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

Abstract

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 °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 °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) × 10 5 s -1 at -90 °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 × 10 5 s -1 at -90 °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.

Theoretical study of the rhenium–alkane interaction in transition metal–alkane σ-complexes

“…The difference between the two extremes (top of Group 5 vs. bottom of Group 7) was remarkable; CpV(CO) 3 (n-heptane) reacted 50,000 times more rapidly than CpRe(CO) 2 (n-heptane). Also, for the choice of alkane, experiment indicates that large alkanes form more stable complexes than smaller alkanes, and that cyclic alkanes form more stable complexes than comparably sized linear alkanes (24,32,35). In agreement with the observed stability trends, several rhenium complexes with rather large alkanes, CpRe(CO) 2 -(cyclopentane), -(cyclohexane), and -(npentane), have now been detected using low-temperature NMR (9-11).…”

“…In these experiments (25), and others similar to them (26)(27)(28)(29), an inverse kinetic isotope effect and H/D exchange between the hydride and alkyl groups before elimination of the alkane indicated the presence of an alkane -complex intermediate. More recently, direct evidence for alkane -complex intermediates was obtained from TRIR flash kinetics studies on the photoinduced COH bond activation by Cp*Rh(CO) 2 in liquid rare gases (30)(31)(32). These experiments showed a reaction intermediate, assigned to be Cp*Rh(CO)(alkane), which became increasingly stable (relative to the reactants) with increasing alkane size, ranging from Ϫ0.9 kcal/mol for ethane to Ϫ2.3 kcal/mol for octane and from Ϫ2.4 kcal/mol for cyclopentane to Ϫ3.5 kcal/mol for cyclooctane (32).…”

“…More recently, direct evidence for alkane -complex intermediates was obtained from TRIR flash kinetics studies on the photoinduced COH bond activation by Cp*Rh(CO) 2 in liquid rare gases (30)(31)(32). These experiments showed a reaction intermediate, assigned to be Cp*Rh(CO)(alkane), which became increasingly stable (relative to the reactants) with increasing alkane size, ranging from Ϫ0.9 kcal/mol for ethane to Ϫ2.3 kcal/mol for octane and from Ϫ2.4 kcal/mol for cyclopentane to Ϫ3.5 kcal/mol for cyclooctane (32). Also studied was the reaction profile of methane activation by CpRe(CO) 2 using computational methods, which predicted a methane -complex intermediate lying 8.7 kcal/mol below reactants in enthalpy, with an activation barrier to COH oxidative addition of 6.6 kcal/mol (33).…”

Understanding the factors affecting the activation of alkane by Cp ′ Rh(CO) 2 (Cp ′ = Cp or Cp*)

“…Irradiation of a mixture of alkane and lKr solution resulted in formation of the noble gas complex CpRh(CO)Kr followed by the formation of the alkane complex Cp*Rh(CO) (RH), which subsequently undergoes C─H bond cleavage to give the final alkyl hydride product. Activation was observed for all alkanes studied except methane (6). In solution at room temperature, CpRh(CO)(alkane) was observed on the picosecond time scale (7) and CpRh(CO)(alkyl)H, and the only information for the rate of C─H activation was given by the slight decay on the picosecond time scale providing an estimate for the formation of the alkyl hydride, k obs ¼ 4.0 × 10 8 s −1 (τ act ¼ 2.5 ns).…”

“…There has been considerable research effort directed toward understanding this key reaction in order to allow the full exploitation of the C─H activation process. The photochemistry of Cp 0 RhðCOÞ 2 [Cp 0 ¼ ðη 5 -C 5 R 5 Þ, R ¼ H (Cp) or CH 3 (Cp*)] has played an important role in developing our understanding particularly because the infrared νðC─OÞ bands are a useful spectroscopic tool for characterizing the reactive intermediates and monitoring the C─H activation reaction (5,6). Photolysis results in CO loss and coordination of the alkane followed by C─H activation to form the alkyl hydride product.…”