Solvent-Induced Structural Diversity of Partially Fluorinated, Stable Pb

Article pubs.acs.org/crystal

Solvent-Induced Structural Diversity of Partially Fluorinated, Stable Pb(II) Metal−Organic Frameworks and Their Luminescence Properties Atanu Santra and Parimal K. Bharadwaj* Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh 208016, India S Supporting Information *

ABSTRACT: Rigid, linear partially fluorinated ligand 2,2′-bis-trifluoromethyl-biphenyl-4,4′-dicarboxylic acid (H2L) was synthesized and used to construct five metal organic frameworks with Pb(II) through hydro- and solvothermal techniques. Depending on the nature of the solvent or solvent mixture, five different coordination polymers, {[Pb6(L)5(OH)2]· 2H2O}n (1), {[Pb3(L)2(O)(EtOH)]·(EtOH)}n (2), {[Pb3(L)2(O)(PrOH)]·(PrOH)}n (3), {[Pb3(L)2(O)(i-PrOH)]·(i-PrOH)}n (4), and {[Pb6(L)4(O)2]·(benzylalcohol)2(H2O)2}n (5), were synthesized. All of these compounds were characterized by single-crystal X-ray diffraction, infrared spectroscopy, thermogravimetric analysis, elemental analysis, and powder X-ray diffraction measurements. Complex 1, which is a 3D MOF, can be synthesized hydrothermally. However, isostructural complexes 2−4 can be obtained solvothermally using ethanol, propanol, and isopropanol, respectively, with water. The alcohols (ethanol, propanol, and isopropanol) are incorporated in the final 3D structures. Complex 5, having a 2D layer structure, was synthesized using a benzyl alcohol and water mixture. All of the complexes exhibit rare rod-type topology. Solid-state photoluminescence studies were carried out for all of the complexes at room temperature.

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INTRODUCTION In recent years, metal−organic frameworks (MOFs)1 have attracted great attention not only for their topological varieties but also because of their potential applications as functional materials in the areas of gas adsorption,2 separation,3 catalysis,4 drug delivery,5 molecular magnetism,6 and photoluminescence.7 In general, practical applications of MOFs are directly related to their structural features. Therefore, the development of new synthetic strategies to produce MOFs has become a great challenge.8 There are various key factors governing the formation of MOFs, such as selection of ligands, pH, solvents, temperatures, and so forth.9 In particular, the nature of the solvent is an important factor because its structure as well as chemical properties can influence the rate of crystal growth and the final structure.10 With regard to the template synthesis of MOFs, there have been several reports on the use of various structuredirecting agents such as alcohols,11 amines,12 alkali metal ions,13 polyoxometalates,14 metal complexes,15 surfactants,16 urea derivatives,17 and so on. A large number of new MOFs have been constructed with many of the templating molecules showing unprecedented structural characteristics and, in some cases, guest-induced properties like enhancement of porosity,18 electrochemical property,19 catalytic activity,20 and so forth. In particular, organic alcohols may be highly efficient as © 2014 American Chemical Society

structure-directing agents because of their excellent H-bond formation abilities, and variation in their bulkiness is capable of controlling the shape and size of the channels.21 The rational design of MOFs is very challenging and depends on the judicious selection of linkers as well as the metal ion. Considering various organic linkers, carboxylate donor ligands have been shown to be of great importance as constructors owing to their strong coordination ability in diverse modes to satisfy geometric requirements of metal centers, leading to frameworks of higher dimensions and interesting topologies.22 The heavy p-block Pb(II) ion, with a lone-pair and large ionic radius, can possess a flexible coordination environment. This offers unique opportunities for the construction of an unusual coordination networks with suitable organic ligands.23 Moreover, such crystalline systems with structural diversity exhibit important properties such as electroluminescence, photovoltaic conversion, fluorescence sensing, and organic light-emitting diodes.24 Hence, there is a great significance in exploring new Pb(II) coordination polymers and developing their potential applications. Herein, we have designed and synthesized a linear, rigid ligand, 2,2′-bis-trifluoromethyl-biphenyl-4,4′-dicarboxylic acid (H2L) Received: January 8, 2014 Revised: February 8, 2014 Published: February 13, 2014 1476

