Mechanochemical Synthesis, Characterization, and Structure

Article pubs.acs.org/crystal

Mechanochemical Synthesis, Characterization, and Structure Determination of New Alkaline Earth Metal-Tetrafluoroterephthalate Frameworks: Ca(pBDC‑F4)·4H2O, Sr(pBDC‑F4)·4H2O, and Ba(pBDC‑F4) Abdal-Azim Al-Terkawi,†,‡,§ Gudrun Scholz,*,† Franziska Emmerling,*,‡ and Erhard Kemnitz*,† †

Department of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-Strasse 2, D-12489 Berlin, Germany Federal Institute for Materials Research and Testing (BAM), Richard-Willstaetter-Strasse 11, D-12489 Berlin, Germany § School of Analytical Sciences Adlershof (SALSA), Humboldt-Universität zu Berlin, Unter den Linden 6, D-10099 Berlin, Germany ‡

S Supporting Information *

ABSTRACT: New fluorinated alkaline earth metal−organic frameworks were successfully synthesized by milling of metal hydroxides M(OH)2 with tetrafluoroterephthalic acid H2pBDC-F4. Both calcium- and strontium-tetrafluoroterephthalates are tetrahydrated, while the barium tetrafluoroterephthalate is free of coordinating water molecules. The two isomorphic structures Ca(pBDC-F4)·4H2O and Sr(pBDC-F4)·4H2O were solved from the powder diffraction data by ab initio structure determination and subsequent Rietveld refinement. The products were thoroughly characterized by elemental analysis, thermal analysis, magicangle spinning NMR, Fourier transform infrared spectroscopy, scanning electron microscopy imaging, and Brunauer−Emmett− Teller measurements. Our findings suggest that the mechanochemical synthesis route is a promising approach for the preparation of new fluorinated alkaline earth metal−organic frameworks.

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The presence of fluorine affects chemical and physical properties of organic molecules and consequently MOFs containing fluorine.24−26 Cheetham and co-workers reported that the replacement of hydrogen with fluorine in an organic ring leads to the formation of MOF structures with higher dimensionalities.27 However, several studies on synthesis, characterization, structures determination, and physicochemical properties of partially fluorinated and fluorinated MOFs (FMOFs) were performed since the first reported structure by Johnson and co-workers in 2004.28−36 These studies were usually done under solvothermal conditions. In the past decade, mechanochemical synthesis of MOFs and FMOFs becomes a greatly broadening area of research.34,35 Nevertheless, structure determination of such hybrid materials from powder X-ray diffraction (PXRD) is limited.36 Comparative studies for structure determination from both single

INTRODUCTION Nowadays, mechanochemical reactions are increasingly utilized for the preparation of new materials. These reactions are considered not only as green and rapid synthesis routes for various new materials but also as an alternative to conventional chemical syntheses. The method is applicable for different materials ranging from nanomaterials to pharmaceutical compounds or metal−organic frameworks (MOFs).1−20 Hence, solvent-free mechanochemical reactions are of high interest from both economical and synthetic perspectives. Alkaline earth metals recently became a matter of interest for building MOFs. These metals promise new topologies of the resulting metal−organic hybrid materials compared to transition metals. The large radii of the heavier alkaline earth metals allow higher coordination numbers and provide increasing structural diversity. Moreover, possible biological applications of hybrid materials containing the nontoxic alkaline earth metals Mg and Ca are another important point of interest. The synthesis of such hybrid materials was usually carried out under solvothermal conditions.21−23 © XXXX American Chemical Society

Received: October 12, 2015 Revised: March 4, 2016

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DOI: 10.1021/acs.cgd.5b01457 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

