Abstract:     The rising cost of oil and fear of high CO₂ levels in the atmosphere are leading scientists to search for cheap alternatives to coal and oil.  Direct aerobic oxidations of methane, the principal component of natural gas to liquid fuels and useful chemicals is an attractive approach.  Unfortunately, the selectivity for partial oxidation of methane with most oxidants, including O₂, are complicated by the increasing reactivity of its oxidation products.

Organometallic reagents offer potentially more attractive selectivity, and among organometallic reagents that activate carbon-hydrogen bonds, the "Shilov System", stoichiometric oxidation of alkanes to alcohols by aqueous Pt(IV), exhibits unusual selectivity and better compatibility with oxidants and protic reagents. Using a variety of kinetics, isotopic labeling and stereochemical studies, we have examined the mechanisms of the individual steps of the Shilov cycle. The rate and selectivity determining step of the cycle is the initial coordination of the C-H bond to Pt(II).  A series of bis(aryl)diimine-ligated Pt(II) methyl cations are found to provide better characterized models for the reactive species in the Shilov System. These react smoothly in trifluoroethanol solvent with the C-H bonds of arenes, alkanes, methanol and dimethylether, and the relative rates for these models reactions have been measured. The relative rate constants are found to vary over a relatively small range for these substrates. This non-selectivity is similar to that established for other C-H bond activations at d⁸ metal centers, and stands in contrast to the higher, but unfavorable selectivity exhibited in autoxidation of methane. Recent results indicate that a truly catalytic variant based on O₂ is possible, although turnover numbers are not yet sufficient to be practical.

Biography:    

Professor Bercaw’s research areas include the early transition metal chemistry (complexes of Sc, Y, Zr, Hf, Nb and Ta) and its applications to the Ziegler-Natta olefin polymerization. Also, he studies the late transition metal chemistry (complexes of Pd and Pt) relevant to alkane oxidation catalysis. His work is includes new compound synthesis and subsequent characterization by multinuclear NMR spectrometry and by single crystal X-ray diffraction methods. A key area of study is reaction mechanism studies using isotopic labeling, characterization of intermediates, dynamic NMR techniques, and monitoring of reaction kinetics and stereochemistry. These studies are ultimately directed toward assessing the roles of transition metals in catalysis and developing new stoichiometric and catalytic reactions for converting readily available molecules such as olefins and alkanes into more valuable products.

Prof. Bercaw has received the American Chemical Society awards in Pure Chemistry (1980), for Organometallic Chemistry (1990), for Distinguished Service in the Advancement of Inorganic Chemistry (1997), the George A. Olah Award for Hydrocarbon or Petroleum Chemistry (1999), and an Arthur C. Cope Scholar Award (2000). He has also been selected a Chemical Pioneer by the American Institute of Chemists (1999). He was selected as a Fellow of the American Association for the Advancement of Science (1986) and was elected a member of the National Academy of Sciences (1990), as a Fellow of the American Academy of Arts and Sciences (1991), and an Honorary Doctorate of Science, University of Chicago (2001).