Iron-Based Metal–Organic Frameworks MIL-88B and NH2-MIL-88B

Article pubs.acs.org/crystal

Iron-Based Metal−Organic Frameworks MIL-88B and NH2‑MIL-88B: High Quality Microwave Synthesis and Solvent-Induced Lattice “Breathing” Mingyan Ma,† Angélique Bétard,‡ Irene Weber,† Noura Saad Al-Hokbany,§ Roland A. Fischer,‡ and Nils Metzler-Nolte*,† †

Ruhr-Universität Bochum, Inorganic Chemistry I − Bioinorganic Chemistry, 44801 Bochum, Germany Ruhr-Universität Bochum, Inorganic Chemistry II − Organometallics and Materials, 44801 Bochum, Germany § Chemistry Department, College of Science, King Saud University, P.O. Box 22452, Riyadh 11495, Saudi Arabia ‡

S Supporting Information *

ABSTRACT: Iron-based MIL-88B and NH2-MIL-88B microcrystals with high dispersibility and uniform size were successfully synthesized by using a rapid microwave-assisted solvothermal method. By carefully controlling the reaction conditions, the microwave method provided superior quality MIL-88B crystals in high yields and excellent phase purity. Framework flexibility was observed for both MIL-88B-Fe and NH2-MIL-88B-Fe frameworks in various solvents, which however significantly differs between the two materials. MIL-88B-Fe shrinks reversibly by about 25% only when it is dispersed in the strongly hydrogen bonding solvents water or methanol. In contrast, NH2-MIL-88B-Fe shrinks up to 33% upon replacement of dimethylformamide (DMF) by any other solvent studied (benzene, chloroform, acetone, acetonitrile, methanol, water). The change in unit cell parameters (shortening of the a axis) can be seen macroscopically, although the overall integrity of the materials is maintained. We suggest that hydrogen bonding between the oxygen atoms of the MIL-88B-Fe framework and solvent molecules plays an important role in the framework shrinkage, while in the NH2-MIL-88B-Fe framework additional hydrogen bonds may form and thus a different breathing behavior is observed.

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INTRODUCTION Metal−organic frameworks (MOFs, also known as porous coordination polymers) have attracted considerable attention in recent years as an important class of hybrid materials with highly porous structures and well-defined tailorable cavities. MOFs were widely studied in the fields of energy (H2/CH4 gas) storage,1,2 molecule adsorption and separation,3 in-pore assembly of nanoparticles,4−6 catalysis,7,8 and drug delivery.9−13 Among the thousands of MOF materials, the highly flexible MIL-88B frameworks (MIL = Matériaux de l’Institut Lavoisier/ Materials from the Lavoisier Institute) have been much sought after since the first report on their exceptionally large “breathing effect” in 2007. “Breathing” in this context describes the capacity of the framework’s unit cell to reversibly swell and shrink under the influence of an external stimulus (temperature, pressure, chemical inclusion...) without harming the framework topology. Such a behavior is unusual for solid crystals and thus of great interest for solid state chemists and physicists especially in the field of host−guest interactions and for the “gateopening/closing” selective adsorption of gases or solvents.14−17 However, subsequent studies on the flexibility of bulk MIL-88B material are rather rare compared with the increasing numbers of studies on other flexible MOF materials with MIL-53 topology.18−21 A deeper understanding of the flexible behavior of MIL-88B frameworks in different environments is thus of great interest. © 2013 American Chemical Society

Very recently, Horcajada and co-workers investigated the influence of various substitutes on the linkers upon the flexible behavior of MIL-88B and MIL-88D frameworks, indicating decreased swelling amplitude after introducing functional groups. However, the crystals were treated by washing (i.e., exchanging the synthesis solvent, DMF, with deionized water or acetone) and drying processes prior to the investigation of the lattice parameters.22 Possible lattice changes during these treatments were therefore overlooked. Thus, a detailed study of the structural behavior of MIL-88B frameworks already starting from solvent exchange is of great interest, especially for understanding the interaction between the frameworks and different solvents. In this communication, we used a rapid microwave-assisted solvothermal synthesis method for the production of iron(III) terephthalate MIL-88B material (hereafter labeled as MIL-88B-Fe) and amino-functionalized MIL88B frameworks (labeled as NH2-MIL-88B-Fe) and investigated the flexible behavior of as-prepared crystals in various polar or apolar solvents. Compared with the previously reported swelling behavior of dried MIL-88B frameworks,15,22,23 this study reveals a different shrinkage behavior for as-synthesized MIL-88B-Fe material after exchanging the Received: November 27, 2012 Revised: March 19, 2013 Published: March 19, 2013 2286

dx.doi.org/10.1021/cg301738p | Cryst. Growth Des. 2013, 13, 2286−2291

Crystal Growth & Design

Article

Figure 1. MIL-88B structure viewed along a axis (left) and c axis (right).

