Synthesis and Structural Characterization of Lithium-Based

Sep 11, 2009 - S.J.K. is grateful for the support from Korea Institute of Science and Technology. (KIST). The authors would like to thank Yu-Sheng Che...
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DOI: 10.1021/cg900705c

Synthesis and Structural Characterization of Lithium-Based Metal-Organic Frameworks

2009, Vol. 9 4922–4926

Debasis Banerjee,† Lauren A. Borkowski,^ Sun Jin Kim,‡ and John B. Parise*,†,§,^ †

Department of Chemistry, Stony Brook University, Stony Brook, New York 11794-3400, Nano-Material Research Center, Korea Institute of Science and Technology, P.O. Box. 131, Cheongryang, Seoul-130-650, Korea, §Department of Geosciences, Stony Brook University, Stony Brook, New York 11794-2100, and ^Mineral Physics Institute, Stony Brook University, Stony Brook, New York 11794-2100

‡

Received June 23, 2009; Revised Manuscript Received August 24, 2009

ABSTRACT: Two lithium-based metal-organic frameworks, Li2(C14H8O4) [Li2(4,40 -BPDC) [1]; ULMOF-2, UL = ultralight; BPDC = biphenyldicarboxylate]; space group P21/c, a = 12.758(2) A˚, b = 5.142(4) A˚, c = 8.00(2) A˚, β = 97.23, V = 520.6(14) A˚3 and Li2(C14H8O6S) [Li2(4,40 -SDB) [2]; ULMOF-3, UL = ultralight; SDB = sulfonyldibenzoate], space group P21/n, a = 5.5480(11) A˚, b = 23.450(5) A˚, c = 10.320(2) A˚, β = 96.47(3), V = 1334.1(5) A˚3, were synthesized. Compounds 1 and 2 were synthesized by solvothermal methods and were characterized using single crystal X-ray diffraction. Structure 1 consists of layers of two-dimensional antifluorite related LiO motif connected by BPDC linkers, whereas structure 2 is constructed by a combination of tetrameric lithium polyhedral clusters connected by the sulfonyldibenzoate linker. The frameworks are stable up to 575 and 500 C, respectively, under N2 atmosphere.

1. Introduction A wide range of metal-centers forms metal-organic frameworks (MOFs) through linkages with multifunctional ligands.1-3 The variety of possible geometries thus formed inspires researchers to test these MOF’s potential uses in gas storage,1,4-15 ion exchange,16-18 catalysis,19-22 and separation,23-25 and to expand exploratory synthetic efforts aimed at uncovering further novel materials tailored for specific uses. The framework topology primarily depends on the metal center, geometry of the functionalized organic linkers, and synthetic conditions (e.g., temperature,26 solvent27). Aromatic polycarboxylate ligands14,28-30 are commonly used as linker molecules due to their structural rigidity and diversity of possible coordination geometries. First row transition metals5,7,8,12,31 are the popular choice in constructing the framework, because of their well-known coordination preference with polycarboxylates. On the other hand, exploratory synthesis directed toward constructing frameworks with lightweight metals (e.g., magnesium32-35 and lithium36-39,56) are not extensively explored. The aim of producing porous frameworks incorporating lightweight metals could be beneficial with respect to increasing gravimetric storage capacity of adsorbed gases. The lightest metal in the periodic table, lithium, is of particular interest in building frameworks due to recent experimental40,41 and theoretical studies42-46 showing enhanced H2 uptake in lithium doped MOFs. The 2-fold increase40,42 in storage resulting from Li-doping is attributed to the strong electrostatic interaction between uncoordinated lithium centers and hydrogen gas molecules. With the aim of producing ultra light metal organic frameworks (ULMOF) having potential for strong interactions with absorbed gases, we embarked on an exploration of *Corresponding author. Address: 255 ESS, Room 238, Stony Brook University, Stony Brook, New York 11794-2100. Phone: (631) 632 8196. Fax: (631) 632 8240 255. E-mail: [email protected]. pubs.acs.org/crystal

