Anchored Aluminum Catalyzed MeerweinâPonndorfâVerley

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Anchored Aluminum Catalyzed Meerwein−Ponndorf−Verley Reduction at the Metal Nodes of Robust MOFs Patrick J. Larson,† Joseph L. Cheney,† Amanda D. French,‡ David M. Klein,‡ Benjamin J. Wylie,† and Anthony F. Cozzolino*,† †

Department of Chemistry & Biochemistry and ‡Department of Environmental Toxicology, Texas Tech University, Box 41061, Lubbock, Texas 79409-1061, United States S Supporting Information *

ABSTRACT: Catalytic Meerwein−Ponndorf−Verley reductions of ketones and aldehydes in the presence of isopropyl alcohol were performed at aluminum alkoxide sites that were postsynthetically introduced into robust metal−organic frameworks (MOFs). The aluminum was anchored at the bridging hydroxyl sites inherent in some MOFs. MOFs in the UiO-66/ 67 family as well as DUT-5 were successfully adapted to this strategy. Incorporation of catalytically active aluminum species greatly enhanced the reactivity of the native MOF at 80 °C in the case of both UiO-66, and was almost solely responsible for catalytic activity in the case of metalated UiO-66 and DUT-5. The site isolation of the catalyst prevented aggregation and complete deactivation of the molecular aluminum catalyst, allowing it to be recovered and recycled in the case of UiO-67. This catalyst also proved to be moderately tolerant to wet isopropyl alcohol.

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INTRODUCTION The Meerwein−Ponndorf−Verley (MPV) reduction of ketones or aldehydes is a selective transfer hydrogenation in the presence of, typically, isopropyl alcohol as both solvent and hydrogen source.1,2 This reaction commonly uses a Lewis acid catalyst and produces acetone as an innocuous coproduct which can be distilled off to drive the reaction equilibrium toward the products. The original catalyst for the MPV reduction, aluminum isopropoxide, stands out from many of the traditional catalysts for transfer hydrogenations because of the high availability of aluminum, as well as its low cost and low toxicity.3,4 Other metals, such as ruthenium,5 tin,6 and zirconium,7 have also been used for this reaction. MPV reductions have been catalyzed using both homogeneous and heterogeneous catalysts, each of which has their own benefits and limitations. Homogeneous catalysts offer the benefit of selectivity, through either enantiospecific or regiospecific activity, as well as milder reaction conditions. They are, however, restricted to batch chemistry and often are supported by complex ligands that are expensive or difficult to prepare.8,9 Active aluminum catalysts are also often very sensitive toward hydrolysis and can aggregate over time in solution; both processes lead to catalyst deactivation. Approaches to overcome catalyst aggregation include freshly preparing the catalyst and sterically protecting the aluminum center.8−10 Heterogeneous catalysts, on the other hand, are more robust and allow for catalyst recovery and reuse as exemplified by a Sn-beta zeolite.6 They lack the tunability that is one of the hallmarks of homogeneous catalysts.11 In addition, while similar reactions © XXXX American Chemical Society

have been catalyzed by zirconia, higher reaction temperatures can make the process less desirable.7 Our approach to prevent catalyst deactivation is to siteisolate a molecular catalyst in a heterogeneous environment, specifically a metal−organic framework (MOF) (Figure 1).

Figure 1. Depiction of incorporating active single site Al catalysts in MOFs through postsynthetic modification of hydroxyl sites at SBUs. Green balls represent SBUs; gray cylinders represent organic linkers.

