Name: development of heterogeneous enantioselective catalysts using chiral metal-organic frameworks (mofs)
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This work was supported by a National Research Foundation of Korea (NRF) Basic Science Research Program NRF-2019R1A2C4070584 and the Science Research Center NRF-2016R1A5A1009405 funded by the Korea government (MSIP). S. Kim was supported by NRF Global Ph.D. Fellowship (NRF-2018H1A2A1062013).

1. Preparation of (S)-KUMOF-1 crystals in three sizes
NOTE: Each step follows the experimental section and supplementary information of previous reports2,24,27. Three different sizes of (S)-KUMOF-1 were prepared: large (S)-KUMOF-1-(L), medium (S)-KUMOF-1-(M), and small (S)-KUMOF-1-(S) with particle sizes &#62;100 &#956;m, &#62;20 &#956;m, and &#60;1 &#956;m, respectively. When out of the solvent, (S)-KUMOF-1 dismantles. Therefore, the crystals should always be kept wet while in use.
<ol>
	<li>Synthesis of small size (S)-KUMOF-1-(S)
	<ol>
		<li>In a 10 mL cell, dissolve Cu(NO3)2 &#8729; 3H2O (0.2 mg, 0.0008 mmol) and (S)-2,2&#39;-dihydroxy-6,6&#39;-dimethyl-[1,1&#39;-biphenyl]-4,4&#39;-dicarboxylic acid2 (0.24 mg, 0.0008 mmol) in 4 mL of DEF/MeOH (DEF = N,N-diethylformamide, 1/1, v/v). 
		NOTE: It is best to use newly prepared DEF and MeOH (methanol). (S) in (S)-KUMOF-1 means that the stereochemical configuration of the ligand used in KUMOF synthesis is S.</li>
		<li>Cap the reaction cell with a PTFE (polytetrafluoroethylene) cap and place it into a microwave reactor (65 &#176;C, 100 psi, 50 W, 20 min). 
		NOTE: To obtain the required number of crystals, repeat the above steps (1.1.1. and 1.1.2.) several times.</li>
		<li>Whisk gently with a small spatula to float the obtained blue cubic crystals (45% yield).</li>
		<li>Pour the floating crystals on filter paper, and wash 3x with 3 mL of hot DEF.</li>
		<li>Exchange the solvent 3x with 3 mL of anhydrous dichloromethane (DCM) for storage. 
		NOTE: Every step requiring DCM in the protocol is DCM distilled over CaH2.</li>
	</ol>
	</li>
	<li>Synthesis of medium size (S)-KUMOF-1-(M)
	<ol>
		<li>Dissolve Cu(NO3)2 &#8729; 3H2O (7.2 mg, 0.030 mmol) in 1.5 mL of MeOH and (S)-2,2&#39;-dihydroxy-6,6&#39;-dimethyl-[1,1&#39;-biphenyl]-4,4&#39;-dicarboxylic acid (9 mg, 0.030 mmol) in 1.5 mL of DEF. 
		NOTE: The compounds and solvents mentioned are for one vial set. Scaling up is needed to obtain the required number of MOFs for catalytic use. Multiply the scales in this step and make stock solutions for each compound. Then subdivide the stock solutions into each vial.</li>
		<li>Combine the two solutions in a 4 mL vial.</li>
		<li>Cover the 4 mL vial with PTFE tape and punch the cover with a needle to make a hole.</li>
		<li>Put this small vial into a 20 mL vial and add 1.0 mL of N,N-dimethylaniline into the space between the small and large vials.</li>
		<li>Cap the large vial tightly and place in an oven at 65 &#176;C for 1 day.</li>
		<li>Whisk gently with a small spatula to float the obtained blue cubic crystals.</li>
		<li>Pour the floating crystals on a filter paper and wash 3x with DEF/MeOH (3 mL/3 mL). 
		NOTE: After pouring the floating crystals, tilt the vial above the filter paper. Then eject the solvent with a syringe to wash down every crystal remaining in the vial.</li>
		<li>Exchange the solvent 3x with 3 mL of anhydrous DCM for storage.</li>
	</ol>
	</li>
	<li>Synthesis of large size (S)-KUMOF-1-(L)
	<ol>
		<li>Use the same procedure as in section 1.2, except at step 1.2.3, leave the 4 mL vial open. 
		NOTE: The yield of the obtained crystal is based on the ligand used. The yield for the medium and large size (S)-KUMOF-1 were almost the same (35% yield) after final washing.</li>
	</ol>
	</li>
</ol>
2. Preparation of Zn/(S)-KUMOF-1 in three sizes
NOTE: Each step follows the experimental section and supplementary information of previous reports2,24,27.
<ol>
	<li>Add dimethylzinc (0.68 mL, 1.2 M in toluene, 0.81 mmol) to a suspension of (S)-KUMOF-1 (102 mg, 0.27 mmol) in 2 mL of DCM at -78 &#176;C and shake 3 h at this temperature. 
	CAUTION: All steps at -78 &#176;C are done with a cryogenic cooling bath (dry ice with acetone). Always be careful when handling this equipment. 
	NOTE: All shaking procedures are done using a plate shaker (180 rpm).</li>
	<li>Decant the supernatant and wash with 3 mL cold DCM several times for complete removal of unreacted dimethylzinc. 
	NOTE: Three sizes of Zn/KUMOF-1 are required for the carbonyl-ene reaction. Follow the same steps as described for the three sizes of KUMOF-1. The number of catalytic sites is calculated assuming that one catalytic site is present in a Cu and a ligand pair. For this reason, the Zn/Cu and Ti/Cu ratios of the prepared crystals were determined as in the previous report using inductive coupled plasma atomic emission spectroscopy (ICP-AES)27. The amounts of Zn and Ti reagents used in this protocol were the same as those used in our previous study27.</li>
</ol>
3. Preparation of Ti/(S)-KUMOF-1 in three sizes
NOTE: Each step follows the experimental section and supplementary information of previous reports2,24,27.
<ol>
	<li>Add Ti(O-iPr)4 (59 &#956;L, 0.20 mmol) to a suspension of (S)-KUMOF-1 (24 mg, 0.063 mmol) in 2 mL of DCM and shake for 5 h at room temperature.</li>
	<li>Decant the supernatant and wash with 3 mL of cold DCM several times for the complete removal of residual Ti(O-iPr)4.</li>
</ol>
4. Carbonyl-ene reaction using the prepared MOFs
NOTE: Prepare a series of substrates according to the method described in our previous report27. All three substrates are used individually in each carbonyl-ene reaction except for the particle size effect determination, in which only the smallest substrate (1a) is used27. Each step follows the experimental section and supplementary information of previous reports2,24,27.
<ol>
	<li>Heterogeneous stoichiometric carbonyl-ene reaction by Zn/(S)-KUMOF-1.
	<ol>
		<li>Add the substrate solution (0.089 mmol) in 0.1 mL of DCM to a suspension of Zn/(S)-KUMOF-1 (102 mg, 0.27 mmol) in 2 mL of DCM at -78 &#176;C.</li>
		<li>Warm the reaction mixture slowly to 0 &#176;C and shake for 3.5 h at this temperature.</li>
		<li>Quench the reaction mixture with 3 mL of an aqueous solution of 6 N HCl.</li>
		<li>Filter the resultant mixture through a diatomaceous silica pad.</li>
		<li>Concentrate the filtrate in vacuo and purify the residue by flash chromatography (n-hexane/ethyl acetate 10:1). 
		NOTE: Silica gel 60 (230&#8211;400 mesh) and an appropriate n-hexane/ethyl acetate mixture as the eluent are used for flash chromatography. The product is a pale yellow oil. Optical purity of all products in this protocol were determined as described previously27. The same procedure should be followed for the three sizes of Zn/(S)-KUMOF-1.</li>
	</ol>
	</li>
	<li>Heterogeneous catalytic carbonyl-ene reaction by Ti/(S)-KUMOF-1.
	<ol>
		<li>Add the substrate solution (0.29 mmol) in 0.1 mL of DCM to a suspension of Ti/(S)-KUMOF-1 (12 mg, 0.029 mmol) in DCM (2 mL) at 0 &#176;C, and shake for 36 h at this temperature.</li>
		<li>Collect the supernatant and wash the resultant crystals 3x with 3 mL of DCM.</li>
		<li>Concentrate the collected supernatant in vacuo and purify the residue by flash chromatography (n-hexane/ethyl acetate 10:1).</li>
	</ol>
	</li>
</ol>

