 2.33−30.59 −24 ≤ h ≤ 24, −20 ≤ k ≤ 24, −14 ≤ l ≤ 12 19033/1241 0.0596 1180 0.9818/0.9579 1.207 R1 = 0.0434, wR2 = 0.1237 R1 = 0.0454, wR2 = 0.1250 0.514/−0.213

2

3

C10H4Li2O8 266.01 triclinic P1̅ 5.0422(3) 7.1471(5) 7.2031(5) 83.237(2) 76.586(2) 81.731(2) 248.91(3) 1 1.775 0.153 134 0.17 × 0.11 × 0.10 2.89−28.51 −6 ≤ h ≤ 5, −7 ≤ k ≤ 9, −9 ≤ l ≤ 8 2076/1249 0.0099 1099 0.9848/0.9744 1.063 R1 = 0.0319, wR2 = 0.0840 R1 = 0.0374, wR2 = 0.0884 0.447/−0.221

C86H101Li11O58 2139.01 trigonal P3̅1c 18.6108(7) 18.6108(7) 19.3734(7) 90 90 120 5811.2(4) 2 1.222 0.102 2228 0.32 × 0.26 × 0.18 1.64−28.33 −20 ≤ h ≤ 24, −24 ≤ k ≤ 24, −25 ≤ l ≤ 17 56137/4835 0.0320 4045 0.9819/0.9681 1.090 R1 = 0.0830, wR2 = 0.2617 R1 = 0.0929, wR2 = 0.2724 1.605/−0.798

5 C64H84Li5N7O27 1418.08 monoclinic P21/n 16.3128(4) 16.4758(5) 29.1404(8) 90 91.924(1) 90 7827.5(4) 4 1.203 0.093 2992 0.30 × 0.28 × 0.16 1.40−27.52 −12 ≤ h ≤ 21, −20 ≤ k ≤ 21, −34 ≤ l ≤ 37 60055/17997 0.0277 12636 0.9853/0.9727 1.365 R1 = 0.1004, wR2 = 0.3227 R1 = 0.1230, wR2 = 0.3444 1.153/−0.743

H2pml2− organic ligands connect these chains into two other directions to form a regular 3D network where each H2pml2− anion coordinates eight Li+ cations. The resulting binodal 8,4-connected network has quite complicated topology, which could be described by point symbol (45.6)2(410.52.610.86) (see Figure S1, Supporting Information). Most interestingly, by looking at the structure along the c axis, we could identify straight square-shaped channels ca. 4.5 Å × 4.5 Å. Free solvent accessible volume in compound 1 derived from the PLATON routine analysis was found to be 38%. The channels are occupied by highly disordered 1,4-dioxane molecules that could not be modeled as a set of discrete atomic sites. We employed the PLATON/SQUEEZE11 procedure to calculate the contribution to the diffraction from the solvent region and thereby produced a set of solvent-free diffraction intensities. The final formula of 1, [Li2(H2pml)]·C4H8O2, was derived from the SQUEEZE results (388e per unit cell) and elemental analysis. The thermogravimetric data show steady 10% weight decrease upon heating to 230 °C corresponding to removal of 0.5 1,4-dioxane molecule per formula unit, followed by the framework decomposition (Figure S11, Supporting Information). Similarly, a prolonged vacuum activation of 1 at 80 °C only results in a partially activated material [Li2(H2pml)]·1/2C4H8O2 with a similar powder diffraction pattern as for the pristine compound 1 and an absence of N2 or CO2 gas adsorption (Figure S5, Supporting Information).15 Structure of [Li2(H2pml)] (2) and a Solvent Template Effect. The crystalline precipitate 2 was obtained in high yield in the solvothermal reaction of H4pml with slight excess of LiOH in methanol/THF mixture at 80 °C. According to the X-ray crystallography data, the asymmetric unit of 2 contains

lengths and angles are listed in Tables S1−S5, Supporting Information. CCDC 994777−994781 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.