Although the stability of MOFs has often been a drawback, a number of applications of MOFs as heterogeneous catalysts have recently been reported.12,13 These efforts have been bolstered by the use of very robust MOFs.14−17 Because of their permanent porosity and large pore sizes, MOFs are an ideal scaffold for site-isolating aluminum for MPV reductions. Any MOF to be considered for this reaction must be (1) stable toward alcoholysis and hydrolysis, (2) thermally stable, and (3) mechanically stable under the reaction conditions. For these reasons, the UiO-66 family (UiO-66, -67, -68, and -69),18 a series of well-studied zirconium frameworks with the formula Received: January 12, 2018

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DOI: 10.1021/acs.inorgchem.8b00119 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Zr6(OH)4O4L6 (where L is a linear dicarboxylate), as well as the MIL-53 series of MOFs, typically made with Al3+, Cr3+, or Fe3+, with the formula Al(OH)L were chosen. Both MOFs have been used for a number of applications and have been synthesized using different linkers through the use of isoreticular design to create MOFs with more desirable characteristics.12,19 This tailored approach is complemented by postsynthetic modification (PSM), which has been successfully used to introduce an otherwise unattainable functionality into MOF linkers20−22 as well as the secondary building units (SBUs) of the MOF.23 The UiO-66 family has Zr6 SBUs which have eight μ3-oxygens, including four μ3hydroxyl groups which can be postsynthetically modified.24−26 The μ3-OH sites facing into the tetrahedral pore of the MOF (Figure 2a,b) were targeted for postsynthetic modification.

Table 1. Catalyst Screening of MOFs for MPV Reduction of Cyclohexanone at 80 °Ca (Conversion in %)

MOF

reaction time (h)

no Al

Ala

UiO-66 UiO-66b MIL-53 UiO-67 DUT-5 Mg-MOF-74

12 96 12 1 1 168

59 0 0 9 3 0

99 86 0 66 94 0

a

0.4 mmol of metal in unactivated MOF in each case was treated with excess AlMe3 (1 mmol) after activation. bReaction performed at 21 °C.

4 h before centrifuging and decanting off the solvent. The MOFs were washed 3 times in total with 2 mL of hexanes. For each wash, the MOFs were allowed to stir for 4 h before centrifuging and decanting the solvent. After 3 washes with hexanes, the materials were washed 3 times with isopropyl alcohol using the same procedure in order to remove any soluble aluminum species. Preparation of Al@UiO-67 and Al@DUT-5. For all other catalytic experiments, 2 dram vials fitted with a Teflon-lined cap were charged with a stir bar and 0.4 mmol of the activated MOF (based on a molecular formula with one zirconium or aluminum atom). In a N2 atmosphere glovebox, to each vial 2 mL of hexanes was added, followed by addition of trimethylaluminum in hexanes, which depended on the experiment (Tables S1 and S2). After stirring for 4 h, the vials were centrifuged and the solution was decanted to remove any unreacted trimethylaluminum. After this, 2 mL of hexanes was added to each vial and the vials were left to stir again. This process was repeated twice, after which the materials were washed in the same manner with isopropyl alcohol three times. After decanting, the material was then subjected to the reaction conditions. General Catalytic Procedure. A typical catalytic reaction involved adding 20 mmol of isopropyl alcohol and 4 mmol of the substrate to the vial containing the prepared catalyst. The reaction mixture was stirred at the specified temperature. After centrifuging, a drop of the reaction mixture (approximately 0.05 mL) was sampled and analyzed by 1H NMR in CDCl3. For recycling experiments, the catalyst was soaked in dry isopropyl alcohol over 4 h before centrifuging the mixture and decanting the solvent prior to reuse.

Figure 2. Hydroxyl sites (hydroxyl hydrogens shown in blue) in the selected MOFs: (a) μ3-OH in UiO-67 (octahedral pore depicted), (b) μ3-OH in UiO-67 (tetrahedral pore depicted), (c) μ2-OH in DUT-5.

