Catalytic carbenes: UV and the metal connection

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  • Published: Oct 1, 2013
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: Catalytic carbenes: UV and the metal connection

Catalytic complexity

a team of scientists led by University of Wisconsin-Madison chemist John Berry have developed a technique that lets them freeze the action long enough during a critical step so that they can see the fine details of the mechanism. Image c/o Berry

Green chemistry is lean chemistry, synthesising a high-yielding, impurity-low product from the starting materials. Often catalysts, such as those based on rhodium and other heavy metals are the most powerful way to drive a chemical reaction in this regard but they are often rare and expensive. Improving the efficacy of metal catalysts, and their recyclability, can offset these issues, but studying the mechanisms by which a dirhodium metal complex catalyses a chemical reaction is often hindered by the fact that that they work so quickly, turning over hundreds of times per second in the reaction. Now, a team of scientists led by University of Wisconsin-Madison chemist John Berry has developed a technique that lets them freeze the action long enough during a critical step so that they can see the fine details of the mechanism.

Most synthetic reactions proceed through several reaction steps from starting material to end product each with the own intermediates, side reactions, transition states, intramolecular rearrangements, intermolecular exchanges and electron transfers. The problem for the mechanistically inclined chemist hoping to understand the tortuous route taken is that intermediates rarely last for more than a fleeting moment, especially if the reaction is kinetically efficient being driven by a catalyst. The team has now chilled their reaction to the relatively warm - for such studies - 0 Celsius, and describes their findings online in Science Express with regard to the isolation and characterization of an intermediate that lasts for many hours at this temperature.

Dirhodium di

"We've provided the first solid fundamental data on these compounds - dirhodium carbene complexes," explains Berry, who led the effort to synthesize the stable version of a normally short-lived molecule. "People have thought about it for forty years, but this is the first time that we can actually see it and say this is definitely what’s going on." He told SpectroscopyNOW that, "Carbenes are transiently stable divalent carbon intermediates that can be stabilized by bonding to metals. The dirhodium complex does not provide a lot of stabilization, which is why the isolation of this compound has been so difficult. The most interesting aspect of the reactivity of this compound, highly relevant to catalysis, is the insertion of the carbene species into carbon-hydrogen bonds."

Berry and graduate student Katherine Kornecki used computational models to predict how the intermediate molecules might be trapped. From those predictions, they were able to identify a suitable dirhodium complex and starting material with the properties needed to stabilize that target intermediate compound long enough to study it in detail. Formation of the reactive intermediate is visible as the green starting material, provided by Huw Davies of Emory University, changes to the colour of the sea over time. However, they used ultraviolet-visible spectroscopy for a more definitive assessment of the changes as the intermediate forms. Davies' starting material was well chosen as being relatively easy to characterise using vibrational spectroscopy and nuclear magnetic resonance (NMR) spectroscopy. Jochen Autschbach from the University of Buffalo, New York, used density function theory (DFT) to provide the spectroscopists with a predicted pattern of NMR peaks for the compound, while Kyle Lancaster of Cornell University carried out X-ray absorption spectroscopy on it.

Wonderful challenge

"This paper is a wonderful example of how big challenges in chemistry can be solved by employing a multidisciplinary, collaborative approach," says organic chemist Davies. In addition to providing evidence of an intermediate previously known only on paper, the finding opens new avenues for the field of catalysis. "Now that we can make the intermediate, we can further explore its reactivity. We can try reactions with substrates that nobody has ever thought of before," Berry says.

"We hope someday to be able to obtain an even more stable version of the carbene complex that we could characterize by crystallographic methods," Berry told us. "There are also other intermediates in this reaction that we do not know anything about yet. We might be able to spot them by going to even lower reaction temperatures. Ultimately, we would like to have an understanding of how these reactions (mainly, the C-H functionalization reactions) occur that is supported by direct experimental data with atomic-scale resolution."

Related Links

Science 2013, online: "Direct Spectroscopic Characterization of a Transitory Dirhodium Donor-Acceptor Carbene Complex"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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