Researchers in LBL's Chemical Sciences Division (CSD) have taken another important step towards harnessing the vast potential of one of nature's most plentiful materials. For the first time, they have measured the reaction rates between a 16-electron organometallic complex and 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 (CHsub4), 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 carbon and hydrogen atoms are so strong as to render alkanes generally unreactive.
In the early 1980s, Robert Bergman, a chemist in LBL's Chemical Sciences Division (CSD) and a professor in the College of Chemistry at UC Berkeley, led the discovery of an unusual new group of organometallic complexes -- compounds of metal atoms, such as iridium or rhodium, sandwiched between organic molecules. These compounds can break carbon-hydrogen bonds in alkanes and insert a metal atom. This leads to the formation of a carbon- metal-hydrogen compound. The carbon-metal and metal-hydrogen bonds in this new compound are much more reactive than the carbon-hydrogen bonds in the alkane, which means the alkane can potentially be much more readily converted into a useful product. However, because these organometallic complexes are themselves chemically altered (which means they can't be used over and over again like catalysts), the process is of limited commercial use so far.
Since discovering this alkane-activating process, which is often called the "C-H oxidative addition reaction," Bergman has been working to better understand it with the ultimate aim of designing a process that is truly catalytic. A problem has been that one of the crucial steps in the process -- the interaction between the alkanes and a highly reactive 16-electron organometallic intermediate -- takes place so quickly that its rate has been impossible to measure.
Bergman and fellow CSD and UCB chemist Brad Moore, working with postdoctoral fellow Bruce Weiller and UCB graduate student Eric Wasserman, have now been able to measure the reaction rates of this crucial step for a number of different alkanes. They have done so using a special time-resolved infra-red flash kinetics spectrometer constructed in Moore's laboratory, and a unique high-pressure sample cell developed by the late George Pimentel.
In their first round of experiments, the researchers utilized the liquefied noble gases krypton and xenon as solvents. This led them to the discovery that rhodium-noble gas and rhodium-alkane "solvate" complexes were formed as intermediates in the C-H oxidative addition process.
"However, there was an elusive 16-electron intermediate, which was believed to be the precursor of these solvates, that we were not able to catch," Bergman says.
This "naked" or unsolvated 16-electron complex was finally detected and the rates of its reaction with alkanes were measured when the researcher carried out time-resolved infra red experiments in the gas phase. Under these conditions, Bergman and his colleagues found that the reaction rates vary according to the size of the alkane, with larger molecules reacting more efficiently than smaller molecules.
"For alkanes of moderate size, such as cyclohexane (Csub6Hsub12), almost every collision between an alkane and the organometallic intermediate results in the activation of the alkane," says Bergman. "For ethane (Csub2Hsub6), roughly one in two collisions leads to activation."
By combining data obtained in the gas phase and the liquefied noble gas solvent experiments, Bergman, Moore and their coworkers have been able to put together a unified picture of the C-H oxidative addition process. Ultraviolet irradiation excites a stable rhodium complex and rapidly produces the unsolvated 16-electron complex which, in turn, reacts with an alkane or a noble gas solvent molecule on nearly every collision.
An important, and unexpected, finding in this study is that larger molecules, such as cyclohexane, bind more strongly than smaller molecules, such as ethane, to the rhodium center in the alkane solvate. Although it is not yet clear whether the physical cause is the same, this observation correlates with similar results in other laboratories, which have revealed that larger molecules also bind more strongly to metal surfaces.
Says Bergman, "This size-binding effect may in part explain why it is so difficult to activate the smallest but often most important alkane, methane. Understanding the factors that cause this difficulty should help us move toward finding a solution for the methane-activation problem."