Indeed, the Lin group recently demonstrated that the hydroxyl groups in the SBUs of UiO-69 can be used to support a reactive magnesium site.25 A similar approach was used to introduce a number of transition metals to the same site.24,27−30 MOFs have also been used in ion capture, showing a versatile ability to postsynthetically incorporate metals.31 MIL-53 and DUT-5 exhibit a 2-dimensional structure with a 1D SBU consisting of aluminum octahedra bridged by μ2-hydroxyl groups. In MIL-53, the pores of the framework can open or close depending on the guest molecules or in response to stimuli.32,33 The larger DUT5 is isostructural but does not display the same structural flexibility.34 This paper reports the heterogenization of an established molecular aluminum catalyst by postsynthetically modifying the available bridging hydroxyl moieties present in the SBUs of known robust MOFs (Figure 2).

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EXPERIMENTAL SECTION

Caution! Trimethylaluminum is pyrophoric, should be used in a glovebox, and excess amounts should be properly quenched. Synthesis of MOFs. UiO-66,35 MIL-53,32 UiO-67,19 DUT-5,36 and Mg-MOF-7437,38 were all synthesized and activated as previously reported. MOFs were characterized by powder X-ray diffraction (PXRD) to confirm phase purity (S3.3), TGA to determine thermal stability (S3.4), BET to confirm porosity (S3.7), IR spectroscopy (S3.5), and Raman spectroscopy (S3.6). Initial Screening of Catalysts. For catalytic experiments described in Table 1, 2 dram vials fitted with a Teflon-lined cap were charged with a stir bar and 0.4 mmol of the unactivated MOF (molecular formula based on 1 metal per unit, see S1.4 in the Supporting Information). The MOFs were thermally activated at 200 °C overnight and slowly brought into a N2 atmosphere glovebox uncapped and covered with a watch glass to prevent loss of MOF. After bringing the MOFs into the glovebox, 2 mL of hexanes was added to each vial, followed by 1 mL of 1 M AlMe3 (this is in excess of the hydroxyl sites in the MOFs). The materials were allowed to stir for

RESULTS AND DISCUSSION The MOFs, UiO-66, MIL-53, UiO-67, and DUT-5, were synthesized according to previously reported literature procedures.19,36 Postsynthetic metalation of the MOFs was accomplished by treatment with trimethylaluminum (Figure 1). The hydroxyl sites inherent in the UiO-66/67 MOFs are quite acidic with pKa’s around 3.5.39 These, and the slightly less acidic hydroxyl sites in MIL-53 and DUT-5, can be readily metalated with AlMe3 with concomitant liberation of methane. Similar chemistry has been demonstrated by metalation of the bridging hydroxyl groups in the SBUs of NU-1000 with AlMe3 or a UiO69 analogue with Me2Mg.25,30 For an initial probe of reactivity, an excess of trimethylaluminum was added to each of the frameworks. After metalation, the newly introduced aluminum sites were converted to the desired alkoxide through an alcoholysis reaction with isopropyl alcohol. These solidB


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Inorganic Chemistry

with hexanes, the ratio decreased following washings with isopropyl alcohol (Table 2). This likely results from the

supported aluminum alkoxides were initially probed as catalysts and tested against nonmetalated MOFs in control reactions. The catalytic ability of the metalated MOFs was evaluated using the reduction of cyclohexanone in excess isopropyl alcohol to give cyclohexanol and acetone (Scheme 1). Initial

Table 2. Ratio of Aluminum to Initial Hydroxyl Sites (Al:OH) in Metalated UiO-67 As Determined by SEM-EDX

Scheme 1. Model MPV Reduction, with the Accepted Homogeneous Transition State Shown40

