Other Alkane Activations

Note that the main difference between zirconium hydride and tantalum hydride is that tantalum hydride is formally a d 8-electron Ta complex. On the one hand, a direct oxidative addition of the carbon-carbon bond of ethane or other alkanes could explain the products such a type of elementary step is rare and is usually a high energy process. On the other hand, formation of tantalum alkyl intermediates via C - H bond activation, a process already ob-... 

The role of adsorbed oxygen species in the mechanism of alkane transformation, on the contrary, is more questionable. The effect induced by the substitution of O2 with N2O and IR indications are in agreement with this interpretation, but, on the other hand, activated electrophilic oxygen species form on reduced sites, preferably in tetrahedral coordination (79). The partial reduction of tetrahedral V =0 with formation of tetrahedral v after propane oxidative dehydrogenation can be observed using UV-Visible diffuse reflectance, ESR and V-NMR spectroscopies. It is thus not possible to assign unequivocally the active species in propane selective activation to a tetrahedral V =0 species or to or V -0-0 species formed in the... 

Unlike higher alkanes, ethane contains only primary C—H bonds, and the dehydrogenation product ethene contains only vinylic C—H bonds. As shown in Table I, these are strong bonds. Thus one would expect that, compared to other alkanes, the activation of ethane would require the highest temperature, but the reaction might be the most selective in terms of the formation of alkene. Indeed, this appears to be the case. 

On the other hand, above 20mol% SbF5, a small but increasing amount of unionized SbF5 can be observed, which may rationalize the change in the mechanism of alkane activation from the protolytic to the oxidative pathway, when the concentration of SbF5 increases over 20mol% (see Section 5.1.1). 

However, attempts to develop similar selective catalysts failed in the case of reactions that require one oxygen atom, like the oxidation of methane, ethane and other alkanes to alcohols, aromatic compounds to phenols, alkenes to epoxides, and many others. These mechanistically simple reactions assume one difficult condition the presence of active sites that upon obtaining two atoms from gas-phase 02 can transfer only one of them to the molecule to be oxidized, reserving the second atom for the next catalytic cycle with another molecule. This problem remains a hard challenge for chemical catalysis. 

The interrelationships between activation of H2 and other a-bonded molecules such as alkanes and silanes are highly significant because catalytic conversion of methane and other alkanes is strongly being pursued (17-19). An important question thus is whether C-H bonds in alkanes, particularly CH4, can bind to superelectrophilic metal centers to form a a alkane complex that can be split heterolytically where proton transfer to a cis ligand (or anion) takes place followed by functionalization of the resultant methyl complex (Eq. (3)). 

Supported noble metals and in particular, palladium, are being widely used for the complete combustion of methane and other alkanes to form CO2 and H2O, environmentally acceptable emission products and extremely low NOx levels [ 1-3], A considerable amount of research effort has been devoted to the process, however, there does not appear to be a consensus with regard to either the mechanism of the reaction or the chemical identity of the active catalytic species [4-8]. This state of affairs is further complicated by the fact that the chemical state of the catalyst is extremely sensitive to the reaction conditions, including time-on-stream and reaction temperature [9-12]. It has also been demonstrated that the nature and form of the support plays a key role in modifying both the activation and deactivation steps encountered with palladium catalyst particles [13-16]. 

The reduced form of RNR reacts with dioxygen to generate the p-oxo diferric core (crystallographically defined (3)) and a tyrosyl radical necessary for the production of a reactive species responsible for the reduction of ribonucleotides. The intimate details of this dioxygen-based chemistry and the structure of the reduced enzyme are still unknown. Despite intensive spectroscopic characterization of the active site of MMO and the recent X-ray structural analysis of the hydroxylase component (4), even less is known concerning its mechanistic pathways responsible for the conversion of methane and other alkanes to their corresponding oxygenated products. 

The reaction probably proceeds througli a carbenoid mechanism. In fact, both heterogeneous and homogeneous catalysts are known to promote an H-D exchange between methane (or other alkanes) and DjO in the DjO-HOAc system. In the homogeneous phase, catalysis by (PtClj)-, DCIO4 and an arc-inatic additive (for example, pyrene) are used to stabilize the active species. 

We have shown that perfluorination of the phthalocyanine ligand enhances the stability and catalytic activity of RuFiePc. Encapsulation of this complex in zeolite NaX by the synthesis method dramatically improves the activity and selectivity of RuFiePc. These results suggest that RuFi Pc-NaX is one of the best alkane oxidation catalysts of its kind. Although cycloalkanes are readily oxidized, the complete range of possible substrates is uncertain at this point. The oxidation of other alkanes and olefins will be the subject of continuing studies. 

In addition to the systems listed in Table I for the oxidative dehydrogenation of ethane, other systems have been tested because they have proven to be active in other alkane oxidations this is particularly the case of many catalysts used in the oxidative coupling of methane, VPO and magnesium phosphate catalysts (butane oxidation and propane dehydrogenation, respectively) and MoVO catalysts. Various zeolites have also been tested. This table, the largest to be presented here, perfectly illustrates the fact that no formulation seems convincingly better than the others. 

Evidence for alkane activation has also been seen by the observation of H/D exchange between two alkanes, an alkane and an arene, or an alkane and THE Using CpRe(PPh3)2H2 as the photo catalyst, thousands of turnovers have been observed. While the intermediate responsible for this catalysis was not identified, it does not appear to be [CpRe(PPh3)H2] undergoing Rem/Rev oxidative addition/reductive elimination, since no deuterium incorporation was observed in the dihydride catalyst [99]. Several other metal hydrides are known to catalyze H/D exchange between alkanes and deuterated benzene, such as Ir(PMe3)2H5 [100],CpMo(dmpe)H3 [101], and Re[P(c-hexyl)3]2H7 [102]. 

A similar approach can be applied for the Y atom insertion into the C-H bond of alkenes and other alkanes. Our calculation by the Cl method in a 6-311-H-G(2d,2p) basis set with a complete active space for 8 electrons in 8 orbitals (Is orbital of carbon atom is frozen) predicts that the vertical S-T excitation energy in methane is around 11 eV (11.37 eV or 262 kcal/mol). Following the above approximation it is equal -2 Jch- From Eq. (9) the activation energy for the yttrium atom insertion reaction, Eq. (1) M=Y, should be 17.6 kcal/mol. This simple estimation is in a good agreement with very accurate ab initio calculations, Ea = 20.7 kcal/mol [15]. 

Other calculations93 97 focus mainly on adducts of CH4 and other small molecules with CpML and unsaturated Pd/Pt fragments because of their extensive use in experimental studies of alkane activation (see below). The binding and activation is similar to Eq. (12.17), and t 2-C,H coordination of CH4 is shown by Hall for CpRh(CO)(CH4) and [ 1 ( )( 3)( 4)]+ and by Smith for CpM(NO)(CH2)(CH4) (M = Mo,W).93,97 The energy profiles are in agreement with... 

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