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Solid-state photoexcitation and emission spectra were recorded using an UV−vis−NIR spectrophotometer (Varian Model Cary 5000) and a Jobin Yvon Horiba Fluorolog-3 spectrofluorimeter at room temperature. Single-Crystal X-ray Studies. Single-crystal X-ray data of 1−5 were collected at 100 K on a Bruker SMART APEX CCD diffractometer using graphite monochromated Mo Kα radiation (λ = 0.71073 Å). The linear absorption coefficients, scattering factors for the atoms, and the anomalous dispersion corrections were from the International Tables for X-ray Crystallography.26 The data integration and reduction were worked out with SAINT27 software. Empirical absorption correction was applied to the collected reflections with SADABS,28 and the space group was determined using XPREP.29 All structures were solved by direct methods using SHELXTL-9730 and refined on F2 by full-matrix least-squares using the SHELXTL-97 program31 package. All non-H atoms were refined anisotropically except for the C1 and C56 atoms of complex 1, C1 and C34 atoms of complex 2, O11, C6, C34, C35, and C36 atoms of complex 3, C2, C3, C33, C34, C35, and C37 atoms of complex 4, and F10, O20, C32, C45, C75, C76, C79, C91, C92, C93, C94, C95, C96, C97, C98, C11B, C12B, and C13B atoms of complex 5. The H-atoms attached to carbon atoms were positioned geometrically and treated as riding atoms using SHELXL default parameters. The H-atoms of two μ3-OH groups in 1 and lattice solvent molecules in 5 could not be located in the successive difference Fourier maps. Several DFIX commands were used for fixing some bond distances in 1−5. Squeeze refinement using PLATON32 indicated two water molecules for 1, one ethanol molecule for 2, one propanol molecule for 3, one isopropanol molecule for 4, and one water molecule for 5 per formula weight. Crystal and refinement data for 1−5 are collected in Table 1, and selected bond distances and bond angles are given in Table S1 (Supporting Information). Synthesis of the Ligand. Ligand 2,2′-bis-trifluoromethyl-biphenyl4,4′-dicarboxylic acid (H2L) was synthesized following a previously reported procedure.33 Synthesis of Complexes. {[Pb6(L)5(OH)2]·2H2O}n (1). A mixture containing H2L (0.03 g, 0.08 mmol), Pb(NO3)2·6H2O (0.094 g, 0.32 mmol), and 4 mL of water was placed in a Teflon-lined stainless steel autoclave. The mixture was heated under autogenous pressure at 180 °C for 72 h and then allowed to cool to room temperature at a rate of 1 °C/min. Colorless block-shaped crystals of 1 were collected by filtration, washed with H2O followed by acetone, and dried in air. Yield ∼43%. Anal. Calcd for C80H36F30O24Pb6: C, 30.08; H, 1.13.

Scheme 1. 2,2′-Bis-trifluoromethyl-biphenyl-4,4′-dicarboxylic acid (H2L)

(Scheme 1), which is composed of a carboxylate group at each terminus with trifluoromethyl groups in the middle to decorate the pores with this group. Frameworks built with partially fluorinated organic linkers may impart a variety of new functional properties, such as enhanced thermal stability and catalytic activity, higher gas affinity and selectivity, enhanced hydrophobicity, and excellent optical and electrical properties.25 In this work, we used the ligand H2L and Pb(NO3)2·6H2O to constructed five new complexes by hydro- and solvothermal techniques: {[Pb6(L)5(OH)2]· 2H2O}n (1), {[Pb3(L)2(O)(EtOH)]·(EtOH)}n (2), {[Pb3(L)2(O)(PrOH)]·(PrOH)} n (3), {[Pb 3 (L) 2 (O)(i-PrOH)]· (i-PrOH)}n (4), and {[Pb6(L)4(O)2]·(benzyl alcohol)2(H2O)2}n (5). Although the structures of these new MOFs are based on the same ligand, they exhibit a remarkable diversity and unique structural features that are clearly induced by the various alcohols used in the reaction medium. The TGA, powder X-ray diffraction patterns, and photoluminescence properties of 1−5 are also reported.