Figure 2. SEM micrographs of (a−c) MIL-88B-Fe, (d−f) NH2-MIL-88B-Fe crystals gained from a microwave heating at 150 °C for 10 min. The jagged crystal edges in (f) are probably caused by a slight shaking of the SEM instrument during the measurement.

producing small-sized MOFs with improved phase purity, macroscopic morphology, and yield.31−34 However, by adopting the microwave synthesis method, small differences in the reaction conditions can be expected to lead to vastly different results. We therefore detailedly investigated the influences of the synthesis conditions, i.e., heating time, heating temperature, and mother solution composition, on the morphology and phase purity of iron-based MIL-88B crystals under microwave heating. NH2-MIL-88B-Fe frameworks were also synthesized by using the same synthesis method. To the best of our knowledge, there is no microwave synthesis study of MIL-88B-Fe and NH2-MIL-88B-Fe crystals up to the present moment.

synthesis solvent DMF by a variety of other solvents, as shown by powder X-ray diffraction studies. Meanwhile, the shrinkage behavior of amino-functionalized framework NH2-MIL-88B was also investigated in the present study, to the best of our knowledge for the first time, and revealed striking differences to MIL-88B. The structure of MIL-88B frameworks viewed along the a axis and c axis is shown in Figure 1. The production of pure and high-quality MIL-88B crystals was found to be quite challenging and thus hampered detailed studies of MIL-88B materials. Often, a mixture of MIL-88B with MIL-53 or MIL101 crystals, which can be produced from the same starting solution, is obtained rather than a single pure crystalline phase.24,25 The development of a reproducible synthetic method for high quality MIL-88B materials is therefore of great significance but as yet has not been achieved. The welldeveloped and most commonly used synthesis method for metal−organic frameworks is the hydro-/solvothermal method, which normally takes hours to days.26 A more effective method, i.e., microwave-assisted solvothermal synthesis, has attracted growing attention recently as a rapid way to synthesize MOF materials within minutes.27−30 Beside its high efficiency, the microwave-assisted method is reported to be superior for

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RESULTS AND DISCUSSION As our initial target, MIL-88B-Fe crystals were produced from a dimethylformamide (DMF) solution of ferric chloride hexahydrate (FeCl3·6H2O) and terephthalic acid (H2BDC) by microwave-assisted heating to 150 °C for 1−10 min; see Supporting Information for detailed procedures. Scanning electron micrographic (SEM) images demonstrated that spindle-shaped crystals with high dispersibility and a uniform size of ca. 2.4 μm in length and 1.2 μm in diameter were 2287



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the morphologies of MIL-88B-Fe products was investigated. Well-crystallized MIL-88B-Fe microcrystals can be obtained by microwave heating for as short as 1 min with a yield of ca. 35% from a typical mother solution with a Fe3+/H2BDC/DMF molar ratio of 1:1:282. Increasing the heating time only slightly increased the yield without changing the product morphology (Figure S1, Supporting Information). Given the high quality of the MIL-88B-Fe crystals gained, the highly efficient microwave heating undoubtedly provides an energy-saving route for the fast synthesis of MIL crystals. For a typical mother solution, and by applying microwave power, crystals can be gained by heating at a temperature as low as 120 °C (Figure S2). Increasing the heating temperature remarkably increased the yield and the crystallinity of products. A heating temperature of 150 °C was thus adopted in a typical procedure to maximize the yield, keeping the reaction mixture just below the boiling point of DMF solvent, which is 152.8 °C. Varying the concentration of the mother solution, our studies showed that MIL-88B products can be isolated in a wide range of concentrations. Increasing the mother solution concentration from a Fe3+/H2BDC/DMF molar ratio of 1:1:282 to 1:1:140 and 1:1:70 yielded the same MIL-88B crystalline phase with only small variations of crystal size and morphology (Figure S3). In comparison, decreasing the mother solution concentration from 1:1:282 to 1:1:565 and 1:1:1130 resulted in the formation of crystals with MIL-101 topologies (Figures S4 and S5). Varying the H2BDC concentration from a H2BDC/Fe3+ molar ratio of 0.2−2 yielded products with the same MIL-88B crystalline phase but with an improved growth of crystalline faces (Figure S6). However, increasing the Fe3+ concentration favored production of MIL-101 crystals rather than MIL-88B crystals (Figures S7 and S8). On the basis of the overall results from the above studies, a DMF solution with a Fe3+/H2BDC/ DMF molar ratio of 1:1:282 and a subsequent microwave heating at 150 °C for 10 min was adopted as a typical synthesis method in our study. In order to probe the generality of the microwave-assisted solvothermal synthesis strategy we developed herein, we