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The authors have nothing to disclose.

After the synthesis of (S)-KUMOF-1, crystals in some vials seem to be powdery and are not appropriate for use in catalysis. Therefore, proper crystals of (S)-KUMOF-1 need to be selected. The yield of (S)-KUMOF-1 is calculated using only those vials in which it was successfully synthesized. When withdrawn from the solvent, (S)-KUMOF-1 dismantles. Therefore, the crystals should always be kept wet. For this reason, weighing of intact (S)-KUMOF-1 crystals dipped into the solvent is difficult. The amount of (S)-KUMOF-1 needs to be determined before its use in catalysis. By synthesizing (S)-KUMOF-1 on a massive scale and abandoning some crystal samples in the vial, a statistical calculation of the yield was possible. The yield was calculated by weighing perfectly dried samples per vial. Samples were selected randomly from the population of synthesized (S)-KUMOF-1 averaged by the number of vials. This method gave a statistically defined amount of (S)-KUMOF-1 in one vial. The amount of (S)-KUMOF-1 required for catalysis was prepared by collecting the crystals in vials (e.g., the required amount of (S)-KUMOF-1 = amount of (S)-KUMOF-1 per vial x number of vials). Subdivision of (S)-KUMOF-1 floating in the solvent is incorrect for matching the calculated equivalents of substrates; the amount of substrate used is calculated from the calculated amount of (S)-KUMOF-1. The crystal structure and characteristics have been reported previously2,27.
Homogeneous carbonyl-ene reactions with Zn and Ti catalysts have been performed previously to prove that there is no discrimination by substrate size in the homogeneous reaction. At this point, the influence of the substrate size on the reaction efficiency can be neglected as the same as in the heterogenous reaction. The temperature required for the carbonyl-ene reaction using Ti/(S)-KUMOF-1 is 0 &#176;C. Owing to the smashing problems of crystals, all reactions should be performed by shaking and not stirring. However, a low temperature shaking incubator chamber was not available. Instead, a polystyrene foam icebox was used. A stainless steel wire test tube rack was installed in the icebox and tightly sealed reaction vials were fixed into the rack. Water was poured to ~1 cm height into the icebox, and ice cubes were added. The lid-covered icebox was put on the shaker and fixed with adhesive tape. New ice cubes were added to replace the melting ice. For the carbonyl-ene reaction using Zn/(S)-KUMOF-1, the reaction vial was kept in a cryogenic cooling bath (dry ice with acetone) before adding the substrate to the solution. After adding the substrate, the reaction vial was moved to the icebox described above.
Additional well-marked data for the reaction site validation of Zn/(S)-KUMOF-1 and Ti/(S)-KUMOF-1 used in the carbonyl-ene reaction can be visualized by Two-Photon Microscopy (TPM) measurements27. Characterization of (S)-KUMOF-1 crystals by TPM has been previously reported. To gauge the cavity size of newly synthesized MOFs, TPM measurements with various size of dyes are available30.

