Large Pore Isoreticular Strontium-Organic Frameworks: Syntheses

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Large Pore Isoreticular Strontium-Organic Frameworks: Syntheses, Crystal Structures, Thermal and Luminescent Properties Afsaneh Khansari, Shane G. Telfer, and Christopher Richardson Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01347 • Publication Date (Web): 11 Dec 2018 Downloaded from http://pubs.acs.org on December 14, 2018

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Crystal Growth & Design

Large Pore Isoreticular Strontium-Organic Frameworks: Syntheses, Crystal Structures, Thermal and Luminescent Properties Afsaneh Khansari†, Shane G. Telfer‡, and Christopher Richardson*†

† School of Chemistry and Biomolecular Science, Faculty of Science, Medicine and Health, University of Wollongong, Wollongong, NSW 2522, Australia

‡ MacDiarmid Institute of Advanced Materials and Nanotechnology, Institute of Fundamental Sciences, Massey University, Palmerston North 4442, New Zealand

Two isoreticular and topologically unique metal-organic frameworks (MOFs) have been synthesized using Sr(NO3)2 and the organic linkers 2-nitro-[1,1'-biphenyl]-4,4'dicarboxylic acid (H2bpdcNO2) and 2,2'-dinitro-[1,1'-biphenyl]-4,4'-dicarboxylic acid (H2bpdc(NO2)2).

The

structures

of

[Sr(bpdcNO2)2(DMF)2(H2O)2]

(WUF-15;

WUF=Wollongong University Framework) and [Sr4(bpdc(NO2)2)4(DMF)2(H2O)4·2DMF]

ACS Paragon Plus Environment

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(WUF-16) were determined by single crystal X-ray diffraction (SCXRD) and are composed of infinite strontium carboxylate SBUs and contain large square (~18 Å) and smaller triangular channels (~9 Å) orientated parallel to each other and lined with nitro functional groups. An in-situ ligand transformation of H2bpdc(NO2)2 to benzo[c]cinnoline3,8-dicarboxylic acid (H2bc) and formation of a non-porous coordination polymer of formula [Sr(bc)(H2O)2] (WUF-17) with interesting photoluminescent properties was discovered. Independent synthesis of H2bc enabled the preparation of WUF-17 crystals suitable for SCXRD structure determination. Powder X-ray diffraction, thermal and elemental analyses support the structures of all complexes.

Introduction Metal-organic frameworks are intensely investigated for the separation and storage of gases,1-3 as heterogeneous catalysts,4-6 and for their magnetism.7 More recent developments have been to explore excitation-emission properties of MOFs in luminescence-based

chemical

sensing,8-10

as

scintillation

materials11-13

and

pyrotechnics.14-15


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The majority of MOFs have been made using d-block metal ions and to a lesser extent, rare-earth metal ions. However, s-block metal ions have also recently been attracting more attention in MOF chemistry.16-18 High coordination numbers and flexible coordination geometries of large s-block ions like Sr(II) and Ba(II) are generally believed to leave little room for control over framework connectivity to engineer porous MOFs. Frameworks constructed from closed-shell alkaline earth metal ions may, however, find applications based on their fluorescence,19-23 semiconducting24 or dielectric properties.2527

Functionalized biphenyl-4,4'-dicarboxylates are popular ligands for porous MOFs based on IRMOF-9,28 UiO-67,29 MIL-88D,30 DUT-531 and manganese32 frameworks. The nitrofunctionalized biphenyl-4,4'-dicarboxylates have been incorporated in aluminum,33-34 and in luminescent and magnetic lanthanide frameworks35 and bpdc(NO2)22 features in several copper-based MOFs investigated for CO2 adsorption36 and for their topological diversity and magnetism.37 Here, we report the syntheses and structures of isoreticular and topologically unique strontium MOFs with large open pore systems in which bpdcNO22 and bpdc(NO2)22 bridge Sr(II) metal centers. During scoping of synthetic


