[(DPEPhos)(bcp)Cu]PF₆: A General and Broadly Applicable Copper-Based Photoredox Catalyst

Photoredox catalysis has emerged over the past decade as one of the most efficient methods to promote radical processes. It is indeed especially easy to implement, user-friendly, highly sustainable, and also it does not rely on the use of toxic or hazardous reagents that are usually used in radical chemistry. The main principle of photoredox catalysis is very simple, since, as you can tell by the name, it is based on the use of light-responsive compound called the photoredox catalyst that can be easily activated by irradiation with visible light in order to initiate the catalytic cycle.

Among all photoredox catalysts that have been reported and utilized to date, most of them are actually based on ruthenium and iridium complexes which pose a rather strong limitation due to the high prices. As a consequence, the development of alternative photoredox catalysts that are based on cheaper and non-noble metals such as copper is therefore highly important not only for the development of less expensive systems but also for new applications. This was a main motivation when we started the synthesis and the development of this new copper-based photoredox catalyst whose synthesis is presented in this video together with synthetic applications.

In addition to the role for chemical syntheis, copper-based catalysts could also play a role in photocatalysis, for instance, in a water reduction or carbon dioxide reduction which is very important for clean energy production and storage and also for the pollution. The procedures will be demonstrated by two PhD students from my lab, Ms.Hajar Baguia and Mr.Jerome Beaudelot. To begin, add 10 millimoles of tetrakisacetonitrile copper(I)hexafluorophosphate and 10 millimoles of DPEPhos to a two liter round bottom flask equipped with a magnetic stir bar.

Fit the round bottom flask with a three neck vacuum adapter connected to a vacuum line and a balloon filled with argon. Turn the adapter to evacuate the flask for one minute and then turn it again to backfill the flask with argon. Repeat this cycle two times.

Replace the three neck vacuum adapter by a rubber septum with a balloon of argon. Through a needle, add 800 milliliters of distilled dichloromethane and wrap the flask with aluminum foil to block light. Place the flask on a magnetic stirrer and stir the reaction mixture for two hours at approximately 23 to 25 degrees Celsius under the argon atmosphere.

After that, add 10 millimoles of bcp to a 500 milliliter round bottom flask equipped with a magnetic stir bar. Fit the round bottom flask with a three neck vacuum adapter connected to a vacuum line and a balloon filled with argon. Evacuate the flask under vacuum and backfill with argon three times.

Replace the three neck vacuum adapter by a rubber septum with a balloon of argon. Through a needle, add 200 milliliters of distilled dichloromethane and gently stir the suspension until complete dissolution of the bcp. Equip a cannula to the two rubber septa on the two liter and 500 milliliter flasks with the end of the canula submerged in the solution in the 500 milliliter flask.

Remove the balloon of argon from the two liter flask to transfer the whole content of the smaller flask to the bigger flask. Then, put the balloon of argon back on the two liter flask. Stir for an additional hour in the dark at approximately 23 to 25 degrees Celsius under an argon atmosphere.

Next, place a pad of celite in a fritted funnel on top of a flask and filter the mixture. Wash with approximately 100 milliliters of distilled dichloromethane. Then, place the flask on a rotary evaporator to concentrate the filtrate to a volume between 50 to 100 milliliters under reduced pressure.

Use an addition funnel to add the concentrate drop-wise to one liter of diethyl ether on a magnetic stirrer with vigorous stirring to induce precipitation of the desired complex. Now, collect the precipitate by filtration through a fritted funnel with a pore size of three micrometers on top of a Buchner flask and wash the precipitate with approximately 100 milliliters of diethyl ether. Dry the bright yellow precipitate under vacuum at room temperature for five hours to recover 10.1 grams equaling 91%yield of the copper complex.

First, in an oven-dried 10 milliliter vial, add 0.05 millimoles of the synthesized catalyst, 0.25 millimoles of dicyclohexyl isobutylamine, one millimole of potassium carbonate, and 0.5 millimoles of 4-iodobenzonitrile. Place a magnetic stir bar. Seal the vial with rubber septum.

Connect the vial to a rubber line connecting to vacuum, evacuate the vial under vacuum for 30 seconds, and backfill with argon three times. Then, through the septum, add five milliliters of freshly distilled and degassed acetonitrile and 890 microliters of n-methylpyrrole to the vial. Replace the rubber septum by a screw cap.

Place the vial in a photoreactor under 420-nanometer-wavelength irradiation and stir the reaction mixture for three days at approximately 23 to 25 degrees Celsius. Blue LED strips or a blue LED lamp at 440 nanometers and 34 watts can be used instead. After three days, filter the reaction mixture through a pad of celite.

Wash with approximately five milliliters of diethyl ether and concentrate the filtrate under reduced pressure on a rotary evaporator. Next, dissolve the concentrate with the minimum amount of solvent and place it on top of a column chromatography to start purifying the crude residue over silica gel. After that, place the content of all tubes containing the pure product in a flask and concentrate on a rotary evaporator.

Place the flask on a vacuum line to dry the pure compound at room temperature for three hours in order to recover 65 milligrams equaling 72%healed of the desired C2 arylated pyrrole. In this protocol, the synthesis of DPEPhos bcp copper(I)hexafluorophosphate is particularly convenient and can be easily performed on a multigram scale. The proton and carbon-13 NMR spectra indicate formation of the pure complex.

The UV and visible light absorption spectrum displays two main absorption bands with two maxima at 385 nanometers and 485 nanometers. The emission spectrum obtained by excitation at 445 nanometers displays maximum at 535 nanometers. As for the characterization of the C2 arylated pyrrole, the proton and carbon-13 NMR spectra indicate formation of the pure compound.

The photocatalyst can be used for the synthesis of other molecules as exemplified with the total synthesis of the anti-cancer agent, luotonin A, which demonstrated a good purity shown by its proton and carbon-13 NMR spectra. One of the most important things to remember is that molecular oxygen can be activated by the photocatalyst and induce side reaction. As a consequence, all reactions should therefore be performed under argon and with strict exclusion of oxygen.

Copper-based photoredox catalyst can be utilized in other areas of photocatalysis and other copper complexes are being developed and studied right now. The photoreactor emits strong light and care should therefore be taken to turn it on only after closing the door. And to avoid any further exposure, UV-filtering goggles should be worn in addition.