Al:OH ratio added

EDX ratio post hexanes

EDX ratio post iPrOH

2.3 1.1 0.8

2.3 1.1 0.8

1.5 0.9 0.8

alcoholysis of weakly bound, or encapsulated Al species. Interestingly, no change in Al incorporation was observed when 0.8 equiv of aluminum was added per hydroxyl, suggesting that all the Al is covalently bound. EDX cannot distinguish between the postsynthetic aluminum and the inherent aluminum in DUT-5, so for the purpose of comparing all the catalysts, the amount of AlMe3 initially added is used hereafter to represent the catalyst loading as this represents the maximum possible catalyst loading. Subjecting these systems to the catalytic conditions revealed no significant enhancement to the conversion beyond a ratio of 0.75 Al:OH for UiO-67 and 0.5 Al:OH for DUT-5 (Figure 3). This lack of enhancement

studies of metalated UiO-66 showed good conversions over 96 h at room temperature, with no reactivity from the nonmetalated control (Table 1). Trials at 80 °C showed better conversions over 12 h (Table 1), but the native MOF also showed reactivity as has been previously reported for Zr MOFs.41−43 The activity of the nonmetalated UiO-66 is attributed to the presence of Lewis acidic sites that result from defects within the framework.44 Subsequent recycles of the metalated UiO-66 species showed good retention of reactivity at 80 °C (Table S1), but the activity of the native MOF cannot be readily decoupled from the introduced aluminum. In contrast, treatment of MIL-53 with AlMe3 did not afford a material that was catalytically active for MPV hydrogen transfer under these conditions. The possibility of a physically encapsulated catalyst was also tested for by using Mg-MOF74 as a control.37,38,45 As is the case with both UiO-67 and DUT-5, MOF-74 represents a chemically and thermally robust MOF with high surface area. However, it lacks the hydroxides which are found in UiO-66 and MIL-53 families that serve as sites for the covalent tethering of the molecular aluminum catalytic sites.37 After treating Mg-MOF-74 with AlMe3, no reactivity was observed, even after 1 week at 80 °C, suggesting that physical encapsulation is not the source of the observed reactivity (Table 1). To avoid the inherent reactivity toward MPV transfer hydrogenation in UiO-66 and increase the accessible space for a larger variety of substrates, isoreticular analogues of these MOFs were utilized. On the basis of observations with other catalytic systems,46,47 it was expected that a larger MOF with proportionally larger pores would allow for faster conversions and might allow for a more thorough study of the reactivity of the molecular catalyst inside the robust framework. UiO-67 was chosen as a larger analogue of UiO-66, and DUT-5 as a larger analogue of MIL-53. Subjecting the two larger MOFs to identical catalytic conditions (Table 1) revealed that these frameworks allow for superior catalytic activity (shorter times, with high conversions) when treated with AlMe3. Little background reactivity was observed from the native MOFs. Due to the enhanced reactivity, subsequent discussions will mainly focus on UiO-67 and DUT-5. Given the superior performance of UiO-67 and DUT-5, the effect of catalyst loading within the MOF was evaluated with the aim of determining an amount that balances catalyst site density with maximizing internal space to allow for efficient mass transport. The initial Al:OH ratio (based on the initial number of OH sites in the idealized formula) was varied from 0.8 to 2.3 for UiO-67. EDX analysis revealed that, although the initial Al:OH ratio was retained after rinsing the framework

Figure 3. Catalytic results of a variety of aluminum loadings in MOFs using 10 mol % unactivated MOF.

beyond these levels can be attributed to removal of loosely or unbound aluminum upon washing with isopropyl alcohol, leading to a leveling of catalyst loading. Subsequent reactions are carried out under initial catalyst loadings of 1 Al:OH for UiO-67 and 0.5 Al:OH for DUT-5. The PXRD patterns of the optimally metalated materials after hexanes washes shows a small decrease in crystallinity in the larger UiO-67 (Figure 4a) and DUT-5 (Figure 4b), and almost no decrease in crystallinity in the case of UiO-66 (see Figure S16). No further decrease in crystallinity was observed following subsequent manipulations. The Di-ATR FTIR spectrum of UiO-67 after activation contains a peak at 3674 cm−1 attributed to the μ3-OH (Figure 5).48 This peak disappears following treatment of the activated framework with AlMe3 (in an Al:OH ratio of 1:1), clearly indicating that this is the site of metalation, and supporting the formation of the new Al−O bond. Subsequent treatment with isopropyl alcohol, which could lead to alcoholysis, does not result in the reappearance of the OH stretch at 3674 cm−1. This is in line with the acidity of the μ3-OH sites.39 The FTIR spectrum of metalated DUT-5 shows little noticeable change upon metalation when monitoring the OH stretch at 3696 cm−1 (Figure S28).32 The lack of change in each spectrum with regards to new Al−O stretches can be attributed to the overlap C


