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Synthesis and Characterization of Phosphine-Functionalized Metal− Organic Frameworks Based on MOF‑5 and MIL-101 Topologies Flavien L. Morel,† Marco Ranocchiari,*,‡ and Jeroen A. van Bokhoven*,†,‡ †

ETH Zürich, Department of Chemistry and Bioengineering, Wolfgang-Pauli Str. 10, CH-8039 Zürich, Switzerland Paul Scherrer Institute, Laboratory for Catalysis and Sustainable Chemistry, CH-5232 Villigen, Switzerland

‡

S Supporting Information *

ABSTRACT: Phosphine metal−organic frameworks (P-MOFs) are an emerging class of coordination polymers that provide novel opportunities for metal−supported catalysis as solid ligands. Here, we present the synthesis and characterization of three novel P-MOFs based on a (diphenylphosphino)terephthalic acid linker. We targeted the structures of MOF-5, MIL-101(Cr), and MIL-101(Al)-NH2 to produce materials with partial diphenylphosphine substitution, which were crystalline and highly porous.

1. INTRODUCTION Metal−organic frameworks (MOFs) are coordination polymers that are produced via self-assembly of multidentate organic building blocks with inorganic clusters.1 They offer unique opportunities in gas sorption,2 sensing applications,3 drug delivery,4 and catalysis5,6 because of their defined and highly porous structures, adaptability to a broad range of functional groups using the concepts of isoreticularity, receptivity to postsynthetic modification (PSM) strategies, and amenability to frameworks with mixed organic linkers (MIXMOFs). We and other groups foresee their opportunities in the field of catalysis because of their chemical and structural versatility.5−8 Despite an ever-increasing number of MOF-related publications, research on MOF catalysts remains in its infancy. Much research focuses on the synthesis of MOFs bearing catalytically active sites with well-defined structures. Many catalytic examples involve the reactivity of unsaturated sites, which are located at the inorganic nodes of the framework or at defect sites. Early works capitalized on the reactivity of transition metals to perform hydrogenation and isomerization reactions.9−11 More recently, the HKUST-112,13 and MIL10114,15 frameworks were used in diverse Lewis acid catalytic reactions such as the isomerization of terpene derivatives and the cyclization of citronellal to isopulegol. The catalytic properties of the organic building blocks were also used in many MOFs. The amino moieties in IRMOF-3 and MIL-53 were used to catalyze the condensation of benzaldehyde with acetates.16 The incorporation of a second metallic species is also compatible with the synthesis of MOFs and was investigated using porphirine struts for their catalytic applications in olefin epoxidation17 and acyl transfer to pyridylcarbinols.18 Metalosalene struts were recently reported for their activity toward olefin epoxidation19 and epoxide resolution20 The most flexible strategy to generate an active site is by PSM, which is an elegant method for including functionalities that cannot be obtained directly by synthesis.21,22 This is a particularly attractive method when one is willing to incorporate a second metallic species into a MOF.23,24 Rosseinsky et al. demonstrated that two PSM steps led to the © XXXX American Chemical Society

coordination of a vanadium catalyst onto the IRMOF-3 framework.25 Since then, the multistep modification of amino-functionalized linkers is commonly pursued, as it provides a controlled strategy for metal incorporation. Yaghi et al. modified the amino groups of UMCM-1-NH2 with 2pyridinecarboxaldehyde to create an iminopyridine moiety that was subsequently metalated with PdCl2(CH3CN)2.26 Cohen et al. used a similar approach to transform the amino moieties of the same material into salene and pyrazine groups, which were subsequently employed as coordinating groups for Fe(III) and Cu(II) complexes. The resultant metalated materials were active in Mukaiyama-aldol reactions.27 Corma et al. used IRMOF-3 to produce a Schiff base complex containing Au(III), which was active in three-component coupling and cyclization reaction of various N-protected ethynylanilines with amines and aldehydes.28 Secondary amine modification is not limited to zinc-based frameworks and can also be applied to the more robust MIL-101 topology, as was recently shown by Canivet et al.29 They modified the amino groups of MIL-101(Fe)-NH2 with a Ni(II) preformed complex to produce an active catalyst for the dimerization of ethylene into 1-butene. The modification of amine groups attached to a MOF framework is a very flexible method to incorporate new functionalities. The popularity of amino functional groups in MOFs presumably arises from their facile synthesis of very stable and crystalline frameworks, attributes that are difficult to secure with other functional building blocks. Few modifications of the aminobiphenyldicarboxylic acid have been reported so far, even though it contains the same functional group as its terephthalate counterpart and in principle can produce structures with bigger pores.30 A good MOF support for catalysis should offer synthetic versatility and be disposed toward coordination of transition Special Issue: Massimo Morbidelli Festschrift Received: October 21, 2013 Revised: January 6, 2014 Accepted: January 7, 2014