MOFs are considered a useful heterogeneous catalyst for chemical reactions. There are many different reported uses of MOFs for enantioselective catalysis1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19. Still, it has yet to be determined whether the reactions take place on the inner or outer surface of the MOFs. Recent studies have raised questions concerning the utilization of the available surface and reduced diffusion20,21,22,23. A more striking issue is that the chiral environment varies with the location of each cavity in the MOF crystal. This heterogeneity of the chiral environment implies that the stereoselectivity of the reaction product depends on the reaction site24. Thus, designing an efficient enantioselective catalyst requires identification of the location where the reaction would take place. To do so, it is necessary to ensure that the reaction occurs either only on the inner surface or only on the outer surface of the MOF while leaving the interior intact. The porous structure of MOFs and their large surface area containing chiral environment active sites can be exploited for enantioselective catalysis. For this reason, MOFs are excellent replacements of solid-supported heterogeneous catalysts25. The use of MOFs as heterogeneous catalysts needs to be reconsidered if the reaction does not occur inside them. The location of the reaction site is important, as well as the size of the cavity. In porous materials, the size of the cavity determines the substrate based on its size. There are some reports of MOF-based catalysts that overlook the cavity size issue25. Many MOF-based catalysts introduce bulky catalytic species (e.g., Ti(O-iPr)4) to the original framework structure3,8,13. There is a change in the cavity size when bulky catalytic species are adopted in the original framework structure. The reduced cavity size caused by the bulky catalytic species makes it impossible for the substrate to fully diffuse into the MOFs. Thus, discrimination of substrate size by the cavity size of the MOFs needs to be considered for these cases. The catalytic reactions by MOFs often make it difficult to support evidence of reactions taking place inside the MOF cavity. Some studies have shown that substrates larger than the MOF cavities are converted to the expected products with ease, which seems contradictory8,13. These results can be interpreted as a contact between the functional group of the substrate and catalytic site initiating the catalytic reaction. In this case, there is no need for the substrate to diffuse into the MOFs; the reaction occurs on the surface of the MOF crystals26 and the cavity size is not directly involved in the discrimination of the substrate based on its size.
To identify the reaction sites of MOFs, a known Lewis-acid promoted carbonyl-ene reaction was selected2. Using 3-methylgeranial and its congeners as substrates, four types of enantioselective carbonyl-ene reactions (Figure 1) were studied27. The reactions, which have been previously reported, were classified into two classes: a stoichiometric reaction using a Zn reagent and catalytic reactions using a Ti reagent27. The reaction of the smallest substrate requires a stoichiometric amount of Zn/KUMOF-1 (KUMOF = Korea University Metal-Organic Framework); it has been reported that this reaction takes place inside of the crystal27. Two kinds of MOFs were used in this method, Zn/KUMOF-1 for the stoichiometric reaction and Ti/KUMOF-1 for the catalytic reaction. Owing to the distinct reaction mechanisms of these two kinds of MOFs, a comparison between the reaction rate versus substrate size is possible2,28,29. The effect of particle size on the carbonyl-ene reaction with Zn/KUMOF-127 demonstrated that, as seen in the previous report, the chiral environment of the outer surface was different from the inner side of the MOF crystal24. This article demonstrates a method that determines the reaction sites by comparing the reactions of three kinds of substrates with two classes of catalysts and the effect of particle size as reported in the previous paper27.

<table><tbody><tr><td>Acetone</td><td>Daejung</td><td>1009-4110</td><td></td></tr><tr><td>Analytical Balance</td><td>Sartorius</td><td>CP224S</td><td></td></tr><tr><td>Copper(II) nitrate trihydrate</td><td>Sigma Aldrich</td><td>61194</td><td></td></tr><tr><td>Dichloromethane</td><td>Daejung</td><td>3030-4465</td><td></td></tr><tr><td>Dimethyl zinc</td><td>Acros</td><td>377241000</td><td></td></tr><tr><td>Ethyl acetate</td><td>Daejung</td><td>4016-4410</td><td></td></tr><tr><td>Filter paper</td><td>Whatman</td><td>WF1-0900</td><td></td></tr><tr><td>Methanol</td><td>Daejung</td><td>5558-4410</td><td></td></tr><tr><td>Microwave synthesizer</td><td>CEM</td><td></td><td>Discover SP</td></tr><tr><td>Microwave synthesizer 10 mL Vessel Accessory Kit</td><td>CEM</td><td>909050</td><td></td></tr><tr><td>N,N-Diethylformamide</td><td>TCI</td><td>D0506</td><td></td></tr><tr><td>N,N-Dimethylaniline</td><td>TCI</td><td>D0665</td><td></td></tr><tr><td>n-Hexane</td><td>Daejung</td><td>4081-4410</td><td></td></tr><tr><td>Normject All plastic syringe 5 mL luer tip 100/pk</td><td>Normject</td><td>A5</td><td></td></tr><tr><td>Pasteur Pipette 150 mm</td><td>Hilgenberg</td><td>HG.3150101</td><td></td></tr><tr><td>PTFE tape</td><td>KDY</td><td>TP-75</td><td></td></tr><tr><td>Rotary Evaporator</td><td>Eyela</td><td>243239</td><td></td></tr><tr><td>Shaker</td><td>DAIHAN Scientific</td><td>DH.WSO04010</td><td></td></tr><tr><td>Silica gel 60 (230-400 mesh)</td><td>Merck</td><td>109385</td><td></td></tr><tr><td>Synthetic Oven</td><td>Eyela</td><td>NDO-600ND</td><td></td></tr><tr><td>Titanium isopropoxide</td><td>Sigma Aldrich</td><td>87560</td><td></td></tr><tr><td>Vial (20 mL)</td><td>SamooKurex</td><td>SCV2660</td><td></td></tr><tr><td>Vial (5 mL)</td><td>SamooKurex</td><td>SCV1545</td><td></td></tr></tbody></table>

development of heterogeneous enantioselective catalysts using chiral metal-organic frameworks (mofs)