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EXPERIMENTAL SECTION

Materials. The metal salt and other reagent-grade chemicals were procured from Sigma-Aldrich and used as received. All solvents were from S. D. Fine Chemicals, India. These solvents were purified following standard methods prior to use. Physical Measurements. Infrared (IR) spectra (KBr disk, 400− 4000 cm−1) were recorded on a PerkinElmer Model 1320 spectrometer. Thermogravimetric analyses (TGA) (5 °C min−1 heating rate under a nitrogen atmosphere) were performed with a Mettler Toledo Star System. Powder X-ray diffraction (PXRD) patterns were acquired using a Bruker D8 Advanced powder diffractometer with Cu Kα radiation.

Table 1. Crystal and Structure Refinement Data for 1−5 complexes empirical formula formula wt crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) U (Å3) Z ρcalc (g/cm3) μ (mm−1) F(000) refl. collected independent refl. GOOF final R indices [I > 2σ(I)] R indices (all data)

1 C80H36F30O24Pb6 3194.18 orthorhombic P212121 12.581(5) 26.347(5) 29.852(5) 90 90 90 9895(5) 4 2.119 10.292 5792 54 048 15 849 1.011 R1 = 0.0391, wR2 = 0.0894 R1 = 0.0500, wR2 = 0.0953

2

3

C36H14F12O11Pb3 1482.05 monoclinic C2/c 32.565(5) 14.331(5) 18.549(5) 90 114.870(5) 90 7854(4) 8 2.429 12.941 5248 21 029 5732 0.994 R1 = 0.0448, wR2 = 0.1072 R1 = 0.0595, wR2 = 0.1157

C38 H28F12O11Pb3 1510.08 monoclinic C2/c 32.486(5) 14.351(5) 18.557(5) 90 114.461(5) 90 7875(4) 8 2.446 12.908 5312 21 026 5815 1.013 R1 = 0.0454, wR2 = 0.1142 R1 = 0.0603, wR2 = 0.1244 1477

4 C40H32F12O11Pb3 1538.11 monoclinic C2/c 32.434(5) 14.341(5) 18.581(5) 90 114.342(5) 90 7874(4) 8 2.470 12.910 5376 20 957 6190 1.055 R1 = 0.0460, wR2 = 0.1180 R1 = 0.0567, wR2 = 0.1261

5 C78H44F24O22Pb6 3032.11 monoclinic C2/c 44.582(5) 14.441(5) 29.792(5) 90 120.920(5) 90 16455(7) 8 2.419 12.360 10 944 58 097 11 397 1.073 R1 = 0.0413, wR2 = 0.1033 R1 = 0.0631, wR2 = 0.1103

