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

Published: May 12th, 2023



1Department of Chemistry and Biochemistry, University of California, San Diego

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, 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.

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 node....

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1. Setting up the Schlenk line

  1. 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.
  2. 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.......

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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.......

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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 .......

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


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NameCompanyCatalog NumberComments
2800 Ultrasonic Cleaner, 3/4 Gallon, 40 kHzBransonCPX2800HUsed for sonicating
Argon, Ultra High PurityMathesonG1901101Used as inert gas source
D8 ADVANCE Powder X-Ray DiffractometerBrukerUsed to collect PXRD patterns
Dewar FlaskChemglass Life SciencesCG159303Dewar used for liquid nitrogen
Flask, High Vacuum Valve, Capacity (mL) 10, Valve Size 0-4 mmSynthware GlassF490010Reaction vessel referred to as "10 mL flask"
Grade 2 Qualitative Filter Paper, Standard, 42.5 mm circleWhatman1002-042Used for product isolation
Methylene Chloride (HPLC)Fisher ScientificMFCD00000881Dried and deoxygenated prior to use
Sn1 (tetratopic phosphine linker)Prepared according to literature procedure (ref. 15)
SuperNuova+ Stirring HotplateThermo Fisher ScientificSP88850190Used to heat oil bath
Tetrakis(triphenylphosphine) palladium(0), 99% (99.9+%-Pd)Strem Chemicals46-2150Commercial Pd(0) source
Toluene (HPLC)Fisher ScientificMFCD00008512Dried and deoxygenated prior to use
Triphenylphosphine, ≥95.0% (GC)Sigma-Aldrich93092Used as a modulator
Weighing PaperFisher Scientific09-898-12BUsed for solid addition

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