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conditions, we obtained a novel Sr(II) coordination polymer that is the result of an in-situ ligand transformation. The structure was determined by SCXRD analysis and showed interesting photoluminescence properties. Experimental Materials and Methods All chemicals used were analytical grade and purchased from Sigma Aldrich, VWR Australia or Ajax Finechem Pty Ltd. 2-Nitro-[1,1'-biphenyl]-4,4'-dicarboxylic acid (H2bpdcNO2),38 2,2'-dinitro-[1,1'-biphenyl]-4,4'-dicarboxylic acid (H2bpdc(NO2)2)39 and benzo[c]cinnoline-3,8-dicarboxylic acid (H2bc)40 were prepared by literature procedures. 1H

NMR spectra were obtained using a Varian Mercury VX-300-MHz NMR

spectrometer operating at 300 MHz for 1H or a Varian Inova NMR spectrometer operating at 500 MHz for 1H. 1H NMR spectra were referenced to the residual protio peaks at 2.50 ppm (d6-DMSO). For 1H NMR analysis, MOF samples (~5 mg) were digested by adding 35% DCl in D2O (2 μL) and DMSO (500 μL) and stirring until a solution was obtained.


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Simultaneous thermogravimetric and differential thermal analysis (TG-DTA) data were recorded using a Shimadzu DTG-60 fitted with a TA-60WS thermal analyzer with measuring parameters of 10 °C per min under a static atmosphere of air. A GBC Scientific MMA X-ray diffractometer was used to record powder X-ray diffraction (PXRD) patterns on samples mounted on 2.5 cm diameter quartz disks in the 2θ angle range of 3–30° with a step size of 0.02° and scan speed of 1° min-1. A Horiba Jobin Yvon fluorescence spectrophotometer (FL3-221, France) was used to record photoluminescence spectra at room temperature on thin layers of powdered sample in the wavelength range 330–600 nm (λex 310 nm). Mass spectra were recorded on a Shimadzu LCMS-2010EV electrospray ionization (ESI) mass spectrometer in MeOH-H2O solutions. Single crystal X-ray crystallography Diffraction data were collected using a Rigaku Spider diffractometer equipped with a MicroMax MM007 rotating anode generator (Cu Kα radiation, λ =1.54180 Å), high-flux Osmic multilayer mirror optics, and a curved image-plate detector at 292 K. The data were integrated and scaled and averaged with FS Process.41 The crystal structures were


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solved by direct methods using SHELXS-97 and refined against F2 on all data by fullmatrix least-squares with SHELXL-97.42 Synthetic procedure for WUF-15 Sr(NO3)2 (25.1 mg, 0.119 mmol) and H2bpdcNO2 (31.1 mg, 0.108 mmol) were dissolved in DMF (6 mL) and H2O (0.3 mL) in a screw top vial and the mixture was heated at 130 °C in an oven for 16 hours. After cooling to RT, the needle-shaped crystals were soaked in fresh DMF containing 5% H2O (2 × 2 mL), DMF (1 × 2 mL) and then CH2Cl2 (3 × 2 mL) and vacuum dried at 70 mbar before elemental analysis. Yield: 29.3 mg (55%). Found: C, 40.38; H, 3.13; N, 4.20. C14H7NO6Sr·3H2O·½DMF requires C, 40.15; H, 3.59; N, 4.53. Synthetic procedure for WUF-16 Sr(NO3)2 (25.1 mg, 0.119 mmol) and H2bpdc(NO2)2 (35.8 mg, 0.108 mmol) were dissolved in DMF (6 mL) and H2O (0.3 mL) in a screw top vial and the mixture was heated at 130 °C in an oven for 16 hours. The resultant orange needle-shaped crystals were soaked in fresh DMF containing 5% H2O (2 × 2 mL), DMF (1 × 2 mL) and then CH2Cl2 (3 × 2 mL) and vacuum dried at 70 mbar before elemental analysis. Yield: 24.6 mg (52%).


