Imine Metathesis by Silica-Supported Catalysts Using the Methodology of Surface Organometallic Chemistry

My protocol is significant because it allows you to make very precise catalyst, very selective, which can do only one catalytic reaction. So the protocol is derived from a concept of well-defined, single-site catalyst, and these single-site catalysts are doing fantastic reactions. This technique is challenging and time-consuming but remains unparalleled in preparing surface or surface complexes that could be used as catalyst.

And the surface organometallic chemistry allows deeper understanding of the catalysis and the preparation of new, single-site, well-defined catalyst. Catalysis represents 90%of the processes in chemical industry and petrochemical industry. So, catalysis is extremely important for energy, environment, fine chemicals.

And of course, new catalyst are very active in this field, can open a lot of doors in chemistry and petrochemistry. Because in KAUST we have the most sophisticated equipment you can find in the world, we are the most equipped catalysis laboratory. And in particular, you have very strong infrared spectroscopy, you have very strong NMR, you have very strong technique that allows you to see catalyst at the atomic level.

We can see atoms on a surface, and we can say which kind of atom you see on the surface of an electron microscope. So visualization is necessary to understand the complexity of the tools that are required to fully characterize catalyst. First, connect a high-vacuum vessel to a high-vacuum line.

Check whether the pressure increases by alternating the dynamic and the static vacuum. In case of a leak, scan the connection with the high-frequency generator to localize leaks and holes. Cover five grams of fumed silica in a 100-milliliter beaker with enough deionized water until it becomes a compact gel.

Then, cover the beaker with aluminum foil, and heat it in the oven at 200 degrees Celsius overnight. On the following day, grind the cooled silica, and transfer one gram to a glass reactor. Close the reactor with a cap, and seal it with grease.

Connect the glass reactor to the port of the high-vacuum vessel, gradually heat it to 700 degrees Celsius, and leave it overnight. Next, prepare a disc pellet from the dehydroxylated silica for an FTIR measurement in the glove box. Once the measurement is complete, observe the isolated silanol signal in the FTIR spectrum.

Prepare a sodium mirror-coated solvent bomb equipped with a Teflon stopcock. Transfer approximately 25 to 50 milliliters of pentane to the solvent bomb. Connect the solvent bomb to the high-vacuum line.

Freeze the solvent using a liquid nitrogen-filled Dewar. Evacuate until the solvent finishes degassing. Then, distill the solvent to another solvent bomb.

Dry a double-Schlenk flask by evacuating it with the vacuum line and heating it with a heat gun. After transferring the dry Schlenk flask to the glove box, add 089 milliliters of the precursor complex to one compartment. Add one gram of the dehydroxylated silica and a stir bar to the other compartment, and seal them with grease.

Close the two necks of the double-Schlenk flask with a cap. Using a T-joint, connect the high-vacuum line to the solvent Schlenk flask on one side and to the double-Schlenk flask on the other side. Ensure that all connections are secured by metallic clips, and evacuate the line and the double-Schlenk flask until reaching a stable high vacuum of 10 to minus five millibar.

Transfer the solvent from the solvent Schlenk flask to the compartment of the double-Schlenk flask containing the metal precursor by distillation. Once the glassware assembly is under static vacuum, use a liquid nitrogen Dewar to cool the compartment, to condense the solvent, and to dissolve the precursor. Next, transfer the solution to the silica compartment by gravitation.

Stir for one to three hours to complete grafting. Then, filter the material by transferring the solvent to the solvent compartment, and distill the solvent to the solid compartment. Remove the waste solvent by distillation using an interceptor.

Prepare a disc pellet of the grafted material for an FTIR measurement in the glove box. Add one gram of the grafted material to a Schlenk flask, and connect it to the high-vacuum line. Start heating it gradually to 200 degrees Celsius, and leave it for four hours.

After allowing the grafted material to cool under vacuum, prepare a disc pellet from 50 to 70 milligrams of the prepared material for an FTIR measurement in the glove box. In an ampule tube, add 12.47 milligrams of the catalyst. Add two imine substrates, 0.5 milliliters of toluene, and a stir bar.

Connect the ampule tube to the high-vacuum line, and freeze it using liquid nitrogen. Next, seal the ampule tube with a flame torch. Place the sealed ampule tube in an oil or sand bath, and heat it to 80 degrees Celsius for up to six hours.

After the reaction is complete, freeze the cooled ampule tube, and cut the top using a glass cutter. Filter the solution into a GC vial, and dilute with one milliliter of toluene for GC-MS analysis. After grafting the complex on dehydroxylated silica, the characteristic FTIR peak for isolated silanol almost completely disappeared, and new peaks appeared in the alkyl region.

After heat treatment of the prepared material, its infrared spectrum showed a new peak for the imido fragment. The carbon cross-polarized magic angle spinning spectrum of the grafted material revealed two overlapping peaks at 37 and 46 ppm, attributed to the nonequivalent methyl group in methylamine. A low-intensity peak at 81 ppm was determined to be correlated with the methylene peak at 2.7 ppm in the heteronuclear correlation spectrum.

The nitrogen NMR spectrum of the grafted material displayed two peaks. The intense signal downfield was assigned to the nitrogen nuclei of the metallaaziridine and imido functionalities. The weak upfield-shifted peak was attributed to a dimethylamine moiety.

For the imido metal fragment in the catalyst that was generated after heat treatment, one broad peak appeared in the proton NMR spectrum, representing the methylene groups. The carbon cross-polarized magic angle spinning spectrum displayed two peaks at 37 and 48 ppm. The imine metathesis with three imine compounds and mass spectra of the resulting products are shown here.

Always check the high-vacuum line, and make sure all the glassware are sealed properly in each step to prevent any air leaks. This technique could potentially bring a better general understanding of the catalysis by the preparation of new, well-defined, single-site, silica-supported catalyst.