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Inorganic Chemistry

Figure 4. PXRD patterns of the activated MOFs, MOFs after treatment with AlMe3 and washing with hexanes, after subsequent washing with isopropyl alcohol, and after going through reaction cycles, with (a) UiO-67 (1 equiv of AlMe3 used) and (b) DUT-5 (0.5 equiv of AlMe3 used).

Figure 6. SS MAS 27Al NMR of (a) activated DUT-5, (b) DUT-5 after addition of 0.5 equiv of AlMe3 per OH and washing with isopropyl alcohol, (c) UiO-67 after addition of 1 equiv of AlMe3 per OH after washing with isopropyl alcohol. Figure 5. Di-ATR FTIR spectra in OH stretching region. OH stretch of UiO-67 after activation, after treatment with 1 equiv of AlMe3 and washing with hexanes, and after subsequent washing with isopropyl alcohol.

DUT-5. Here, the primary coordination environment is octahedral (−50 to 20 ppm).41 Small populations of 5- and 4-coordinate aluminum can also be observed at 33 and 66 ppm, respectively.41 Figure 6b depicts the Al resonances in DUT-5 after metalation. A significant increase in octahedral Al can be observed at ∼6 ppm as well as a modest increase in 5- and 4coordinate aluminum. Fitting of the spectra reveals that the total increase is 50%. This equates to an Al:OH ratio of 0.5, consistent with the initial loading of AlMe3 during the postsynthetic modification. Figure 6c depicts the 27Al NMR for UiO-67 following treatment with AlMe3 and isopropyl alcohol. Here it can be seen that the postsynthetically introduced aluminum occupies very similar chemical environments to the postsynthetically introduced aluminum in DUT-5. In all cases, the lack of narrow resonances associated with encapsulated, and therefore freely

of the vibrational modes of the newly formed Al−O bonds with M−O bonds native to the MOF. There are notable shifts to lower energy for the peaks at ∼1480 observed in the Raman spectra of UiO-67 and DUT-5. These peaks correspond to a combination of C−O and C−C stretches that can be affected by changes at the cluster, consistent with metalation at the bridging hydroxide. These peaks have been reported to shift in response to dehydroxylation (calculated A1 mode for UiO-67 shifts from 1473 to 1485 cm−1)49 (S3.7). Incorporation of aluminum through postsynthetic modification was probed via solid state magic angle spinning (SS MAS) 27 Al NMR. Figure 6a shows the initial Al environments in D


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Inorganic Chemistry rotating, Al species are absent, again indicating that the Al is covalently bound to the framework. To verify that Al was not leaching and acting as a homogeneous catalysts,50 a split test was performed on the metalated MOFs. In all cases, no further reactivity was observed after separating the MOF from the reaction mixture. A side-byside comparison of the nonseparated mixture shows continued reactivity which is consistent with the catalyst being heterogeneous (Figure 7).

Figure 8. Reaction recycles of metalated MOFs in dry and wet isopropyl alcohol with 2.3 mol % catalyst loading (based on Al added), and 1 h reaction time. For the last cycle, the reactions were left for 20 h before sampling again.

