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 alkanes (RH) in liquid Kr and liquid Xe solution has been studied by time-resolved infrared spectroscopy. Irradiation leads to the formation of a transient species absorbing at 1947 cm(-1) in liquid Kr. Reaction rates for the conversion 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 methane; for all other hydrocarbons, the dependence of the reaction rate on the alkane concentration follows saturation kinetics. Analysis of the kinetic data was carried out using the assumption, established in earlier work, that the observed transient is a mixture of two solvates, a krypton complex Cp*Rh(CO). Kr (2) and an alkane complex Cp*Rh(CO). RH (3), both having essentially the same CO stretching frequency in the IR. For each alkane, the rate law 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 and 3. Within each series (linear and cyclic), as alkane size increases, the measured free energy of binding of the alkane to the coordinatively unsaturated Rh center in the Cp*Rh(CO) fragment becomes increasingly thermodynamically favorable, ranging from -0.9 (ethane) to -2.3 kcal/mol (octane) and from -2.4 (cyclopentane) to -3.5 kcal/mol (cyclooctane) at -90 degrees C relative 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 series of linear alkanes. Propane, hexane, and octane each react with a rate constant 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 series. 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 cycloheptane and cyclooctane are slower by an order of magnitude. Possible explanations are discussed for this unexpected alkane structure dependence in both steps of the reaction.