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Found: C, 37.99; H, 2.98; N, 7.23. C14H6N2O8Sr·2H2O·½DMF requires C, 37.95; H, 2.78; N, 7.14. Synthetic procedure for WUF-17 Sr(NO3)2 (25.1 mg, 0.119 mmol) and H2bc (28.9 mg, 0.108 mmol) were dissolved in DMF (4.8 mL) and water (1.5 mL) in a screw top vial and heated in an oven at 100 °C for 16 hours. The needle-shaped crystals were collected by filtration and washed with dry DMF and acetone. Yield: 14.7 mg (35%). Found: C, 43.22; H, 2.57; N, 7.14. C14H6O4N2Sr·2H2O requires C, 43.14; H, 2.59; N, 7.19. Results and discussion Syntheses and structural descriptions of WUF-15 and WUF-16 Crystals of WUF-15 and WUF-16 formed by reacting equimolar quantities of Sr(NO3)2 with H2bpdcNO2 or H2bpdc(NO2)2, respectively, in DMF solutions containing 5% water at 130 °C. Single crystal X-ray diffraction (SCXRD) analysis shows WUF-15 crystallizes in the space group Ima2 with an asymmetric unit consisting of one whole and two half bpdcNO22 linkers and three aqua ligands coordinated to two Sr centers, giving the framework formulation of [Sr2(bpdc-NO2)2(H2O)3] (Fig. S1). The analysis of the strontium–


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carboxylate binding modes described here follows on from that of others.43-45 A carboxylate group chelates a central Sr atom while bridging two others (µ3-(κ1 : κ2 : κ1)) in mode a (Figure 1). To simplify the following structural descriptions, aC refers to the central chelating relationship and aT to coordinating strontium at the termini of this mode. In mode b, the carboxylate bridges two Sr atoms, binding each in a monodentate fashion (µ2-(κ1 : κ1) and in mode c, the carboxylate binds a Sr atom in a monodentate fashion.

Figure 1. Strontium-carboxylate binding modes a-c. Terminal (aT) and central motifs (aC) of mode a are represented by green and magenta, respectively.


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Sr1 is seven-coordinate SrO7 with four oxygen atoms in an equatorial plane (O10, O13, O30 and O31), but is best thought of as distorted octahedral in geometry by considering the chelating carboxylate in an apical site (O1 and O2, mode aC) as a single vertex point (Figure 2). The atoms O10 and O13 are trans and coordinate via mode b; likewise, O30 and O31 bind via mode aT and are also trans. The chelating carboxylate is trans to a monodentate ligand that was crystallographically modelled as an aqua ligand (O70), but a mixture of DMF and water occupies this site based on elemental and 1H NMR spectroscopic analysis (Figure S7). Sr2 is eight-coordinate SrO8 but if a carboxylate binding in mode aC is again simplified to a single vertex point the geometry is pentagonal bipyramidal (Figure 2). This moiety is

trans to a coordinating water (O40i) in the other apical site. The second coordinating water (O50i) is necessarily in a cis relationship to the first and is flanked by O1i and O2i that bind via mode aT and the pentagonal plane is completed by coordination of O11i and O12i via mode b. This arrangement of bridging carboxylates binds the Sr centers alternately in chains that run parallel to the crystallographic c-axis (Figure S2).


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Figure 2. A view of the coordination environments, with selected atomic labeling, around the strontium centers in WUF-15. The symmetry operation +X, ½-Y, ½+Z generates the atoms bearing superscripted symbols.

There is a dihedral angle of 54 ° between the mean planes of the phenyl rings of the whole bpdcNO2 linker contained in the asymmetric unit. Each carboxylate group is very nearly coplanar with its connected aromatic ring with angles of 7-8 ° between carboxylatephenyl mean planes and an overall angle of 36 ° between carboxylate mean planes. The two half ligands have dihedral angles between phenyl ring mean planes of 0 ° and 52 °, which result from proximities to two-fold screw and two-fold axes, respectively. The


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carboxylate groups are nearly coplanar with the connected rings and this is reflected in the values for their dihedral angles of 0 ° and 53 °, respectively. Expanding the structure and viewing parallel to the c-axis reveals two shapes of channel (Figure 3). The triangular channels share edges to form layers that separate the larger square-shaped channels. These square channels are approximately 18 Å across (atomcenter-to-atom-center) and are one of the largest pore sizes found in strontium MOFs to date. To the best of our knowledge, this topology of framework is new.

Figure 3. Views parallel to the c-axes highlighting the isotopological nature of (a) WUF15 and (b) WUF-16. Hydrogen atoms are omitted, for clarity.