successfully prepared (Figure 2a−c). We found that this procedure yields highly reproducible results with only slight variation of the crystal size among different batches. The phase purity of the products was determined by powder X-ray diffraction (PXRD). As shown in Figure 3a,b, the reflection

Figure 3. PXRD patterns of (b) as-prepared MIL-88B crystals isolated from DMF and (d) as-prepared NH2-MIL-88B crystals isolated from DMF. The vertical bars in (a) and (c) separately present the 2θ positions in the corresponding calculated pattern.

patterns of as-prepared materials, which were isolated from the DMF solution, could be indexed in the hexagonal space group of MIL-88B (P6̅2c), which indicates that pure products with MIL-88B topology were obtained. The influence of modifying the microwave-assisted synthesis conditions, i.e., (i) heating time, (ii) heating temperature, and (iii) mother solution concentration on the crystalline phase and

Figure 4. PXRD patterns of as-synthesized MIL-88B-Fe after isolating from various solvents. I: (a) C6H6, (b) CHCl3, (c) Acet., (d) DMF, (e) DMSO, (f) MeCN, (g) EtOH, (h) MeOH, (i) H2O. II: enlarged PXRD patterns of MIL-88B-Fe after isolating from (b) DMF, (d) MeOH, (f) H2O. The short bars in (a), (c), (e) separately present the 2θ positions in the corresponding calculated pattern. Note the products isolated from DMF are MIL-88B-Fe-as. The values for relative polarity are normalized from measurements of solvent shifts of absorption spectra and were taken from Reichardt, C. Solvents and Solvent Effects in Organic Chemistry, 3rd ed.; Wiley-VCH Publishers: New York, 2003. 2288



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applied the same synthesis method to Fe3+/NH2−BDC solutions aiming to produce NH2-MIL-88B-Fe. The amino groups in the framework would not only facilitate covalent modification of the material, such as fluorescent labeling or peptide conjugating,35,36 but may also affect the structural flexibility. SEM images show the successful production of high quality products. Contrasting with MIL-88B-Fe crystals, needleshaped crystals with a size of ca. 1.5 μm in length and 300 nm in diameter were formed (Figure 2d−f). The phase purity of the products was again determined by powder X-ray diffraction (PXRD). As shown in Figure 3c,d, the reflection patterns of asproduced products could also be indexed in the space group P6̅2c. The interaction of MOF frameworks with organic liquids after synthesis, in terms of host−guest interaction, is important because many materials show selective uptake of guest molecules, and this property is highly desirable for applications in the separation or purification of organics. Kitagawa et al. have proposed that structural changes that occur upon exposure to external stimuli (i.e., gases or liquid) are governed by the host− guest interactions.37 Herein, we dispersed the MIL-88B-Fe-as (as = as synthesized, i.e., isolated from DMF solution, Figure 4I, d) products in various polar or apolar solvents, aiming at studying the response of the MIL-88B-Fe framework to different liquid environments. As shown in Figure 4-I, a−g, dispersing the MIL-88B-Fe-as crystals in a variety of solvents including benzene (C6H6), chloroform (CHCl3), acetone (Acet.), dimethyl sulfoxide (DMSO), acetonitrile (MeCN), and ethanol (EtOH), yielded PXRD patterns similar to the initial MIL-88B-Fe-as product. This indicates that no lattice variation is caused by treatment with these solvents. In comparison, however, drastically different PXRD patterns were recorded upon dispersing the MIL-88B-Fe-as products in methanol (MeOH, Figure 4-I, h) or water (H2O, Figure 4-I, i), indicating significant variations of the lattices (Figure 4-I, d,h,I are enlarged in Figure 4-II). Table 1 shows the calculated