Published on Web 09/11/2009

frameworks based on lithium metal centers. Our initial study suggests a rich vein of lithium-based frameworks of diverse topology depending primarily on the reaction condition and organic linker used. Herein, we report the solvothermal synthesis and characterization of two lithium-based three-dimensional (3-D) frameworks Li2(4,40 -BPDC) [1, ULMOF-2] and Li2(4,40 -SDB) [2, ULMOF-3] as part of our ongoing study. 2. Experimental Procedures 2.1. Synthesis. All compounds were synthesized under solvothermal conditions using Teflon -Lined Parr stainless steel autoclaves. Starting materials include lithium nitrate (LiNO3, 99þ%, AcrosOrganic), 4,40 -biphenyl dicarboxylic acid (C14H10O4, 4,40 -BPDC, 97%, Sigma-Aldrich), 4,40 -sulfonyldibenzoic acid (C14H10O6S, 4,40 SDB, 98%, Sigma-Aldrich), ammonium fluoride (NH4F, 98%, Sigma-Aldrich), lithium hydroxide (LiOH 3 H2O, 98%, Alfa-Aesar), N,N-dimethylformamide (C3H7NO, DMF, 99%, Sigma-Aldrich), and ethanol (C2H5OH, 99%, Sigma-Aldrich) and were used without any further purification. Structure [1]: Li2(C14H8O4); ULMOF-2. ULMOF-2 was synthesized using a mixture of 0.002 mol of LiNO3 (0.172 g), 0.002 mol of 4,40 -BPDC (0.625 g) and 0.0008 mol of NH4F (0.031 g). This mixture was dissolved in 15 g of DMF and stirred for 4 h to achieve homogeneity [molar ratio of metal salt: ligand: solvent = 1:1:100]. The resultant solution was heated for 5 days at 180 C. The product obtained was needle-shaped crystal (yield: 55% based on lithium) and was recovered by filtration and subsequently washed by ethanol. Structure [2]: Li2(C14H8O6S); ULMOF-3. A typical synthesis of ULMOF-3 includes a mixture of 0.002 mol of LiNO3 (0.172 g), 0.002 mol of 4,40 -SDB (0.67 g) and 0.001 mol of LiOH (0.042 g). The mixture was dissolved in 6.5 g of DMF and stirred for 4 h [molar ratio of metal salt/ligand/solvent = 1:1:45]. The resultant solution was heated at 180 C for 5 days. The product obtained as colorless needle-shaped crystal (yield: 45% based on lithium) and was recovered by filtration and subsequent washing by DMF and ethanol. 2.2. X-ray Crystallography. A suitable crystal of each compound 1 and 2 was selected from the bulk samples and were mounted on a glass fiber using epoxy. Reflections for compound 1 were collected at ChemMatCars (Sector 15) at the Advanced Photon r 2009 American Chemical Society

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Table 1. Crystallographic Data and Structural Refinement Details of Compounds 1 and 2 1 empirical formula formula weight collection temperature (K) wavelength (A˚) space group a (A˚) b (A˚) c (A˚) R () β () γ () volume Z calculated density (g/cm3) absorption coefficient (mm-1) F(000) crystal size (mm) Θ range of data collection index range total reflection independent reflection goodness of fit refinement method data/restraints/parameter R1 (on Fo2, I > 2σ(I)) wR2 (on Fo2, I > 2σ(I))

2

C7H4LiO2 127.04 100(2)

C14H8O6SLi2 318.14 298(2)

0.41328 P21/c (No. 14) 12.753(6) 5.138(2) 8.420(4) 90 97.218(13) 90 547.4(4) 4 1.541

0.71073 P21/n (No. 14) 5.5480(11) 23.450(5) 10.320(2) 90 96.47(3) 90 1334.1(5) 4 1.584

0.027

0.269

260 0.08  0.03  0.01 1.87-17.21

648 0.16  0.08  0.03 1.74-25.02

-18 e h e 18 0eke7 0 e l e 12 1544 1544 [R(int) = 0.0000] 0.836 full matrix least-squares on F2 1544/0/93 0.0788 0.1770

-6 e h e 6 -27 e k e 27 -11 e l e 12 7670 2364 [R(int) = 0.0997] 0.986 full matrix least-squares on F2 2364/0/209 0.0607 0.1257

