Chemistry Department Seminar

"Nanosecond Time-Resolved Infrared Spectroscopy: A Valuable Tool for the Investigation of Photoinduced Small Molecular Activations and Hydrogen Atom Transfers"

Presented by David Grills, Brookhaven National Laboratory

Tuesday, November 15, 2005, 1:30 pm — Hamilton Seminar Room, Bldg. 555

Our research examines the fundamental chemistry underlying the efficient capture and storage of solar energy in useful chemical forms, with a focus on the mechanistic chemistry of transient species involved in homogeneous catalysis. In particular, we are interested in the photochemical activation of small, abundant molecules such as CO2, N2, hydrocarbons and H2O. An invaluable tool that we use to investigate these processes is nanosecond time-resolved step-scan FTIR spectroscopy (TR-S2FTIR).
Bond formation between a transition metal center and a substrate is an important fundamental step in many catalytic reactions. In order to investigate the nature of these interactions with low-valent metal centers, we have investigated the thermodynamics, kinetics and isotope effects of photoinduced W-L bond cleavage and formation as a function of temperature and pressure, for the series of complexes, W(CO)3(PCy3)2L (Cy = cyclohexyl; L = H2, D2, C2H4, N2 [1] or CH3CN).
The cleavage of C-H bonds is an important step in the catalytic conversion of hydrocarbons to higher value chemicals. Hydrogen atom transfer reactions are fundamentally important in a wide range of catalytic processes and there are many examples of metal-to-carbon H-atom transfers. However, the reverse reaction (carbon-to-metal H-atom transfer) has never been directly observed. We have used the metal radical, Cp(CO)2Os•, which was photogenerated from the metal-metal bonded dimer, [Cp(CO)2Os]2 to cleave C-H bonds of 1,4-cyclohexadiene. Using TR-S2FTIR, we were able to directly monitor, for the first time, the transfer of hydrogen atoms from a hydrocarbon to a metal center [2]. Kinetic studies revealed that the rate constant for this H-atom transfer process is surprisingly high, kH = 2.1 x 106 M-1 s-1.
We have previously shown [3] that photolysis of the CO2 reduction catalyst, [Re(CO)3(dmb)]2 (dmb = 4,4’-dimethyl-2,2’-bipyridyl) in the presence of CO2 produces the radical species, Re(CO)3(dmb)S (S = coordinating

Hosted by: John Miller

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