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Found: C, 30.17; H, 1.21. IR (cm−1): 3465 (broad), 2924 (m), 1613 (s), 1582 (s), 1520 (s), 1420 (s), 1306 (s), 776 (s), 696 (s). {[Pb3(L)2(O)(EtOH)]·(EtOH)}n (2). When the solvent was changed from pure water to aqueous ethanol (1:1, v/v) with everything else kept the same as in 1, colorless block-shaped crystals of 2 were formed. These were collected by filtration, washed with H2O followed by EtOH, and dried in air. Yield ∼47%. Anal. Calcd for C36H24F12O11Pb3: C, 29.17; H, 1.63. Found: C, 29.26; H, 1.71. IR (cm−1): 3441 (broad), 3073 (m), 2974 (m), 1613 (s), 1582 (s), 1520 (s), 1420 (s), 1305 (s), 776 (s), 696 (s). {[Pb3(L)2(O)(PrOH)]·(PrOH)}n (3). Following the procedure adopted for 1 and changing the solvent to a water and n-propanol mixture (1:1, v/v) afforded 3 as colorless block-shaped crystals. These were collected by filtration, washed with H2O followed by acetone, and dried in air. Yield ∼38%. Anal. Calcd for C38H28F12O11Pb3: C, 30.22; H, 1.87. Found: C, 30.34; H, 1.95. IR (cm−1): 3442 (broad), 3074 (m), 2975 (m), 1613 (s), 1583 (s), 1521 (s), 1420 (s), 1380 (s), 776 (s), 696 (s). {[Pb3(L)2(O)(i-PrOH)]·(i-PrOH)}n (4). When a water and isopropanol mixture (1:1, v/v) was used as the solvent, compound 4 was formed in ∼45% yield as colorless block-shaped crystals. These were collected by filtration, washed with H2O followed by acetone, and dried in air. Anal. Calcd for C40H32F12O11Pb3: C, 31.23; H, 2.09. Found: C, 31.16; H, 2.16. IR (cm−1): 3438 (broad), 2975 (m), 1613 (s), 1582 (s), 1520 (s), 1420 (s), 1379 (s), 777 (s), 696 (s). {[Pb6(L)4(O)2]·(benzylalcohol)2(H2O)2}n (5). Compound 5 was obtained following the procedure as in 1 in aqueous benzyl alcohol (1:1, v/v) in ∼37% yield as colorless block-shaped crystals. These crystals were collected by filtration, washed with H2O followed by acetone, and dried in air. Anal. Calcd for C78H44F24O22Pb6: C, 30.89; H, 1.46. Found: C, 30.96; H, 1.52. IR (cm−1): 3363 (broad), 3064 (m), 3030 (m), 2875 (m), 1612 (s), 1582 (s), 1453 (s), 1419 (s), 1380 (s), 734 (s), 696 (s). Use of MeOH or t-BuOH either alone or mixed with water did not give any product of definite composition and hence the use of alcohols other than those mentioned above was not attempted.

Figure 1. Perspective view of the asymmetric unit of 1 (H atoms are omitted for clarity).

All Pb−O bond distances are comparable within statistical errors to those reported earlier.23 The carboxylate groups bind the metal ions in different fashion: μ6:η2:η2:η2:η2, μ5:η2:η1:η1:η1, μ4:η1:η2:η2:η1, and μ7:η2:η2:η2:η2 (Figure 3a−d). This connectivity pattern is repeated infinitely to create Pb−C−O rods along the crystallographic a direction (Figure 4b). Topological simplification with TOPOS software37 shows parallel packing ladderlike rods as building units (Figure 4c). These rods are further linked by the L2− ligands, which connect each rod to six neighboring rods, generating a 3D framework. This framework contains fluorine-decorated nanoporous channels along the crystallographic a axis (Figure 4a). Complex 2 crystallizes in monoclinic space group C2/c, and the asymmetric unit consists of three crystallographically independent Pb(II) ions, two L2− ligands, a μ3-O2− ion, and one coordinated ethanol molecule (Figure 5). Among the three metal ions, Pb1 is hexacoordinated, Pb2 is tetracoordinated, and Pb3 is heptacoordinated. The Pb1 ion is bonded to four carboxylate O atoms from three L2− ligands and two O atoms from two bridging μ3-O2− ions, forming a distorted octahedral geometry. Pb2, however, is coordinated by three carboxylate O atoms from three L2− ligands and one O atom from a bridging μ4-O2− ion and shows distorted tetrahedral coordination geometry. Lastly, Pb3 is coordinated to five carboxylate O atoms from four L2− ligands, one O atom from a bridging μ4-O2− ion, and one O from a weakly bonded ethanol molecule. Like 1, all of the Pb(II) centers are in the hemidirected geometry, indicating the lone-pairs in Pb(II) to be stereochemically active. Interestingly, this structure has a [Pb6(μ4-O)2(O2C)10(EtOH)2] cluster, which contains a centrosymmetric [Pb6(μ4-O2−)2]8+ octahedral core in which six Pb2+ ions occupying the apexes are linked by two equivalent O2− anions with a μ4-tetrahedral bridging mode (Figure 6d). The structure of 2 can be viewed as comprising infinite Pb2+-oxo clusters in which the [Pb6(μ4O)2(O2C)10(EtOH)2] clusters are attached by four bridging carboxylates, producing a 1D chain along the crystallographic c axis (Figure 6b). The Pb−O distances for the [PbII6(μ4-O2−)2]8+ octahedral core are in the range of 2.298(7)−2.322(7) Å, which are significantly shorter than those of Pb−O carboxylate (2.461(7)−2.711(7) Å, Table S1, Supporting Information). All of these structural parameters are comparable to those found in the compounds containing [[PbII6(μ4-O2−)2]8+ units.23b The two