■

4

C24H27LiN6O11 582.46 monoclinic P21/c 4.9748(2) 37.2189(12) 16.0246(5) 90 92.582(1) 90 2964.05(18) 4 1.305 0.104 1216 0.30 × 0.10 × 0.08 1.68−28.43 −3 ≤ h ≤ 6, −42 ≤ k ≤ 49, −21 ≤ l ≤ 20 19675/7344 0.0301 5391 0.9917/0.9695 1.069 R1 = 0.0746, wR2 = 0.2127 R1 = 0.0961, wR2 = 0.2254 1.053/−0.680

RESULTS AND DISCUSSION

Syntheses of Coordination Compounds 1−5. All Li− organic coordination polymers were synthesized by solvothermal methods. In the typical syntheses, mixed solvent systems of alcohol and aprotic polar solvent were used. As a lithium source, LiOH was chosen to facilitate the deprotonation of the carboxylic acids. All reactions were carried out at 80−100 °C, and increase of the temperatures did not give any improvements in the yields. Crystal Structure of [Li2(H2pml)]·C4H8O2 (1). The solvothermal reaction of LiOH and H4pml in methanol/ dioxane mixture at 80 °C results in a crystalline precipitate, the structure of which was solved by single-crystal X-ray crystallography. The asymmetric unit of 1 contains one Li+ cation and one benzenetetracarboxylate anion. Two hydrogen atoms were found on the partially deprotonated H2pml2− ligands with intramolecular hydrogen bonds (d(O···O) = 2.392(1) Å) between neighboring carboxylate groups (Figure 1). Lithium cations have tetrahedral coordination environment with four oxygen atoms of four different H2pml2− ligands. The Li−O bond lengths (1.997(2) and 1.8798(19) Å) are common for carboxylate complexes of lithium with coordination number 4.14 The Li+ cations are connected through bridging μ2-RCOO-O,O′ groups into helical chains, running along the c crystallographic direction of the tetragonal unit cell (Figure 1). The C



Article

topological isomers since they have the same composition of the metal−organic framework [Li2(H2pml)] but drastically different structures. Both compounds were reproducibly isolated as phase pure crystalline materials with high yield and no indication of the alternative isomer, according to XRD or TGA data. Such high selectivity is especially remarkable considering very similar reaction conditions where the solvent composition is the only significant distinction between two procedures. We can conjecture that the formation of open channels in 1 is strongly attributed to a guest molecule template effect. Moreover, it must be very sensitive to the size of the endotemplate because THF molecules, having a similar nature but slightly smaller size than 1,4-dioxane, do not direct the self-assembly of the Li−carboxylate framework to an open structure, resulting in the nonporous 2. The template effect phenomenon is one of the most fundamental in supramolecular synthesis of zeolites; however, for porous metal−organic 3D frameworks, such examples are rather uncommon16 since the design of coordination networks mostly relies on shape-persistent secondary building units with robust structure-directing coordination geometries.17 Crystal Structure of [Li(H2tatab)]·5H2O (3). The metal− organic coordination polymer 3 was crystallized from the methanol/dioxane solution of LiOH and H3tatab ligand at 100 °C. The X-ray analysis revealed one Li+ cation and one H2tatab− anion per the monoclinic unit cell. Lithium cations are in tetrahedral coordination by four oxygen atoms of four different H2tatab− ligands. The Li−O bond lengths (1.909(5)− 1.968(5) Å) found are typical for tetrahedral carboxylate complexes of lithium. Lithium cations are connected by bridging carboxylate groups into chain-like units, running along the a direction (Figure 3). Another monodentate protonated carboxylic group, RCOOH, is connected to each Li+ center, completing their tetrahedral environment. There is an intermolecular hydrogen bond between the OH function of the carboxylic groups and an oxygen atom of a bridging carboxylate with relatively short O···O interatomic distance 2.581(3) Å. Interestingly, only two of three carboxylic groups of the tatab ligands are connected to Li cations. The other protonated RCOOH group forms an intermolecular double Hbond with a complementary part of the other tatab ligand (Figure 3). The corresponding interatomic distances are 2.664(3) Å for N···H−O and 2.919(3) Å for N−H···O. The anionic H2tatab− ditopic ligands connect the Li−carboxylate chains into two other directions to form a 3D metal−organic coordination network with open structure and relatively wide channels of ca. 8 Å × 10 Å size, running along the a axis. The solvent accessible volume in 3 derived from PLATON analysis was calculated to be 31%. The interstitial part of the crystal structure is filled with disordered guest solvent molecules, which could be removed by heating. The calculated weight loss in the TG curve (14%) corresponds to a loss of 4.5 H2O molecules. The overall framework topology of 3 could be described as a rod-like net with a square-grid pattern of the interconnected organic ligands.18 Crystal Structure of (H3O)[Li11(H2O)5(Html)6]·8C4H8O2 (4). The heating of LiOH and trimellitic acid (H3tml) in ethylcellosolve/1,4-dioxane mixture produces colorless crystals in high yield the structure of which was investigated by X-ray crystallography. The asymmetric unit of 4 contains three Li cations and one partly protonated Html2− anion. Lithium cations are assembled into unprecedented undecanuclear {Li11} units5f through bridging oxygen atoms of the μ3-RCOO-O,O,O′ and μ2-RCOO-O carboxylate groups (Figure 4). These

