Iridium methane oxidative addition

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/. 

Janowicz AH, Periana RA, Buchanan JM, Kovac CA, Stryker JM, Wax MJ, Bergman RG (1984) Oxidative addition of soluble iridium and rhodium complexes to Carbon-Hydrogen bonds in methane and higher alkanes. Pure Appl Chem 56 13-23... 

Electron-richer dM compounds can also be considered as H2-activating alternatives to compounds with the unfavorable dM configuration. In the case of the bis-dppm bridged Rh(I)Ir(-I) complex 14, the d d configuration has been found to result in a metal-metal bonded species in which the coordination around the rhodium center is similar to that in planar homovalent d compounds. [47] The kinetic product of dihydrogen addition to 14 is consistent with the occurrence of a single-metal oxidative addition to the Rh(I) (Scheme 12). This kinetic product is thermally unstable and reductively eliminates methane from the iridium center. The overall reaction constitutes a clear example of bimetallic cooperation, since the oxidative addition to one center provokes a reductive elimination in the other metal. 

The metal atom substitutes only the meta and para positions of the benzene ring. In the case of X = NH2, only the meta isomer is formed. The authors propose that the reaction proceeds as DMSO is replaced by the arene with subsequent oxidative addition of the C-H bond to indium to produce an iridium(IV) derivative. Then reductive elimination of methane and addition of DMSO gives the final product. A synchronous elimination of methane and addition of the arene cannot be ruled out however. 

In 1993 Bergman discovered that an iridium (iii) methyl cation was capable of undergoing an exchange of the methyl group with other alkanes in a process that looked similar to the electrophilic activation of alkanes by Shilov s Pt(n) complex (Equation (17)). Theoretical treatment of this system provided evidence that the actual pathway involved oxidative addition of the alkane to give an Ir(v) dialkylhydride that then underwent reductive elimination of methane. ... 

Finally, this analysis of the data in Table 6.2 shows that the oxidative addition of the C-H bond of methane and the C-C bond of ethane to this iridium compoimd is thermodynamically unfavorable. Althougji die reaction is predicted to be sligjitly favored enthalpicaUy, the free energy is positive because of the large positive TAS term for a process that generates one product from two reactants. The entropy for a reaction of this t37pe is usually dominated by translational entropy, and this value is t5 icaUy about -35 eu. Near room temperature, TAS will then equal about -10 kcal/mol, and the free energy for addition of methane and ethane to this Ir(I) complex is positive. 

Oxidative Addition to Low Valent Metal Complexes. The H/D exchange and oxidative addition of aromatic compounds by low valent coordina-tively unsaturated complexes are well known. On the other hand, clear examples of the oxidative addition of saturated hydrocarbons were first reported in 1982, in which 16-e iridium(I) and rhodium(I) complexes, which are generated through the photoassisted dissociation of CO or H2 were utilized. Similar methodologies are applicable to methane activation. Thus, 16-e cyclopentadienyl iridium(I) complexes, which are generated thermally or under photolysis, easily react with methane and result in methyl hydrido complexes at relatively low temperatures (eq. (D) (2,3). 

Inversion of configuration was reported for the oxidative addition of // a 5-l-bromo-2-fluorocyclohexane to rrfl 5-[IrCl(CO)(PMe3)2], in dichloro-methane. Recently an attempt to confirm this observation failed, and it was claimed that there was no reaction between these two compounds. This is not very surprising in the light of the generally low reactivity of other acyclic halides towards iridium(i) compounds. 

The reactivity of [Ir(GO)2l3Me] with other species has also been investigated, in particular, reactions leading to methane, a known byproduct of iridium-catalyzed carbonylation. Methane formation occurs on reaction of [Ir(GO)2l3Me] either with carboxylic acids or with H2 at elevated temperature. In both cases, the reaction is inhibited by the presence of GO, suggesting that GO dissociation from the reactant complex is required. For the protonolysis reaction with carboxylic acids, a mechanism was proposed (Scheme 8(a)) in which the acid coordinates to a vacant site created by GO loss, and methane is then liberated via a cyclic transition state. The hydrogenolysis reaction, which leads cleanly to [Ir(GO)2l3H] , could proceed via oxidative addition of H2 or an rf-Hz complex as shown in Scheme 8(b). 

Accelerated oxidation of alcohols was found when even 5% ruthenium was added to platinum [241]. It was noticed that alloys containing ruthenium have greater activity than pure platinum in the oxidation of propane, and alloys of platinum with rhodium and iridium were found to be less active. However, the oxidation rate of methane [100] decreased when even small amounts of ruthenium were added to the platinum. Accelerated oxidation of formic acid [224] and of hydrogen— carbon monoxide mixture [242] was observed in the presence of platinum and ruthenium alloys. An improvement in the characteristics of platinum electrodes with the addition of ruthenium was observed by Vielstich [224]. 

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