Synthesis. [Ca(pBDC-F4)·4H2O] (1). Calcium tetrafluoroterephthalate tetrahydrate was obtained by milling H2pBDC-F4 with Ca(OH)2 (molar ratio = 1:1) and 130 μL of distilled water for 1 h. The synthesis conditions were adapted from Scholz et al.56 [Sr(pBDC-F4)·4H2O] (2). Strontium tetrafluoroterephthalate tetrahydrate was obtained under different milling conditions either by milling H2pBDC-F4 with Sr(OH)2 for 4 h, by milling H2pBDC-F4 with Sr(OH)2 and 130 μL distilled water for 1 h, or by milling H2pBDC-F4 with Sr(OH)2·8H2O for 1 h. The molar ratio between reactants was always maintained as 1:1. [Ba(pBDC-F4)] (3). Barium tetrafluoroterephthalate was obtained under different milling conditions either by milling H2pBDC-F4 with Ba(OH)2 for 1 h, by milling H2pBDC-F4 with Ba(OH)2 and 130 μL of distilled water for 1 h, by milling H2pBDC-F4 with Ba(OH)2·H2O for 4 h, or by milling H2pBDC-F4 with Ba(OH)2·8H2O for 1 h. The molar ratio between reactants was always maintained as 1:1. Powder X-ray Diffraction. X-ray diffractograms were recorded with a XRD-3003-TT diffractometer (Seiffert & Co., Freiberg, Germany) with Cu−Kα radiation (λ = 1.542 Å; step scan: 0.05°, step time: 5 s) and with a Stoe Stadi MP diffractometer (STOE & Cie. GmbH, Darmstadt, Germany) operated in transmission geometry with Cu−Kα1 radiation (λ = 1.54056 Å, step scan 0.015°), equipped with a Mythen 1K detector. The samples were measured in a 2θ range of 5− 65° Reflections were compared with diffractograms of the JCPDS-PDF database.68 Powder diffraction measurements for structure solution were performed with a D8 Discover diffractometer (Bruker AXS, Karlsruhe, Germany) operated in transmission geometry (Cu−Kα1 radiation, λ = 1.54056 Å), equipped with a Lynxeye detector. Samples were prepared in glass capillaries (diameter 0.5 mm) and were measured in a 2θ range of 5−65° with a step size of 0.009° and 5 s per step. Structure determinations were carried out based on the PXRD patterns. Indexing and structure solution were performed using the open source program FOX.69 The unit cell and space group were confirmed using CHEKCELL.70 FOX uses global-optimization algorithms to solve the structure performing trials in direct space. This search algorithm uses random sampling coupled with simulated temperature annealing to locate the global minimum of the figure-ofmerit factor. To reduce the total degrees of freedom, parts of the molecules were added to a rigid group. The tetrafluoroterephthalic acid was set rigid with the exception of the carboxyl oxygen. The crystal structures of both compounds were solved by the simulated annealing procedure on a standard personal computer. To complete the structure determination, the structural solution obtained from MC/SA was subsequently subject to a Rietveld refinement employing the TOPAS software.71 Thermal Analysis. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) measurements were performed simultaneously on a thermobalance TAG 24 (SETARAM, Caluire, France). For this purpose, the samples (initial mass ≈ 12 mg) were filled into 100 μL corundum-crucibles with corundum cover and heated under a flow of argon and synthetic air at a rate of 10 K/min up to 400 °C. For simultaneous analysis of evolved gases a mass spectrometer (Balzers Quad-star 421) was coupled by a heated (180 °C) quartz glass capillary (measurements were performed in MID (multiple ion detection) mode). Elemental Analysis. An EURO EA equipment (HEKAtech GmbH) was used for carbon and hydrogen content determinations. The fluorine content was determined with a fluoride sensitive electrode after conversion of the solids with Na2CO3/K2CO3 into a soluble form. The results are shown in Table 2. MAS NMR. 1H, 19F, and 1H → 13C CP MAS NMR spectra were recorded on a Bruker AVANCE 400 spectrometer (Larmor frequencies: ν1H = 400.1 MHz, ν13C = 100.6 MHz and ν19F = 376.4 MHz) using both a 2.5 mm and a 4 mm double-bearing MAS probe (Bruker Biospin). The applied different rotation frequencies are given in the figure captions. 1 H MAS studies were made with a π/2 pulse length of 3.6 μs and a recycle delay of 5 s. Values of the isotropic chemical shifts of 1H and

crystal and powder data show that determining structures from powder diffraction data is considered as reliable as solving structures via single crystals.36−39 In combination with other analytical techniques, solid-state NMR is a powerful approach for assisting setting up initial structural model and for validating structures.40−43 Gagliardi and co-workers, and Van de Streek and Neumann, reported on the combination of PXRD and quantum mechanical electronic structures that allows accurate crystal structures to be determined from powder diffraction data.44,45 The number of reported FMOFs using alkaline earth metals as a metal source is very limited. The synthesis of new coordination polymers (CPs) and FMOFs using (pBDC-F4)2− anions as organic linkers is restricted due to the low solubility of H2pBDC-F4 in some solvents. This problem can be partly circumvented using metal salts of tetrafluoroterephthalic acid [MI2(pBDC-F4)].46−48 Against this background, a solvent-free mechanochemical synthesis, where the reactants are introduced in their solid state, is a promising alternative. In general, the mechanochemical synthesis of coordination polymers with alkaline earth metals is rarely described in the literature. In previous studies, the alkaline earth metals are typically introduced as oxides, hydroxides, carbonates, acetates, hydrides, or halides, and the possibilities of liquid assisted grinding (LAG) are explored.49−59 Mechanochemical synthesis in the form of LAG is utilized for improving the crystallinity of product and for reducing milling time.60−67 The successful formation of Ca-, Sr-, and Ba-terephthalate compounds via mechanochemical reactions and the ab initio structure determination of the hydrated and dehydrated strontium terephthalate55,56 encouraged us to apply the mechanochemical route also for fluorinated MOFs using alkaline earth metal compounds as reactants. Here we report about the first mechanochemically prepared fluorinated alkaline earth MOFs. The structures of calcium and strontium tetrafluoroterephthalate tetrahydrate were determined ab initio based on the powder X-ray patterns. The structure and properties of the new compounds were determined by applying different methods such as PXRD, 1H, 1 H−13C CP, and 19F magic-angle spinning NMR, Fourier transform infrared spectroscopy (FT-IR), elemental analysis, thermal analysis, scanning electron microscopy (SEM) imaging, and Brunauer−Emmett−Teller (BET) surface measurements.

