The effect of alkane structure on rates of photoinduced C-H bond activation by Cp*Rh(CO)(2) in liquid rare gas media: An infrared flash kinetics study

C-H bond activation via photoinduced reaction of Cp*Rh(CO)(2) (1) with alka nes (RH) in liquid Kr and liquid Xe solution has been studied by time-resol ved infrared spectroscopy. Irradiation leads to the formation of a transien t species absorbing at 1947 cm(-1) in liquid Kr. Reaction rates for the con version of this species to the final C-H activation product Cp*(CO)Rh(R)(H) (4) have been measured in the -80 to -110 degrees C temperature range for a series of linear and cyclic alkanes. No reaction was observed with methan e; for all other hydrocarbons, the dependence of the reaction rate on the a lkane concentration follows saturation kinetics. Analysis of the kinetic da ta was carried out using the assumption, established in earlier work, that the observed transient is a mixture of two solvates, a krypton complex Cp*R h(CO). Kr (2) and an alkane complex Cp*Rh(CO). RH (3), both having essentia lly the same CO stretching frequency in the IR. For each alkane, the rate l aw supports a deconvolution of the overall reaction into an alkane binding step and an oxidative addition step. For the binding step, the parameter K- eq and its associated free energy characterize a preequilibrium between 2 a nd 3. Within each series (linear and cyclic), as alkane size increases, the measured free energy of binding of the alkane to the coordinatively unsatu rated Rh center in the Cp*Rh(CO) fragment becomes increasingly thermodynami cally favorable, ranging from -0.9 (ethane) to -2.3 kcal/mol (octane) and f rom -2.4 (cyclopentane) to -3.5 kcal/mol (cyclooctane) at -90 degrees C rel ative to Kr. The second step, oxidative addition of the C-H bond across the Rh center to convert 3 to 4, proceeds with an absolute rate characterized by the parameter k(2). This rate exhibits very little variance in the serie s of linear alkanes. Propane, hexane, and octane each react with a rate con stant of roughly (6-7) x 10(5) s(-1) at -90 degrees C, while ethane reacts about a factor of 3 more rapidly. More variance is observed in the cyclic s eries. Oxidative addition of cyclopentane proceeds with a rate constant of 6.8 x 10(5) s(-1) at -90 degrees C, while the oxidative addition rates of c ycloheptane and cyclooctane are slower by an order of magnitude. Possible e xplanations are discussed for this unexpected alkane structure dependence i n both steps of the reaction.