The Mechanism of a C-H Bond Activation Reaction in Room-Temperature Alkane Solution

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confidence: 80%

Structural and dynamic properties of propane coordinated to TpRh(CNR) from a confrontation between theory and experiment

Clot

Eisenstein

Jones

2007

“…T here is considerable interest in sigma-bonded organometallic alkane complexes, particularly since they have been identified as key intermediates in the transition metal-mediated COH activation process (1)(2)(3). Although such complexes generally are very short-lived intermediates (4), they have been known for over 30 years.…”

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confidence: 99%

“…Subsequently, several examples of alkane complexes in solution have been reported, and studies on the mechanism of the COH activation process have clearly demonstrated the role of these complexes in oxidative addition reactions (1,8,9). Time-resolved infrared (TRIR) spectroscopy has proved to be a powerful tool for the study of metal carbonyl alkane complexes.…”

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confidence: 99%

Time-resolved infrared (TRIR) study on the formation and reactivity of organometallic methane and ethane complexes in room temperature solution

Cowan

Portius

Kawanami

et al. 2007

We have used fast time-resolved infrared spectroscopy to characterize a series of organometallic methane and ethane complexes in solution at room temperature: W(CO)5(CH4) and M( 5 OC5R5)(CO)2(L) [where M ‫؍‬ Mn or Re, R ‫؍‬ H or CH3 (Re only); and L ‫؍‬ CH4 or C 2 H 6 ]. In all cases, the methane complexes are found to be short-lived and significantly more reactive than the analogous nheptane complexes. Re(Cp)(CO)2(CH4) and Re(Cp*)(CO)2(L) [Cp* ‫؍‬ 5 OC5(CH3)5 and L ‫؍‬ CH4, C2H6] were found to be in rapid equilibrium with the alkyl hydride complexes. In the presence of CO, both alkane and alkyl hydride complexes decay at the same rate. We have used picosecond time-resolved infrared spectroscopy to directly monitor the photolysis of Re(Cp*)(CO)3 in scCH4 and demonstrated that the initially generated Re(Cp*)(CO)2(CH4) forms an equilibrium mixture of Re(Cp*) ( T here is considerable interest in sigma-bonded organometallic alkane complexes, particularly since they have been identified as key intermediates in the transition metal-mediated COH activation process (1-3). Although such complexes generally are very short-lived intermediates (4), they have been known for over 30 years. Early experiments involved the photolysis of complexes such as Cr(CO) 6 and Fe(CO) 5 to generate the unstable intermediates Cr(CO) 5 or Fe(CO) 4 in low-temperature matrices, where coordination to cocondensed CH 4 results in the formation of Cr(CO) 5 (CH 4 ) and Fe(CO) 5 (CH 4 ) (5, 6).Flash photolysis experiments have demonstrated that the photolysis of Cr(CO) 6 in cyclohexane solution at room temperature forms Cr(CO) 5 (cyclohexane) (7). Subsequently, several examples of alkane complexes in solution have been reported, and studies on the mechanism of the COH activation process have clearly demonstrated the role of these complexes in oxidative addition reactions (1,8,9). Time-resolved infrared (TRIR) spectroscopy has proved to be a powerful tool for the study of metal carbonyl alkane complexes. Their reactivity decreases on going both across and down groups 5, 6, and 7 (10-12), and these observations led to the identification of a very long-lived alkane complex, Re(Cp)(CO) 2 (nheptane) (Cp ϭ 5 OC 5 H 5 ), which has a lifetime of Ϸ25 ms at room temperature (13). The relative stability of Re(Cp)(CO) 2 (alkane) complexes allowed Re(Cp)(CO) 2 (C 5 H 10 ) to be observed at 180 K by NMR spectroscopy (14), and subsequent NMR studies have been carried out to determine the binding modes of a series of related alkanes to the Re(CpЈ)(CO) 2 moiety (15, 16).The activation of methane is of particular interest because of the potential of using this abundant hydrocarbon as both an energy source and chemical feedstock (17). Organometallic methane complexes have been characterized in low-temperature matrix isolation experiments (5,6,18). In solution, the existence of methane complexes has been inferred by examining product ratios and from the rates of COH activation and reductive elimination reactions in isotopic labeling experiments (19).The lifetime of the...

“…The related chemistry of the RhðTp 3;5-Me ÞðCOÞ 2 [Tp 3;5-Me ¼ HBð3;5-dimethylpyrazolylÞ 3 ] complex has been studied (8,9), and photolysis of RhðTp 3;5-Me ÞðCOÞ 2 led to the ejection of a CO ligand and the formation of a monocarbonyl complex, which was rapidly solvated to form Rhðκ 3 -Tp 3;5-Me ÞðCOÞðRHÞ. This species then decayed to form the more stable Rhðκ 2 -Tp 3;5-Me ÞðCOÞðRHÞ complex, which is able to undergo C─H activation to the final oxidative addition product, Rhðκ 3 -Tp 3;5-Me ÞðCOÞðRÞH.…”

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confidence: 99%

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

George

Hall²,

Jina³

et al. 2010

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...