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EXPERIMENTAL SECTION

Materials. Commercially accessible powders of Ca(OH)2, Sr(OH)2, Sr(OH)2·8H2O, Ba(OH)2, Ba(OH)2·H2O, and Ba(OH)2· 8H2O (Aldrich, chemical purity 95−98%) were used as alkaline earth metal sources. Tetrafluoroterephthalic acid (H2pBDC-F4, Aldrich, chemical purity 97%) was used as organic precursor. All chemicals were used without further purification. Planetary Mill. All mechanochemical reactions were performed in a commercial planetary mill “Pulverisette 7 premium line” (Fritsch, Germany) under access of air. Each silicon nitride grinding bowl (volume: 45 mL) was filled with 1 g of a powder mixture and assembled with five silicon nitride balls (12 mm in diameter, 2.8 g each). A ball to powder mass ratio of 14 was ensured. All samples were milled with a rotational speed of 600 rpm. Typical reaction time ranged between 1 and 4 h. The short syntheses (1 h milling) were not interrupted. In the case of the reactions run over 4 h, the reaction was interrupted for 30 min every 60 min to cool down the vials. The milling direction was not reversed in every cycle. The temperature rise in the vial is estimated to be around 100 K. B



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Scheme 1. Mechanochemical Reactions of Hydrated and Non-Hydrated Alkaline Earth Metal Hydroxides with Tetrafluoroterephthalic Acid (H2pBDC-F4)

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C are given with respect to TMS. A contact time of 10 ms was used for the 1H → 13C CP MAS NMR experiments. 19 F MAS NMR spectra were made with a π/2 pulse duration of 3.6 μs, a spectrum width of 400 kHz, a recycle delay of 5 s, and accumulation number of 64. The isotropic chemical shifts δiso of 19F resonances are given below with respect to the CFCl3 standard. Existent background signals of 19F were suppressed with the application of a phase- cycled depth pulse sequence according to Cory and Ritchey.72 BET. Gas adsorption experiments were carried out on an ASAP 2010 (Micromeritics) using a 1 Torr pressure transducer with a resolution of 5−10−5 Torr (0.007 Pa). The adsorptive used was nitrogen at a temperature of 77 K. Prior to the measurements, the samples were degassed at room temperature for 2−4 h in a vacuum. FT-IR. Infrared spectra were recorded with an Equinox 55 IR microscope (Bruker), using the potassium bromide (KBr) pellet technique in the range of 4000−400 cm−1. The sample weight was 1 mg in a pellet of 500 mg KBr.

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RESULTS AND DISCUSSION The mechanochemical reactions of the alkaline earth metal hydroxides (Ca, Sr, or Ba) with tetrafluoroterephthalic acid H2pBDC-F4 led to the formation of new fluorinated metal− organic hybrid frameworks (see Scheme 1). The powder X-ray diffraction patterns of the products Ca(pBDC-F4)·4H2O (1), Sr(pBDC-F4)·4H2O (2), and Ba(pBDC-F4) (3) are depicted in Figure 1c, Figure 2c, and Figure 3c. The comparison with the PXRD pattern of the reactants clearly indicates the completeness of the reactions; i.e., reflections of all reactants disappeared. For a given metal cation, the mechanochemical reactions led to the formation of the same powder product regardless of the variation of the water content of the inorganic precursors (metal hydroxides with different water content in the crystal structure or by adding a small amount of water as shown in Scheme 2). However, due to the implementation of water into the crystal structure at least for the Ca- and Sr-compound, an increasing water content of the inorganic precursors not only reduces the milling time but also improves the crystallinity of the final product. Figure 2II shows the PXRD patterns of the same compound 2 recorded after the milling of H2pBDC-F4 with different strontium hydroxide precursors: (c) Sr(OH)2· 8H2O and milling for 1 h, (d) Sr(OH)2 and addition of 130 μL distilled water, and milling for 1 h, or (e) Sr(OH)2 and milling for 4 h. Milling of H2pBDC-F4 with Sr(OH)2·8H2O for 1 h

Figure 1. Powder X-ray diffraction patterns of the two reactants H2pBDC-F4 (a), Ca(OH)2 (b), and the product Ca(pBDC-F4)·4H2O (1) (c) obtained by milling.

