synthesis and isolation of the mof product

Synthesis of Low-Valent MOFs from Multitopic Phosphine Linkers

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.

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?

Watch this Scientific Journal Video about Synthesis and Isolation of the MOF Product at JoVE.com

Synthesis and Isolation of the MOF Product  

To begin, cool the cold trap of the Schlenk line by placing a Dewar flask filled with liquid nitrogen around it. Use a towel to cover the top of the Dewar flask. This will slow down the evaporation of the liquid nitrogen during the experiment, open the bubbler to a light flow of inert gas, either nitrogen or argon.Roll a piece of weighing paper into a cone to use as a solid addition funnel and place it in the tap opening of the 10-milliliter flask. Ensure the bottom of the cone is inserted far enough that it extends past the hose attachment. Weigh the tetrakis triphenylphosphine palladium-0 into the 10-milliliter flask.Repeat this step for triphenylphosphine. After disposing the weighing paper cone, screw the polytetrafluoroethylene, or PTFE tap, onto the 10-milliliter flask. Similarly, weigh the tetratopic phosphine linker tin-1 into a separate 10-milliliter flask.To place the reagents under an inert atmosphere, connect hose from the Schlenk line to each of the 10-milliliter flasks. Open the PTFE tap just enough that the vessel is open to the hose. Open both the flasks to the vacuum and wait for five minutes.Close the tap on each flask and then close each hose to the vacuum. Switch the hoses to the inert gas and then slowly open the tap on each flask to backfill it with inert gas. Repeat these steps two more times for a total of three cycles.Remove the PTFE tap and replace it with a septum for each flask under a positive pressure of inert gas sufficient to prevent air from entering the flask. Use a syringe and needle to transfer 1.5 milliliters of dry and deoxygenated toluene into the flask containing the tetrakis triphenylphosphine palladium-0 and triphenylphosphine. Then, transfer 1.5 milliliters of dry and deoxygenated methylene chloride to the flask.Swirl the flask until all the solids have dissolved. Next, transfer three milliliters of dry and deoxygenated methylene chloride into the flask containing the tetratopic phosphine linker tin-1 and swirl the flask until all the solid has dissolved. Transfer the entire tin-1 linker solution into the flask containing the tetrakis triphenylphosphine palladium-0 and triphenylphosphine and swirl the solution for 30 seconds to thoroughly mix it.Replace the septum with the PTFE tap under a positive pressure of inert gas and seal the flask. Sonicate the reaction solution at 40 kilohertz for an additional 30 seconds. Place the sealed flask into a preheated oil bath at 60 degrees Celsius and leave it for 24 hours without agitating it.To isolate the MOF product, remove the flask from the oil bath and allow it to cool to room temperature. Set up a vacuum filtration apparatus using a small Buchner funnel and filter paper. Remove the PTFE tap from the flask and then use a pipette to transfer the total volume of the suspension to the filter.Rinse the solid with two milliliters of deoxygenated 3:1 methylene chloride/toluene solution. Repeat the step two more times and allow the solid to dry on the filter paper for three minutes. Scrape the solid into a pre-weighed vial and then weigh the vial to obtain the yield of the low-valent MOF.The synthesis of three-dimensional low valent MOFs using tetratopic phosphine ligands as linkers, palladium-0 and platinum-0 as nodes, and triphenylphosphine as a modulator is shown here. The central atom E can be silicon or tin.

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Research

JoVE Journal

Chemistry

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.

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, ...

Characterization of the MOF Product by Powder X-Ray Diffraction (PXRD)

Transfer approximately 20 to 30 milligrams of the crystalline solid MOF product to a silicon PXRD sample holder and place it in the diffractometer. Close the door of the instrument and collect the PXRD pattern from 4 to 40 2-theta. Compare the data to the simulated powder pattern of silicon palladium low-valent MOF.The PXRD pattern of the pristine tin-palladium low-valent MOF is shown in this figure. Here, blue is the experimental PXRD pattern, and black is the simulated PXRD pattern of the silicon palladium low-valent MOF obtained from its crystalled structure. The PXRD pattern obtained for an amorphous sample of the tin-palladium low-valent MOF is shown here.The sample was prepared without any triphenylphosphine modulator, resulting in an amorphous or poorly crystalline material.