alcohol is typically one of the requirements for homogeneous MPV catalysis, as even small amounts of water lead to the hydrolysis and deactivation of the Lewis acid catalysts.9 Wet isopropyl alcohol was used in order to evaluate the impact of increased amounts of water on the catalytic reduction of cyclohexanone at 80 °C. With Al@UiO-67, a lower yield as compared to the first cycle with dry isopropyl alcohol was observed. Notably, it was on par with the yield of the second cycle with dry isopropyl alcohol, indicating that water might indeed be responsible for the initial decrease in yield from cycle 1 to 2 with dry isopropyl alcohol. Further recycles with wet isopropyl alcohol revealed small decreases in conversion similar to that observed with dry isopropyl alcohol. These small subsequent decreases in reactivity can be attributed to attrition of the catalyst through manipulation. The last reaction cycle of Al@UiO-67 with wet isopropyl alcohol was continued for a total of 20 h, and quantitative conversion was observed. A more drastic decrease in reactivity was observed upon recycling with wet isopropyl alcohol using DUT-5. The initial cycle only yielded 81% of the product after an hour, 12% less than when the reaction was performed with dry isopropyl alcohol. Subsequent recycles demonstrated a rapid loss in activity, indicating that the catalytic sites in Al@DUT-5 are sensitive to water. A split test was performed to determine if catalyst leaching was occurring in the presence of water. Again, no additional conversion was observed after hot filtration. If the water is leading to leaching of the aluminum, then it is in an inactive form. It is more likely that the water is leading to hydrolysis at the more basic alkoxide rather than hydrolysis at the less basic bridging oxido. Five additional substrates were evaluated with UiO-67 and DUT-5 catalysts in order to determine what, if any, restrictions the catalysts have. Table 3 depicts these and compares the results with the catalytic activity of in situ prepared Al(OiPr)3, a more active form of the catalyst then isolated Al(OiPr)3.8 Valeraldehyde and hexanal showed conversion as expected, whereas dodecanal was not reduced under the same conditions. This contrasts with the results of Al(OiPr)3 which catalyzed the reduction of all three. This suggests that, even though conversion should have been expected, the larger substrate, dodecanal, was sterically prevented from interacting with the catalyst (Table 3). One of two cases are anticipated: (1) the reagent is too large to enter the framework or (2) the reagent is too bulky to enter the coordination sphere of the catalyst. In addition, benzyl ketones were not as easily reduced in the frameworks, suggesting the added bulk impacts the kinetics.

Figure 7. Split test for the reduction cyclohexanone in both wet and dry isopropyl alcohol using (a) Al@UiO-67, (b) Al@DUT-5. Hollow circles represent results from sample that has been separated from the MOF after 10 min.

One of the major advantages of a heterogeneous catalyst can be the ease of recovery and reuse. To test this, the reaction time was maintained at 1 h to allow differences to be observed. While the larger MOFs demonstrate high initial reactivity, UiO67 shows a 22% decrease in conversion after 5 reaction cycles. Continuation of the reaction for a total of 20 h on the fifth cycle revealed quantitative conversion. DUT-5, on the other hand, shows an 83% loss in conversion (Figure 8) after 4 reaction cycles. Furthermore, continuation of the reaction for 20 h on the fourth cycle only resulted in a yield of 76%. None of the MOFs showed any significant loss of crystallinity after 4 reaction cycles when compared to the postsynthetically modified samples (Figure 4). This suggests that the loss of reactivity in the case of DUT-5 is not due to framework decomposition or pore collapse. One possible reason for the decline in reactivity is the presence of adventitious water in the reaction mixture and washes between recycles. Dry isopropyl E


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Inorganic Chemistry

approach to anchoring homogeneous catalysts to hydroxyl sites within metal−organic frameworks. The stability of the catalyst might be improved by defining multidentate sites within the framework for supporting the catalyst. In addition, different OH sites in MOFs may offer similar platforms for this approach to be used.

