Ultrafast UV pump/IR probe studies of C-H activation in linear, cyclic, and aryl hydrocarbons

The photochemical C-H activation reactions of η³-Tp*Rh(CO)₂ (Tp* = HB-Pz₃*, Pz* = 3,5-dimethylpyrazolyl) and CpRh(CO)₂ (Cp = C₅H₅) have been studied in a series of linear, cyclic, and aromatic hydrocarbon solvents on a femtosecond to microsecond time scale. These results have revealed that the structure of the hydrocarbon substrate affects the final C-H bond activation step, which is in accordance with the known preference of bond activation toward primary C-H sites. In the case of aromatic C-H activation, the reaction is divided into parallel channels involving σ- and π-solvated intermediates. Results for the analogous CpRh(CO)₂ molecule have shown that the coordination of the cyclopentadienyl ligand does not play a direct role in the dynamics of the reaction, in contrast to the C-H activation mechanism observed in η³-Tp*Rh(CO)₂ studies.