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RESULTS AND DISCUSSION Once isolated, all of the coordination polymers were stable in air and insoluble in common organic solvents and water. The IR spectra of 1−5 show strong absorption bands between 1400 and 1620 cm−1 attributable to coordinated carboxylate groups.34 The broad peak in the region 3363−3465 cm−1 indicates the presence of hydroxyl groups and both coordinated and noncoordinated water molecules.35 Structural Description. Complex 1 crystallizes in the orthorhombic space group P212121, and the asymmetric unit consists of six crystallographically independent Pb(II) ions, five L2− ligands, and two μ3-OH groups (Figure 1). Out of the six Pb(II) ions, both Pb1 and Pb5 are heptacoordinated, whereas the remaining metal ions exhibit hexacoordination. As shown in Figure 2a, Pb1 is ligated by seven carboxylate O atoms (Pb−O = 2.414(7)−2.750(8) Å) from five L2− ligands, forming a distorted PbO7 polyhedron, whereas Pb5 is coordinated to five carboxylate oxygen atoms (Pb−O = 2.673(7)−2.741(7) Å) from three different L2− ligands and two O atoms from two bridging μ3-OH groups (Pb−O = 2.367(7) and 2.467(7) Å). In the cases of Pb2, Pb4, and Pb6, each metal ion is ligated by a μ3-OH group (Pb−O = 2.298(7)− 2.366(7) Å) and five carboxylate O atoms (Pb−O = 2.438(8)− 2.674(8) Å) from five different L2− ligands in a distorted octahedral coordination geometry. The Pb3 ion is also hexacoordinated, but in this case, five carboxylate O atoms from four different L2− ligands in addition to one μ3-OH group are bonded to the metal ion. Here, all of the Pb(II) centers are in the hemidirected geometry, indicating the lone-pair electrons of Pb(II) are stereochemically active.36 1478

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Figure 2. Metal coordination modes found in the complexes.

Figure 3. Coordination modes of L2− in complexes 1−5.

crystallographically independent L2− ligands display different coordination modes as μ 6 :η 2 :η 2 :η 1 :η 2 and μ 4 :η 2 :η 1 :η 1 :η 1 (Figure 1d,e). Topological simplification with TOPOS software37

shows parallel packing ladderlike rods as building units, which is similar to 1. These rods are further linked by the L2− ligands, which connect each rod to six neighboring rods generating a 3D 1479

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Figure 4. Representation of (a) 3D view of the framework, (b) the 1D Pb−O−Pb chain, and (c) parallel packing of ladderlike rods in complex 1.

Figure 5. Perspective view of asymmetric unit of complex 2 (H atoms are omitted for clarity).

