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

Summary

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.

Abstract

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.

Introduction

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 structure^6,8,9,10. 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 coupling^17,18, 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.

Protocol

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 should be a closed system under a dynamic vacuum.
3. Wait at least 5 min while the atmosphere of the Schlenk line is evacuated.
   NOTE: Some Schlenk lines may be fitted with a barometer to determine the lowest pressure the apparatus will reach under a dynamic vacuum. If that pressure has been reached before 5 min have passed, proceed to the next step.
4. 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 and slow the evaporation of the liquid nitrogen during the experiment.
   CAUTION: Contact with liquid nitrogen can cause severe damage to the skin and eyes and should only be handled by those trained to use it safely. Wear skin and eye protection.
   NOTE: Often, it is easier and safer to first place the empty Dewar flask around the cold trap and then use a second Dewar to fill the trap Dewar flask with liquid nitrogen.
5. Open the bubbler to a light flow (approximately 3 bubbles/s) of inert gas (N_2(g) or Ar_(g)).

2. Measuring out the solid reagents

1. Adding tetrakis(triphenylphosphine)palladium(0) and triphenylphosphine modulator into the reaction flask.
   1. 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 mL flask. Ensure the bottom of the cone is inserted far enough that it extends past the hose attachment.
      NOTE: Using an empty NMR tube or a similarly small tubular object to roll the weighing paper over is helpful to attain the small diameter required to fit in the tap opening.
   2. Weigh by differences the tetrakis(triphenylphosphine)palladium(0) (0.084 g, 0.073 mmol, 1 equiv.) into the 10 mL flask.
      CAUTION:  Tetrakis(triphenylphosphine)palladium(0) is harmful to the body, especially if swallowed, and may ignite if finely dispersed in the air. Avoid dust formation and all forms of contact, and wear personal protective equipment.
      NOTE: The flask and weighing paper cone can be gently tapped to ensure all the solid is transferred into the bottom of the flask.
   3. Repeat step 2.1.2 with triphenylphosphine (1.23 g, 4.67 mmol, 64 equiv.).
      CAUTION: Triphenylphosphine is harmful to the body and central nervous system. Avoid all forms of contact, and wear personal protective equipment, including chemical-resistant gloves.
   4. Dispose of the weighing paper cone, and screw the poly(tetrafluoroethylene) (PTFE) tap onto the 10 mL flask.
2. Measure the tetratopic phosphine linker into a separate 10 mL flask.
   1. Repeat step 2.1.1 with a second 10 mL flask.
   2. Using the second 10 mL flask, repeat step 2.1.2 with the tetratopic phosphine linker Sn1 (0.085 g, 0.073 mmol, 1 equiv.).
      CAUTION: The hazardous properties of Sn1 are unknown. As it is an Sn(IV) compound and a tertiary phosphine, assume it is acutely toxic, and avoid all forms of contact. Wear personal protective equipment, including chemical-resistant gloves.
   3. Repeat step 2.1.4 with the second 10 mL flask.

3. Putting the reagents under an inert atmosphere

1. Connect a hose (black rubber vacuum tubing, 3/16 in inner diameter x 3/16 in wall) from the Schlenk line to each of the 10 mL flasks.
2. Open the PTFE tap just enough that the vessel is open to the hose.
   NOTE: If the tap is too wide open, the solids may be pulled into the hose during evacuation.
3. Open both the 10 mL flasks to the vacuum. Wait for 5 min.
4. Close the tap on each 10 mL flask, and then close each hose to the vacuum. Switch the hoses to the inert gas, and then slowly open the tap on each 10 mL flask to backfill with inert gas.
   NOTE: When switching from the vacuum to the inert gas, ensure that the bubble flow of the inert gas is high enough to prevent oil from being pulled into the Schlenk line but low enough not to disturb the solids in the flask. Never open the system to vaccum and inert gas at the same time.
5. Repeat steps 3.3-3.4 two more times for a total of three cycles.

4. Adding solvent to the reagents under an inert atmosphere

1. Under a positive pressure of inert gas sufficient to prevent air from entering the flask, remove the PTFE tap, and replace it with a septum for each 10 mL flask.
2. Add toluene and methylene chloride to the palladium and phosphine mixture.
   1. Use a syringe and needle to transfer 1.5 mL of dry and deoxygenated toluene into the flask containing the tetrakis(triphenylphosphine)palladium(0) and triphenylphosphine.
      CAUTION: Toluene is both toxic and flammable. Avoid all forms of contact, keep away from heat sources, work in the fume hood, and wear personal protective equipment.
      NOTE: Solvents can be dried by passing them through an activated aluminum column under inert gas and deoxygenated by sparging them with inert gas for 30 min. Be sure to purge the syringe and needle with inert gas three times before drawing the solution.
   2. Repeat step 4.2.1 with 1.5 mL of dry and deoxygenated methylene chloride.
      CAUTION: Methylene chloride is toxic and carcinogenic. Avoid all forms of contact, work in a fume hood, and wear personal protective equipment.
   3. Swirl the flask until all the solids have dissolved (approximately 30 s).
3. Add methylene chloride to the tetratopic phosphine linker.
   1. Use a syringe and needle to transfer 3.0 mL of dry and deoxygenated toluene into the flask containing the tetratopic phosphine linker Sn1.
   2. Swirl the flask until all the solid has dissolved (approximately 30 s).

5. Adding the linker to the palladium and phosphine mixture

1. Use a syringe and needle to transfer the entire Sn1 linker solution into the flask containing the tetrakis(triphenylphosphine)palladium(0) and triphenylphosphine.
2. Swirl the solution for 30 s to thoroughly mix it, then replace the septum with the PTFE tap under a positive pressure of inert gas sufficient to prevent air from entering the flask, and seal the flask.
3. Sonicate (40 kHz) the reaction solution for an additional 30 s.

6. Heating the reaction

1. Place the sealed flask into a pre-heated oil bath at 60 °C, and leave it for 24 h without agitating it.

7. Isolating the MOF product

1. Remove the flask from the oil bath, and allow it to cool to room temperature.
   CAUTION: When handling hot glassware and/or surfaces, be sure to wear heat-resistant gloves.
2. Set up a vacuum filtration apparatus using a small Buchner funnel and filter paper (8 µm pore size).
3. Remove the PTFE tap from the flask, and then use a pipette to transfer the total volume of the suspension to the filter.
   NOTE: A light flow of inert gas over the top of the filter can help to avoid the decomposition of the oxygen-sensitive MOF product.
4. Rinse the solid with 2 mL of deoxygenated 3:1 methylene chloride/toluene solution. Repeat this step two more times, and allow the solid to dry on the filter paper for 3 min.
5. Scrape the solid into a pre-weighed vial, and then weigh the vial to obtain the yield of Sn1-Pd.
   ​NOTE: Store the LVMOF material under inert gas or a dynamic vacuum in order to avoid decomposition in the presence of oxygen in the air.

8. Characterization of the MOF product by powder X-ray diffraction (PXRD)

1. Transfer approximately 20-30 mg of the crystalline solid to a silicon PXRD sample holder.
   NOTE: While Sn1-Pd is sufficiently stable in air for the characterization by PXRD, more air-sensitive LVMOF materials can be transferred to an inert atmosphere glovebox and loaded into a sealable capillary PXRD sample holder.
2. Place the sample holder in a diffractometer, and close the door to the instrument.
3. Collect the PXRD pattern from 4 to 40 2θ (scan speed of 0.5 s/step, step size of 0.0204° 2θ), and compare the data to the simulated powder pattern of Sn1-Pd.

Results

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...

Discussion

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 ...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
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