cell parameters after isolating the product from methanol and water, compared to DMF. The lattice constants of MIL-88BFe-as were determined to be a(b) =12.69 Å, c = 18.44 Å, V = 2570.15 Å3. After contact with methanol or water, a shrinkage of the cell volume over 23% with an atom displacement around 2 Å was observed, which is in stark contrast to the swelling behavior of MIL-88B-Cr material in methanol/DMF.15 One remark to be noticed is that the MIL-88B-Fe crystals are less stable in H2O for prolonged periods of time compared to all other solvents, and thus extended immersion in H2O is not suggested. A deformation of the macro crystals is expected as a result of the lattice shrinkage after isolation from methanol and water, which was indeed proven by SEM. As shown in Figure 5, a decrease in crystal diameter of ca. 100−130 nm and an increase of the crystal length of ca. 100 nm were recorded, corresponding to the decrease of the a axis and the increase of the c axis, respectively. No crystal collapse or face crack was observed during the framework shrinkage. MIL-88B-Fe with “gate-closing” function could thus serve as a drug delivery material, which is loaded in organic solvents and releases its load only slowly under physiological condition in water by disintegration of the framework. One may notice that the MIL-88B-Fe-as framework shrank only in methanol and H2O, both of which can possibly form OH---O hydrogen bonds with the terminal water molecules on the iron octahedrons or the oxygen atoms of the bridging carboxylate groups. This might be the cause of the lattice shrinkage since a simple removal of guest DMF molecules from the pores did not result in any deformation of the framework (Figure S10). The guest −OH groups are more likely trapped in the ab plane with more, closer, and compressed O atoms, forming strong hydrogen bonds and leading to the decrease of cell parameter a (which equals b), and the subsequent increase of the c axis, which matches the observed experimental data (Figure S11). In comparison, ethanol did not cause a related lattice variation, although it also has a −OH group (Figure 4-I, g). We speculate that this is due to the weaker polarity of ethanol molecules which does not result in strong enough hydrogen bonds to trigger the lattice shrinkage. It is clear that the −OH---O hydrogen bonds play an important role in the lattice shrinkage of MIL-88B-Fe. Similar to observations made by Loiseau et al., the hydrogen bonds between guest hydroxyl groups (from water in that case) and the oxygen atoms of the framework are responsible for the framework shrinkage as shown by solid-state NMR studies on MIL-53-Al.19 However,

Table 1. Calculated Cell Parameters of MIL-88B-Fe Frameworks after Isolation from DMF, MeOH, and H2O isolated from

a [Å]

c [Å]

V [Å3]

space group

DMF MeOH H2O

12.7 10.9 10.8

18.4 19.3 19.3

2570.2 1966.2 1944.4

P6̅2c P6̅2c P6̅2c

Figure 5. SEM micrographs of (a) MIL-88B-Fe-as crystals isolated from DMF, (b) same sample after dispersing in MeOH, and (c) after dispersing in H2O. Scale bar = 2 μm. More pictures are shown in Figure S9. 2289



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Figure 6. PXRD patterns of as-synthesized NH2-MIL-88B-Fe after isolating from various solvents. I (left): (a) C6H6, (b) CHCl3, (c) Acet., (d) DMF, (e) DMSO, (f) MeCN, (g) EtOH, (h) MeOH, (i) H2O. II (right): Enlarged PXRD patterns of NH2-MIL-88B-Fe after isolation from (a, b) DMF, (c, d) EtOH. In II, the short bars in (a, c) separately present the 2θ positions in the corresponding calculated pattern. Note the products isolated from DMF are NH2-MIL-88B-Fe-as.

We thus assume that the shrinkage of the NH2-MIL-88B-Fe-as framework is not caused by the interaction between the exchanging solvents and the framework. Nevertheless, removal of the guest DMF molecules by calcination at 150 °C under a vacuum for 48 h resulted in shrinkage of the NH2-MIL-88B-Fe framework, concomitant with slight decomposition of the product as witnessed by decreased PXRD quality (Figure S14). Hence, we suggest that during solvent exchange or activation (removal of DMF molecules) -HNH---NH2 or -HNH---O hydrogen bonds inside the pores become possible and lead to the observed framework shrinkage. As discussed above for the original MIL-88B-Fe framework, steric interactions might also influence this behavior as amino subsitution of the organic ligand will decrease not only the available space inside the pores but also close some of the gaps necessary for solvent exchange or removal. In summary, we have successfully developed a rapid microwave-assisted solvothermal method for the synthesis of iron-based MIL-88B and NH2-MIL-88B crystals with high dispersibility and uniform size distribution. By carefully controlling the reaction conditions, the microwave method provides superior quality MIL-88B crystals in high yield, devoid of contaminations of other MIL materials with similar stoichiometry. We further observed a flexible behavior of MIL-88B-Fe framework in various polar or apolar liquids. Notably, the unit cell shrinks reversibly by almost 25% upon dispersing the crystals in water or methanol. This seems to suggest that hydrogen bonding between the oxygen atoms in the MIL-88B-Fe framework and solvent molecules plays an important role in the crystal shrinkage. The reversible “breathing” between the swelling in DMF and the shrinkage in hydrogen bonding solvents without collapse of the crystal lattice makes MIL-88B-Fe a potential “gate opening/closing” material for selective adsorption and separation or controlled molecule loading/delivery applications. The NH2-MIL-88B-Fe framework behaves differently due to the presence of pendant amino groups, which can form hydrogen bonds within the framework. For NH2-MIL-88B-Fe, the lattice shrinks upon