Substrate size discrimination by the pore size and homogeneity of the chiral environment at the reaction sites are important issues in the validation of the reaction site in metal-organic framework (MOF)&#8211;based catalysts in an enantioselective catalytic reaction system. Therefore, a method of validating the reaction site of MOF-based catalysts is necessary to investigate this issue. Substrate size discrimination by pore size was accomplished by comparing the substrate size versus the reaction rate in two different types of carbonyl-ene reactions with two kinds of MOFs. The MOF catalysts were used to compare the performance of the two reaction types (Zn-mediated stoichiometric and Ti-catalyzed carbonyl-ene reactions) in two different media. Using the proposed method, it was observed that the entire MOF crystal participated in the reaction, and the interior of the crystal pore played an important role in exerting chiral control when the reaction was stoichiometric. Homogeneity of the chiral environment of MOF catalysts was established by the size control method for a particle used in the Zn-mediated stoichiometric reaction system. The protocol proposed for the catalytic reaction revealed that the reaction mainly occurred on the catalyst surface regardless of the substrate size, which reveals the actual reaction sites in MOF-based heterogeneous catalysts. This method for reaction site validation of MOF catalysts suggests various considerations for developing heterogeneous enantioselective MOF catalysts.

The enantioselective carbonyl-ene reaction using the Zn reagent is stoichiometric because of the difference in the binding affinities of the alkoxy and carbonyl groups to the metal (Figure 2). For this reason, the substrates were converted into the products at the reaction site and remained there. The desired products were obtained by dismantling the crystals, as detailed in section 4 of the protocol. The results of the heterogeneous enantioselective carbonyl-ene reaction of substrates by Zn/(S)-KUMOF-1 (Table 1) showed that the smallest substrate (1a) could diffuse inside the crystal and convert to the product in a high yield (92%), proving that all reaction sites of the MOF were available. The yield and enantiomeric excess (ee) decreased as the substrate size increased, which suggests that the larger substrates could not access the reaction sites inside the MOF crystal. The largest substrate (1c) did not undergo the reaction in this system. It is plausible that the reaction channel was blocked by the corresponding reaction products in this case (Figure 3). When the size of the substrate is sufficiently small in comparison to the size of the void, additional substrates can penetrate the crystal. If the size of substrate is too large, the surface reaction site makes the first contact and directly blocks the entrance of the channel, which makes it impossible for other substrates to penetrate (Animation 1). As the reaction takes place near the surface, the ee is lower24 and the blockage of the reaction site decreases the reaction yield.
Particle size effect results (Table 2) showed that the larger crystals were better than the small crystals in utilizing the reaction sites inside the crystal, clearly demonstrating the identification of the reaction site in this system. The yields in the carbonyl-ene reaction of 1a using the three sizes of Zn/(S)-KUMOF-1 were similar, which indicates that the efficacies of the three MOFs are identical. The optical purity dramatically decreased with the decreasing size of the crystals because their surface area increased. In contrast, a larger sized crystal had much lower surface area, which allowed 1a to penetrate deeply and have better access to the inner reaction sites.
Unlike the Zn-mediated system, the Ti-catalyzed system provided more information about the events occurring at the catalytic reaction sites. The results of the heterogeneous catalytic carbonyl-ene reaction by Ti/(S)-KUMOF-1 (Table 3) revealed no discrimination by the substrate size; indeed, the effect of the substrate size on the yield was marginal. The optical purity of 2a was much lower compared to the product obtained via the Zn-mediated reaction. Most of the product was found in the reaction solution, and the amount inside the crystal was negligible. These results indicate that most reactions occurred on or beneath the surface and the products were immediately removed to the solution (Figure 4) (Animation 2). The substrate that is larger than the cavity size undergoes the reaction upon contact with the reaction site on the surface. The product dissociates quickly from the catalytic site without penetrating the crystal.
Based on these results, the reaction sites of MOFs can either be on the outer surface or the inner side of MOFs. However, as previously reported, the chiral environment of the reaction site varies by its location. A reaction that is catalytic with MOFs should follow the method proposed in this article to determine the location of the reaction site. Therefore, if the reaction is catalytic, claims of the reaction occurring inside the channel should be reconsidered.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/60624/60624fig1a.jpg" /> 
Figure 1: Two classes of enantioselective carbonyl-ene reactions. Lewis acid Cat I and II were used for a homogeneous model reaction in a previous report27. This figure has been reprinted with permission from Han et al.27 <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/60624/60624fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/60624/60624fig2a.jpg" /> 