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SCXRD analysis shows WUF-16 is isotopological with WUF-15 although it crystallizes in the space group Cc with the four unique Sr centers in the asymmetric unit coordinating two complete ligands, four half-ligands, two coordinated DMF and four coordinated water molecules (Figure S3). The SCXRD analysis also revealed the presence of two DMF molecules within the triangular pores. The four strontium centers are arranged in pairs that run parallel to the c-axis with Sr1 and Sr2 alternating in an infinite chain and Sr3 and Sr4 alternating in another chain. Sr1 and Sr3 are seven-coordinate and each bind a DMF ligand that protrudes noticeably into the triangular channels, which now contain twice the density of nitro groups by virtue of the di-functionalized bridging ligand. Sr2 and Sr4 are eight-coordinate with each directing two aqua ligands into the larger square channels. Four unique and large dihedral angles exist between phenyl rings in the bpdc(NO2)22 linkers in WUF-16 (53 °, 64 °, 84 °, 72 °). This is consistent with the increased steric influence by two ortho-nitro groups about the inter-ring bond in the ligand structure. In each of these ligands, however, the angle between the respective coordinating carboxylates is systematically smaller (10 °, 47 °, 50 °, 49 °, respectively). This suggests


12

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that the torsional flexibility afforded by such biphenyl-type linkers is important in forming this MOF topology. The large open channels in these structures prompted us to explore generating solvent free materials for gas adsorption. Solvent exchange studies established that acetone and water substituted DMF bound to the metal nodes by immersion in dry or wet acetone, respectively, as signals for these molecules are seen in the NMR digestion spectra of dried samples (Figures S10-11). PXRD showed that samples remained crystalline during these solvent exchanges but underwent significant loss of crystallinity upon freeze-drying or conventional drying under dynamic vacuum (Figure S12). The materials were examined for CO2 uptake at 273 K following activation procedures outlined in Table S2. It was found that WUF-16 takes up 22.5 cm3g-1 at 1 bar (Figure S13) after sequential solvent exchange to wet acetone and then cyclohexane and activation by a combination of low temperature freeze-drying and mild heating under dynamic vacuum. Neither WUF15 nor WUF-16 adsorbed N2 at 77 K. Synthetic scope of reactions of H2bpdc(NO2)2 with Sr(NO3)2


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Aiming to synthesize MOFs de novo with no coordinated DMF, we scoped synthetic conditions of H2bpdc(NO2)2 with Sr(NO3)2 by increasing the percentage of water in the solvent and using higher temperatures (Table 1). Raising the water content to 16% at a reaction temperature of 130 °C resulted in the formation of WUF-16 as determined by PXRD and 1H NMR spectroscopy (Figure 4a). Raising the water content in the solvent to 24% or 32% at 130 °C resulted in no crystal formation after 72 h. However, crystalline material began forming around 72 hours for reactions with 24% or 32% water contents at 150 °C. Termination of these reactions at 96 hours and analysis of the recovered solids by PXRD showed a new crystalline phase. The crystals were not of sufficient quality for single crystal structure determination and therefore were digested and analyzed by 1H NMR spectroscopy and ESI mass spectrometry (Figure S14-18). Table 1. Reaction conditions for H2bpdc(NO2)2 and Sr(NO3)2 in DMF-H2O.

Entry

Water content (%)

Temp. (°C)

Time (h)

Framework formulation

1

16

130

16

[Sr4(bpdc-(NO2)2)4(DMF)2(H2O)4]

2

24

130

72

No crystal formation


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3

32

130

72

No crystal formation

4

24

150

96

Srbc0.51(bc-NO)0.49

5

32

150

96

Srbc0.76(bc-NO)0.24

6

40

150

96

Srbc

1H

NMR data obtained after digestion of recovered crystals from entry 4 (24% water;

150 °C; 96 h) revealed an organic ligand composition of 51% benzo[c]cinnoline-3,8dicarboxylate (bc) and 49% of the corresponding N-oxide (bcNO) (Figure 4a). Entry 5 shows increasing to 32% water content gives a bc2:bcNO2 ratio of 76:24 in the solid and further increase in water content to 40% results in bc2 as essentially the only type of organic ligand in the crystalline phase. Clearly, water accelerates the in-situ conversion of H2bpdc(NO2)2 to H2bc via intermediary H2bcNO, and the bc / bcNO structural similarity allows co-crystallization in the Sr polymer lattice (Figure 4b). A control experiment confirmed, however, that H2bc can be isolated in 50% yield from the conditions of entry 6 (40% water; 150 °C; 96 h) without Sr(NO3)2. Reductive cyclizations of 2,2′dinitrobiphenyls to benzo[c]cinnolines is a well-established synthetic pathway46-47 but this appears as a unique preparation without added base.