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dx.doi.org/10.1021/ie403549v | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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Scheme 1. Synthesis of LSK-11, LSK-12, and LSK-15 from PPh2-bdc Presented in This Publication

The resulting material catalyzed the hydration of phenylacetylene in the presence of trifluoroacetic acid, and the cyclization of N-(prop-2-yn-1-yl)benzamide into the corresponding oxazole. Kaskel et al. demonstrated the potential of phosphine SL as support for rhodium complexes.40 A common feature of the existing P-MOFs is that they are synthesized from tridentate organic building blocks. Such structures are not always crystalline or do not maintain porosity upon solvent removal. The tuning of steric and electronic properties of the phosphine group is limited in an organic linker of this type, and the framework stability is dependent on the pyramidal geometry of the tridentate phosphine. Organophosphine linkers based on linear dicarboxylic acids should not suffer from these limitations. With this in mind, we recently reported the synthesis of LSK-3, a zinc MOF based on 2(diphenylphosphino)-[1,1′-biphenyl]-4,4′-dicarboxylic acid (PPh2-bpdc) with IRMOF-9 topology.41 The PPh2-bpdc linker produced a MOF in which phosphine moieties were oriented directly into the pores of the framework. An organophosphine linker based on terephthalic acid would allow access to more numerous, stable, and porous structures. Goesten and coworkers recently investigated a postsynthetic chloromethylation method as a route to incorporate diphenylphosphine moieties into MIL-101(Cr) and MIL-53(Al)-NH2 frameworks.42 Here, we present the synthesis of novel P-MOFs with MOF-5, MIL101(Cr), and MIL-101(Al)-NH2 topologies that are based on 2-(diphenylphosphino)terephthalic acid (PPh2-bdc) (Scheme 1).

metals in a single site fashion. In pursuit of heterogeneous catalysts with single sites, we propose a novel synthetic strategy for preparing solid ligands (SLs) capable of triggering single coordination events with transition metal atoms. SLs with alcohol, catechol, and imidazolium salts (furnishing Nheterocyclic carbene (NHC) sites)31−33 are known, but their ability to coordinate transition metals is strongly limited and few catalytic applications are described. A good SL for catalysis should create strong bonds with transition metals under mild conditions and must produce single atom active sites selectively. Phosphine ligands are commonly found in homogeneous catalysts as they have a strong affinity for transition metals and can be used for coordination without prior modification, in contrast to secondary amines and other functional groups.34 In addition to their good transition metal ligating properties, organophosphines are widely used for their chemical flexibility, which allows fine-tuning of their steric and electronic properties in homogeneous applications.34 MOFs containing phosphine moieties (P-MOFs) were recently investigated for their metal-binding properties. In an early work, Lin et al. created two different zirconium-based frameworks using diphenylphosphinobinaphthyl ruthenium chiral complexes to produce heterogeneous hydrogenation catalysts.35 These materials were stable and recyclable, but their structures were never fully resolved due to their partially amorphous nature. The first crystalline MOF containing an organophosphine linker, PCM-1, was reported a few years later by Humphrey et al.36 The carboxylate groups on 4,4′,4″phosphinetriyltribenzoic acid (ptbc) were coordinated to zinc to form Zn4O clusters, yielding a framework with free phosphorus groups that proved unstable upon solvent removal. The use of tri(para-carboxylated) triphenylphosphine37 and phosphine oxide38 linkers has led to more robust materials. We recently reported the catalytic properties of a highly stable ptbczirconium MOF called LSK-1.39 The phosphine anchoring groups coordinated single Au(I) atoms onto the framework.