leads to the highest crystallinity of compound 2. Figure 3II shows the PXRD patterns of compound 3 obtained after milling of H2pBDC-F4 with different barium hydroxide precursors: by milling H2pBDC-F4 with Ba(OH)2·8H2O for 1 h (c), with Ba(OH)2·H2O for 4 h (d), with Ba(OH)2 and 130 μL distilled water for 1 h (e), or with Ba(OH)2 for 1 h (f). The milling of H2pBDC-F4 with Ba(OH)2·H2O for 4 h (d) or with Ba(OH)2·8H2O for 1 h (c) lead to the highest crystallinity of compound 3. These results are different from previous work with terephthalic acid H2pBDC.55,56 For instance, the milling of H2pBDC with Sr(OH)2 led to the formation of strontium terephthalate monohydrate, while Sr(OH)2·8H2O as inorganic precursor formed strontium terephthalate trihydrate. Water-free barium terephthalate was formed by milling H2pBDC with Ba(OH)2 or Ba(OH)2·H2O but in the case of Ba(OH)2·8H2O, a hydrated barium terephthalate was formed.56 MII(pBDC-F4)·4H2O (Compounds 1 and 2). Starting from the PXRD patterns, the determination of the new structures Ca(pBDC-F4)·4H2O (1) and Sr(pBDC-F4)·4H2O (2) followed by Rietveld refinement was performed. The results of the C



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Figure 2. (I) Powder X-ray diffraction patterns of the two reactants H2pBDC-F4 (a), Sr(OH)2·8H2O (b), and the product Sr(pBDC-F4)·4H2O (2) (c) obtained by milling; (II) X-ray diffraction patterns of Sr(pBDC-F4)·4H2O (2) obtained by milling H2pBDC-F4 with (c) Sr(OH)2·8H2O (milling time: 1 h), (d) Sr(OH)2 with 130 μL of H2O (milling time: 1 h) or (e) Sr(OH)2 (milling time: 4 h).

Figure 3. (I) Powder X-ray diffraction patterns of the two reactants H2pBDC-F4 (a), Ba(OH)2·8H2O (b), and the product Ba(pBDC-F4) (3) (c) obtained by milling; (II) X-ray powder patterns of Ba(pBDC-F4) (3) obtained by milling H2pBDC-F4 with different hydrated Barium hydroxide precursors: (c) Ba(OH)2·8H2O (milling time: 1 h), (d) Ba(OH)2·H2O (milling time: 4 h) (e) Ba(OH)2 with 130 μL of H2O (milling time: 1 h), or (f) Ba(OH)2 (milling time: 4 h).

monodentate (pBDC-F4)2− anions and seven oxygen atoms (O3, O4, O5) from water molecules (see Figure 5a). The MII− O distances range from 2.3879(1) Å to 2.6001(0) Å for 1 and from 2.4118(1) Å to 2.6471(1) Å for 2. The MIIO9 polyhedra are connected via common faces to adjacent MIIO9 polyhedra resulting in a chain structure. These chains are connected by (pBDC-F4)2− anions to form layers of MII-tetrafluoroterephthalate units wherein, the MII ions are linked by monodentate (pBDC-F4)2− anions. The M···M distances to adjacent chains amount to 12.5377(2) Å in 1 and 12.6252(5) Å in 2. The layers are stacked parallel to the caxis (see Figure 5b). The layered six-member rings of the (pBDC-F4)2− anions are stacked by weak π−π interactions

Rietveld refinement are shown in Figure 4 indicating a good agreement between the simulated and measured powder patterns. The indexing of the PXRD patterns was possible with the lattice constants and unit cell parameters given in Table 1. The only differences between compounds 1 and 2 in the unit cell parameters result from the small difference in the average size between Ca and Sr cations. On the basis of the cell volume, a composition of MII(pBDCF4)·4H2O is reasonable. Both elemental analysis and DTA-TG measurements confirm the formation of tetrahydrates. Both compounds crystallize isomorphous. The MII ion is 9-fold coordinated in a tricapped trigonal prism. The coordination polyhedron MIIO9 in both compounds comprises two carboxylate oxygen atoms (O1) from two D



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Scheme 2. Reaction Paths for the Formation of MII(pBDC-F4)·nH2O Starting from Different Hydrated or Nonhydrated Metal Hydroxides M(OH)2·nH2O, (M = Ca, Sr, or Ba)a

a

Milling time is represented on arrows.