Table 3. Percent Conversion (and TON) for MPV Reduction of Select Substrates at 80 °C over 1 h

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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.inorgchem.8b00119. Experimental details, as well as PXRD, EDAX, IR, TGA, surface area analysis, and NMR data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 806-834-1832. ORCID

Patrick J. Larson: 0000-0001-5152-040X Anthony F. Cozzolino: 0000-0002-1100-0829 Author Contributions

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

We are grateful for financial support from the Robert A. Welch Foundation (D-1838), Texas Tech University, and from the National Science Foundation (NMR instrument grant CHE1048553).

a

% conversion is reported because of the formation of unwanted side products. b24 h reaction time.

Notes

The authors declare no competing financial interest.

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This can result from either slower diffusion or congestion at the catalytically active site. Lastly, in the cases of the smaller aldehydes in both metalated and nonmetalated UiO-67, a side reaction was observed, leading to a mixture of aldol condensation products, which was not observed in the case of metalated DUT-5. It should be noted, as well, that, if the reaction mixture was left at elevated temperatures for 24 h instead, the metalated UiO-67 gave only the expected alcohol, with no byproducts, whereas the nonmetalated case gave a mixture of products, with little preference for the MPV product. This suggests that the Zr SBU of the MOF may interact differently from the incorporated Al depending on the substrate.

ACKNOWLEDGMENTS We gratefully acknowledge Clemens Krempner for advice, the Hope-Weeks group for TGA assistance, the Fatib-Khatib group for BET assistance, Bo Zhao for SEM-EDAX analysis, and Kendall Larson for graphic design assistance.

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REFERENCES

(1) de Graauw, C. F.; Peters, J. A.; van Bekkum, H.; Huskens, J. Meerwein-Ponndorf-Verley Reductions and Oppenauer Oxidations: An Integrated Approach. Synthesis 1994, 1994 (10), 1007−1017. (2) Shiner, V. J.; Whittaker, D. The Mechanism of the MeerweinPonndorf-Verley Reaction. J. Am. Chem. Soc. 1963, 85 (15), 2337− 2338. (3) Meerwein, H.; Schmidt, R. Ein Neues Verfahren Zur Reduktion von Aldehyden Und Ketonen. Liebigs Ann. Chem. 1925, 444, 221−238. (4) Verley, A. Sur l’echange de Groupements Fonctionnels Entré Deux Molecules. Passage de La Fonction Alcool à la Fonction Aldehyde et Inversement. Bull. Soc. Chim. Fr. 1925, 37, 537−542. (5) Dani, P.; Karlen, T.; Gossage, R. A.; Gladiali, S.; van Koten, G. Hydrogen-Transfer Catalysis with Pincer-Aryl Ruthenium(II) Complexes. Angew. Chem., Int. Ed. 2000, 39 (4), 743−745. (6) Corma, A.; Domine, M. E.; Nemeth, L.; Valencia, S. Al-Free SnBeta Zeolite as a Catalyst for the Selective Reduction of Carbonyl Compounds (Meerwein−Ponndorf−Verley Reaction). J. Am. Chem. Soc. 2002, 124 (13), 3194−3195. (7) Battilocchio, C.; Hawkins, J. M.; Ley, S. V. A Mild and Efficient Flow Procedure for the Transfer Hydrogenation of Ketones and Aldehydes Using Hydrous Zirconia. Org. Lett. 2013, 15 (9), 2278− 2281.

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CONCLUSIONS Molecular aluminum catalysts have been covalently tethered inside chemically and thermally stable MOFs for MPV reductions of ketones and aldehydes. Mapping a homogeneous catalyst into a MOF served as a strategy to prevent the aggregation and deactivation of the catalyst that is observed in the homogeneous system. At elevated temperatures, UiO-66 itself and the introduced Al work in concert to achieve excellent conversions, while only the postsynthetically introduced Al is responsible for catalysis at lower temperatures. In both UiO-67 and DUT-5, however, the introduced Al alone is responsible for reactivity under the reaction conditions. Al@UiO-67 showed continued activity upon recycling and was tolerant to wet isopropyl alcohol, which contrasted with DUT-5, which quickly became less active after recycling. This study outlines a general F


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Inorganic Chemistry

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