2. EXPERIMENTAL SECTION 2.1. Synthesis of P-MOFs. 2.1.1. MIXMOF-5-PPh2 (LSK11). PPh2-bdc (0.264 g, 0.75 mmol), terephthalic acid (0.250 g, 1.5 mmol), and Zn(NO3)2·4H2O (1.773 g, 6.77 mmol) were combined in a round-bottom flask under an argon atmosphere. N,N-dimethylformamide (DMF, 80 mL) was added and the mixture was stirred for 10 min at 50 °C. The solution was B



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divided into ten 20 mL scintillation sealed vials and these were placed in a nitrogen-flushed oven at 95 °C (isothermal mode) for 48 h. After cooling to room temperature, the yellow crystals of MIXMOF-5-PPh2 (LSK-11) were transferred to a single vial and suspended in DMF (3 × 10 mL) over a period of three days. Prior to nitrogen adsorption, the sample was exchanged with acetone (3 × 10 mL), dried using a Tousimis Samdri 931 critical point dryer, and activated at 150 °C for 24 h under a reduced vacuum. The recovered mass was 0.230 g. 2.1.2. MIXMIL-101(Cr)-PPh2 (LSK-12). PPh2-bdc (0.100 g, 0.29 mmol) and terephthalic acid (0.285 g, 1.71 mmol) were added to an aqueous solution of tetramethylammonium hydroxide (TMAOH, 10 mL, 0.05 M) and stirred for 10 min. Cr(NO3)3·9H2O (0.800 g, 2.00 mmol) was added and the mixture was stirred for an additional 20 min. The mixture was placed in a PTFE-lined autoclave and heated at 210 °C for 24 h under a nitrogen atmosphere. After cooling to ambient temperature, the green powder was collected using centrifugation and washed with water (3 × 20 mL) and DMF (3 × 10 mL) and placed in a Soxhlet extractor for 24 h (ethanol, 200 mL) to yield MIL-101(Cr)-PPh2 (LSK-12). Prior to the nitrogen adsorption experiment, the sample was activated at 150 °C for 24 h under a reduced vacuum. The recovered mass was 0.235 g. 2.1.3. MIXMIL-101(Al)-NH2-PPh2 (LSK-15). PPh2-bdc (0.175 g, 0.5 mmol) and aminoterephthalic acid (0.181 g, 1.0 mmol) were dissolved in DMF (40 mL) under an argon atmosphere at 110 °C. A solution of AlCl3·6H2O in DMF (0.724 g, 3.0 mmol in 20 mL) was added dropwise over a period of 1 h with stirring. The mixture was stirred for 3 h and then permitted to stand without stirring for 18 h. The solid was isolated by centrifugation and washed with DMF (3 × 20 mL) and ethanol (3 × 20 mL) to give MIL-101(Al)-NH2-PPh2 (LSK-15). Prior to analysis, the sample was activated by Soxhlet extraction for 24 h (ethanol, 200 mL) and dried at 150 °C for 24 h under a reduced vacuum. The recovered mass was 0.142 g. 2.2. Characterization. Magic-angle spinning (MAS) NMR spectra were recorded on a Bruker AVANCE AMX-400 spectrometer. The spectrometer was calibrated to external NH4H2PO4, NH4Al(SO4)2, and adamantane references for 31P, 27 Al, and 13C NMR spectra, respectively. 31P spectra were recorded at 20 kHz in a 2.5 mm zirconium oxide rotor. 27Al and 1 H/13C CP spectra were recorded at 10 kHz in a 4 mm zirconium oxide rotor. Powder X-ray diffraction patterns were recorded at room temperature on a PANanalytical Empyrean diffractometer at 45 kV, 40 mA with Cu Kα (λ = 1.541 Å) radiation. Measurements were carried out at a diffraction angle 2θ = 2−70° with a step of 0.033°. Nitrogen adsorption isotherms were recorded on a Micromeritics Tristar II 3020 at 77 K. BET surface areas were calculated in a relative pressure range of p/p0 = 0.05 to 0.3. MOF samples were digested using a DCl/DMSO-d6 mixture under sonication.

