Name: Author Spotlight: Experimental Approaches for the Synthesis of Low-Valent Metal-Organic Frameworks from Multitopic Phosphine Linkers
Uploaded: 2023-05-12T14:20:00
Description: Watch this Scientific Journal Video about Experimental Approaches for the Synthesis of Low-Valent Metal-Organic Frameworks from Multitopic Phosphine Linkers at JoVE.com

This work was supported by a grant from the National Science Foundation, Division of Chemistry, under Award No. CHE-2153240.

1. Setting up the Schlenk line
<ol>
	<li>Ensure all the taps are closed, then secure the cold trap to the Schlenk line using an O-ring (size 229 was used in our set up, although the size may vary depending on the specific Schlenk line used), and clamp.</li>
	<li>Turn on the vacuum pump (gas-ballast closed), and then open the taps of the Schlenk line such that the whole apparatus is open to vacuum. 
	NOTE: Do not open any taps to the hoses or any other taps that are open to the air; the apparatus s...

Synthesis of Low-Valent MOFs from Multitopic Phosphine Linkers

<ol>
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G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+metal-organic+framework+with+cooperative+phosphines+that+permit+post-synthetic+installation+of+open+metal+sites.">A metal-organic framework with cooperative phosphines that permit post-synthetic installation of open metal sites.</a> Angewandte Chemie - International Edition. 57 (30), 92959299 (2018).</li><li>Sikma, R. E., Balto, K. P., Figueroa, J. S., Cohen, S. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Metal-organic+frameworks+with+low-valent+metal+nodes.">Metal-organic frameworks with low-valent metal nodes.</a> Angewandte Chemie - International Edition. 61 (33), e202206353 (2022).</li><li>Agnew, D. W., Gembicky, M., Moore, C. E., Rheingold, A. L., Figueroa, J. 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There are several critical steps in the protocol that must be followed in order to achieve the desired phosphine-based LVMOF product with sufficient crystallinity. The first is that the metal precursor and modulator mixture (in this case, tetrakis(triphenylphosphine)palladium(0) and triphenylphosphine, respectively) must be dissolved independently of the multitopic phosphine linker (in this case, Sn1). This is to avoid the rapid and irreversible formation of amorphous coordination polymers, which occurs ...