solvent molecules like DMF or DMSO are also considerably bigger than water and MeOH, which might prevent optimal formation of hydrogen-bonding interactions in the constriction of the MOF pores. To study the reversibility of this shrinkage process, MIL-88BFe-as crystals were dispersed in methanol (or H2O) and then redispersed in DMF. Calculated lattice parameters based on the PXRD patterns show that the MIL-88B-Fe lattice swelled back to the original open (Figure S12 and Table S1). The flexible behavior of the NH2-MIL-88B-Fe framework in different solvents was also studied by using again PXRD analysis and computer calculation. To this end, NH2-MIL-88BFe-as (isolated from DMF solution, Figure 6-I, d) products were dispersed in the same series of solvents (hereafter labeled as exchanging solvents) as used in the above MIL-88B-Fe study. Differently, shrinkage between 28−35% was observed in all solvents, although these solvents have different polarities and structures (see Figure 6 and Table 2). This is different from the behavior of MIL-88B-Fe-as, which only shows lattice shrinkage induced by water and methanol. Removing the exchanging solvents from the framework pores by vacuum or calcination did not lead to any further lattice variation, which suggests that the interaction between these exchanging solvents and the NH2-MIL-88B-Fe-as framework is rather weak (Figure S13). Table 2. Calculated Cell Parameters of NH2-MIL-88B-Fe Frameworks after Isolation from Various Solvents isolated from

a [Å]

c [Å]

V [Å3]