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CO2H

CO2H

Sr(NO3)2

O 2N NO2

CO2H

DMF / H2O 150 oC

Page 16 of 30

CO2H

N N

O

N N

H2bcNO

H2bc

CO2H

CO2H co-polymerization

[Sr(bcNO)x(bc)1-x.2H2O]

Figure 4. (a) 1H NMR spectra of the aromatic regions of H2bpdc(NO2)2 (black) and from digestions of the solids obtained from entry 1 (blue), 4 (magenta), 5 (red), and 6 (green). The signal marked with an * is from DMF. (b) A scheme showing the conversion to H2bcNO and H2bc and co-polymerization.

Structural description of WUF-17


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Single crystals of WUF-17 suitable for analysis by SCXRD were prepared by reacting Sr(NO3)2 and H2bc in DMF containing 24% water at 100 °C for 16 h. The structure is a non-porous coordination polymer crystallizing in the space group Cc with an asymmetric unit comprised of one Sr center, one bc2 ligand and two aqua ligands, giving the formulation [Sr(bc)(H2O)2] (Figure S5). The eight coordinate Sr atom has a pentagonal plane consisting of an aqua ligand (O10) flanked by carboxylates binding via mode aT and completed by symmetry equivalent bridging aqua ligands (O11, O11ii) (Figure 5a). Capping the equatorial plane on one face is a chelating carboxylate (O3 and O4; mode aC) and a monodentate carboxylate donor on the other (O1i, mode c). The uncoordinated oxygen atom of this carboxylate hydrogen bonds to a bridging aqua ligand (O2i···H11aO11, 2.07(10) Å). Thus, the ligand binds to strontium in mode a at one end and mode c at the other (Figure S6a). In contrast to the nitro-functionalised biphenyl linkers, the fused benzo[c]cinnoline is planar and the torsional angle between the carboxylate moieties is only 13 ° and this allows close layers in the structure. The SrO8 units assemble into 1D infinite chain SBUs by triangular face-sharing oxygen atoms (e.g. O3, O4ii and O11ii). The near-planar bc2 ligands are stacked in an AB pattern


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with close inter-ligand distances (3.256(19) Å) (Figure S6) and align in the same direction, giving the polymeric structure remarkable directionality (Figure 5b).

Figure 5. (a) A perspective view, with selected atomic labeling, of the coordination environment of the strontium centers in WUF-17. The symmetry operations ½+X, ½+Y, +Z and -½+X, 3/2-Y, -½+Z generate the atoms bearing superscripted symbols i and ii, respectively. (b) Part of the extended structure of WUF-17, as viewed parallel to the c axis.


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Thermal Analysis and Photoluminescence Thermal properties of the complexes were assessed using thermogravimetricdifferential thermal analysis (TG-DTA) under a static atmosphere of air from room temperature to 700 °C (Figure 6). The porous MOFs, WUF-15 and WUF-16, were filtered, washed with DMF and acetone and dried in air very briefly before TG-DTA. The mass loss of 32.5% out to 280 °C for WUF-15 matches to approximately two residual DMF guests per formula unit after washing. A noticeable exothermic mass loss of 7.3% between 340-450 °C corresponds to devolatilization of the nitro groups48 after which there is major decomposition to SrCO3 (27.5% found; 25.3% calc.). The TG curve of WUF-16 shows continual mass loss out to 320 °C, at which point the exothermic devolatilization of the nitro groups from ligand backbones is evident. The next event is exothermic framework decomposition starting at 430 °C. WUF-17 loses all terminal and bridging aqua ligands in an endothermic process centered at 200 °C (9.3% found; 9.2% calc.) with no further events until the onset of exothermic decomposition at 460 °C to SrCO3 (36.9% found; 37.9% calc.).