Figure 1. Crystal structure of 1. (a) Side view of the helical Li− carboxylate chain. Hydrogen intramolecular bonds through the disordered protons are marked by pink dotted lines. (b) The perspective projection of the structure along the c axis. Li, green; O, red; C, gray; H, white.

one Li+ cation and one H2pml2− anion. The Li centers adopt a tetrahedral coordination environment with four oxygen carboxylate atoms. The Li−O bond lengths (1.917(2)− 1.976(2) Å) found fall within the common values for tetrahedral carboxylate complexes of lithium. Two lithium cations are joined via two oxygen atoms of two bridging carboxylate groups forming a binuclear building unit, {Li2(μ2-RCOO)2(RCOO)4} (Figure 2). Such Li2 units are connected to six independent organic ligands, while each ligand is connected to six {Li2} units. The overall six-connected 3D metal−organic network adopts a primitive cubic topology (pcu) or, more specifically, NaCl, emphasizing a different nature of the alternating sixconnected nodes (Figure S2, Supporting Information). The overall charge of the framework is neutral due to a partial protonation of organic ligands and a formation of an intramolecular hydrogen bond between neighboring carboxylate groups of H2pml2− ligand (d(O···O) = 2.469(1) Å). The metal−organic framework 2 is densely packed with no apparent solvent-accessible cavities. According to TGA, no weight loss is observed up to 120 °C (Figure S12, Supporting Information). Irreversible thermolysis takes place above 220 °C. Strikingly, both compounds 1 and 2 are D



Article

Figure 2. Crystal structure of 2. (a) Coordination geometry of Li cations and the structure of the {Li2(μ2-RCOO)2(RCOO)4} building unit. (b) Local environment and connectivity of the H2pml2− anion. The intramolecular hydrogen bonds are marked by pink dotted lines. (c) The perspective projection of the structure along the a axis. Li, green; O, red; C, gray; H, white.

Figure 3. Crystal structure of 3. (a) View of Li−carboxylate chain along the a axis. Intermolecular hydrogen bonds are shown by dotted pink lines. (b) View of the crystal packing along the a axis. Guest solvent moelcules are omitted. The intermolecular H-bonds are indicated by dotted pink lines. Inset shows the scheme of the double H-bond formation between tatab ligands.

polynuclear units are located on 3-fold axes of the crystal structure and adopt the S3 point group symmetry. The Li(1)

cation has a common tetrahedral coordination environment containing oxygen atoms of four Html2− ligands with normal E



Article

Figure 4. Crystal structure of 4. (a) Structure of the {Li11}−carboxylate complex. Disordered Li+ positions are shown light-green. (b) Connectivity of two adjacent {Li11} units through double Html− linkers. (c) Projection of the metal−organic framework along the c axis. Li, green; C, gray; O, red; hydrogen atoms and solvent molecules are omitted.

{Li11(H2O)5(RCOO)18}. Each Html2− ligand connects two such units in a double bridge fashion (Figure 4). There are intermolecular hydrogen bonds between carboxylic groups of neighboring Html2− ligands with relatively short O···O distance 2.495(2) Å. Overall, each {Li11} complex is connected to six neighboring complexes via 12 bridging Html2− ligands, and the geometry of such {Li11} building units could be adequately described as trigonal prism. The connection of trigonal prisms results in a 3D metal−organic coordination framework with wellknown acs topology (as a subnet of the NiAs niccolite mineral; Figure S3, Supporting Information). There are relatively wide hexagonal channels running along the c direction (diameter ca. 7 Å), intersecting through smaller windows (ca. 3 Å × 8 Å) (Figure 4). These channels are occupied by disordered 1,4-dioxane molecules and H3O+ cations. The solvent accessible volume in guest-free compound 4 reaches 54% as estimated by PLATON. The assynthesized 4 released all six guest 1,4-dioxane molecules and 5 H2O corresponding to 32% weight decrease until 150 °C (Figure S14, Supporting Information). Unfortunately, guest removal by vacuum activation of 4 results in amorphization of the crystalline compound. However, the porosity of 4 was confirmed by a number of solvent exchange experiments (vide inf ra).

