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May 2013

When Modifying Hydrocarbons, the Road Taken Makes All the Difference

Determining mechanisms of partial oxidation

Paul Giokas

This tungsten complex can take multiple routes to its alcohol form. The molecules above and below the arrows explain the conditions that cause the complex to proceed along each path. Copyright 2013 American Chemical Society

Researchers at the Center for Catalytic Hydrocarbon Functionalization, or CCHF, found three pathways to add oxygen atoms, known as partial oxidation, to hydrocarbonyl ligands in tungsten complexes to ultimately create alcohol, which is a valuable chemical intermediate and fuel. A ligand is a group or molecule that binds to a metal center in a complex. In collaborative work between the Gunnoe (University of Virginia) and Cundari (University of North Texas) groups, Jiajun Mei and coworkers have been investigating the oxidization of hydrocarbons, which is an extremely important process in the fossil fuel industry. The burning of fossil fuels is a common example of hydrocarbon oxidation; however, selective partial oxidation without radical or highly reactive intermediates is a difficult problem.

One of the most difficult aspects of determining the mechanism or route that a chemical reaction takes is determining the transition states—the high-energy hills it must climb along the way. Transition states are fleeting, and because these intermediate structures exist for short periods of time, they can be extremely difficult—many times nearly impossible—to characterize experimentally.

Mei and his team focused on inserting an oxygen atom into the hydrocarbonyl ligand of the tungsten complex and forming a carbon-oxygen (C-O) bond. This C-O bond is ultimately split off and converted to an alcohol (C-OH). Through a battery of experimental reactions under variable conditions, in concert with mathematical calculations to determine bond lengths and the geometries of suspected intermediates at key points in the reaction, the CCHF chemists proposed three distinct routes. In addition to a detailed understanding of reaction mechanisms, by varying the oxidizing agent chemical that provides the oxygen and reaction conditions, the researchers determined which conditions result in the fastest reactions. The information obtained here can help guide the design of metal complexes for hydrocarbon functionalization using other systems and applications.

More Information

Mei J, KM Carsch, CR Freitag, TB Gunnoe, and TRJ Cundari. 2013. “Variable Pathways for Oxygen Atom Insertion into Metal-Carbon Bonds: The Case of Cp*W(O)2(CH2SiMe3).” Journal of the American Chemical Society 135:424-435. DOI: 10.1021/ja309755g 

Acknowledgments

This work was solely supported as part of the Center for Catalytic Hydrocarbon Functionalization, an Energy Frontier Research Center funded by the Department of Energy, Office of Science, Office of Basic Energy Sciences.

About the author(s):

  • Paul Giokas is a member of the Center for Solar Fuels at UNC Chapel Hill. He is a physical chemistry graduate student studying chromophore binding at semiconductor interfaces, as well as ultrafast dynamics with nonlinear spectroscopy in the Moran Group.

Hydrocarbons Choose Between Three Clear Paths in Their Journey to Change

Insights for new catalyst designs that could drive fuel manufacturing

This tungsten complex can take multiple routes to its alcohol form.The molecules above and below the arrows explain the conditions that cause the complex to proceed along each path. Copyright 2013 American Chemical Society

Adding an oxygen atom at just the right spot to change a simple hydrogen-and-carbon-packed molecule into an alcohol that can act as a fuel is a difficult proposition. The steps in between the start and finish can waste time and resources. Directing the reaction to the fastest, most efficient path could lead to new fuel sources and less wasteful manufacturing. Scientists discovered three routes that add an oxygen atom in the right spot:

  • A route that relies on traditional base or Brønsted acid.
  • A route that uses an intermediate or transition state promoted by a traditional base.
  • A rare reaction, known as the organometallic Baeyer-Villiger path.

These routes avoid radical molecules that can cause problems elsewhere. Understanding these reactions aids scientists in designing catalysts to efficiently turn hydrocarbons into fuels and chemicals. The scientists are at the Center for Catalytic Hydrocarbon Functionalization, led by the University of Virginia.

More Information

Mei J, KM Carsch, CR Freitag, TB Gunnoe, and TRJ Cundari. 2013. “Variable Pathways for Oxygen Atom Insertion into Metal-Carbon Bonds: The Case of Cp*W(O)2(CH2SiMe3).” Journal of the American Chemical Society 135:424-435. DOI: 10.1021/ja309755g 

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.