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 MOSs prepared from low-valent metal ions.

Perhaps the most notable are those by Professor Joshua Figueroa here at UC San Diego, who has described MOSs prepared from bulky isocyanate ligands and zero or monovalent metals. Because there are so few descriptions of low-valent MOSs, 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 MOSs materials with distinct characteristics from most MOSs 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 MOSs that do this. We are interested in opening up new opportunities to make low-valent MOSs, 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 MOSs, and can we mimic the reactivity of low-valent compounds using these solid state materials? To begin, cool the cold trap of 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 the 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 a hose from 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. Transfer approximately 20 to 30 milligrams of the crystalline solid MOF product to a silicon PXRD sample holder and place it in a diffractometer. Close the door of the instrument, and collect the PXRD pattern from 4 to 42 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 crystallined material.