Li(1)−O bond lengths (1.909(3)−1.929(4) Å). The Li(2) cation features a tetragonal pyramidal coordination environment containing five oxygen atoms of four Html2− anions (equatorial positions) and one aqua ligand (axial position). The Li(2)−O(carboxylate) bond lengths are 2.019(2) and 2.153(2) Å, which is typical for Li−O distances with the coordination number of Li+ being 5, while the Li(2)−O(H2O) bond length is somewhat shorter (1.920(7) Å).14 The third lithium cation is disordered over two half occupied positions (Li(3) and Li(4)) along the 3-fold axis. Both Li(3) and Li(4) have a tetrahedral coordination environment containing four oxygen atoms of three Html2− anions and one aqua-ligand. The H2O ligand of the Li(3) cation is located in the center of the {Li11} cage, while the H2O ligand of the Li(4) cation is directed outward from the {Li11} unit along the 3-fold axis. Since only one of the disordered Li(4) position exists in the polynuclear unit, the corresponding coordinated water molecules must be disordered over two half occupied symmetry related positions. The Li− O(carboxylate) bond lengths are 1.953(3) and 1.949(5) Å and the Li−O(H2O) distances are 1.87(1) or 1.74(2) Å for Li(3) and Li(4), respectively. Taking the disorder into account, the overall formula of the {Li11} complex unit could be written as F



Article

Figure 5. Crystal structure of 5. (a) Structures of the Li−carboxylate building units. Only one oxygen atom of the disordered NMP ligand of the Li(3) cation is presented. (b) Perspective view of the crystal structure along the a direction. (c) Perspective view of the crystal structure along the b direction. Hydrogen atoms and solvent molecules are omitted for clarity. Pink dotted bonds indicate hydrogen bonds between partly protonated carboxylate groups.

Crystal Structure of [Li{Li(nmp)}{Li(H2O)(nmp)}{Li(nmp)2}2(H2tms)(Htms)2]·NMP·EtOH (5). The single crystals were isolated after heating of the ethanol/NMP mixture of LiOH and trimesic acid H3tms at 80 °C. The single-crystal X-ray analysis of 5 revealed five Li+ cations, one H2tms− and two Htms2− anions, six coordinated NMP molecules, and one aqua ligand in the asymmetric unit. All the lithium cations have tetrahedral coordination environments of oxygen atoms. The structure is composed of three Li−carboxylate units of different nuclearity and connectivity. The Li(1) and Li(2) cations form a binuclear 3-connected unit (Figure 5) where Li(1) is coordinated by two oxygen atoms of NMP ligands and two oxygen atoms of carboxylate groups and the Li(2) is coordinated by one oxygen atom of an NMP ligand and three carboxylic oxygen atoms. Two of these carboxylic groups form bridges between Li cations in μ2-COO-O,O′ and μ2COO-O fashion. Cations Li(3) and Li(4) make up another binuclear four-connected building unit (Figure 5). The Li(3) is coordinated by two oxygen atoms of NMP ligands (one being disordered over two positions) and two carboxylate anions. The Li(4) is coordinated by four oxygen atoms of four different carboxylate anions. These Li cations are bridged by two μ2-O atoms of carboxylate groups. Finally, Li(5) cations form a mononuclear unit, coordinated by two carboxylate anions, one oxygen atom of the NMP ligands, and a H2O molecule. The Li−O(carboxylate) bond lengths (1.902(4)−2.022(5) Å), Li−O(NMP) distances (1.856(7)−2.065(9) Å), and Li(5)−