Source using a three-circle Bruker D8 diffractometer equipped with an APEXII detector at 100 K using synchrotron X-ray radiation (λ = 0.41328 A˚) and 0.5 j scans. The raw intensity data were analyzed using the APEXII47 suite of software at which time it was determined that the crystal contained more than one component. CELL_NOW48 was used to determine the nonmerhedral twin law [1 0 0 0 -1 0 -0.381 0 -1] relating the two major components by a 180.0 rotation about the a axis. The data were then integrated using two components and were corrected for absorption using TWINABS.49 The major component contained 38% of the sample intensity. The structure was solved using direct methods and was refined using SHELXL.50 All non-hydrogen atoms were refined anisotropically with the hydrogen atoms were placed in idealized positions. Reflections for compound 2 were collected using a Bruker four circle P4 single crystal diffractometer equipped with a SMART 1K CCD detector at room temperature (298 K) using Mo KR radiation (λ = 0.71073 A˚) and j and ω scans. The raw intensity data for compound 2 were collected and integrated with software packages, SMART51 and SAINT.52 An empirical absorption correction was applied using SADABS.50 The crystal structure was solved using direct methods (SHELXS).50 All non-hydrogen atoms were refined anisotropically, and the hydrogen atoms were added to the biphenyl rings using geometrical constraints (HFIX command). The crystallographic details for both structures can be found in Table 1. Bulk sample identification and phase purity were determined using powder X-ray diffraction. The data were collected using a Scintag Pad-X diffractometer equipped with Cu KR (λ = 1.5405 A˚) radiation within a range of 5 e 2θ e 40 (step size: 0.02, counting time: 1 s/step). Comparison of the observed and calculated powder X-ray diffraction patterns for both 1 and 2 confirmed phase purity (Figures S1 and S2, Supporting Information). Combined TGA-DSC experiments for compounds 1 and 2 were performed using a Netzsch 449C Jupiter instrument. The samples were heated from room temperature to 750 C under N2 atmosphere (Figures 6 and 7) and air (Figures S8-S9, Suppporting Information).

Figure 1. View of Li2(4,40 -BPDC) from the [010] direction showing the connectivity of the organic linkers with the alternating antifluorite type LiO layers.

Figure 2. ORTEP plot of ULMOF-2 [1] illustrating the numbering scheme. Ellipsoids are shown at the 50% probably level. Hydrogen atoms have been omitted for clarity. Symmetry related atoms are shown to complete the coordination sphere of the lithium center. Symmetry operators: i (-x, -y, -z þ 1); ii (-x, -y þ 1, -z þ 1); iii (x, y - 1, z); iv (-x þ 1, -y þ 1, -z þ 1); v (-x þ 1, y - 1/2, -z þ 1/2).

3. Results and Discussion 3.1. Structural Description of [1], Li2(4,40 -BPDC)-ULMOF-2. ULMOF-2 consists of a combination of alternating antifluorite type LiO layers connected by aromatic biphenyl bridging units (Figure 1) to form a 3-D framework. ULMOF-2 is isoreticular with our previously reported ULMOF-1,56 which contains 2,6-naphthalene dicarboxylic acid as the organic linker. The lithium center is present in a distorted tetrahedral environment (Table 2) which has been observed in previously reported structures.38,56 The asymmetric unit (Figure 2) of ULMOF-2 consists of a crystallographically unique lithium center (Li1) connected to four carboxylate oxygens from four independent 4,40 -BPDC groups. The average Li-O distance is found to be 1.966 A˚, which is consistent with reported values36,37 forming the tetrahedral motifs (Figure S4, Supporting Information). Each carboxylate group of the 4,40 -BPDC ligand is connected to a total of four lithium centers connecting the twodimensional (2-D) layers into an overall 3-D structure. The bond valence sum53 of 1.14 valence units (v.u.) per lithium center matches the expected value of þ1. The tetrahedrally coordinated lithium centers form a 2-D LiO layer (Figure 3) consisting of dimers of edge

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Figure 5. Ellipsoidal plot of the asymmetric unit of compound [2] with ellipsoids shown at the 50% probably level. Hydrogen atoms have been omitted for clarity.

Figure 3. Polyhedral view of 2-D-antifluorite type LiO layer with square planar vacancies.

Figure 4. 3-D network of ULMOF-3 along the [100] direction showing the connectivity of organic linkers with the tetrameric lithium cluster.

sharing lithium tetrahedra that are corner-shared to form the layered structure. The distance between the lithium atoms in each dimer is 2.73(1) A˚, while the distance between the lithium atoms of corner-shared interaction is 3.104(8) A˚. The 2-D LiO layer forms an antifluorite type structural motif, which is common in lithium oxide-based compounds. The basic difference between the Li2O antifluorite motif and the observed motif in ULMOF-2 is the presence of square planar vacancies in the LiO layer. The antifluorite layers, 11.03 A˚ apart, stack along the [100] direction. The layers are separated by the biphenyl dicarboxylate groups with the biphenyl rings layered along the [100] direction. The distance