Figure 1. Microwave-assisted synthesis of POPh2-bdc by a two-step strategy.

bdc is carried out in water, does not require anoxic conditions, has no major byproducts, and involves simple purification steps. This makes it highly environmentally friendly and, so far, the only route to POPh2-bdc. High resolution mass spectrometry (HRMS) and NMR spectroscopy confirmed the structure of the desired product (Figures S1−S3, Supporting Information). Crystallization of POPh2-bdc with a mixture of ethanol and water led to the formation of single crystals that were suitable for analysis by X-ray diffraction. The structural analysis (Figure 2) revealed that the ligand cocrystallizes with two molecules of

Figure 2. Representation of POPh2-bdc cocrystallized with water after refinement of the single-crystal XRD structure (ORTEP drawings; thermal ellipsoids at 30%, hydrogen atoms omitted for clarity). C atoms are shown in gray, O in red, and P in orange.

water (refined as O(6) and O(7)). The unit cell was triclinic with the following dimensions: a = 9.2111(7) Å, b = 9.9850(8) Å, c = 10.5450(8) Å, α = 95.976(1)°, β = 90.822(1)°, γ = 105.171(1)°. The structure was refined in P(−1) and indicated that the POPh2-bdc ligand adopted a tetrahedral geometry around the phosphorus atom, as expected for a triarylphosphine.34 Reduction of the phosphine oxide POPh2-bdc to its free phosphine derivative PPh2-bdc was necessary in order to synthesize the desired P-MOFs. Beller et al.46 recently reported a metal-free reduction of functionalized phosphine oxides, a method we adapted to POPh2-bdc. Reduction of POPh2-bdc in the presence of methyldiethoxysilane and diphenylphosphate catalyst in toluene at 110 °C for 72 h produced the sought phosphine PPh2-bdc in 61% isolated yield (Figure 3). Slow oxidation of the phosphine was observed in solution, and the product was stable in its solid form. The final product used in the MOFs syntheses contained less than 5% of phosphine oxide as verified by NMR spectroscopy. The structure of PPh2-bdc was confirmed by HRMS and NMR spectroscopy (Figures S4− S6, Supporting Information). Although this reduction method

3. RESULTS AND DISCUSSION 3.1. Synthesis of the Organic Linker PPh2-bdc. A P−C coupling reaction between 3-iodo-4-methylbenzoic acid and diphenylphosphine oxide, catalyzed by Pd/C in water under microwave irradiation43,44 (Figure 1), afforded 3-(diphenylphosphoryl)-4-methylbenzoic acid in 91% yield. Oxidation of 3iodo-4-methylbenzoic acid with KMnO4 in water produced the desired diphenylphosphine oxide terephthalic acid POPh2bdc.45 Advantageously, the preparative procedure for POPh2C



