Kinetics of photochemical alkane dehydrogenation catalyzed by Rh(PMe₃)₂(CO)Cl: Implications concerning the C-H bond activation step

The mechanism of photochemical alkane dehydrogenation catalyzed by Rh(PMe₃)₂(CO)Cl has been further probed with an emphasis on characterizing the initial C-H activation step and understanding the effect of added CO on selectivity. While pure cyclooctane and pure cyclohexane are dehydrogenated at the same rate (same quantum yields), cyclooctane shows much greater reactivity in mixtures of the two solvents. The product ratio (cyclooctene:cyclohexene) is highly dependent upon the partial pressure of CO, ranging from 12 in the absence of CO, to 75 in the limit of high CO pressure (>ca. 400 torr). The kinetic isotope effect for the dehydrogenation of c-C₆H₁₂/c-C₆D₁₂ is also found to be dependent upon CO pressure, ranging from 10 in the absence of CO to 4.2 under high CO pressure. The results support our earlier conclusion that the intermediate responsible for C-H activation is ground state [Rh(PMe₃)₂Cl]. It is also concluded that inhibition of the reaction by CO operates primarily via addition of CO to the intermediate alkyl hydrides, (R)(H)Rh(PMe₃)₂Cl. Addition of CO prior to C-H bond addition is apparently not a kinetically significant process, even under high CO pressure.