Figure 4. Rietveld refinement of the crystal structure of (a) Ca(pBDC-F4)·4H2O (1). The R-values are Rp = 3.88%, Rwp = 5.07%, and (b) Sr(pBDCF4)·4H2O (2). The R-values are Rp = 4.19%, Rwp = 6.44%. Scattered X-ray intensity for MII (pBDC-F4)·4H2O at ambient conditions as a function of diffraction angle 2θ. The observed pattern (black circles), the best Rietveld fit profile (red line), the reflection positions (blue tick marks), and the difference curve (gray line) between observed and calculated profiles are shown. Rp and Rwp refer to the Rietveld criteria of fit for profile and weighted profile defined by Langford and Löuer.90

pose at a lower temperature of 254 °C in a one-step mass loss 36.8%. These findings of the formation of hydrated compounds are confirmed by the carbon and hydrogen content determined by elemental analysis (see Table 2) and in addition by the MAS NMR spectra. 1 H−13C CP MAS NMR spectra (Figure 6a) depict two carbon signals belonging to the carboxylic group at δ = 167 ppm in both compounds 1 and 2 and the aromatic carbon atoms at δ = 118.7 ppm. Compound 1 contains residues of the precursor H2pBDC-F4. This is indicated by the two shoulders at δ = 114 ppm and δ = 165 ppm. In the spectrum of the precursor H2pBDC-F4, the carbon signal at about δ = 147 ppm can be assigned to residues of C−H bonds in benzene rings. This signal shifted to δ = 143 ppm in both compounds 1 and 2. The existence of the C−H signal indicates the incomplete conversion to the tetrafluoro-compound46 and may be one reason for the low measured fluorine content. The low measured fluorine content is also expected as a consequence

along the a-axis with a distances of 3.7259(1) Å in compound 1 and 3.9249(2) Å in compound 2 (see Figure 5c). Each two adjacent metal cations from two layers bridged by three water molecules to form chains of a 2D layered framework and introducing unique polymeric chains {[MII(H2O)3]2}n. The formed chains are parallel to a-axis. The MII··· MII distances separated by the triple aqua bridges amount to 3.7259(1) Å in compound 1 and 3.9249(2) Å in compound 2. The thermoanalytical behavior of both compounds is comparable and confirms the hypothesis of MII(pBDC-F4)· 4H2O formation. The thermoanalytical curves of Ca(pBDCF4)·4H2O and Sr(pBDC-F4)·4H2O indicate two consecutive mass loss steps in both samples, which correspond to the mass release of the four water molecules (see Figures S2 and S3, Supporting Information). The release of water molecules from the Sr-compound is completed at 254 °C. In the Ca-compound, the four water molecules were completely released at 259 °C. The decomposition of the Ca(pBDC-F4) framework begins with two mass losses of 35.2% and 0.5% at 259 and 365 °C, respectively. The Sr(pBDC-F4) framework begins to decomE



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Table 1. Crystal Data of MII (pBDC-F4)·4H2O Ca(pBDC-F4)·4H2O Synthesis method solvent inorganic educt molar ratio of educts M(OH)2·xH2O: H2pBDC-F4 Structure method coordination oxygen-water oxygen-carboxylate cell parameters crystal system space group cell volume (Å3) unit cell parameters, a, b, c (Å), β (deg) Z λ (Å) M−O (carboxylate) (Å) M−O (water) (Å) M-(pBDC-F4 linker)-M (layer) (Å) M-(water linker)-M (chain) (Å)

Sr(pBDC-F4)·4H2O

mechanochemistry 130 μL H2O Ca(OH)2 1:1

mechanochemistry solvent-free Sr(OH)2·8H2O 1:1

ab initio CaO9 7 2 (from two (pBDC-F4)2− anions)

ab initio SrO9 7 2 (from two (pBDC-F4)2− anions)

monoclinic P21/m (11) achiral 563.07(3) a = 3.7259(1), b = 22.4805(5), c = 6.7332(2), β = 93.5435(19)° 2 1.54056 (Cu−Kα1) 2.6001(0) 2.3879(1)−2.5714(0) 12.5377(2) 3.7259(1)

monoclinic P21/m (11) achiral 602.85 a = 3.9249(2), b = 22.6208(11), c = 6.8030(3), β = 93.5435 (19)° 2 1.54056 (Cu−Kα1) 2.6191(0) 2.4118(1)−2.6472(1) 12.6252(5) 3.9249(2)