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distribution inside the crystal, but the systematic presence of both PPh2-bdc and bdc in analyzed samples indicates that LSK11 is a unique MIXMOF phase and not the result of a mixture of two independent phases. Standard 31P solid-state MAS NMR may be used to directly assess the phosphorus nuclei inside LSK-11 because of the high natural abundance of the 31P isotope. In contrast, 15N and 13C solid-state NMR measurement often requires isotopically enriched samples or application of advanced NMR techniques, such as dynamic nuclear polarization (DNP), to achieve a satisfactory signal-to-noise ratio.50 Figure 4 presents the 31P NMR spectrum of LSK-11, in which two signals associated with phosphine oxide and free phosphine are evident at +37 ppm and −6 ppm, respectively. These signals are consistent with reported values in P-MOFs and other solid or soluble phosphines materials.34,36 The sharp phosphine peak at −6 ppm supports the crystalline nature of LSK-11. The PPh2-bdc organophosphine linker partially oxidizes when in contact with residual oxygen during the solvothermal synthesis. This is the most probable cause of the phosphine oxide peak in the 31 P NMR spectrum of LSK-11. Triarylphosphines are known for their resistance toward oxidation, particularly in their solid-state forms.51 The resulting material resisted oxidation when stored under an inert atmosphere at room temperature. Partial oxidation occurred upon exposure in air, even more so if suspended in aerated solvents (Figure S10, Supporting Information). For this reason, LSK-11 was stored in dry, degassed solvent, and the crystals could be stored for more than a month without oxidation or loss of crystallinity. Phosphine oxidation reduces the number of coordination sites available for metal atoms but does not impact negatively on the framework. The ratio between phosphine and nonphosphine linker was varied to evaluate how the amount of phosphine inside LSK-11 affected the properties of the material (Table S1, Supporting Information). We observed that a PPh2-bdc to bdc ratio up to 2:1 did not affect the crystallinity of the material, although a reduced BET surface area from 2320 to 1490 m2/g was noted because of the increased occupation of the bulky phosphine ligand inside the pores of LSK-11. At PPh2-bdc to bdc ratios below 1:3, a new amorphous phase formed. A PPh2-bdc to bdc ratio below 1:5 did not yield crystalline material. 3.3. MIXMIL-101(Cr)-PPh2 (LSK-12). MIL-101(Cr) is wellknown for its large pores and its excellent chemical and thermal

Figure 3. Reduction of POPh2-bdc to the corresponding organophosphine PPh2-bdc.

was slow compared to other reported procedures, its high selectivity and product yield opens new perspectives in the synthesis of PPh2-bdc on a large scale. 3.2. MIXMOF-5-PPh2 (LSK-11). The PPh2-bdc organophosphine linker was used to produce a framework with MOF5 topology, keeping its further application as a SL for catalysis in mind. We diluted the bulky diphenylphosphino functional groups in the MOF by synthesizing a MIXMOF with both terephthalic acid (bdc) and PPh2-bcd organic linkers (Scheme 1). By doing so, we hoped to reduce functional group crowding in the framework, thereby leaving enough free space to accommodate metal catalysts and to allow reactants and products to diffuse through the pores. The initial synthesis of LSK-11 was performed with PPh2-bdc and bdc linkers in a molar ratio of 1:2 in DMF under solvothermal conditions at 95 °C for 24 h. Light-yellow cubic crystals were formed, which were characterized by powder X-ray diffraction (Figure 4). Comparison of the resultant spectrum with the MOF-5 diffraction pattern confirmed the cubic structure of LSK-11.47 We dried LSK-11 using supercritical CO2, which is an established technique for removing solvent in zinc MOFs,48 followed by an activation period under vacuum at 150 °C. Nitrogen adsorption experiments indicated that LSK-11 had a BET surface area of 2320 m2/g, comparable to functionalized materials of MOF-5 topology.49 We digested the sample in a mixture of DCl/DMSO-d6 to ascertain the amount of phosphine present inside LSK-11. An 1H NMR spectrum of the sample revealed that the PPh2-bdc to bdc ratio inside the framework was 1:3.5 (Figure S7, Supporting Information). We also observed that the organophosphine linker oxidized into phosphine oxide under digestion conditions. To verify that the organophosphine crystallized together with bdc in a unique phase, we analyzed digested single crystals via liquid chromatography (Figure S8, Supporting Information). This method does not provide an indication regarding linker

Figure 4. Characterization of LSK-11. Powder X-ray diffraction pattern (left), nitrogen adsorption at 77 K (middle) and 31P MAS NMR (right), displaying the signals of free phosphine inside LSK-11 (A) and phosphine oxide (B). D