between two consecutive biphenyl rings are on the average 5 A˚ in the [010] direction. 3.2. Structural Description of [2], Li2(4,40 -SDB) - ULMOF3. ULMOF-3 consists of tetrameric clusters of tetrahedral lithium centers connected by 4,40 -SDB to form a 3-D framework (Figure 4). The asymmetric unit (Figure 5) of ULMOF3 consists of the organic ligand (4,40 -SDB) along with two crystallographically independent lithium centers (Li1, Li2). Each lithium center is present in distorted tetrahedral coordination with oxygen atoms also observed in ULMOF-2. The bond length of Li1 center tetrahedrally connected with three carboxylate oxygen (O1, O2, O3) and one sulfonyl oxygen (O4) varies between 1.923(9) A˚ to 2.08(9) A˚. In case of Li2, it is coordinated tetrahedrally with four carboxylate oxygen atoms (O1, O2, O3, O5), and the bond length varies between 1.873(9) A˚ to 2.04(9) A˚ (Table 3). The bond valence sums53 of the two lithium centers (Li1 and Li2) are 1.02 v.u. and 1.04 v.u., respectively, matching the expected value of þ1. One of the main structural characteristic of ULMOF-3 is the formation of isolated tetrameric clusters consisting of corner shared pairs of edge sharing lithium polyhedra. The isolated cluster so formed is isostructural with the basic building unit of antifluorite type extended LiO layer in ULMOF-2 with a square planar vacancy. Each of the isolated clusters of lithium polyhedra are connected with each other by the organic linker forming an overall 3-D network (Figure 4). The 4,40 -SDB molecule is not planar as compared to 4,40 BPDC, but rather V-shaped in nature due to the presence of the sulfur atom in between the two phenyl group, which allows for a torsion angle of 104.1(2). Each organic linker is associated with three tetrameric lithium polyhedral cluster using both the carboxylate oxygens and sulfonyl oxygen. The 4,40 -SDB ligand is connected to a total of eight lithium centers (Figure S7, Supporting Information). Among the carboxylate oxygens, only one oxygen center (O5) is associated with only one lithium center, whereas the other carboxylate oxygens are associated with two lithium metal centers each. In case of sulfonyl oxygen, O4 is associated in bonding interaction with lithium center (Li1), while O6 remains uncoordinated. The preference in coordination between the sulfonyl oxygens is mainly due to the geometrical constraint imposed by the V-shaped ligand. The lengths of the sulfur-oxygen bonds are consistent with reported values51,52 (Table 3). The phenyl rings of 4,40 SDB are stacked on each other along the [100] and [001] direction. The average distances between the phenyl rings of each successive layer are 5.5 and 10.32 A˚ along the [100] and [001] directions, respectively. 3.3. Discussions. Both ULMOF-2 and ULMOF-3 have similar types of ligand backbones with two phenyl rings. The

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Figure 6. TGA-DSC plot of ULMOF-2. The blue line represents TGA plot of ULMOF-3, while the red line shows the DSC signal associated with it.

Figure 7. Combined TGA-DSC plot of ULMOF-3. The blue line represents TGA plot of ULMOF-3, while the red line shows the DSC signal associated with it. Table 2. Selected Bond Lengths (A˚) and Angles () for Compound 1a Li(1)-O(1) Li(1)-O(2) Li(1)-O(2)#2 Li(1)-O(1)#1 Li(1)-Li(1)#1

1.923(8) 1.971(8) 1.983(7) 1.987(8) 2.725(14)

O(2)#2-Li(1)-O(1)#1 O(1)-Li(1)-O(2)#2 O(1)-Li(1)-O(1)#1 O(2)-Li(1)-O(1)#1

112.1(3) 103.0(4) 91.7(3) 101.5(4)

a Symmetry code: #1 -x þ 1, -y þ 1, -z þ 1, #2 -x þ 1, y - 1/2, -z þ 1/2.

Table 3. Selected Bond Lengths (A˚) and Angles () for Compound [2]a Li(1)-O(3)#2 Li(1)-O(2)#4 Li(1)-O(4)#5 Li(1)-Li(2)#6 Li(1)-Li(2)#1 Li(2)-O(3)#4 Li(2)-O(2)#3 Li(2)-O(1)#5

1.923(9) 1.961(8) 2.080(9) 2.759(12) 3.074(12) 1.956(9) 1.999(9) 2.040(9)

S(1)-O(6) S(1)-O(4 C(8)-S(1)-C(7) O(3)#2-Li(1)-O(1) O(3)#2-Li(1)-O(2)#4 O(5)-Li(2)-O(2)#3 O(3)#4-Li(2)-O(1)#5

1.432(3) 1.447(3) 104.1(2) 117.9(4) 91.5(4) 113.8(4) 99.4(4)

a Symmetry code: #1 x - 1/2, -y þ 1/2, z - 1/2, #2 -x þ 3/2, y þ 1/2, -z þ 5/2, #3 -x þ 3/2, y - 1/2, -z þ 5/2, #4 x þ 1, y, z, #5 x þ 1/2, -y þ 1/2, z þ 1/2, #6 -x þ 5/2, y þ 1/2, -z þ 5/2.

