Activating Alkanes

Researchers in LBL's Chemical Sciences Division (CSD) took an important step towards harnessing the vast potential of one of nature's most plentiful materials when they determined how metals can chemically interact with the large class of naturally occurring hydrocarbons known as alkanes.

Alkanes are compounds of carbon and hydrogen atoms held together by single bonds. The simplest and most abundant is methane, the primary constituent of natural gas. Chemists have long coveted the use of alkanes as feedstock for clean-burning fuels and a host of petrochemicals, including plastics, solvents, synthetic fibers, and pharmaceutical drugs. The problem has been that the bonds between an alkane's hydrogen and carbon atoms are so strong as to render alkanes generally unreactive.

LBL chemists have shown that it is possible to insert the metal centers of certain metal complexes into the carbon-hydrogen (C-H) bonds of alkanes to form weaker carbon-metal bonds that are much more chemically useful. This illustration shows the energy barrier associated with the insertion of an iridium metal center (the blue iridium atom) into the C-H bond (red carbons and white hydrogens) of an "alkane complex." (The C-H insertion proceeds from left to right.) In the transition state of the reaction, one can see the C-H bond stretching as the iridium-
carbon and iridium-hydrogen bonds start to form.

CSD researchers, led by chemist and UC Berkeley professor Robert Bergman, winner of an E.O. Lawrence Award in 1993 for his alkane studies, have been working with organometallic complexes that can break carbon-hydrogen (C-H) bonds in alkanes and insert a metal atom (the metals that they have used so far are iridium, rhodium and rhenium). This leads to the formation of carbon-metal-hydrogen complexes that are much more chemically reactive than alkanes and better able to be converted into products.

Utilizing liquefied krypton and xenon as solvents so that the C-H activation process could be carried out at low temperatures (to slow the process down), Bergman and his group discovered that metal noble gas and metal alkane "solvate" complexes were formed as intermediates in the activating reaction. These weakly bound "alkane complexes" are formed before the C-H bonds are broken.

By combining the data obtained in their liquefied noble gas experiments, with gas phase data obtained in experiments conducted by LBL-UCB chemist Brad Moore and his group of researchers, Bergman and his group have been able to put together a unified picture of the C-H activation process. This picture shows that larger alkane molecules, such as cyclohexane, bind more strongly to the metal center in the solvate than smaller alkanes, such as ethane. This size-binding effect may in part explain why it is so difficult to activate methane, the smallest but most important alkane. Understanding the factors behind this difficulty could provide a solution for activating methane and converting it into useful products.

This past year, Bergman and his group were also able to measure the energy barrier that must be crossed in order for the C-H bond-breaking reaction to occur in the alkane complex. The energy barrier was found to be approximately 5 kilocalories per mole.

-- Lynn Yarris