at 3409 and 3593 cm−1 in the IR spectrum of compound 2, while the crystal water band appears at 3247 cm−1.73 In the lower wavenumber region, the IR spectrum of the precursor H2pBDC-F4 shows the O−H stretching of a protonated carboxylic group at 2800 cm−1. The vibration band of the CO stretching in COOH is in the range of 1712 cm−1. The CO stretching and OH deformation of COOH resonate at 1225 cm−1. The asymmetric and symmetric stretching of CO at 1482 and 1414 cm−1, respectively. The absorption band at 1001 cm−1 can be assigned to the C−F stretching. For compounds 1 and 2, the IR spectra show the expected bands resulting from the coordination of calcium and strontium cations to the carboxylate oxygen atoms in (pBDCF4)2− anions. The conversion of protonated carboxylic groups of H2pBDC-F4 leads to the absence of the vibration at 2800 cm−1 and at 1712 cm−1 in the formed MII(pBDC-F4)·4H2O samples. The absorption at 1225 cm−1 (C−O stretching and OH deformation of COOH) is shifted to weak vibration bands at 1260 cm−1 in both compounds 1 and 2. The vibrations of the deprotonated carboxylic groups appear at 1604 and 1611 cm−1 in 1 and 2, respectively. The typical band of the deformation vibration of water, which usually appears at ∼1600 cm−1,74,75 could be covered by the strong vibration of carboxylate at the vibration range 1604−1611 cm−1. The vibration band assigned to the C−F stretching in the coordinated tetrafluoroterephthalate is shifted to 994 cm−1 in both compounds 1 and 2. For more information see Figure S1 (Supporting Information). Finishing our work on these compounds, the calcium tetrafluoroterephthalate structure was recently published. Different calcium-tetrafluoroterephthalate compounds were synthesized under solvothermal conditions. These compounds include [Ca(pBDC-F4)(MeOH)2]n and {[Ca4(pBDC-F4)4(H2O)4]·4H2O}n containing 7- and 6-fold coordinated calcium ions.76 The latter complex forms, after further dissolution in water, a new crystal [Ca(pBDC-F4)(H2O)4]n, wherein Ca is 9fold oxygen-coordinated. The last structure is very similar to the structure of compound 1 obtained from powder X-ray

of the difficult splitting of the strong C−F bond in an organic ring (see Table 2). The 1H MAS NMR spectra of both compounds 1 and 2 (see Figure 6b) indicate the coordination of carboxylic groups to the metal cations by the absence of a proton-carboxylic peak at 13.5 ppm, visible in the spectrum of H2pBDC-F4. However, the small peak at δ = 13.5 ppm in compound 1 indicates some residues of H2pBDC-F4 in agreement with the 13C spectrum (Figure 6a). The broad contributions at about 5 ppm can be assigned to protons from coordinating water molecules in both compounds 1 and 2. The 19F MAS NMR spectra (see Figure 6c) depict signals at a chemical shift of δ = −142 ppm in compound 1 and of δ = −144 ppm in compound 2. Two signals at δ = −130 ppm and −136 ppm of low intensity in compound 1 refer to unreacted residues of H2pBDC-F4 (not visible in Figure 6c). The solved crystal structures of both compounds 1 and 2 point to two crystallographically distinguishable positions of fluorine atoms (see Figure 5b). These two positions are not resolved in the NMR spectra, but the shape and broadness of the fluorine signals at δ = −142 ppm and at δ = −144 ppm of compound 1 and compound 2 respectively indicate that the two crystallographic positions of fluorine atoms may be hidden within the broad peak. The FT-IR spectra give an overview about the chemical binding environment and allow the assignment of crystal water molecules. The fraction of broad vibration bands above 3000 cm−1 in the IR spectra of the formed complexes indicates the presence of water molecules (see Figure S1, Supporting Information). Two peaks and a shoulder appear in the spectrum of compound 1 at approximately 3572 cm−1, 3410 cm−1, and 3192 cm−1, indicating the presence of different water moieties in the sample. The broad peak at 3410 cm−1 results from water at the surface, whereas the small overlapped band at 3192 cm−1 could be assigned to contributions of coordinating water molecules. The vibration bands of water at surface appear F



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Figure 5. Crystal structure of MII(pBDC-F4)·4H2O (M = Ca (1) or Sr (2)). (a) The coordination environment of MII atom. The polyhedron MIIO9 comprises seven oxygen atoms from water molecules (O3, O4, and O5) and two oxygen atoms from two monodentate (pBDC-F4)2− anions (O1). The dark lines represent the tricapped trigonal prism with the MII ion (yellow) in center. (b) The 2D layers of MII(pBDC-F4) framework. (c) The layered chains of the tetra-hydrated MII(pBDC-F4) compound.

diffraction with small differences in cell parameters and cell volume, which could result from using two different synthesis

approaches. However, the coordination number nine for Ca is rare.77 Though, only few compounds with a 9-fold coordination G