space group

H2O DMSO MeOH DMF MeCN Acet. CHCl3 EtOH C6H6

10.9 11.6 10.9 14.3 11.0 11.5 11.1 11.0 11.2

19.6 19.2 19.7 17.4 19.6 19.3 19.3 19.0 19.7

2016.7 2212.4 2033.8 3073.8 2060.8 2185.6 2055.7 1998.7 2149.4

P6̅2c P6̅2c P6̅2c P6̅2c P6̅2c P6̅2c P6̅2c P6̅2c P6̅2c 2290



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(14) Surble, S.; Serre, C.; Mellot-Draznieks, C.; Millange, F.; Ferey, G. Chem. Commun. 2006, 284. (15) Serre, C.; Mellot-Draznieks, C.; Surble, S.; Audebrand, N.; Filinchuk, Y.; Férey, G. Science 2007, 315, 1828. (16) Férey, G.; Serre, C. Chem. Soc. Rev. 2009, 38, 1380. (17) Scherb, C.; Schödel, A.; Bein, T. Angew. Chem., Int. Ed. 2008, 47, 5777. (18) Bourrelly, S.; Moulin, B. a.; Rivera, A.; Maurin, G.; DevautourVinot, S.; Serre, C.; Devic, T.; Horcajada, P.; Vimont, A.; Clet, G.; Daturi, M.; Lavalley, J.-C.; Loera-Serna, S.; Denoyel, R.; Llewellyn, P. L.; Férey, G. J. Am. Chem. Soc. 2010, 132, 9488. (19) Loiseau, T.; Serre, C.; Huguenard, C.; Fink, G.; Taulelle, F.; Henry, M.; Bataille, T.; Férey, G. Chem.Eur. J. 2004, 10, 1373. (20) Devic, T.; Horcajada, P.; Serre, C.; Salles, F.; Maurin, G.; Moulin, B.; Heurtaux, D.; Clet, G.; Vimont, A.; Greneche, J.-M.; Ouay, B. L.; Moreau, F.; Magnier, E.; Filinchuk, Y.; Marrot, J.; Lavalley, J.-C.; Daturi, M.; Férey, G. J. Am. Chem. Soc. 2010, 132, 1127. (21) Salles, F.; Ghoufi, A.; Maurin, G.; Bell, R. G.; Mellot-Draznieks, C.; Férey, G. Angew. Chem., Int. Ed. 2008, 47, 8487. (22) Horcajada, P.; Salles, F.; Wuttke, S.; Devic, T.; Heurtaux, D.; Maurin, G.; Vimont, A.; Daturi, M.; David, O.; Magnier, E.; Stock, N.; Filinchuk, Y.; Popov, D.; Riekel, C.; Férey, G.; Serre, C. J. Am. Chem. Soc. 2011, 133, 17839. (23) Scherb, C.; Koehn, R.; Bein, T. J. Mater. Chem. 2010, 20, 3046. (24) Bauer, S.; Serre, C.; Devic, T.; Horcajada, P.; Marrot, J.; Ferey, G.; Stock, N. Inorg. Chem. 2008, 47, 7568. (25) Scherb, C.; Schödel, A.; Bein, T. Angew. Chem., Int. Ed. 2008, 47, 5777. (26) Ma, M.; Zacher, D.; Zhang, X.; Fischer, R. A.; Metzler-Nolte, N. Cryst. Growth Des. 2011, 11, 185. (27) Taylor-Pashow, K. M. L.; Rocca, J. D.; Xie, Z.; Tran, S.; Lin, W. J. Am. Chem. Soc. 2009, 131, 14261. (28) Taylor, K. M. L.; Rieter, W. J.; Lin, W. J. Am. Chem. Soc. 2008, 130, 14358. (29) Jhung, S. H.; Lee, J. H.; Yoon, J. W.; Serre, C.; Férey, G.; Chang, J. S. Adv. Mater. 2007, 19, 121. (30) Centrone, A.; Harada, T.; Speakman, S.; Hatton, T. A. Small 2010, 6, 1598. (31) Seo, Y.-K.; Hundal, G.; Jang, I. T.; Hwang, Y. K.; Jun, C.-H.; Chang, J.-S. Microporous Mesoporous Mater. 2009, 119, 331. (32) Haque, E.; Khan, N. A.; Park, J. H.; Jhung, S. H. Chem.Eur. J. 2010, 16, 1046. (33) Ni, Z.; Masel, R. I. J. Am. Chem. Soc. 2006, 128, 12394. (34) Diring, S. p.; Furukawa, S.; Takashima, Y.; Tsuruoka, T.; Kitagawa, S. Chem. Mater. 2010, 22, 4531. (35) Ma, M.; Gross, A.; Zacher, D.; Pinto, A.; Noei, H.; Wang, Y.; Fischer, R. A.; Metzler-Nolte, N. CrystEngComm 2011, 13, 2828. (36) Liu, B.; Ma, M.; Zacher, D.; Betard, A.; Yusenko, K.; MetzlerNolte, N.; Woell, C.; Fischer, R. A. J. Am. Chem. Soc. 2011, 133, 1734. (37) Kitagawa, S.; Kitaura, R.; Noro, S.-i. Angew. Chem., Int. Ed. 2004, 43, 2334.

displacement of the in situ captured DMF molecules from the pores by any other solvent, or by heating under a vacuum. The study of the flexible behavior of MIL-88B-Fe and NH2-MIL88B-Fe frameworks in different solvents and the understanding of the host−guest interaction between the MIL-88B-Fe framework and guest molecules will contribute to our understanding of material design and usage. Considering the good biocompatibility of iron carboxylate MIL-88B-Fe materials9 and the availability of coordinatively unsaturated metal sites (CUSs) in MIL-88B frameworks, MIL-88B-Fe and NH2-MIL-88B-Fe materials are promising candidates for controlled drug delivery, gas-selective adsorption, and catalysis applications.

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ASSOCIATED CONTENT

* Supporting Information S

Synthetic procedures as well as additional SEM micrographs and PXRD data. This information is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Fax: ++49 − (0)234 − 32 14378. E-mail: nils.metzler-nolte@ rub.de. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Dr. Kirill Yusenko for valuable discussions regarding the PXRD data. The authors also gratefully acknowledge support from the Research Department Interfacial Systems Chemistry at Ruhr-Universität Bochum. Financial support from KSU to N.S.A. and N.M.-N. is also gratefully acknowledged.

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REFERENCES

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Iron-Based Metal–Organic Frameworks MIL-88B and NH2-MIL-88B

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