Femtosecond Infrared Studies of Chemical Bond Activation

The identification of the intermediates observed in bond activation reactions involvingorganometallic complexes on time scales from femtoseconds to milliseconds has beenaccomplished through the use of ultrafast infrared spectroscopy. C—H bond activationby the molecule Tp * Rh(CO) 2 showed a final activation time of 200 ns in cyclic solvents,indicating a reaction barrier of 8.3 kcal/mol. An important intermediate is the partiallydechelated η2-Tp * Rh(CO)(S) solvent complex, which was formed 200 ps after the initialphotoexcitation. Si—H bond activation by CpM(CO) 3 (M=Mn, Re) showed someproduct formation in less than 5 ps, indicating that the Si—H activation reaction isbarrierless. The activated product was formed on several timescales, from picoseconds tonanoseconds, suggesting that there are different pathways for forming final productwhich are partitioned by the initial photoexcitation.