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The IR spectrum of compound 3 shown in Figure S1 (Supporting Information) depicts the vibration bands of water at the surface at 3443 cm−1. The shoulder at 3343 cm−1 cannot clearly be assigned to a contribution of crystal water. In the lower wavenumber region, the weak vibration bands at 1266 cm−1 refer to C−O stretching and OH deformation vibrations of COOH. The vibrations of the deprotonated carboxylic groups appear at 1604 cm−1. The vibration band assigned to the C−F stretching vibration is shifted to 987 cm−1. In the literature, the formation of water-free bariumdicarboxylate aryls was reported. Lo et al. have synthesized 3D barium terephthalate.85 The formed compound was free of coordinating water molecules, although the synthesis was processed under aqueous condition. The water-free barium terephthalate was also obtained mechanochemically by milling H2pBDC with Ba(OH)2 or Ba(OH)2·H2O.56 Comparison of Compounds 1, 2, and 3. The thermal analysis of the three compounds 1, 2, and 3 indicates that the three frameworks exhibit a similar trend of decompositions. They are stable up to 259 °C, 254 °C, and 243 °C, respectively. These temperature ranges of decomposition are higher than the measured decomposition temperature of the precursor H2pBDC-F4. The latter begins to decompose at 220 °C up to 290 °C by releasing CO2.46 The thermal analysis of the DMF solvate of lithium tetrafluoroterephthalate shows a release of CO2 at 330 °C.48 The total decomposition ended at 500 °C by the release of LiF species. Here, the formed compounds 1, 2, and 3 are stable in air and maintain their structural integrity at ambient conditions up to 250 °C. The thermal post-treatment of the two tetrahydrated compounds 1 and 2 at 250 °C for 1 h results in new powder X-ray patterns as shown in Figures S5 and S6, respectively (Supporting Information). The MAS NMR spectra confirm the different structural properties of compound 3 in comparison to the two isomorphic compounds 1 and 2. The 1H−13C CP MAS NMR spectrum of compound 3 shows a chemical shift of the carboxylic groups, which is different from the corresponding one in the compounds 1 and 2. The solved structures of compounds 1 and 2 indicate four crystallographic positions of carbon (C1, C2, and C3 in the ring and C4 in the carboxylic group, see Figure 5b). The three positions of carbon in the ring could not be resolved in NMR spectra due to their similarities, but the shape and broadness of carbon signals at δ = 118.7 ppm in both compounds 1 and 2 suggest that these positions are covered by the broad peak. The comparatively broader carbon signal in

Table 2. Results of Elemental Analysis and BET Measurements of MII(pBDC-F4)·nH2O

C% (calc.) C% (exp.) H% (calc.) H% (exp.) F% (calc.) F% (exp.) SBET (m2·g−1)

Ca(pBDC-F4)· 4H2O

Sr(pBDC-F4)· 4H2O

Ba(pBDCF4)

27.6 28.4 2.3 2.0 21.8 17.9 2.6 ± 0.02

24.1 24.8 2.0 1.7 19.2 16.1 12.1 ± 0.1

25.7 24.5 0.0 0.2 20.3 13.3 5.3 ± 0.05

of Ca atoms are found in the literature.78−80 A few compounds including the 9-fold coordination of strontium atoms were also reported.81−84 Ba(pBDC-F4) (3). The structure determination of compound 3 is difficult due to the low crystallinity of the sample and the low quality of the measured PXRD pattern. However, the hypothesis of the formation of water-free Ba(pBDC-F4) framework was confirmed by different analytical methods including elemental analysis, thermal analysis, and the spectroscopic methods MAS NMR and FT-IR. The elemental analysis indicates the presence of only 0.2% of hydrogen, which is below the detection limit and could result from moisture (see Table 2). The thermoanalytical curves show no water release confirming the assumption of being free of coordinating water molecules (see Figure S4, Supporting Information). The decomposition of the framework begins at 243 °C with a total mass loss of 39.8%. The MAS NMR spectra support the deviating structural properties of compound 3 in comparison to compounds 1 and 2. In the 1H−13C CP MAS NMR spectrum, the signal of the carboxylic group is almost at the same position as for the precursor H2pBDC-F4 at δ = 165 ppm (see Figure 6a). The signal assigned to carbon in the aromatic ring in compound 3 has a chemical shift of 119.3 ppm. The comparatively narrow signal at a chemical shift 4.4 ppm in the 1H MAS NMR spectrum (see Figure 6b) indicates the presence of mobile water at the surface of the sample. The two fluorine signals at δ = −140.5 ppm and δ = −142.7 ppm in addition to the peak at δ = −135.5 ppm in the 19F MAS NMR spectrum refer at least to three or even more crystallographic positions of fluorine atoms (see Figure 6c).