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stability, which is essential for catalytic applications. We produced a phosphine material with MIL-101 topology called LSK-12. Its synthesis was adapted from the HF-free synthesis of MIL-101(Cr) reported by Hatton et al.52,53 LSK-12 was synthesized mixing PPh2-bdc and bdc in a ratio of 1:6 in an aqueous solution of TMAOH at 210 °C for 24 h. Powder X-ray diffraction (Figure 5) showed that the resulting material was

Figure 6. Comparison of the physical properties of LSK-12 synthesized with different PPh2-bdc to bdc ratios. Top: Powder Xray diffraction patterns. Bottom: Nitrogen adsorption isotherm of LSK-12 with ratios of 1:6 (●), 1:2 (□), and 1:1 (△).

the framework. Digested samples were analyzed by liquid chromatography to compare their respective phosphine loading (Table S2, Supporting Information). Because LSK-12 could only be obtained as a microcrystalline powder, characterization was only performed using bulk analysis techniques, and therefore, cannot give insights regarding the relative distribution of the two organic linkers at the crystal scale. Linker homogeneity is a general issue with MIXMOFs, which was addressed in the past on a case-by-case fashion such as in MIL53(Al)-NH2,56 but a general method has yet to be developed. 3.4. MIXMIL-101(Al)-NH2-PPh2 (LSK-15). To demonstrate the versatility of the PPh2-bdc linker toward different metallic nodes, we attempted to synthesize a phosphinefunctionalized MIL-101(Al) framework. The amino MIL101(Al)-NH2 is already reported and represents an interesting alternative to MIL-101(Cr), as it contains amino functional groups that can undergo PSM for subsequent metal incorporation.19 The synthesis reported by Hartmann and Fischer57 is carried out at milder conditions than its chromium analogue, which helps to prevent the formation of phosphine oxide. It also contains Al(III) centers, which are diamagnetic unlike Cr(III), thus allowing solid state NMR spectroscopy characterization to be performed.58 We first attempted the synthesis using PPh2-bdc and bdc linkers (see Supporting Information for detailed synthetic procedure) and recovered a semicrystalline solid whose powder XRD pattern is similar to the MIL-53(Al) (Figure 7).59 To the best of our knowledge, the nonamino functionalized MIL-101(Al) phase has not yet been reported. We overcame this problem by replacing bdc

Figure 5. Powder X-ray diffraction pattern of LSK-12 and nitrogen adsorption isotherms of LSK-12.

highly crystalline and possessed a pattern similar to that of MIL-101(Cr). LSK-12 displayed the same thermal and chemical stability as MIL-101(Cr) (Figure S12, Supporting Information) and could be washed with ethanol and activated at 150 °C without loss of crystallinity. Analysis of the digested sample by liquid chromatography showed that the PPh2-bdc to bdc ratio inside LSK-12 was 1:19 (Figure S13, Supporting Information). The nitrogen adsorption isotherm of LSK-12 (Figure 5) confirmed the high porosity of the material. The shape of the isotherm is comparable to that of MIL-101(Cr) synthesized via the TMAOH route53 as well as the HF route.54 The BET surface area (3020 m2/g) was lower than that of the phosphinefree material, a quality that is attributed to the presence of the bulky phosphine ligand. We investigated the variation in phosphorus loading inside LSK-12 by changing the PPh2-bdc to bdc ratio in the mother liquor. Ratios of 1:2 and 1:1 were compared with the original synthesis procedure (1:6). Although the X-ray diffraction patterns showed that all materials were crystalline, the BET surface area decreased significantly from 3020 m2/g for the 1:6 ratio to 1860 m2/g and 835 m2/g for 1:2 and 1:1, respectively (Figure 6). We did not observe the formation of a MIL-53(Cr) phase, which can occur when similar conditions are used.55 The loading of phosphine functional groups can, therefore, be easily tuned in these materials without having a detrimental effect on E



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Figure 8. Top: 31P NMR showing the signals of free phosphine in LSK-15 at −10 ppm (A1) and −6 ppm (A2) and phosphine oxide at +35 ppm (B). Bottom: 27Al MAS NMR spectrum of LSK-15. Sidebands are signaled by (*).