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Figure 6. TG-DTA traces for as-synthesized WUF-15 (black), WUF-16 (orange) and WUF-17 (magenta) to 700 °C. Solid lines represent the TGA and dotted lines the DTA.

The solid-state emission spectra of the complexes and ligands were recorded at room temperature with an excitation wavelength of 310 nm. The nitro-functionalized ligands H2bpdcNO2 and H2bpdc(NO2)2 and their strontium MOFs are weakly fluorescent and the data are shown in Figure S20 with accompanying commentary. Figure 7 shows the interesting results of the photoluminescence spectra for H2bc and WUF-17. Peaks at 374 nm and 524 nm in the emission spectrum of H2bc likely arise from π*→π electronic transitions in the planar aromatic structure. It is also possible that this photoluminescence is via a ligand-to-ligand-charge transfer in the solid state. In addition to peaks at similar wavelengths to H2bc, the spectrum of WUF-17 shows it is highly emissive and contains a


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new feature appearing as a shoulder at 486 nm. We proffer the extensive, close and directional π-π stacking arrangement (3.256(19) Å) between bc2 linkers in the WUF-17 structure gives rise to a new emission pathway.

Figure 7. Photoluminescence spectra (λex 310 nm) for H2bc (blue) and WUF-17 (red).

Conclusions In summary, unique isotopological MOFs with large pores have been synthesized using Sr(II) and linear nitro-functionalized biphenyldicarboxylate linkers. Apart from one crystallographically-imposed planar linker, the bpdcNO2 and bpdc(NO2)2 bridging ligands have considerable torsional angles, and this contrasts with the case of the planar H2bc linker, where a non-porous Sr coordination polymer formed. These results point to the importance of torsional flexibility in biphenyl ligands to modify MOF structures. Water


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content in the solvent medium was found to play a crucial role in promoting formation of crystals large enough for X-ray structural analyses. Water acts as a terminal ligand in WUF-15 and WUF-16 and in terminal and bridging modes in WUF-17. We also found that for WUF-16 a water content of 24% or above resulted in no crystal growth. All structures reported here feature the recurring structural element of 1D strontium-carboxylate chains. Binding mode a is particularly efficient in satisfying the high coordination numbers required for the large strontium atom at the same time as forming charge-neutral frameworks. We have shown that monodentate ligands, provided by solvent molecules, that complete the coordination spheres are substituted easily with retention of the MOF structure, pointing to potential application for these types of Sr(II) framework as heterogeneous Lewis acid catalysts. Further work is in progress in our laboratory exploring the coordination of Sr(II) with a range of functionalized dicarboxylates.

ASSOCIATED CONTENT


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Supporting Information. Comparative 1H NMR spectra for H2bpdcNO2, H2bpdc(NO2)2, H2bc, WUF-15, WUF-16, WUF-17, ESI-MS data, TG-DTA data, SCXRD data, PXRD patterns, gas sorption isotherms, fluorescence spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author * E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources

Notes


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ACKNOWLEDGMENT

AK acknowledges the Australian Government for an Australian Government Research Training Program Award. CR thanks the University of Wollongong for financial support.

ABBREVIATIONS bc, benzo[c]cinnoline-3,8-dicarboxylate; bpdc, 4,4′-biphenyl-dicarboxylate; DMF, N,N′dimethylformamide; DMSO, dimethylsulfoxide; DUT, Technical University of Dresden; ESI, electrospray ionization; IRMOF, isoreticular metal-organic framework; MMA, minimaterials analyzer; MOF, metal-organic framework; UiO, University of Oslo; MIL, Materiaux de l’Institute Lavosier; MS, mass spectrometry; NMR, nuclear magnetic resonance; PXRD, powder X-ray diffraction; SCXRD, single crystal X-ray diffraction; TG-DTA, thermogravimetric and differential thermal analysis; WUF, Wollongong University Framework.

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Large Pore Isoreticular Strontium-Organic Frameworks: Syntheses, Crystal Structures, Thermal and Luminescent Properties

Afsaneh Khansari, Shane G. Telfer, and Christopher Richardson*

Nitro-functionalized biphenyldicarboxylate bridging ligands assemble with Sr(II) into topologically unique MOFs with large pores.


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Large Pore Isoreticular Strontium-Organic Frameworks: Syntheses

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