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, weighi...

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 border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<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...

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 ...

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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

Preparation of Hydrophobic Metal-Organic Frameworks via Plasma Enhanced Chemical Vapor Deposition of Perfluoroalkanes for the Removal of Ammonia

Plasma enhanced chemical vapor deposition (PECVD) of perfluoroalkanes has long been studied for tuning the wetting properties of surfaces. For high surface area microporous materials, such as metal-organic frameworks (MOFs), unique challenges present themselves for PECVD treatments. Herein the protocol for development of a MOF that was previously unstable to humid conditions is presented. The protocol describes the synthesis of Cu-BTC (also known as HKUST-1), the treatment of Cu-BTC with PECVD of perfluoroalkanes, the aging of materials under humid conditions, and the subsequent ammonia microbreakthrough experiments on milligram quantities of microporous materials. Cu-BTC has an extremely high surface area (~1,800 m2/g) when compared to most materials or surfaces that have been previously treated by PECVD methods. Parameters such as chamber pressure and treatment time are extremely important to ensure the perfluoroalkane plasma penetrates to and reacts with the inner MOF surfaces. Furthermore, the protocol for ammonia microbreakthrough experiments set forth here can be utilized for a variety of test gases and microporous materials.

Herein the procedures for plasma enhanced chemical vapor deposition of perfluoroalkanes on microporous materials such as metal-organic frameworks to enhance their stability and hydrophobicity are described. Furthermore, breakthrough testing of milligram quantities of samples is described in detail.

Plasma enhanced chemical vapor deposition (PECVD) of perfluoroalkanes has long been studied for tuning the wetting properties of surfaces. For high ...

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 ...

Synthesis of a Water-soluble Metal&#8211;Organic Complex Array

We demonstrate a method for the synthesis of a water-soluble multimetallic peptidic array containing a predetermined sequence of metal centers such as Ru(II), Pt(II), and Rh(III). The compound, named as a water-soluble metal-organic complex array (WSMOCA), is obtained through 1) the conventional solution-chemistry-based preparation of the corresponding metal complex monomers having a 9-fluorenylmethyloxycarbonyl (Fmoc)-protected amino acid moiety and 2) their sequential coupling together with other water-soluble organic building units on the surface-functionalized polymeric resin by following the procedures originally developed for the solid-phase synthesis of polypeptides, with proper modifications. Traces of reactions determined by mass spectrometric analysis at the representative coupling steps in stage 2 confirm the selective construction of a predetermined sequence of metal centers along with the peptide backbone. The WSMOCA cleaved from the resin at the end of stage 2 has a certain level of solubility in aqueous media dependent on the pH value and/or salt content, which is useful for the purification of the compound.

Synthesis of a Water-soluble Metal&amp;#8211;Organic Complex Array

A potential general method for the synthesis of water-soluble multimetallic peptidic arrays containing a predetermined sequence of metal centers is presented.

We demonstrate a method for the synthesis of a water-soluble multimetallic peptidic array containing a predetermined sequence of metal centers such as ...

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. ...

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. ...

A Technical Guide for Performing Spectroscopic Measurements on Metal-Organic Frameworks

Metal-organic frameworks (MOFs) offer a unique platform to understand light-driven processes in solid-state materials, given their high structural tunability. However, the progression of MOF-based photochemistry has been hindered by the difficulty in spectrally characterizing these materials. Given that MOFs are typically larger than 100 nm in size, they are prone to excessive light scatter, thereby rendering data from valuable analytical tools like transient absorption and emission spectroscopy nearly uninterpretable. To gain meaningful insights of MOF-based photo-chemical and physical processes, special consideration must be taken toward properly preparing MOFs for spectroscopic measurements, as well as the experimental setups that garner higher quality data. With these considerations in mind, the present guide provides a general approach and set of guidelines for the spectroscopic investigation of MOFs. The guide addresses the following key topics: (1) sample preparation methods, (2) spectroscopic techniques/measurements with MOFs, (3) experimental setups, (3) control experiments, and (4) post-run stability characterization. With appropriate sample preparation and experimental approaches, pioneering advancements toward the fundamental understanding of light-MOF interactions are significantly more attainable.

Here, we use a polymer stabilizer to prepare metal-organic framework (MOF) suspensions that exhibit markedly decreased scatter in their ground-state and transient absorption spectra. With these MOF suspensions, the protocol provides various guidelines to characterize the MOFs spectroscopically to yield interpretable data.

Metal-organic frameworks (MOFs) offer a unique platform to understand light-driven processes in solid-state materials, given their high structural ...

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, ...

Magnetometric Characterization of Redox-Active MOFs Solid-State Electrochemistry

Magnetometric Characterization of Intermediates in the Solid-State Electrochemistry of Redox-Active Metal-Organic Frameworks

Electrochemical energy storage has been a widely discussed application of redox-active metal-organic frameworks (MOFs) in the past 5 years. Although MOFs show outstanding performance in terms of gravimetric or areal capacitance and cyclic stability, unfortunately their electrochemical mechanisms are not well understood in most cases. Traditional spectroscopic techniques, such as X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS), have only provided vague and qualitative information about valence changes of certain elements, and the mechanisms proposed based on such information are often highly disputable. In this article, we report a series of standardized methods, including the fabrication of solid-state electrochemical cells, electrochemistry measurements, the disassembly of cells, the collection of MOF electrochemical intermediates, and physical measurements of the intermediates under the protection of inert gases. By using these methods for quantitatively clarifying the electronic and spin state evolution within a single electrochemical step of redox-active MOFs, one can provide clear insight into the nature of electrochemical energy storage mechanisms not only for MOFs, but also for all other materials with strongly correlated electronic structures.

Author Spotlight: Magnetometric Characterization of Intermediates in the Solid-State Electrochemistry of Redox-Active Metal-Organic Frameworks  

Ex situ magnetic surveys can directly provide bulk and local information on a magnetic electrode to reveal its charge storage mechanism step by step. Herein, electron spin resonance (ESR) and magnetic susceptibility are demonstrated to monitor the evaluation of paramagnetic species and their concentration in a redox-active metal-organic framework (MOF).

Electrochemical energy storage has been a widely discussed application of redox-active metal-organic frameworks (MOFs) in the past 5 years. Although MOFs ...

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. ...

Using Nitrogen Porosimetry to Characterize Metal-Organic Frameworks

Determining Surface Areas and Pore Volumes of Metal-Organic Frameworks

The surface area and pore volume of a metal-organic framework (MOF) can provide insight into its structure and potential applications. Both parameters are commonly determined using the data from nitrogen sorption experiments; commercial instruments to perform these measurements are also widely available. These instruments will calculate structural parameters, but it is essential to understand how to select input data and when calculation methods apply to the sample MOF. This article outlines the use of the Brunauer-Emmett-Teller (BET) method and Barrett-Joyner-Halenda (BJH) method for the calculation of surface area and pore volume, respectively. Example calculations are performed on the representative MOF UiO-66. Although widely applicable to MOFs, sample materials and adsorption data must meet certain criteria for the calculated results to be considered accurate, in addition to proper sample preparation. The assumptions and limitations of these methods are also discussed, along with alternative and complementary techniques for the MOF pore space characterization.

Author Spotlight: Characterizing Porous Materials for Aiding the Development of Robust Metal-Organic Frameworks with Adsorption Behavior 

This article describes the use of nitrogen porosimetry to characterize metal-organic frameworks, using UiO-66 as a representative material.

The surface area and pore volume of a metal-organic framework (MOF) can provide insight into its structure and potential applications. Both parameters are ...