O(H2O) distance (1.964(8) Å) fall within the characteristic range for tetrahedral carboxylate complexes of lithium. The organic trimesate ligands were found to be partly protonated (H2tms− and Htms2−) with hydrogen bonds existing between neighboring carboxylic groups (O···O distances are in the range 2.404(3)−2.456(3) Å) (Figure 5). These organic ligands serve as three-connectors between the inorganic lithium-based building units, forming a complicated 3,4-connected 3D topology with point symbol (5.8.9)2(5.92)(5.8.10)(5.8.92.102). Alternatively, the coordination net in 5 could be described as 2D alternating layers with fes pattern, interconnected into a 3D structure (Figure S4, Supporting Information). The interstitial volume of the crystal compound is occupied by coordinated and guest solvent molecules. The latter are highly disordered and could not be modeled as a set of discrete atomic sites. The hypothetic removal of NMP and EtOH guest molecules from 5 results in an accessible volume ca. 24%. The final formula of 5, [Li{Li(nmp)}{Li(H2O)(nmp)}{Li(nmp)2}2(H2tms)(Htms)2]· NMP·EtOH, was calculated from the SQUEEZE results (362e per unit cell) combined with elemental (C, H, N) analysis data. The TG analysis of 5 (Figure S15, Supporting Information) shows ca. 25% mass decrease related to the solvent removal (up to 150 °C), followed by a framework destruction at higher temperatures. Guest-Exchange and Photoluminescence Studies. The dioxane solvent molecules were substituted with toluene, benzene, nitromethane, or nitrobenzene by immersing the G



Article

absorption above 320 nm; thus, no overlap takes place between nitrobenzene absorption and the framework luminescence peaks. We can also note that a similar electron transfer mechanism could, at least partly, take place for nitromethane guest molecules, which results in somewhat lower luminescence intensity of 4·8CH3NO2 compared with pristine 4·6C4H8O2. Such interesting luminescence properties of porous coordination framework 4 with remarkable sensitivity toward the nature of the guest molecules could be implemented in sensing devices for detection of explosive nitroaromatics and other dangerous species.8

crystals 4·6C4H8O2 into the corresponding solvents. The XRD indicates the stability of the metal−organic framework 4 during the exchange, while chemical analyses revealed the following formulas: 4·5C6H5CH3, 4·5C6H6, 4·8CH3NO2, 4·6C6H5NO2, respectively (Figure S10, Supporting Information). Photoluminescence of compound 4·6C4H8O2, having the largest channels, was investigated. The excitation and emission spectra are displayed in Figure 6. For the crystalline samples of

■

CONCLUSIONS In conclusion, five new 3D metal−organic frameworks based on lithium carboxylates of rigid linkers were obtained. All of them were synthesized by a solvothermal method in a mixed system alcohol/polar aprotic solvent. Compounds 1 and 2 feature the same framework composition but different guestcontrolled structures and topology. Compounds 1 and 3 have highly opened structures with pore sizes of 4.5 Å × 4.5 Å and 8 Å × 10 Å, correspondingly. The framework 4 is based on a undecanuclear Li−carboxylate complex {Li11(H2O)5(RCOO)18}. Assembly of these units leads to an open structure with pore diameter 7 Å. This impressive unique {Li11} fragment is an interesting example of rare lithium polynuclear complexes. Furthermore, compound 4 is capable of a reversible solvent exchange process. The photoluminescence properties of this porous compound were shown to depend on the nature of the guest molecules, providing the opportunity for the selective detection of dangerous explosive aromatic molecules.

Figure 6. Excitation and emission spectra of compound 4 (red), 4· 5C6H5CH3 (blue), 4·5C6H6 (green), 4·8CH3NO2 (black), 4· 6C6H5NO2 (pink): dotted lines, excitation spectra; continuous lines, emission spectra.

■

ASSOCIATED CONTENT

S Supporting Information *

as-synthesized 4·6C4H8O2 with 1,4-dioxane guest molecules, emission spectra display an intensive blue broad band with a maximum at 440 nm, which could be assigned to an intraligand π* → π transition in the 1,2,4-benzenetricarboxylate anion. The emission spectra of the guest-exchanged series 4·{guests} show the broad band at 440 nm indicating the similar nature of the benzenetricarboxylate-centered electron transition. Remarkably, the intensity of the photoluminescence turned out to be quite different, depending on the nature of the guest molecules. By rough estimation, the inclusion of benzene and toluene increases the intensity of the luminescence by ca. 1.5 and 3.5 times, respectively, while the inclusion of nitromethane decreases it 2.5 times. Preliminarily, we can explain such changes by incident light adsorption by aromatic molecules (benzene or toluene), followed by the energy or electron transfer to the framework thus exciting the benzenetricarboxylate ligand. Assuming this mechanism, the stronger increase of the luminescence for G = toluene compared with G = benzene is apparent, taking the electron-donor properties of methyl group into account. This process is accompanied by the nonemissive relaxation of the excitation through the complex framework dynamics and interatomic vibrations. Such framework relaxation must depend on the guest nature and composition, although it is quite hard to rationalize. Most interestingly, the inclusion of nitrobenzene completely quenches the luminescence of the framework 4, contrary to the other guest molecules. The nature of luminescence quenching can be attributed to the electron transfer from the excited π* ligand orbital to the electron-deficient nitroaromatic system of the guest molecule with subsequent nonemissive energy dissipation. The alternative energy transfer quenching mechanism is unlikely because the nitrobenzene UV absorbance spectrum has a maximum at λ = 260 nm and almost no

