Name: Author Spotlight: Functionalizing Metal-Organic Frameworks: Advancements, Challenges, and the Power of Post-Synthetic Ligand Exchange
Uploaded: 2023-06-23T14:05:00
Description: Watch this Scientific Journal Video about Functionalizing Metal-Organic Frameworks: Advancements, Challenges, and the Power of Post-Synthetic Ligand Exchange at JoVE.com

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF-2022R1A2C1009706).

The reagents required to prepare MOFs and the ligands are listed in the Table of Materials.
1. Setting up the post-synthetic ligand exchange (PSE) process
<ol>
	<li>Completely dry the pre-synthesized UiO-66 MOFs under vacuum to remove any unreacted metal-salts and ligands in the pores, and remaining solvent residues overnight. 
	NOTE: See Supplementary File 1 for the synthesis procedure of UiO-66 MOFs.</li>
	<li>Prepare functionalized l...

Triazole and Tetrazole Functionalized Zr-MOFs Synthesis and Characterization

<ol>
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The PSE process with functionalized BDC ligands toward Zr-based UiO-66 MOFs is a simple and versatile method to obtain MOFs with chemical tags. The PSE process is best conducted in aqueous media, requiring the initial step of solvating the ligand in an aqueous medium. When using pre-synthesized BDC with functional groups, direct dissolution in a basic solvent, such as a 4% KOH aqueous solution, is recommended. Alternatively, sodium or potassium salt of benzene-1,4-dicarboxylate may be used. Neutralization to pH 7 is crit...

