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

Abstract

Fast time-resolved infrared spectroscopic measurements have allowed precise determination of the rates of activation of alkanes by Cp′Rh(CO) (Cp 0 ¼ η 5 -C 5 H 5 or η 5 -C 5 Me 5 ). We have monitored the kinetics of C─H activation in solution at room temperature and determined how the change in rate of oxidative cleavage varies from methane to decane. The lifetime of CpRh(CO)(alkane) shows a nearly linear behavior with respect to the length of the alkane chain, whereas the related Cp*Rh(CO)(alkane) has clear oscillatory behavior upon changing the alkane. Coupled cluster and density functional theory calculations on these complexes, transition states, and intermediates provide the insight into the mechanism and barriers in order to develop a kinetic simulation of the experimental results. The observed behavior is a subtle interplay between the rates of activation and migration. Unexpectedly, the calculations predict that the most rapid process in these Cp′Rh (CO)(alkane) systems is the 1,3-migration along the alkane chain. The linear behavior in the observed lifetime of CpRh(CO)(alkane) results from a mechanism in which the next most rapid process is the activation of primary C─H bonds (─CH 3 groups), while the third key step in this system is 1,2-migration with a slightly slower rate. The oscillatory behavior in the lifetime of Cp*Rh(CO)(alkane) with respect to the alkane's chain length follows from subtle interplay between more rapid migrations and less rapid primary C─H activation, with respect to CpRh(CO)(alkane), especially when the CH 3 group is near a gauche turn. This interplay results in the activation being controlled by the percentage of alkane conformers.A lkanes are generally unreactive molecules and the lack of ability to utilize such feedstock has thwarted the widespread use of methane, the main component of natural gas, as a feedstock to produce synthetically useful compounds even though this inexpensive source is widely available (1). The facile activation of methane is considered a "holy grail" for chemists (2). The use of transition metals in order to provide a way to activate carbon─hydrogen (C─H) bonds in hydrocarbons offers the potential to address this problem, and useful processes have been developed including alkane dehydrogenation, arene borylation, and alkane metathesis.The early reports of alkane activation involved an initial photodissociation of a ligand, from a five-coordinate cyclopentadienyl rhodium(I) or iridium(I) complex to form a coordinatively unsaturated intermediate (3,4). This reactive species subsequently attacks and oxidatively adds a C─H bond to form the alkyl hydride product. 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 charac...

Contrasting electronic requirements for CH binding and CH activation in d6 half‐sandwich complexes of rhenium and tungsten

“…A second aspect that has received significant attention is the intramolecular dynamic process, termed alkane chain walking in many metal‐alkane complexes . The use of 1 H NMR spectroscopy revealed a slow migration of the metal from one CH bond to another in the alkane by Ball and coworkers .…”

“…The three aspects of metal‐alkane chemistry are intertwined as their energetics is comparable. Recently, George et al found that the primary CH activation is more favorable than the secondary CH activation in the d 8 complex [Cp′Rh(CO) (alkane)] (Cp ′ = η 5 ‐C 5 H 5 or η 5 ‐C 5 Me 5 ) . From kinetic simulation studies, they showed that the CH activations are slower than the exchange processes, and so the lifetimes of the complexes vary as the length of the alkane.…”

Computational Kinetics of Cobalt‐Catalyzed Alkene Hydroformylation

“…Hence protocols for interpreting free-energy profiles have been suggested, [20] and the value of kinetic simulations for understanding mechanisms is becoming increasingly recognized. [21] We use TST to predict rate constants based on the calculated activation free energies. Differences in free energy between stable species, also computed from the ab initio data (except for the equilibrium with 23 where we do not have CCSD(T) values and base our calculations on experiment [17] rather than the less accurate DFT value), are used to compute equilibrium constants.…”

Recent computational studies on transition‐metal carbon–hydrogen bond activation of alkanes

“…1.3‐migrations has also been found to be faster that 1,2‐migrations in the CH bond activation of a series of linear alkanes catalyzed by Cp'Rh(CO) 2 (Cp' = Cp or Cp*) by George, Hall and their coworkers employing the combination of high precision fast time‐resolved infrared (TRIR) spectroscopy and DFT calculations [39a] . They found that the lifetimes of Cp'Rh(CO)(alkane) always increase with chain lengths, but with a nearly linear behavior for CpRh(CO) and a more obvious oscillatory behavior for Cp*Rh(CO), as shown in Figure .…”

“…Mechanistic and kinetic simulations based on density functional and coupled cluster calculations indicate an interplay between the rates of activation and migration that renders the different lifetimes, where the migrations along the alkane chain are more rapid than primary CH activations, which are less rapid in Cp*Rh(CO) than the CpRh(CO) counterpart, and the competition between migrations and primary CH activation controls the kinetics of the CH activations in the linear alkanes. Further investigations elucidating the balance between electronic and steric factors that tunes the alkane CH activation and migration reactivities are being undertaken in the same group [39b,c] …”