Figure 2: Possible mechanism of the homogeneous stoichiometric carbonyl-ene reaction. Difference of binding affinity between the alkoxy and carbonyl group to metal makes the Zn-mediated carbonyl-ene reaction stoichiometric. This figure has been reprinted with permission from Han et al.27 <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/60624/60624fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/60624/60624fig3a.jpg" /> 
Figure 3: Schematic presentation of the heterogeneous stoichiometric carbonyl-ene reaction. Pink colored molecules represent the substrates while greens indicate the products attached to the reaction sites. (A) 1a is small enough to penetrate the crystal regardless of blockage by 2a. (B) 1b suffers from blockage of 2b but still diffuses into the channel. (C) 1c makes first contact with the reaction site at the surface and directly blocks the entrance of the channel by 2c, which makes it impossible for another substrate to penetrate. This figure has been reprinted with permission from Han et al.27 <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/60624/60624fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/60624/60624fig4a.jpg" /> 
Figure 4: Schematic presentation of the heterogeneous catalytic carbonyl-ene reaction. Pink colored molecules represent the substrates while greens indicate the products. (A) to (D) illustrate the steps of the reaction. Dissociation of the products from the reaction site is very fast and penetrating the crystal is not necessary. This figure has been reprinted with permission from Han et al.27 <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/60624/60624fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<table border="1" fo:keep-together.within-page="1">
	<tbody>
		<tr>
			<td>entry</td>
			<td>substrate</td>
			<td>t (h)</td>
			<td>yield (%)</td>
			<td>ee (%)</td>
		</tr>
		<tr>
			<td>1</td>
			<td>1a</td>
			<td>3.5</td>
			<td>92</td>
			<td>50</td>
		</tr>
		<tr>
			<td>2</td>
			<td>1b</td>
			<td>3.5</td>
			<td>52</td>
			<td>5</td>
		</tr>
		<tr>
			<td>3</td>
			<td>1c</td>
			<td>20</td>
			<td>NR</td>
			<td>NA</td>
		</tr>
	</tbody>
</table>
Table 1: Heterogeneous stoichiometric carbonyl-ene reaction of 1. NR = no reaction, under detection limit; NA = not applicable. This table has been reprinted with permission from Han et al.27
<table border="1" fo:keep-together.within-page="1">
	<tbody>
		<tr>
			<td>entry</td>
			<td>catalyst</td>
			<td>yield (%)</td>
			<td>ee (%)</td>
		</tr>
		<tr>
			<td>1</td>
			<td>Zn/(S)-KUMOF-1-(L)</td>
			<td>92</td>
			<td>70</td>
		</tr>
		<tr>
			<td>2</td>
			<td>Zn/(S)-KUMOF-1-(M)</td>
			<td>89</td>
			<td>50</td>
		</tr>
		<tr>
			<td>3</td>
			<td>Zn/(S)-KUMOF-1-(S)</td>
			<td>91</td>
			<td>0</td>
		</tr>
	</tbody>
</table>
Table 2: Result of particle size effect determination. Only 1a was used for this determination. Entries 1&#8211;3 correspond to large, medium, and small sized particles, respectively. This table has been reprinted with permission from Han et al.27
<table border="1" fo:keep-together.within-page="1">
	<tbody>
		<tr>
			<td></td>
			<td></td>
			<td colspan="2">product obtained from solution</td>
			<td colspan="2">product obtained from MOF</td>
		</tr>
		<tr>
			<td>entry</td>
			<td>substrate</td>
			<td>yield (%)</td>
			<td>ee (%)</td>
			<td>yield (%)</td>
			<td>ee (%)</td>
		</tr>
		<tr>
			<td>1</td>
			<td>1a</td>
			<td>85</td>
			<td>24</td>
			<td>2.8</td>
			<td>NA</td>
		</tr>
		<tr>
			<td>2</td>
			<td>1b</td>
			<td>89</td>
			<td>7</td>
			<td>0.7</td>
			<td>NA</td>
		</tr>
		<tr>
			<td>3</td>
			<td>1c</td>
			<td>83</td>
			<td>0</td>
			<td>0.2</td>
			<td>NA</td>
		</tr>
	</tbody>
</table>
Table 3: Heterogeneous catalytic carbonyl-ene reaction of 1. NA = not applicable. This table has been reprinted with permission from Han et al.27
<img alt="Animation 1" src="/files/ftp_upload/60624/60624animation1a.jpg" /> 
Animation 1: Animated illustration of the heterogeneous stoichiometric carbonyl-ene reaction. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/60624/Animation_1.mp4" target="_blank">Please click here to view this video. (Right-click to download.)</a>
<img alt="Animation 2" src="/files/ftp_upload/60624/60624animation2a.jpg" /> 
Animation 2: Animated illustration of the heterogeneous catalytic carbonyl-ene reaction. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/60624/Animation_2.mp4" target="_blank">Please click here to view this video. (Right-click to download.)</a>

Watch this Scientific Journal Video about Development of Heterogeneous Enantioselective Catalysts using Chiral Metal-Organic Frameworks MOFs at JoVE.com

Development of Heterogeneous Enantioselective Catalysts using Chiral Metal-Organic Frameworks MOFs

Here, we present a protocol for active site validation of metal-organic framework catalysts by comparing stoichiometric and catalytic carbonyl-ene reactions to find out whether a reaction takes place on the inner or outer surface of metal-organic frameworks.

Development of Heterogeneous Enantioselective Catalysts using Chiral Metal-Organic Frameworks (MOFs)

Substrate size discrimination by the pore size and homogeneity of the chiral environment at the reaction sites are important issues in the validation of ...

development-heterogeneous-enantioselective-catalysts-using-chiral

Nanyang Technological University

Research

JoVE Journal

Chemistry

7.2K Views.  Korea University. Here, we present a protocol for active site validation of metal-organic framework catalysts by comparing stoichiometric and catalytic carbonyl-ene reactions to find out whether a reaction takes place on the inner or outer surface of metal-organic frameworks.