Metal-organic frameworks (MOFs) are three-dimensional porous materials that are formed through coordination bonds between metal clusters and multi-topic organic ligands. MOFs have garnered significant attention due to their permanent porosity, low density, and ability to associate organic and inorganic components, which enables diverse applications1,2. Moreover, the vast range of metal nodes and strut organic linkers offer MOFs theoretically unlimited structural combinations. Even with identical framework structures, MOFs&#39; physical and chemical properties can be modified through ligand functionalization with chemical tags. This modification process offers a promising route to tailor the properties of MOFs for specific applications3,4,5,6,7,8,9.
Both the pre-functionalization of ligands prior to MOF synthesis and post-synthetic modification (PSM) of MOFs have been employed to introduce and/or modify functional groups in MOF ligands10,11. In particular, covalent PSMs have been extensively studied to introduce new functional groups and generate a range of MOFs with diverse functionalities12,13,14. For instance, UiO-66-NH2 can be converted to amide-functionalized UiO-66-AMs with different chain lengths (ranging from the shortest acetamide to the longest n-hexyl amide) through acylation reactions with appropriate acyl halides (such as acetyl chloride or n-hexanoyl chloride)15,16. This approach demonstrates the versatility of covalent PSMs to introduce specific functional groups onto MOF ligands, paving the way for a broad range of applications.
In addition to covalent PSMs, post-synthetic ligand exchange (PSE) is a promising strategy for modifying MOFs (Figure 1). Since MOFs are composed of coordination bonds between metals and ligands (such as carboxylates), these coordination bonds can be replaced with external ligands from a solution. Exposing MOFs to a solution containing the desired ligand with chemical tags can be incorporated into the MOFs via PSE17,18,19,20,21,22. Since the PSE process is accelerated by the existence of coordinative solvents, the phenomenon is also called solvent-assisted ligand exchange (SALE)23,24. This approach offers a flexible and facile method for functionalizing MOFs with a wide range of external ligands, enabling a broad spectrum of applications25,26,27,28,29.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/65619/65619fig01.jpg" /> 
Figure 1: Synthesis of triazole and tetrazole-functionalized H2BDC ligands and preparation of triazole- and tetrazole-functionalized UiO-66 MOF through PSE. <a href="https://www.jove.com/files/ftp_upload/65619/65619fig01large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
The progress of the PSE process can be controlled by adjusting the ligand ratio, exchange temperature, and time. Notably, room temperature PSE can be employed to obtain functionalized MOFs by exchanging ligands from a solution into MOF solids20. The PSE strategy is particularly useful for introducing both thermally unstable functional groups (such as azido groups) and coordinative functional groups (such as phenol groups) into MOF structures18. In addition, the PSE strategy has been applied to various MOFs with metal and coordination bond variations. This exchange is a universal process in the chemistry of MOFs30,31,32. In this study, we present a detailed protocol for PSE to obtain functionalized MOFs from pristine, non-functionalized MOFs, and we provide a characterization strategy to confirm successful functionalization of the MOFs. This method demonstrates the versatility and convenience of PSE for modifying MOFs with diverse functional groups.
Tetrazole-containing benzene-1,4-dicarboxylic acid (H2BDC-Tetrazole)33, and triazole-containing benzene-1,4-dicarboxylic acid (H2BDC-Triazole) are synthesized as target ligands and utilized in the PSE of UiO-66 MOFs to obtain novel, coordination-free, triazole-containing MOFs. Both triazoles and tetrazoles possess acidic N-H protons on their heterocyclic rings and can coordinate with metal cations, thus enabling their use in constructing MOFs34,35. However, there are limited studies on incorporating coordination-free tetrazoles and triazoles into MOFs and related structures. In case of triazole-functionalized Zr-MOFs, UiO-68 type MOFs were investigated to photophysical properties through direct solvothermal synthesis with benzotriazole functionalities36. For tetrazole-functionalized Zr-MOFs, the mixed direct synthesis was employed33. These heterocycle-functionalized MOFs could provide potential coordinating sites in MOF pores for catalysis, selective molecular uptake by binding affinity, and energy-related applications, such as proton conduction in fuel cells.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>2-Bromoterephthalic acid</td>
			<td>BLD Pharm</td>
			<td>BD5695</td>
			<td>reagent for BDC-Triazole</td>
		</tr>
		<tr>
			<td>Azidotrimethylsilane</td>
			<td>Simga Aldrich</td>
			<td>155071</td>
			<td>reagent for BDC-Triazole</td>
		</tr>
		<tr>
			<td>Bis(triphenylphosphine)palladium(II) dichloride</td>
			<td>TCI</td>
			<td>B1667</td>
			<td>reagent for BDC-Triazole</td>
		</tr>
		<tr>
			<td>Copper(I) cyanide</td>
			<td>Alfa-Aesar</td>
			<td>12135</td>
			<td>reagent for BDC-Tetrazole</td>
		</tr>
		<tr>
			<td>Copper(I) iodide</td>
			<td>Acros organics</td>
			<td>20150</td>
			<td>reagent for BDC-Triazole</td>
		</tr>
		<tr>
			<td>Digital Orbital Shaker</td>
			<td>Daihan Scientific</td>
			<td>SHO-1D</td>
			<td>PSE</td>
		</tr>
		<tr>
			<td>Formic Acid</td>
			<td>Daejung chemical</td>
			<td>F0195</td>
			<td>reagent for BDC-Tetrazole</td>
		</tr>
		<tr>
			<td>Hybrid LC/Q-TOF system</td>
			<td>Bruker BioSciences</td>
			<td>maXis 4G</td>
			<td>HR-MS</td>
		</tr>
		<tr>
			<td>Lithum hydroxide monohydrate</td>
			<td>Daejung chemical</td>
			<td>5087-4405</td>
			<td>reagent for BDC-Triazole</td>
		</tr>
		<tr>
			<td>Magnesium sulfate</td>
			<td>Samchun chemical</td>
			<td>M1807</td>
			<td>reagent for BDC-Triazole</td>
		</tr>
		<tr>
			<td>Methyl alcohol</td>
			<td>Daejung chemical</td>
			<td>M0584</td>
			<td>reagent for BDC-Tetrazole</td>
		</tr>
		<tr>
			<td>N,N-Dimethylformamide</td>
			<td>Daejung chemical</td>
			<td>D0552</td>
			<td>reagent for BDC-Tetrazole</td>
		</tr>
		<tr>
			<td>Nuclear Magnetic Resonance Spectrometer-500 MHz</td>
			<td>Bruker</td>
			<td>AVANCE 500MHz</td>
			<td>NMR</td>
		</tr>
		<tr>
			<td>Polypropylene cap (22 mm, Cork-Backed Foil Lined)</td>
			<td>Sungho Korea</td>
			<td>22-200</td>
			<td>material for digestion</td>
		</tr>
		<tr>
			<td>Potassium cyanide</td>
			<td>Alfa-Aesar</td>
			<td>L13273</td>
			<td>reagent for BDC-Tetrazole</td>
		</tr>
		<tr>
			<td>PVDF Synringe filter (13 mm, 0.45 &#181;m)</td>
			<td>LK Lab Korea</td>
			<td>F14-61-363</td>
			<td>material for digestion</td>
		</tr>
		<tr>
			<td>Scintillation vial (20 mL, borosilicate glass)</td>
			<td>Sungho Korea</td>
			<td>74504-20</td>
			<td>material for digestion</td>
		</tr>
		<tr>
			<td>Sodium azide&#160;</td>
			<td>TCI</td>
			<td>S0489</td>
			<td>reagent for BDC-Tetrazole</td>
		</tr>
		<tr>
			<td>Sodium bicarbonate</td>
			<td>Samchun chemical</td>
			<td>S0343</td>
			<td>reagent for BDC-Triazole</td>
		</tr>
		<tr>
			<td>Tetrabutylammonium fluoride (1 M THF solution)</td>
			<td>Acros organics</td>
			<td>20195</td>
			<td>reagent for BDC-Triazole</td>
		</tr>
		<tr>
			<td>Triethylamine</td>
			<td>TCI</td>
			<td>T0424</td>
			<td>reagent for BDC-Triazole</td>
		</tr>
		<tr>
			<td>Triethylamine hydrochloride</td>
			<td>Daejung chemical</td>
			<td>8628-4405</td>
			<td>reagent for BDC-Tetrazole</td>
		</tr>
		<tr>
			<td>Trimethylsilyl-acetylene</td>
			<td>Alfa-Aesar</td>
			<td>A12856</td>
			<td>reagent for BDC-Triazole</td>
		</tr>
		<tr>
			<td>Triphenylphosphine</td>
			<td>TCI</td>
			<td>T0519</td>
			<td>reagent for BDC-Triazole</td>
		</tr>
		<tr>
			<td>X RAY DIFFRACTOMETER SYSTEM</td>
			<td>Rigaku</td>
			<td>MiniFlex 600</td>
			<td>PXRD</td>
		</tr>
		<tr>
			<td>Zirconium(IV) chloride</td>
			<td>Alfa-Aesar</td>
			<td>12104</td>
			<td>reagent for BDC-Tetrazole</td>
		</tr>
	</tbody>
</table>