Figure 7. Top: Powder XRD patterns of the semicrystalline phase (A), MIL-101(Al)-NH2 (B) and LSK-15 (C). Bottom: Nitrogen adsorption isotherms at 77 K of LSK-15 (●), MIL-101(Al)-NH2 (□), and the semicrystalline phase (△).

4. CONCLUSION We synthesized PPh2-bdc, a terephthalic acid derivative bearing a diphenylphosphino moiety. Our multistep approach generated the target linker in high yield without the use of expensive catalysts and tedious separation procedures. This modular method, involving sequential oxidation of 3(diphenylphosphoryl)-4-methylbenzoic acid and reduction of the phosphine oxide group, can be employed to generate finely tuned phosphine sites on a range of terephthalic acid derivatives. PPh2-bdc was successfully used to prepare new MOFs with MOF-5 and MIL-101 topologies. By doing so, we bridged the gap between P-MOFs and existing structures that are often employed in metal-supported catalysis. The former class of materials possesses interesting metal coordination properties, the full potential of which remains unexplored due to stability issues. The three novel materials LSK-11, LSK-12, and LSK-15 were synthesized using a mixture of ligands to avoid excessive pore hindrance by the bulky diphenylphosphino moiety, and to preserve high crystallinity and phase purity. All three materials were highly porous and did not collapse upon solvent removal. We have demonstrated that these materials can incorporate different amounts of phosphine, which may be varied to a certain extent without modification of the synthetic conditions. In pursuit of solid porous ligands, with high chemical stability (particularly toward protic solvents), we have investigated organophosphine-containing MOFs with chromium- and aluminum-based inorganic units. These materials demonstrate potential in single-site catalysis due to their easy synthesis and their ability to coordinate discrete transition metal nuclei.

with the aminoterephthalic acid (NH2-bdc). After the addition of aluminum chloride hexahydrate into a solution of PPh2-bdc and NH2-bdc in DMF at 110 °C, we recovered a pale yellow crystalline material of the expected MIL-101(Al)-NH2-PPh2 topology (LSK-15). Analysis by powder X-ray diffraction confirmed that LSK-15 has a similar structure to MIL101(Al)-NH2 (Figure 7). An 1H NMR spectrum of the digested sample shows that the PPh2-bdc to NH2-bdc ratio was 1:7.3 (Figure S14, Supporting Information). The semicrystalline solid displayed a lower nitrogen adsorbed volume compared to the two amino-functionalized materials and a BET surface area of 907 m2/g (Figure 7). In comparison, the LSK-15 isotherm shape and BET surface area (2690 m2/g) is comparable to the MIL-101(Al)-NH2 framework.58 The 31P MAS NMR spectrum of LSK-15 revealed three distinct signals at +35, −6, and −10 ppm (Figure 8). Signals A1 and A2 (∼85% of the total phosphorus integral) were attributed to the free phosphine sites and signal B to the phosphine oxide. Due to the complex topology of MIL-101, we believe that two preferential, nonequivalent positions are occupied by the organophosphine, which contribute to the two separate signals in the 31P MAS NMR spectrum (Figure 8, top). 13C/1H crosspolarization NMR experiments confirmed the presence of amino groups and did not show extra-framework carboxylic acid groups (Figure S15, Supporting Information). The framework integrity of LSK-15 was further verified by 27Al NMR spectroscopy (Figure 8, bottom) in which octahedral alumina appeared as a lone signal at −0.46 ppm, thereby ruling out P−O−Al coordination. F



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ASSOCIATED CONTENT

S Supporting Information *

Full synthetic details, including compound characterization (NMR spectroscopy, nitrogen adsorption isotherms, thermogravimetric analysis) and crystallographic data in CIF format are presented in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*J. A. van Bokhoven. E-mail: [email protected]. ch. Tel.: +41 44 632 5542 *M. Ranocchiari. E-mail: [email protected]. Tel.: +41 56 310 5843. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Kim Meyer for proofreading the manuscript. We gratefully thank ETH Zürich (ETHIIRA proposal ETH-33 11-1) for funding.

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REFERENCES

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