Oxidative addition of methane

Actually, a similar approach was used in studying the oxidative addition of methane to an iridium complex. Hydrocarbon solvents would have reacted faster than methane with the photochemically produced unsaturated iridium species, therefore J.K. Hoyano et al chose perfluorinated hexane as being an inert solvent. The elevated pressure was necessary in order to increase the concentration of the methane in the solution sufficiently to shift equilibrium (15) to the right /20/. 

In this chapter, we will study the elementary reaction steps of these mechanisms focusing primarily on the anthraphos systems. This chapter begins with a description of the impact of different methods (coupled cluster, configuration interaction and various DFT functionals), different basis sets, and phosphine substituents on the oxidative addition of methane to a related Ir system, [CpIr(III)(PH3)Me]+. Then, it compares the elementary reaction steps, including the effect of reaction conditions such as temperature, hydrogen pressure, alkane and alkene concentration, phosphine substituents and alternative metals (Rh). Finally, it considers how these elementary steps constitute the reaction mechanisms. Additional computational details are provided at the end of the chapter. 

Diefenbach and Bickelhaupt have shown"" that the direct oxidative addition of methane to cz5-[Pdl2(CO)2] through a r -C,H transition state is the... 

A hierarchical series of ab initio methods was applied to obtain benchmark potential energy surface for the oxidative addition of methane to the Pd atom. The best estimate of kinetic and thermodynamic parameters is -8.1 kcal mol for the formation of the reactant complex, 5.8 kcal mol for the activation energy relative to the separate reactants, and 0.8 kcal mol for the reaction energy. The values agree well with those obtained by Siegbahn et al. with the PCI-80 method. Calculations were based on a model reaction involving the f --C,H transition state. 

Figure 58. Reversible oxidative addition of methane between two [Rh°(por)] metalloradicals and the approximate relative energy of the OEP, TXP, and TMP species.

Calculations for the oxidative addition reactions between methane and the whole sequence of second row transition metal atoms from yttrium to palladium have been carried out [53]. The lowest barrier for the C-H insertion has been found for the rhodium atom. Palladium has the lowest methane elimination barrier. In another paper [54], the formation of complexes [CHi-Fc] (17 = +1, 0, -1) and the oxidative addition of methane to Fe has been studied by using the MINDO/SR-UHF method. The potential energy curves for the oxidative addition were calculated for a symmetry (structure VI-14). 

An investigation of the oxidative addition of ChUand CD4[57a], as well as ethane [57b], to a bare palladium atom has demonstrated that quantum tunneling plays a very important role in the process. The barrier of insertion of different transition metal atoms into a C-C bond has been found to be 14-20 kcal mol higher than the barrier for insertion into a C-H bond [57c], Calculations for the activation of the C-H bond in ethylene by second row transition metal atoms showed that the oxidative addition barrier is lowest for the atoms to the right (for rhodium there is no barrier and for palladium the barrier is almost zero) [57d], The activation energy for B2 insertion into methane has been predicted to be 4.1 kcal mol while this value increases to 16.2 kcal mol for insertion of B [58], Two mechanisms have been considered by the SCF CNDO/S method for the oxidative addition of methane to the palladium cluster Pd2 [59a], In the first possible reaction, the C-H bond oxidatively adds to different palladium atoms ... 

The oxidative addition of methane to CpRhCl(CO) was investigated with various computational methods [11-15], and the energy changes were compared with experimental results [16].This reaction proceeds through a precursor complex and a transition state, to afford a product, CpRh(H)(CH3)(CO), as shown in Fig. 1, where geometries were optimized with the MP2 method [12]. The DFT... 

Fig.l MP2-optimized geometry changes in the oxidative addition of methane to Pt(PH3)2 and to CpRh(CO). Bond length in A and bond angle in degrees. From [8f, 13] with permission of American Chemical Society... 

Although the oxidative addition of the C-H o-bond to the transition metal complex is difficult, the oxidative addition of methane derivative to a palla-dium(O) complex was experimentally proposed [19], where a chelate phosphine was employed as a ligand and two electron-withdrawing substituents were introduced to the sp C atom. The C-H o-bond activation of methane by palladium(O) and platinum(O) chelate phosphine complexes was theoretically investigated [8e, 20], as shown in Fig. 3. When monodentate phosphine was employed, the C-H bond is lengthened very much in the transition state and the geometry of the transition state is similar to that of the product. When the chelate phosphine was employed, the Pd-C, Pd-H, and C-H distances in the... 

Table 2 Bond energiesrelated to oxidative addition of methane and benzene to M(PH3)2 (M=Pdor Pt) [9,20]...

The similar C-H o-bond activation by platinum(II) sulfonate complex was theoretically investigated [36-38]. This is a model of the methane-to-methanol conversion by the platinum(II) complex in dry sulfuric acid experimentally reported [3]. There are two possible reaction courses in the C-H o-bond activation one is the oxidative addition of the C-H o-bond to the platinum(II) complex and the other is the metathesis of the C-H o-bond with the Pt-OSOjH moiety. Hush et al. proposed that the C-H o-bond activation took place through the oxidative addition to the platinum(II) complex [36]. This is not surprising because the oxidative addition of methane to the coordinatively unsaturated platinum(II) complex is not very difficult, as discussed above. However, Ziegler et al. reported that the metathesis could take place with the similar activation barrier to that of the oxidative addition [37]. Recently, Goddard et al. clearly concluded that the metathesis more easily proceeded than the oxidative addition in sulfuric acid [38]. They investigated this reac-... 

As observed with reaction (43), the oxidative addition of methane to an unsaturated metal center, reaction (60), has a very negative entropy term, (60) a —44 kJ moF ... 

Theoretical studies on the oxidative addition of methane to second row metal atoms show that Rh has the lowest activation energy the potential surface for Rh atoms is remarkably similar to that of RhCl(PR3)2-based C-H activating complexes.268 The dissociative chemisorption of acrolein and allyl alcohol on a Rh(l 11) surface has been investigated. ... 

The direct oxidative addition of methane to ethylene shown in the equation below is a thermodynamically favorable reaction (Gibbs free energy of-69 Kcal/mole). 

Fig. 9 Geometries of the RC, TS, and PC along the energy profile for oxidative addition of methane to Pd(PH3)2, Pd(Pp3)2, Pd(PCl3)2, Pd(PBr3)2, and Pd(Pl3)2. Computed at dispersion-corrected ZORA-BLYP-D3/TZ2P...

It seems likely that in reaction 19, the interaction of 22 with methane would be similar to that found in the unsymmetrical methyllutetium dimer where a methyl group of one monomer donates electron density to the second monomer. On the other hand, orbital analysis of oxidative addition of methane on d ML and CpML fragments indicated that,... 

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