Metal-organic frameworks, or MOFs, are a class of crystalline, porous materials1. MOFs are constructed from metal ions or metal ion cluster nodes, often referred to as secondary building units (SBUs), and multitopic organic linkers to give two- and three-dimensional network structures2. Over the past three decades, MOFs have been studied extensively due to their potential use in gas storage3 and separation4, biomedicine5, and catalysis6. The overwhelming majority of MOFs reported are composed of high-oxidation state metal nodes and hard, anionic donor linkers, such as carboxylates2. However, many homogeneous catalysts utilize soft, low-valent metals in combination with soft donor ligands, such as phosphines7. Therefore, expanding the scope of MOFs that contain low-valent metals can increase the range of catalytic transformations to which MOFs can be applied.
The established strategies for the incorporation of low-valent metals into MOFs using embedded soft donor sites are limited in scope and reduce the free pore volume of the parent MOF structure6,8,9,10. An alternative approach is to use low-valent metals directly as nodes or SBUs in combination with multitopic soft donor ligands as linkers to construct the MOF. This strategy not only provides a high loading of low-valent metal sites in the MOF but may also reduce or prevent metal leaching into the solution as a result of the stability of the framework structure11. For example, Figueroa and co-workers used multitopic isocyanide ligands as soft donor linkers and Cu(I)12 or Ni(0)13 as low-valent metal nodes to produce two- and three-dimensional MOFs. Similarly, Pederson and co-workers synthesized MOFs containing zero-valent group 6 metal nodes using pyrazine as a linker14. More recently, our laboratory reported tetratopic phosphine ligands as linkers for the construction of MOFs containing Pd(0) or Pt(0) nodes (Figure 1)15. These MOFs are particularly interesting due to the prevalence of phosphine-ligated low-valent metal complexes in homogeneous catalysis7. Nevertheless, low-valent MOFs (LVMOFs) as a general class of materials are relatively underexplored in the MOF literature but have great promise for applications in heterogeneous catalysis for reactions such as azide-alkyne coupling16, Suzuki-Miyaura coupling17,18, hydrogenation17, and others11.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/65317/65317fig01v2.jpg" /> 
Figure 1: Synthesis of LVMOFs using phosphine linkers. Sikma and Cohen15 reported the synthesis of three-dimensional LVMOFs, E1-M, using tetratopic phosphine ligands, E1, as linkers, Pd(0) and Pt(0) as nodes, and triphenylphosphine as a modulator. The central atom, E, can be Si or Sn. <a href="https://www.jove.com/files/ftp_upload/65317/65317fig01largev2.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
While the differences in the nature of the linkers and nodes of LVMOFs may give them unique properties compared to conventional MOF materials, these differences also introduce synthetic challenges. For example, many of the metal precursors and linkers that are commonly used in the MOF literature can be used in air2. In contrast, the successful synthesis of phosphine-based LVMOFs requires the exclusion of both air and water15. Similarly, the types of modulators used to promote crystallinity and the solvents used in the synthesis of phosphine-based LVMOFs are unusual compared to those used in most of the MOF literature15. As a result, the synthesis of these materials requires equipment and experimental techniques that even experienced MOF chemists may be less familiar with. Therefore, in an effort to minimize the impact of these obstacles, a step-by-step method for the synthesis of this new class of materials is provided here. The protocol outlined here covers all aspects of the synthesis of phosphine-based LVMOFs, including the overall experimental procedure, air-free techniques, the required equipment, the proper storage and handling of LVMOFs, and characterization methods. The choice of the metal precursor, modulator, and solvent are also discussed. Enabling the entry of new researchers into this field will help accelerate the discovery of novel LVMOFs and related materials for applications in catalysis.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>2800 Ultrasonic Cleaner, 3/4 Gallon, 40 kHz</td>
			<td>Branson</td>
			<td>CPX2800H</td>
			<td>Used for sonicating</td>
		</tr>
		<tr>
			<td>Argon, Ultra High Purity</td>
			<td>Matheson</td>
			<td>G1901101</td>
			<td>Used as inert gas source</td>
		</tr>
		<tr>
			<td>D8 ADVANCE Powder X-Ray Diffractometer</td>
			<td>Bruker</td>
			<td></td>
			<td>Used to collect PXRD patterns</td>
		</tr>
		<tr>
			<td>Dewar Flask</td>
			<td>Chemglass Life Sciences</td>
			<td>CG159303</td>
			<td>Dewar used for liquid nitrogen</td>
		</tr>
		<tr>
			<td>Flask,&#160;High Vacuum Valve, Capacity (mL) 10, Valve Size 0-4 mm</td>
			<td>Synthware Glass</td>
			<td>F490010</td>
			<td>Reaction vessel referred to as &#34;10 mL flask&#34;</td>
		</tr>
		<tr>
			<td>Grade 2 Qualitative Filter Paper, Standard, 42.5 mm circle</td>
			<td>Whatman</td>
			<td>1002-042</td>
			<td>Used for product isolation</td>
		</tr>
		<tr>
			<td>Methylene Chloride (HPLC)</td>
			<td>Fisher Scientific</td>
			<td>MFCD00000881</td>
			<td>Dried and deoxygenated prior to use</td>
		</tr>
		<tr>
			<td>Sn1 (tetratopic phosphine linker)</td>
			<td></td>
			<td></td>
			<td>Prepared according to literature procedure (ref. 15)</td>
		</tr>
		<tr>
			<td>SuperNuova+ Stirring Hotplate</td>
			<td>Thermo Fisher Scientific</td>
			<td>SP88850190</td>
			<td>Used to heat oil bath</td>
		</tr>
		<tr>
			<td>Tetrakis(triphenylphosphine) palladium(0), 99% (99.9+%-Pd)</td>
			<td>Strem Chemicals</td>
			<td>46-2150</td>
			<td>Commercial Pd(0) source</td>
		</tr>
		<tr>
			<td>Toluene (HPLC)</td>
			<td>Fisher Scientific</td>
			<td>MFCD00008512</td>
			<td>Dried and deoxygenated prior to use</td>
		</tr>
		<tr>
			<td>Triphenylphosphine, &#8805;95.0% (GC)</td>
			<td>Sigma-Aldrich</td>
			<td>93092</td>
			<td>Used as a modulator</td>
		</tr>
		<tr>
			<td>Weighing Paper</td>
			<td>Fisher Scientific</td>
			<td>09-898-12B</td>
			<td>Used for solid addition</td>
		</tr>
	</tbody>
</table>

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  

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

Our research aims to explore the synthesis and properties of metal organic frameworks constructed from low-valent metal ions. How to prepare these materials and their characteristics are not widely understood, but they likely hold tremendous opportunities for solid-state catalysis and other applications. Beyond our own work, there have been only a handful of reports on MOFs prepared from low-valent metal ions.Perhaps the most notable are those by Professor Joshua Figueroa here at UC San Diego who has described MOFs prepared from bulky isocyanate ligands and zero or monovalent metals. Because there are so few descriptions of low-valent MOFs, our protocol is providing a first baseline protocol that works for making these materials. That stated, our protocol is specifically designed for phosphine ligands and may not be readily translatable to different ligand systems such as isocyanates.Our findings open up opportunities to explore MOF materials with distinct characteristics from most MOFs described to date. For example, we have a new paper in press reporting a solid-state analog of Vaska's complex, which binds oxygen reversibly in a unique manner compared to the few other MOFs that do this. We are interested in opening up new opportunities to make low-valent MOFs, particularly as new solid-state catalysts.We want to answer questions such as:What other low-valent metal ions can be used to make LV MOFs, and can we mimic the reactivity of low-valent compounds using these solid-state materials?

The successful synthesis of Sn1-Pd produces a bright yellow, crystalline solid. The Pd(0) MOF products using analogous tetratopic phosphine linkers are also yellow. The most effective way to determine if the reaction was successful is to collect the PXRD pattern and evaluate the crystallinity of the sample. For example, Figure 2 shows the PXRD pattern of crystalline Sn1-Pd. The key features to verify that the sample is crystalline are that the reflection pea...

Watch this Scientific Journal Video about Experimental Approaches for the Synthesis of Low-Valent Metal-Organic Frameworks from Multitopic Phosphine Linkers at JoVE.com

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, ...