Video: Development of Heterogeneous Enantioselective Catalysts using Chiral Metal-Organic Frameworks MOFs

Synthesis and Characterization of Functionalized Metal-organic Frameworks

Metal-organic frameworks have attracted extraordinary amounts of research attention, as they are attractive candidates for numerous industrial and technological applications. Their signature property is their ultrahigh porosity, which however imparts a series of challenges when it comes to both constructing them and working with them. Securing desired MOF chemical and physical functionality by linker/node assembly into a highly porous framework of choice can pose difficulties, as less porous and more thermodynamically stable congeners (e.g., other crystalline polymorphs, catenated analogues) are often preferentially obtained by conventional synthesis methods. Once the desired product is obtained, its characterization often requires specialized techniques that address complications potentially arising from, for example, guest-molecule loss or preferential orientation of microcrystallites. Finally, accessing the large voids inside the MOFs for use in applications that involve gases can be problematic, as frameworks may be subject to collapse during removal of solvent molecules (remnants of solvothermal synthesis). In this paper, we describe synthesis and characterization methods routinely utilized in our lab either to solve or circumvent these issues. The methods include solvent-assisted linker exchange, powder X-ray diffraction in capillaries, and materials activation (cavity evacuation) by supercritical CO2 drying. Finally, we provide a protocol for determining a suitable pressure region for applying the Brunauer-Emmett-Teller analysis to nitrogen isotherms, so as to estimate surface area of MOFs with good accuracy.

Synthesis, activation, and characterization of intentionally designed metal-organic framework materials is challenging, especially when building blocks are incompatible or unwanted polymorphs are thermodynamically favored over desired forms. We describe how applications of solvent-assisted linker exchange, powder X-ray diffraction in capillaries and activation via supercritical CO2 drying, can address some of these challenges.

Metal-organic frameworks have attracted extraordinary amounts of research attention, as they are attractive candidates for numerous industrial and ...

Preparation of Highly Porous Coordination Polymer Coatings on Macroporous Polymer Monoliths for Enhanced Enrichment of Phosphopeptides

We describe a protocol for the preparation of hybrid materials based on highly porous coordination polymer coatings on the internal surface of macroporous polymer monoliths. The developed approach is based on the preparation of a macroporous polymer containing carboxylic acid functional groups and the subsequent step-by-step solution-based controlled growth of a layer of a porous coordination polymer on the surface of the pores of the polymer monolith. The prepared metal-organic polymer hybrid has a high specific micropore surface area. The amount of iron(III) sites is enhanced through metal-organic coordination on the surface of the pores of the functional polymer support. The increase of metal sites is related to the number of iterations of the coating process.
The developed preparation scheme is easily adapted to a capillary column format. The functional porous polymer is prepared as a self-contained single-block porous monolith within the capillary, yielding a flow-through separation device with excellent flow permeability and modest back-pressure. The metal-organic polymer hybrid column showed excellent performance for the enrichment of phosphopeptides from digested proteins and their subsequent detection using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. The presented experimental protocol is highly versatile, and can be easily implemented to different organic polymer supports and coatings with a plethora of porous coordination polymers and metal-organic frameworks for multiple purification and/or separation applications.

A procedure for the preparation of porous hybrid separation media composed of a macroporous polymer monolith internally coated by a high surface area microporous coordination polymer is presented.

We describe a protocol for the preparation of hybrid materials based on highly porous coordination polymer coatings on the internal surface of macroporous ...

HKUST-1 as a Heterogeneous Catalyst for the Synthesis of Vanillin

Vanillin (4-hydoxy-3-methoxybenzaldehyde) is the main component of the extract of vanilla bean. The natural vanilla scent is a mixture of approximately 200 different odorant compounds in addition to vanillin. The natural extraction of vanillin (from the orchid Vanilla planifolia, Vanilla tahitiensis and Vanilla pompon) represents only 1% of the worldwide production and since this process is expensive and very long, the rest of the production of vanillin is synthesized. Many biotechnological approaches can be used for the synthesis of vanillin from lignin, phenolic stilbenes, isoeugenol, eugenol, guaicol, etc., with the disadvantage of harming the environment since these processes use strong oxidizing agents and toxic solvents. Thus, eco-friendly alternatives on the production of vanillin are very desirable and thus, under current investigation. Porous coordination polymers (PCPs) are a new class of highly crystalline materials that recently have been used for catalysis. HKUST-1 (Cu3(BTC)2(H2O)3, BTC = 1,3,5-benzene-tricarboxylate) is a very well known PCP which has been extensively studied as a heterogeneous catalyst. Here, we report a synthetic strategy for the production of vanillin by the oxidation of trans-ferulic acid using HKUST-1 as a catalyst.

The conversion of trans-ferulic acid to vanillin was achieved by heterogeneous catalysis. HKUST-1 was employed in this synthesis and the essential step in the catalytic process was the generation of unsaturated metal sites. Thus, when the catalyst was activated under vacuum, full vanillin conversion (yield of 95%) was obtained.

Vanillin (4-hydoxy-3-methoxybenzaldehyde) is the main component of the extract of vanilla bean. The natural vanilla scent is a mixture of approximately ...

