12 Apr 2019

Reactive Research and Modifying Metals

Publishing as first author is a notable feat. Recently, Okinawa Institute of Science and Technology Graduate University (OIST) PhD student Sébastien Lapointe published a paper in Organometallics as first author, making it the first in his PhD. Lapointe is in the Coordination Chemistry and Catalysis Unit led by Prof. Julia Khusnutdinova.

The scientists modified pincer ligands – a molecule that’s a chelating agent – which binds with a metal center. Lapointe wanted to see if he could stabilize reactive complexes with his ligand, and his results from the experiments showed that he could.

Nickel complexes are often catalysts that are used for polymerization and making fine chemicals in industry, but the intermediates in catalysis are poorly understood. This is because one of the things that make nickel complexes so active, unpaired electrons, also make it impossible for chemists to use their most important spectroscopic technique (NMR) to follow along with the reactions and see short-lived intermediates.

“The main factor that determines the stability of our complexes are the presence of the methyl groups on the arm. They have three roles, blocking unwanted reactions on the ligand, increasing the size of the ligand and donating electron density to the metal.” Lapointe said.  

The experiment’s results can be visually seen. Follow the photos below to get a taste of the research.

PhD student Sébastien Lapointe in the Coordination Chemistry and Catalysis Unit prepares his experiment. The experiments are done in the glovebox, in order to protect the chemicals from air.

The two powders on the left are the starting Nickel (II) molecules, while the two on the right are the final Nickel(I) products after the experiment. The difference in color for the starting Nickel (II) molecules simply comes from the size of the ligands. The red complex as a ligand which is larger in size than the yellow complex.

Pictured is the solution with the starting material Sebastien Lapointe did his reaction on. This is before adding the reductant to the reaction.

“To check how our stabilizing system performs, we used a reductant, which is a molecule that can give an electron to our complex,” Sébastien Lapointe said. “Many ligand systems do not survive this and decompose when you try to isolate the reactive complex.”

Seconds after adding the reductant, the once yellow solution turns to a murky brown. This color change signifies that there is a reaction occurring between the chemicals.

“The change in color is a very interesting way to observe how the molecule changes via a simple reduction reaction,” Sébastien Lapointe said. “The wide variety of colors that metal complexes can have is one of the reasons I love chemistry so much.”

Pictured is another solution containing a slightly larger starting molecule before applying the same reductant.

Moments after applying the reductant, the solution turns to a deep red color, which needs to be purified by filtration. The pure final product is obtained without any further manipulations.

“The complex was easily reduced without any complications or undesired reactions on the ligand itself. We can now comfortably study how these normally very reactive complexes behave with things like oxygen and other small molecules.”

PhD student Sébastien Lapointe holds out the solution after the experiment. He kept the solutions in a fridge for about a week. Over time the solution forms crystals that can be further analyzed, toward the top you can see crystallization. The bottom of the bottle, crystals are everywhere.  

Holding the crystals in his hand, one can faintly see a molecule in the background. At a microscopic level, crystals are a repetition of the molecule in all directions, and we can get the exact structure of that molecule by using an X-ray diffractometer. This technique is one of the most useful technique in chemistry, and we are grateful that we can use this technique every day.

The reduction of the stable Nickel (II) complex (left molecule) using cobaltocene as a reductant (middle molecule) to afford the reactive Nickel(I) complex (right molecule). Shown in the image are 2D and 3D representations of the molecules studied by Sebastien Lapointe. After the reaction, the bromine atom adopts a bent position.

 

By Erica Overfelt


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