2.290(6)−2.314(7) Å, which are significantly shorter than those of Pb−O carboxylate (2.493(7)−2.715(6) Å, Table S1, Supporting Information). The rhombohedral channels with dimensions of 3.91 × 12.33 Å2 (excluding van der Waals radii) along the crystallographic b axis are occupied by weakly coordinated isopropanol molecules (Figure 8b). Complex 5 also crystallizes in monoclinic space group C2/c. The asymmetric unit consists of six crystallographically independent Pb(II) ions, four L2− ligands, two μ3-O2− ions, two benzyl alcohol molecules, and one water molecules in the lattice (Figure 9). For the six crystallographically independent Pb(II) ions, Pb1 and Pb3 are pentacoordinated, Pb2, Pb4, and Pb5 are hexacoordinated, and Pb6 is tetracoordinated. As depicted in Figure 2c, Pb1 and Pb3 are coordinated by three carboxylate O atoms from two L2− ligands and two O atoms from two bridging μ3-O2− ions. Pb2, Pb4, and Pb5, however, are coordinated by five carboxylate O atoms from four different L2− ligands and one O atom from a bridging μ3-O2− ion, forming a distorted octahedral geometry around each metal ion. Lastly, Pb6 is ligated by three carboxylate O atoms from three different L2− ligands and one O atom from a bridging μ3-O2− ion. Here also, all of the

framework. The 3D framework contains rhombohedral channels with the dimension of 3.77 × 12.29 Å2 (excluding van der Waals radii) along the crystallographic b axis decorated by fluorine atoms. These channels are occupied by weakly coordinated ethanol molecules (Figure 6a). Complex 3 also crystallizes in monoclinic space group C2/c and is isostructural with 2. The contents of the asymmetric unit is similar to that of 2 except that a propanol molecule is coordinated to Pb(II) instead of ethanol (Figure 7a). Here, the Pb−O distances for the [PbII6(μ4-O2−)2]8+ octahedral core are in the range of 2.296(7)−2.316(7) Å, which are significantly shorter than those of Pb−O carboxylate (2.449(7)−2.726(7) Å, Table S1, Supporting Information). Complex 3 contains rhombohedral channels with the dimension of 3.87 × 12.31 Å2 (excluding van der Waals radii) along the crystallographic b direction occupied by weakly coordinated propanol molecules (Figure 7b). Complex 4 is also isostructural with 2 and likewise crystallizes in monoclinic space group C2/c. The asymmetric unit is similar to that of 2 except that isopropanol is present in place of ethanol (Figure 8a). As before, the Pb−O distances for the [PbII6(μ4-O2−)2]8+ octahedral core are in the range of 1480

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Figure 6. Perspective view of (a) 1D fluorine-decorated channels along the b axis, where channels are occupied by weakly coordinatrd ethanol molecules, (b) 1D Pb−O−Pb chain in 3, (c) [Pb6(μ4-O)2(O2C)10(EtOH)2] cluster, and (d) centrosymmetric [PbII6(μ4-O2−)2]8+ octahedral core in 2.

Figure 7. Perspective view of (a) asymmetric unit and (b) 1D fluorine-decorated channels along the b axis, where channels are occupied by weakly coordinated propanol molecules in 3.

crystallographic b axis (excluding van der Waals radii), and the channels contain the benzyl alcohol and water molecules. The benzyl alcohols are also intercalated between the 2D layers of the framework (Figure 11). Topological simplification with TOPOS software37 shows parallel packing ladderlike rods as building units, which is similar to 1. Thermal Stability Analyses. To investigate thermal stabilities of 1−5, thermogravimetric analyses (TGA) were carried out (Supporting Information, Figures S6−S10). For 1, a weight loss of ∼1.1% (expected = 1.18%) was observed between 50 and 120 °C, corresponding to the loss of two water molecules. Decomposition of 1 is achieved only beyond 400 °C. For 2−4, the weight loss corresponding to the removal of lattice- and metal-bound alcohol molecules was observed between 80 and

