SCIENCE/TECHNOLOGY
Oxidative Addition of Alkanes to Metal Complexes Probed in Gas Phase esearch is shedding light on the mechanism of an intriguing and potentially important class of re actions between organometallic com plexes and alkanes. Specifically, chem ists have measured for the first time the rates at which the 16-electron organo metallic intermediates react with alkane carbon-hydrogen bonds. The research was carried out by Uni versity of California, Berkeley, chemis try professors Robert G. Bergman and C. Bradley Moore and graduate stu dent Eric P. Wasserman. It was sup ported by the Department of Energy. Research on organometallic com plexes capable of inserting a metal cen ter into a C-H bond in an alkane (R-H) in what is known as an "oxidative ad dition reaction" has been going on in Bergman's lab for a number of years. The reactions lead to stable, isolable products in which an alkane C-H bond has been broken and new metal-hydro gen (M-H) and metal-carbon (M-C) bonds have been formed to produce a hydrido(alkyl)metal complex, R-M-H. Bergman points out that such reac tions are important because they repre sent a method for activating the C-H bonds in alkanes. These bonds are among the strongest single bonds known. Because alkanes and alkyl chains make up a large percentage of natural materials, chemists very much want to develop techniques for con verting these "unactivated" C-H bonds into "more reactive organic functional groups," Bergman says. The chemists studied the gas-phase irradiation of CpRh(CO)2, where Cp is an abbreviation for the π-bound cyclopentadienyl ligand (r|5-C5H5) [Science, 255, 315 (1992)]. In such organometallic systems, the rhodium atom coordinates directly to the π electrons of the cyclopentadienyl group. Gas-phase irradiation of CpRh(CO)2 produces "naked," or unsolvated, CpRh(CO), a 16-electron species thought to be the intermediate that re
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JANUARY 27,1992 C&EN
Wilder and the late George Pimentel, the Berkeley chemists earlier carried out infrared flash kinetics studies of Cp*Rh(CO)2 in liquid xenon. (Cp* is an abbreviation for the pentamethylcyclopentadienyl group; complexes of Cp* and rhodium exhibit the same type of chemistry as complexes of Cp and Rh.) In these studies, Cp*Rh(CO) could not be detected directly, Bergman says, be cause it reacts very rapidly to form both noble gas and alkane complexes. In this system, the reaction observed directly is the conversion of the short lived Cp*(CO)Rh(alkane) σ-complex to the o x i d a t i v e - a d d i t i o n p r o d u c t Cp*(CO)Rh(R)(H). At temperatures near -100 °C, this reaction proceeds with an activation energy of about 4.5 kcal per mole for cyclohexane. The data from these experiments lead Bergman, Moore, and Wasserman to propose a somewhat unusual reac tion pathway for these oxidative addi tion reactions. In it, the system passes from the separated reactants—CpRh(CO) and RH—to a pair of potential wells. The first well, estimated to be at least 10 kcal per mole lower in energy than the separated reactants, represents the energy of the short-lived σ-com plex. The second well represents the energy of the alkyl hydride oxidative
acts with alkanes. (The rhodium atom provides nine electrons, Cp provides five electrons, and CO provides two electrons.) The Berkeley chemists de tected this species spectroscopically, as has been done earlier by other workers under matrix isolation conditions. For the first time, however, they also measured directly the rates of reactions between CpRh(CO) and a number of alkanes in the gas phase. The overall C-H activation process occurs at rates close to the gas-kinetic values, Berg man says, for alkanes of moderate size such as cyclohexane. That is, almost ev ery collision between CpRh(CO) and cyclohexane, for example, results in production of Cp(CO)Rh(H)(C6Hn). In the gas phase, the reaction be tween CpRh(CO) and alkanes proceeds directly to the oxidative addition prod uct, Bergman points out. However, previous research in Bergman's labora tory and independently by other re searchers has demonstrated that com plexes of CpRh(CO) and the alkane are also important as intermediates in these reactions. Bergman calls these in termediates "σ-complexes"; other re searchers refer to them as "alkane com plexes" and "agostic complexes." For example, in a project that also in volved postdoctoral fellow Bruce H.
C-H bond activation may involve intermediate complex
m
Rh(CO) + RH