only difference is the presence of a sulfonyl group between the two phenyl groups in 4,40 -SDB. The presence of sulfur forces the two phenyl group to be out of plane with each

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other at an angle of approximately 104. This angular deviation from planarity affects the orientation of the linker carboxylate groups in both ends of the phenyl ring resulting in the formation of completely different network topology about the lithium centers. ULMOF-2 forms layers of antifluorite type lithium polyhedra, isoreticular with ULMOF1,56 which contains 2,6-naphthalenedicarboxylic acid as the linker. The carboxylate groups are present at an angle of 180 in 2,6-naphthalenedicarboxylic acid similar to 4,40 -BPDC. In another recent example, Liu et al.38 reported a lithium-based network using 1,4 benzenetricarboxylic acid as a linker, which had a similar antifluorite motif. ULMOF-3 contains isolated tetrameric clusters consisting of corner shared pairs of edge sharing lithium polyhedra connected by organic linkers forming the overall network. The tetrameric cluster of lithium polyhedra present in ULMOF-3 is also the basic building unit of the 2-D LiO layer in ULMOF-2. The presence of the same basic building unit in both of the frameworks emphasizes the thermodynamic favorability in its formation. The presence of this basic building unit in the extended 2-D inorganic LiO layer of ULMOF-2 imposes unprecedented thermal stability comparable to ULMOF-156 (ULMOF-1 is stable up to 610 C under N2 atmosphere). TGA-DSC experiments performed on both of the materials support this conclusion. Because of the nature of packing of the aromatic rings as well as the lithium-oxygen tetrahedra, both 3-D networks have no solvent accessible void space.57 As a result, no subsequent gas adsorption studies have been performed on the compounds. Although both ULMOF-2 and -3 contain stacked aromatic phenyl rings in their structures, the distance between the successive phenyl rings is considerably larger (>5 A˚) than the idealized π-π interaction between aromatic rings. ULMOF-2 and -3 are not soluble in any common organic solvent. ULMOF-2 and -3 showed very high thermal stability under N2 atmosphere (Figures 6 and 7). TGA data for ULMOF-2 under N2 atmosphere shows no weight loss until 575 C indicating retention of the network up to that temperature. This is supported by the subsequent DSC measurement, which did not show any exo- or endothermic signal up to 575 C (Figure 6). ULMOF-2 showed a steady decomposition after that temperature due to the decomposition of the network. The end product is recovered as black poorly crystalline powder. No further characterization was pursued on this material. ULMOF-3 shows a nearly identical thermal behavior under N2 atmosphere except it has a lower stability than ULMOF-2. ULMOF-3 shows no weight/phase change until 500 C, after which it shows a gradual decomposition indicating destruction of the framework. The lower thermal stability of ULMOF-3 in comparison to ULMOF-2 can be explained by the absence of extended LiO type antifluorite layer in ULMOF-3. The presence of sulfonyl unit in the organic linker may also be the reason of lower stability of ULMOF-3 due to possible formation of SO2 upon decomposition of the organic linker molecule. The thermal behaviors of ULMOF-2 and -3 are recorded under air (Figures S8-S9, Supporting Information). ULMOF-2 shows a lower thermal stability under air than N2 atmosphere (decomposition temperature: 520 C). ULMOF-3 on the other hand shows identical decomposition temperature under both environments (500 C).

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4. Conclusions Two lithium-based 3-D frameworks were synthesized using 4,40 -BPDC (ULMOF-2) and 4,40 -SDB (ULMOF-3) as organic linkers. ULMOF-2 consists of a 2-D LiO antifluorite layer connected by organic linkers, while ULMOF-3 is constructed by a V-shaped 4,40 -SDB linker connecting tetrameric clusters of lithium polyhedra. The detailed structural characterization through single crystal X-ray diffraction enables us to understand the underlying chemistry of this potentially interesting new class of MOFs containing ultralight metal centers. Currently, we are exploring the chemistry of ultralight metals using more diverse conditions and ligands of a varying nature. Acknowledgment. This work is supported by the National Science Foundation (DMR-0800415). S.J.K. is grateful for the support from Korea Institute of Science and Technology (KIST). The authors would like to thank Yu-Sheng Chen at ChemMatCars, APS, for his assistance during the singlecrystal data collection. ChemMatCars (Sector 15) is principally supported by the National Science Foundation/ Department of Energy under Grant Number CHE-0535644. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. Supporting Information Available: Figures S1-S7 and crystallographic information files (CIFs) are available for ULMOF-2 and ULMOF-3. This material is available free of charge via the Internet at http://pubs.acs.org.

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