Pb(II) centers are in the hemidirected geometry, indicating that the lone-pair of Pb(II) is stereochemically active.36 The carboxylate groups of the four independent L2− ligands are coordinated to the metal ions in different fashion as μ6:η2:η2:η2:η1, μ3:η1:η1:η1:η1, and μ4:η1:η2:η2:η1 (Figure 2e, g, and h). Complex 5 has a [Pb6(μ4-O)2(O2C)9] cluster where six Pb(II) ions inhabiting the apex positions are linked by two equivalent O2− anions with a μ4-η4 tetrahedral bridging mode (Figure 10c,d). Two such cluster are attached by two bridging carboxylates to generate a bigger cluster [Pb12(μ4-O)4(O2C)16] acting as the SBU (Figure 10b). Each SBU is connected to the six neighboring SBUs by 16 L2− ligands to generate a 2D framework (Figure 10a). The overall structure shows fluorine-decorated rhombic channels with dimensions of 5.6 × 8.15 Å2 along the 1481

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Figure 8. Perspective view of (a) asymmetric unit and (b) 1D fluorine-decorated channels along the b axis, where channels are occupied by weakly coordinated isopropanol molecules in 4.

Figure 9. Perspective view of asymmetric unit of 5 (H atoms are omitted for clarity).

180 °C. Decomposition of the compound in each case is achieved beyond 400 °C. For 5, however, loss of two water molecules and one benzyl alcohol molecule takes place up to 220 °C, and rapid decomposition of the compound was observed at 260 °C. Thus, thermally, 1−4 are quite stable compounds. The phase purity of 1−5 was confirmed by their powder X-ray diffraction (PXRD) patterns, which are in excellent agreement with the corresponding simulated patterns (Supporting Information, Figures S7−S12). Photoluminescence Properties. Complexes showing luminescence properties are very important because of their potential applications in photochemistry, chemical sensors, and electroluminescent displays.38 Compared to transition and lanthanide metals, less attention has been paid to the photoluminescence of coordination polymers of main-group metals such as Pb(II), although it has interesting photochemical and photophysical properties.23 Solid-state photoluminescence properties of complexes 1−5 and that of the free ligand (H2L)

were studied at room temperature, and the results are shown in Figure 12. It was observed that the metal-free H2L exhibits an emission band at 421 nm with excitation at 275 nm, attributable to the π → π* transition.39 For each of the complexes, a broad unsymmetrical emission was observed along with the disappearance of the ligand emission. The emission of the complexes can be attributed to ligand-to-metal charge transfer (LMCT) between the delocalized π-bond of carboxylate groups and p orbitals of Pb(II) centers.40

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CONCLUSIONS We have successfully constructed five new Pb(II) metal−organic frameworks based on rigid and partially fluorinated carboxylate tecton by using water and water/alcohol (ethanol, propanol, isopropanol, and benzyl alcohol) mixtures. The results suggest that the solvent plays an important role in the assembly of the frameworks, as change of solvents lead to the formation of different frameworks. Interestingly, all of the complexes show the 1482

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Figure 10. (a) Perspective view of the 2D network with SBUs linked via ligands, (b) [Pb12(μ4-O)4(O2C)16] cluster as the SBU, (c) [Pb6(μ4-O)2(O2C)9] cluster, and (d) a view of the centrosymmetric [PbII6(μ4-O2−)2]8+ octahedral core in 5.

Figure 11. Perspective view of fluorine-decorated 2D channels along the b axis occupied by benzyl alcohol and water molecules in 5. The benzyl alcohol also intercalated between two layers.

unusual rod-type topology. Further research to explore the influence of the solvent on the final architecture of the MOFs is in

progress. Solid-state emissions of the complexes were recorded at room temperature. 1483

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Figure 12. Solid-state luminescence spectra at room temperature.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic data in CIF format; selected bonds and distances for complexes 1−5 as well as their complete data for IR, TGA analysis, PXRD; and solid-state UV/vis of ligand H2L. This material is available free of charge via the Internet at http://pubs. acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support received from the Department of Science and Technology, New Delhi, India (to P.K.B.) and SRF from the CSIR to A.S.

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REFERENCES

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