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 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.
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. Please click here to view a larger version of this figure.
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.
1. Setting up the Schlenk line
2. Measuring out the solid reagents
3. Putting the reagents under an inert atmosphere
4. Adding solvent to the reagents under an inert atmosphere
5. Adding the linker to the palladium and phosphine mixture
6. Heating the reaction
7. Isolating the MOF product
8. Characterization of the MOF product by powder X-ray diffraction (PXRD)
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 peaks are relatively sharp and the baseline is flat. Peak broadening is often indicative of an amorphous material. To illustrate, Figure 3 shows the PXRD pattern of a sample of Sn1-Pd in which no triphenylphosphine modulator was used in the synthetic procedure. In this case, the diagnostic reflection signals were noticeably broad compared to the pristine sample for which 64 equiv. of triphenylphosphine modulator were used in the synthesis. This broadening effect is also observed upon decomposition in the presence of oxygen, especially after more than 72 h of exposure to ambient air conditions. Therefore, it is critical that the samples are stored under inert gas or under a dynamic vacuum to prevent the decomposition and degradation of the crystallinity. If the crystal structure of the desired LVMOF product or an analogous structure is known, a simulated PXRD pattern can be generated for comparison to the experimental powder pattern. If the two PXRD patterns match well, then the quality of the LVMOF sample should be satisfactory (Figure 2). It should be noted that even though the experimental LVMOF PXRD pattern may not perfectly match the simulated PXRD pattern for an analogous LVMOF; however, if the most prominent reflections at a low angle are conserved, then this provides strong evidence that the newly-synthesized LVMOF is isostructural to the LVMOF from which the simulated PXRD pattern was generated.
Figure 2: PXRD pattern of pristine Sn1-Pd. The PXRD pattern obtained for a pristine sample of Sn1-Pd is shown in blue. This sample was prepared using 64 equiv. of triphenylphosphine modulator to achieve a crystalline material. Below the experimental PXRD pattern in black is the simulated PXRD pattern of Si1-Pd obtained from the crystal structure15. Please click here to view a larger version of this figure.
Figure 3: PXRD pattern of amorphous Sn1-Pd. The PXRD pattern obtained for an amorphous sample of Sn1-Pd is shown. This sample was prepared without any triphenylphosphine modulator, which results in an amorphous or poorly crystalline material. Please click here to view a larger version of this figure.
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 when the effective concentration of the modulator relative to the linker is too low or there is no modulator present at all15. Relatedly, all the reagents should be fully dissolved and homogeneous before mixing such that the effective concentration of the reagents relative to one another matches the stoichiometry of the reaction. Another key step is ensuring that no oxygen is present within the reaction flask (or solvent) before mixing and heating the reagents. Not only is the Pd(0) precursor sensitive to oxygen, but the phosphine modulator and phosphine linker are both susceptible to oxidation to the corresponding phosphine oxide in the presence of oxygen, especially when heated. Any of these decomposition events will negatively impact the yield and/or crystallinity of the desired LVMOF product15. Similarly, the filtration to isolate the MOF product should be carried out quickly in order to limit the O2 exposure.
If all the steps are followed and a negative result is obtained (no precipitate is observed or the solid formed is amorphous), several parameters may be adjusted. Too few equivalents of the modulator may result in poorly crystalline material, but too many equivalents may prevent the formation of the MOF altogether. Thus, the equivalents of the modulator can be varied to improve the yield and crystallinity. Pairing metal precursors and solvent identities and/or ratios that produce a homogeneous solution prior to reaction with the linker is another important consideration. The effects of changing other parameters can be less intuitive, but the reaction temperature, concentration, reaction scale, and stoichiometry of the metal and linker can all influence the yield and crystallinity as well. This represents a limitation of the current method, as deviations in the identity of any of the reagents in order to target a new material often require the re-optimization of all the aforementioned parameters15. However, the empirical nature of their synthesis is a common feature among MOFs in general19.
Despite its limitations, this method is significant as there is currently no other known method to synthesize crystalline, three-dimensional LVMOFs using multitopic phosphine linkers15. Indeed, it is our aim that our laboratory and others can use this method as a starting point to guide the exploration of this rare class of materials and access LVMOFs with varied topology and diverse low-valent metal nodes. This will aid the MOF, catalysis, inorganic, and organometallic chemistry communities in the development of new materials with applications in heterogeneous catalysis.
The authors have nothing to disclose.
This work was supported by a grant from the National Science Foundation, Division of Chemistry, under Award No. CHE-2153240.
2800 Ultrasonic Cleaner, 3/4 Gallon, 40 kHz | Branson | CPX2800H | Used for sonicating |
Argon, Ultra High Purity | Matheson | G1901101 | Used as inert gas source |
D8 ADVANCE Powder X-Ray Diffractometer | Bruker | Used to collect PXRD patterns | |
Dewar Flask | Chemglass Life Sciences | CG159303 | Dewar used for liquid nitrogen |
Flask, High Vacuum Valve, Capacity (mL) 10, Valve Size 0-4 mm | Synthware Glass | F490010 | Reaction vessel referred to as "10 mL flask" |
Grade 2 Qualitative Filter Paper, Standard, 42.5 mm circle | Whatman | 1002-042 | Used for product isolation |
Methylene Chloride (HPLC) | Fisher Scientific | MFCD00000881 | Dried and deoxygenated prior to use |
Sn1 (tetratopic phosphine linker) | Prepared according to literature procedure (ref. 15) | ||
SuperNuova+ Stirring Hotplate | Thermo Fisher Scientific | SP88850190 | Used to heat oil bath |
Tetrakis(triphenylphosphine) palladium(0), 99% (99.9+%-Pd) | Strem Chemicals | 46-2150 | Commercial Pd(0) source |
Toluene (HPLC) | Fisher Scientific | MFCD00008512 | Dried and deoxygenated prior to use |
Triphenylphosphine, ≥95.0% (GC) | Sigma-Aldrich | 93092 | Used as a modulator |
Weighing Paper | Fisher Scientific | 09-898-12B | Used for solid addition |