Figure 6. MAS NMR spectra of the reactant H2pBDC-F4, the products Ca(pBDC-F4)·4H2O, Sr(pBDC-F4)·4H2O, and Ba(pBDC-F4), where (a) 1H NMR spectra, νrot = 20 kHz; (b) 1H−13C CP MAS NMR spectra, νrot = 10 kHz; (c) 19F MAS NMR spectra, νrot = 20 kHz. H



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Figure 7. SEM images of Ca(pBDC-F4)·4H2O, Sr(pBDC-F4)·4H2O, and Ba(pBDC-F4).

compound 3 at δ = 119.3 ppm may indicate more crystallographic positions of aromatic carbon atoms in comparison with compounds 1 and 2. Compared to the 1H MAS NMR spectra of compounds 1 and 2, which indicate the presence of coordinating water molecules, the relatively narrow signal in the spectrum of compound 3 indicates the existence of mobile water at the surface. There is no signal of water molecules embedded within the crystal structure of compound 3 (see Figure 6b). The three signals in the 19F MAS NMR spectrum of compound 3 indicate the different coordination environment of the fluorine atoms in comparison to compounds 1 and 2 (see Figure 6c). The solubility of compounds 1, 2, and 3 in organic solvents such as ethanol, DMF, or acetone is very poor. Experiments to obtain single crystals from the powder products prepared by milling were so far not successful. The alkaline earth metals Mg, Ca, Sr, and Ba were used in several studies to investigate the effect of the metal cation size on the structure and stability of extended hybrid materials of aryl-dicarboxylate frameworks.86,87 Mg2+ cations tend to have a 6-fold coordinated environment. Sr2+ and Ba2+ cations are usually 8-fold coordinated, but the intermediate sized Ca2+ cations can be 6-, 7-, or even 8-fold coordinated. Also 9- and 10-fold coordinated Ba2+ cations were reported.87,88 The length of the MII−O bond increases as the radius of the metal cation increases. In addition, the M-O bond length increases by increasing the coordination number of the metal cation.23,89 The binding mode of alkaline earth metals to the aryl polycarboxylate anions confirms the predominant building block as a chain of metal centers bridged either by carboxylate moieties alone or by both carboxylate ligands and solvent molecules. However, all these samples described in the literature were prepared by solvothermal syntheses. In the structures of the two compounds 1 and 2, the parallel chains of the metal centers are linked by carboxylate ligands forming a 2D layered structure. The compounds 1 and 2 are isomorphous with 9-fold coordinated metal cations. The small difference in size between the Ca cation and the bigger Sr cation results in

different MII−O bond lengths in their coordination to (pBDCF4)2− anions and water molecules (see Figure 5 and Table 1). The SEM images shown in Figure 7 depict similar morphologies, platelike structures, and particle sizes for all three compounds. The compounds 1, 2, and 3 have very small surface areas (see Table 2). Even the calculated surface areas are in the region of 15 m2/g only, the calculated porosity being 3.8% for the calcium and 6.9% for the strontium compound. Their adsorption/desorption isotherms are shown in Figures S7, S8, and S9 (see Supporting Information).

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CONCLUSION New alkaline earth metal−organic hybrid frameworks MII(pBDC-F4)·4H2O (MCa, Sr) and Ba(pBDC-F4) were prepared by milling powders of alkaline earth metal hydroxides M(OH)2 with tetrafluoroterephthalic acid H2pBDC-F4 in a stoichiometric ratio of 1:1 in a planetary mill (see Scheme 2). The structures of Ca(pBDC-F4)·4H2O and Sr(pBDC-F4)· 4H2O were solved ab initio from powder diffraction data and refined by Rietveld refinement. It is surprising that the variation of water content in the inorganic precursors of Ca-, Sr-, and Bahydroxides did not influence the number of coordinated water molecules in the final products. This is in contrast to the mechanochemical synthesis of alkaline earth metal-terephthalate compounds,56 where the number of coordinated water molecules of the final products was influenced by the water content of the inorganic precursor compounds. However, the water content of the reactants affects at least the crystallinity of the new formed frameworks, which increases in the case of using metal hydroxide octahydrates of Sr and Ba or by adding a small fraction of water (130 μL) to the Ca(OH)2. Along with these findings, the water content of the reactants has a strong influence on the necessary milling time. The lower the water content the longer is the required milling time. The easy access of these alkaline earth metal tetrafluoroterephthalate by milling is encouraging for possible syntheses of further coordination polymers with alkaline earth metals to study their behavior and local coordination with different fluorinated organic ligands. I



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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01457. FT-IR spectra, thermoanalytical curves, powder X-ray patterns, adsorption and desorption pore volume isotherm (PDF) Accession Codes

CCDC 1443539−1443540 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*(G.S.) E-mail: [email protected]. *(F.E.) E-mail: [email protected]. *(E.K.) Email: [email protected]. Funding

This research was supported by the Excellence Initiative of the German Research Foundation (DFG Project: GSC 1013). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. S. Reinsch (BAM Berlin) and Dr. M. Feist (HU Berlin) for DTA-TG measurements, Mrs. A. Zimathies and Mr. C. Prinz (BAM Berlin) for gas adsorption and BET measurements, Dr. K. Scheurell (HU Berlin) for her contribution in MAS NMR measurements, Ms. S. Bäßler (HU Berlin) for fluorine analysis, and Mrs. U. Kätel and Mrs. J. Odoj (HU Berlin) for elemental analysis.

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Mechanochemical Synthesis, Characterization, and Structure

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