Crystallographic data in CIF format, additional structural figures, IR spectra, PXRD data, and TGA data for the all compounds. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Fax: +7 (383) 330-94-89. Tel: +7 (383) 330-94-90. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work is supported by grant of the Government of the Russian Federation (PN 14.Z50.31.0006, leading scientist M. Schröder). REFERENCES

(1) Topics in Current Chemistry; Schröder, M., Ed.; Springer: Berlin, 2010; Vol. 293, 264 pp. (2) Recent special issues on metal−organic frameworks: (a) Microporous Mesoporous Mater. 2012, 157, 1−146. (b) J. Mater. Chem. 2012, 22, 10093−10358. (c) Chem. Soc. Rev. 2009, 38, 1201− 1508. (d) Chem. Rev. 2012, 112, 673−1268. (e) Inorg. Chim. Acta 2012, 157, 1−145. (3) (a) Lin, X.; Champness, N. R.; Schröder, M. In Topics in Current Chemistry; Schröder, M., Ed.; Springer: Berlin, 2010; Vol. 293, pp 35− 76. (b) Suh, M. P.; Park, H. J.; Prasad, T. K.; Lim, D.-W. Chem. Rev. 2012, 112, 782−835. (c) Murray, L. J.; Dincǎ, M.; Long, J. R. Chem. Soc. Rev. 2009, 38, 1294−1314. (4) Rowsell, J. L. C.; Yaghi, O. M. Angew. Chem., Int. Ed. 2005, 44, 4670−4679. H