Surface Functionalization of Metal-Organic Frameworks for Improved Moisture Resistance

Metal-organic frameworks (MOFs) are a class of porous inorganic materials with promising properties in gas storage and separation, catalysis and sensing. However, the main issue limiting their applicability is their poor stability in humid conditions. The common methods to overcome this problem involve the formation of strong metal-linker bonds by using highly charged metals, which is limited to a number of structures, the introduction of alkylic groups to the framework by post-synthetic modification (PSM) or chemical vapour deposition (CVD) to enhance overall hydrophobicity of the framework. These last two usually provoke a drastic reduction of the porosity of the material. These strategies do not permit to exploit the properties of the MOF already available and it is imperative to find new methods to enhance the stability of MOFs in water while keeping their properties intact. Herein, we report a novel method to enhance the water stability of MOF crystals featuring Cu2(O2C)4&#160;paddle-wheel units, such as HKUST (where HKUST stands for Hong Kong University of Science &#38; Technology), with the catechols functionalized with alkyl and fluoro-alkyl chains. By taking advantage of the unsaturated metal sites and the catalytic catecholase-like activity of CuII ions, we are able to create robust hydrophobic coatings through the oxidation and subsequent polymerization of the catechol units on the surface of the crystals under anaerobic and water-free conditions without disrupting the underlying structure of the framework. This approach not only affords the material with improved water stability but also provides control over the function of the protective coating, which enables the development of functional coatings for the adsorption and separations of volatile organic compounds. We are confident that this approach could also be extended to other unstable MOFs featuring open metal sites.

Robust functional catechol coatings were produced in one step by direct reaction of the material known as HKUST with synthetic catechols under anaerobic conditions. The formation of homogeneous coatings surrounding the entire crystal is ascribed to the biomimetic catalytic activity of Cu(II) dimers on the external surface of the crystals.

Metal-organic frameworks (MOFs) are a class of porous inorganic materials with promising properties in gas storage and separation, catalysis and sensing. ...

Electrophoretic Crystallization of Ultrathin High-performance Metal-organic Framework Membranes

We report the synthesis of thin, highly intergrown, polycrystalline metal-organic framework (MOF) membranes on a wide range of unmodified porous and non-porous supports (polymer, ceramic, metal, carbon, and graphene). We developed a novel crystallization technique, which is termed the ENACT approach: the electrophoretic nuclei assembly for the crystallization of highly intergrown thin films (ENACT). This approach allows for a high density of heterogeneous nucleation of MOFs on a chosen substrate via the electrophoretic deposition (EPD) directly from the precursor sol. The growth of well-packed MOF nuclei leads to a highly intergrown polycrystalline MOF film. We show that this simple approach can be used for the synthesis of thin, intergrown zeolite imidazole framework (ZIF)-7 and ZIF-8 films. The resulting 500 nm-thick ZIF-8 membranes show a considerably high H2 permeance (8.3 x 10-6 mol m-2 s-1 Pa-1) and ideal gas selectivities (7.3 for H2/CO2, 15.5 for H2/N2, 16.2 for H2/CH4, and 2655 for H2/C3H8). An attractive performance for C3H6/C3H8 separation is also achieved (a C3H6 permeance of 9.9 x 10-8 mol m-2 s-1 Pa-1 and a C3H6/C3H8 ideal selectivity of 31.6 at 25 &#176;C). Overall, the ENACT process, owing to its simplicity, can be extended to synthesize intergrown thin films of a wide range of nanoporous crystalline materials.

A simple, reproducible, and versatile approach for the synthesis of intergrown, polycrystalline metal-organic framework membranes on a wide range of unmodified porous and non-porous supports is presented.

We report the synthesis of thin, highly intergrown, polycrystalline metal-organic framework (MOF) membranes on a wide range of unmodified porous and ...

Synthesis of Single-Crystalline Core-Shell Metal-Organic Frameworks

Because of their designability and unprecedented synergistic effects, core-shell metal-organic frameworks (MOFs) have been actively examined recently. However, the synthesis of single-crystalline core-shell MOFs is very challenging, and thus a limited number of examples have been reported. Here, we suggest a method of synthesizing single-crystalline HKUST-1@MOF-5 core-shells, which is HKUST-1 at the center of MOF-5. Through the computational algorithm, this pair of MOFs was predicted to have the matched lattice parameters and chemical connection points at the interface. To construct the core-shell structure, we prepared the octahedral- and cubic-shaped HKUST-1 crystals as a core MOF, in which the (111) and (001) facets were mainly exposed, respectively. Via the sequential reaction, the MOF-5 shell was well-grown on the exposed surface, showing a seamless connect interface, which resulted in the successful synthesis of single-crystalline HKUST-1@MOF-5. Their pure phase formation was proved by optical microscopic images and powder X-ray diffraction (PXRD) patterns. This method presents the potential of and insights into the single-crystalline core-shell synthesis with different kinds of MOFs.

Here, we demonstrate a protocol for the two-step synthesis of single-crystalline core-shells using a non-isostructural metal-organic framework (MOF) pair, HKUST-1 and MOF-5, which have well-matched crystal lattices.

Because of their designability and unprecedented synergistic effects, core-shell metal-organic frameworks (MOFs) have been actively examined recently. ...

Synthesis of Low-Valent MOFs from Multitopic Phosphine Linkers

Experimental Approaches for the Synthesis of Low-Valent Metal-Organic Frameworks from Multitopic Phosphine Linkers

Metal-organic frameworks (MOFs) are the subject of intense research focus due to their potential applications in gas storage and separation, biomedicine, energy, and catalysis. Recently, low-valent MOFs (LVMOFs) have been explored for their potential use as heterogeneous catalysts, and multitopic phosphine linkers have been shown to be a useful building block for the formation of LVMOFs. However, the synthesis of LVMOFs using phosphine linkers requires conditions that are distinct from those in the majority of the MOF synthetic literature, including the exclusion of air and water and the use of unconventional modulators and solvents, making it somewhat more challenging to access these materials. This work serves as a general tutorial for the synthesis of LVMOFs with phosphine linkers, including information on the following: 1) the judicious choice of the metal precursor, modulator, and solvent; 2) the experimental procedures, air-free techniques, and required equipment; 3) the proper storage and handling of the resultant LVMOFs; and 4) useful characterization methods for these materials. The intention of this report is to lower the barrier to this new subfield of MOF research and facilitate advancements toward novel catalytic materials.