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 

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

Our main research topic is functionalizing the porous metal-organic frameworks, MOFs, with interesting functional groups through various techniques. This study is focused on heterocycle installation in MOFs. PSE, post-synthetic ligand exchange is the main technique in this work.The ligand with target functional group could be installed in presynthesized MOFS throughout the solid-solution exchange process. Especially, some of the unstable ligands and ligands with coordinating groups could be efficiently incorporated into MOFs through the PSE techniques. Both triazole and tetrazole functional groups have multiple coordinating nitrogen grooves.Therefore, there is competition for coordination between metal carboxylate and metal triazolate or tetrazolate. The incorporation ratio of target ligands is easily controlled in the PSE process and multiple targets could be installed in a single MOF through this approach.

The successful synthesis of exchanged UiO-66 MOFs, UiO-66-Triazole, and UiO-66-Tetrazole produced colorless microcrystalline solids. Both H2BDC-Triazole and H2BDC-Tetrazole ligands also exhibited a colorless solid state. The standard method used to determine the success of the exchange involved measuring the PXRD patterns and comparing the crystallinity of the sample with pristine UiO-66 MOF. Figure 2 displays the PXRD patterns of exchanged UiO-66-Triazole and UiO-66-Te...

Watch this Scientific Journal Video about Functionalizing Metal-Organic Frameworks: Advancements, Challenges, and the Power of Post-Synthetic Ligand Exchange at JoVE.com

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. ...

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Chemistry

Setting up the Post-synthetic Ligand Exchange (PSE) Process, Exchanged MOF Isolation and Washing Process

Begin by measuring 23.3 milligrams of H2BDC triazole and place it into the scintillation vial. To dissolve the H2BDC ligand in the aqueous solution, transfer 1.0 milliliters of the 4%potassium hydroxide solution into the vial using a glass pipette. Next, sonicate the mixture until all solids completely dissolve.Add one molar hydrochloric acid solution into the decarboxylate-containing vial and stir until a pH of 7 is reached. Add 33 milligrams of UiO-66 MOF to the scintillation vial. Then incubate the vial with the MOF and ligand at room temperature and 120 RPM in a shaker for 24 hours.Next, isolate the solid MOF from the mixture by centrifugation at 1166g for five minutes. at room temperature. Add 10 milliliters of fresh methanol to the obtained solid MOF and shake the vial, creating a heterogeneous mixture to dissolve any remaining unchanged BDC ligands.Isolate the solid once again by centrifuging the mixture at 1166g for five minutes at room temperature and dry it.

Characterization of the MOF by Powder X-Ray Diffraction (PXRD) and Nuclear Magnetic Resonance (NMR) After Digestion

Transfer approximately 10 milligrams of the MOF solid exchanged with UiO-66-Triazole to a powder X-ray diffraction sample holder. Place the sample holder in the diffractometer, and collect the powder X-ray diffraction pattern. To begin NMR characterization, transfer around 30 milligrams of the exchanged MOF solid to a fresh four milliliter vial.Then move 400 microliters of DMSO-d6 to the MOF sample using a micro pipette. Transfer 200 microliters of 4.14 molar ammonium fluoride deuterium oxide solution to a DMSO-d6 suspension of MOF powder using a micro pipette. Sonicate the heterogeneous mixture for 30 minutes until the MOF is dissolved in the dimethyl sulfoxide deuterium oxide mixed solvent after digestion.Once done, remove any remaining insoluble solids by filtering the solution through a polyvinylidene difluoride syringe filter while transferring it from the four milliliter vial to a nuclear magnetic resonance tube. To determine the success of the exchange, powder X-ray diffraction patterns were measured and compared to pristine UiO-66 MOF for crystallinity. The position of reflection peaks, relative intensity, and broadness in the powder X-ray diffraction patterns of exchanged UiO-66-Triazole and UiO-66-Tetrazole matched with pristine UiO-66 MOFs, indicating that the ligand exchange did not impact the framework structure.The exchange ratios were determined by comparing the integration of nonfunctionalized BDC and BDC-Triazole or BDC-Tetrazole. The exchange ratios for Triazole and Tetrazole were found to be 33%and 30%respectively from post synthetic ligand exchange analysis.