Article

(5) (a) Zhao, X.; Wu, T.; Zheng, S.-T.; Wang, L.; Bu, X.; Feng, P. Chem. Commun. 2011, 47, 5536−5538. (b) Guo, Y.; Zhang, J.; Jiang, L.; Shi, X.; Yang, L.; Fang, Q.; Fang, H.; Wang, K.; Jiang, K. Chem. Commun. 2012, 48, 10639−10641. (c) Abrahams, B. F.; Grannas, M. J.; Hudson, T. A.; Robson, R. Angew. Chem., Int. Ed. 2010, 49, 1087− 1089. (d) Zheng, S.-T.; Li, Y.; Wu, T.; Nieto, R. A.; Feng, P.; Bu, X. Chem.−Eur. J. 2010, 16, 13035−13040. (e) Wu, T.; Zhang, J.; Zhou, C.; Wang, L.; Bu, X.; Feng, P. J. Am. Chem. Soc. 2009, 131, 6111− 6113. (f) Clough, A.; Zheng, S.-T.; Zhao, X.; Lin, Q.; Feng, P.; Bu, X. Cryst. Growth Des. 2014, 14, 897−900. (g) Osta, R. E.; Frigoli, M.; Marrot, J.; Guillou, N.; Chevreau, H.; Walton, R. I.; Millange, F. Chem. Commun. 2012, 48, 10639−10641. (6) (a) Allendorf, M. D.; Bauer, C. A.; Bhaktaa, R. K.; Houka, R. J. T. Chem. Soc. Rev. 2009, 38, 1330−1352. (b) Cui, Y.; Yue, Y.; Qian, G.; Chen, B. Chem. Rev. 2012, 112, 1126−1162. (c) Kreno, L. E.; Leong, K.; Farha, O. K.; Allendorf, M.; Van Duyne, R. P.; Hupp, J. T. Chem. Rev. 2012, 112, 1105−1125. (7) Recent selected examples of porous coordination polymers with guest-dependent luminescence: (a) Zhan, C.; Ou, S.; Zou, C.; Zhao, M.; Wu, C.-D. Anal. Chem. 2014, 86, 6648−6653. (b) Li, X.; Xu, H.; Kong, F.; Wang, R. Angew. Chem., Int. Ed. 2013, 52, 13769−13773. (c) Lan, Y.-Q.; Jiang, H.-L.; Li, S.-L.; Xu, Q. Adv. Mater. 2011, 23, 5015−5020. (d) Sapchenko, S. A.; Samsonenko, D. G.; Dybtsev, D. N.; Melgunov, M. S.; Fedin, V. P. Dalton Trans. 2011, 40, 2196−2203. (e) An, J.; Shade, C. M.; Chengelis-Czegan, D. A.; Petoud, S.; Rosi, N. L. J. Am. Chem. Soc. 2011, 133, 1220−1223. (f) Stylianou, K. C.; Heck, R.; Chong, S. Y.; Bacsa, J.; Jones, J. T. A.; Khimyak, Y. Z.; Bradshaw, D.; Rosseinsky, M. J. J. Am. Chem. Soc. 2010, 132, 4119−4130. (8) (a) Pramanik, S.; Zheng, C.; Zhang, X.; Emge, T. J.; Li, J. J. Am. Chem. Soc. 2011, 133, 4153−4155. (b) Nagarkar, S. S.; Joarder, B.; Chaudhari, A. K.; Mukherjee, S.; Ghosh, S. K. Angew. Chem., Int. Ed. 2013, 52, 2881−2885. (c) Kim, T. K.; Lee, J. H.; Moon, D.; Moon, H. R. Inorg. Chem. 2013, 52, 589−595. (d) Kim, Y.; Song, J. H.; Lee, W. R.; Phang, W. J.; Lim, K. S.; Hong, C. S. Cryst. Growth Des. 2014, 14, 1933−1937. (e) Qian, J.-J.; Qiu, L.-G.; Wang, Y.-M.; Yuan, Y.-P.; Xie, A.-J.; Shen, Y.-H. Dalton Trans. 2014, 43, 3978−3983. (f) Wu, Z.-F.; Tan, B.; Feng, M.-L.; Lan, A.-J.; Huang, X.-Y. J. Mater. Chem. A 2014, 2, 6426−6431. (9) Becker, H.; Berger, W. Organikum; MIR Publishing Group: Moscow, 1979; Vol. 2, Chapter E. (10) Fang, Q.-R.; Yuan, D.-Q.; Sculley, J.; Li, J.-R.; Han, Z.-B.; Zhou, H.-C. Inorg. Chem. 2010, 49, 11637−11642. (11) Bruker Advanced X-ray Solutions; Bruker AXS Inc.: Madison, WI, 2004. (12) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112−122. (13) Spek, A. L. Acta Crystallogr. 2009, D65, 148−155. (14) According to the analyses of the Cambridge Structural Database, the average Li−O distances are 1.95(7) for CN = 4 and 2.06(13) for CN = 5. (15) The exchange of 1,4-dioxane with other volatile solvents does not take place at room temperatures. To remove guest 1,4-dioxane molecules, compound 1 was heated in vacuum (10−3 mPa) at 80 °C within 15 h. According to powder XRD analysis, the framework retains its stability after the activation procedure. Unfortunately, neither N2 at 77 K nor CO2 at 273 K were adsorbed. According to the chemical analysis, 0.5 1,4-dioxane molecule was removed from formula unit. Anal. Calcd for C12H8Li2O9 (%): C, 46.48; H, 2.60. Found: C, 46.55; H, 2.80. (16) (a) Tanaka, D.; Kitagawa, S. Chem. Mater. 2008, 20, 922−931. (b) Stock, N.; Biswas, S. Chem. Rev. 2012, 112, 933−969. (17) (a) O’Keeffe, M. Chem. Soc. Rev. 2009, 38, 1215−1217. (b) Tranchemontagne, D. J.; Mendoza-Cortés, J. L.; O’Keeffe, M.; Yaghi, O. M. Chem. Soc. Rev. 2009, 38, 1257−1283. (c) Perry, J. J., IV; Perman, J. A.; Zaworotko, M. J. Chem. Soc. Rev. 2009, 38, 1400−1417. (d) Cook, T. R.; Zheng, Y.-R.; Stang, P. J. Chem. Rev. 2013, 113, 734− 777. (18) Rosi, N. L.; Kim, J.; Eddaoudi, M.; Chen, B.; O’Keeffe, M.; Yaghi, O. M. J. Am. Chem. Soc. 2005, 127, 1504−1518.

I


Syntheses and Structural Characterization of Lithium Carboxylate

Recommend Documents