Author Spotlight: Experimental Approaches for the Synthesis of Low-Valent Metal-Organic Frameworks from Multitopic Phosphine Linkers  

Here, we describe a protocol for the synthesis of low-valent metal-organic frameworks (LVMOFs) from low-valent metals and multitopic phosphine linkers under air-free conditions. The resulting materials have potential applications as heterogeneous catalyst mimics of low-valent metal-based homogeneous catalysts.

Metal-organic frameworks (MOFs) are the subject of intense research focus due to their potential applications in gas storage and separation, biomedicine, ...

Triazole and Tetrazole Functionalized Zr-MOFs Synthesis and Characterization

Synthesis of Triazole and Tetrazole-Functionalized Zr-Based Metal-Organic Frameworks Through Post-Synthetic Ligand Exchange

Metal-organic frameworks (MOFs) are a class of porous materials that are formed through coordination bonds between metal clusters and organic ligands. Given their coordinative nature, the organic ligands and strut framework can be readily removed from the MOF and/or exchanged with other coordinative molecules. By introducing target ligands to MOF-containing solutions, functionalized MOFs can be obtained with new chemical tags via a process called post-synthetic ligand exchange (PSE). PSE is a straightforward and practical approach that enables the preparation of a wide range of MOFs with new chemical tags via a solid-solution equilibrium process. Furthermore, PSE can be performed at room temperature, allowing the incorporation of thermally unstable ligands into MOFs. In this work, we demonstrate the practicality of PSE by using heterocyclic triazole- and tetrazole-containing ligands to functionalize a Zr-based MOF (UiO-66; UiO = University of Oslo). After digestion, the functionalized MOFs are characterized via various techniques, including powder X-ray diffraction and nuclear magnetic resonance spectroscopy.

Author Spotlight: Functionalizing Metal-Organic Frameworks: Advancements, Challenges, and the Power of Post-Synthetic Ligand Exchange 

Post-synthetic ligand exchange (PSE) is a versatile and powerful tool for installing functional groups into metal-organic frameworks (MOFs). Exposing MOFs to solutions containing triazole- and tetrazole-functionalized ligands can incorporate these heterocyclic moieties into Zr-MOFs through PSE processes.

Metal-organic frameworks (MOFs) are a class of porous materials that are formed through coordination bonds between metal clusters and organic ligands. ...

Synthesis and Discovery of Phosphonate-Based MOFs: A Solvothermal Approach

Discovery and Synthesis Optimization of Isoreticular Al(III) Phosphonate-Based Metal-Organic Framework Compounds Using High-Throughput Methods

High-throughput (HT) methods are an important tool for the fast and efficient screening of synthesis parameters and the discovery of new materials. This manuscript describes the synthesis of metal-organic frameworks (MOFs) from solution using an HT reactor system, resulting in the discovery of various phosphonate-based MOFs of the composition [Al2H12-x(PMP)3]Clx&#8729;6H2O (H4PMP = N,N &#39;-piperazine bis(methylenephosphonic acid)) for x = 4, 6, denoted as Al-CAU-60-xHCl, containing trivalent aluminum ions. This was accomplished under solvothermal reaction conditions by systematically screening the impact of the molar ratio of the linker to the metal and the pH of the reaction mixture on the product formation. The protocol for the HT investigation includes six steps: a) synthesis planning (DOE = design of&#160;experiment) within the HT methodology, b) dosing and working with in-house developed HT reactors, c) solvothermal synthesis, d) synthesis workup using in-house developed filtration blocks, e) characterization by HT powder X-ray diffraction, and f) evaluation of the data. The HT methodology was first used to study the influence of acidity on the product formation, leading to the discovery of Al-CAU-60&#8729;xHCl (x = 4 or 6).

Author Spotlight: Accelerating Discovery in Microporous Material Chemistry 

The targeted synthesis of new metal-organic frameworks (MOFs) is difficult, and their discovery depends on the knowledge and creativity of the chemist. High-throughput methods allow complex synthetic parameter fields to be explored quickly and efficiently, accelerating the process of finding crystalline compounds and identifying synthetic and structural trends.

High-throughput (HT) methods are an important tool for the fast and efficient screening of synthesis parameters and the discovery of new materials. This ...

Organocatalysis

Source: Vy M. Dong and Faben Cruz, Department of Chemistry, University of California, Irvine, CA
This experiment will demonstrate the concept of organocatalysis by illustrating the proper setup of a reaction that utilizes enamine catalysis. Organocatalysis is a form of catalysis that uses substoichiometric amounts of small organic molecules to accelerate reactions. This type of catalysis is complementary to other forms of catalysis such as transition metal or biocatalysis. Transition metal catalysis involves transition metals as catalysts and biocatalysis uses enzymes as catalysts. Some advantages of organocatalysis include the low toxicity and cost of the organocatalysts in comparison to many metal catalysts. In addition, most organocatalysts are not sensitive to air and moisture, unlike metal catalysts. In contrast to enzymes found in living organisms, the small molecules that act as organocatalysts are typically easy to access. Furthermore, organocatalysis offers complementary and new reactivity not observed with other forms of catalysis.