Iridium reaction with methane

Reactions of Rhodium and Iridium Complexes with Methane... 

The activation of methane in solution by an organometallic complex presents some experimental difficulties because any solvent that is likely to be chosen will be more reactive than methane. In addition, insolubility of the complex in liquid methane may preclude reaction with the pure hydrocarbon. These problems were overcome in the case of the reaction of CH4 with the iridium complex of Eq. 15.106 by taking advantage of the fact that the desired hydndo methyl complex Is thermodynamically more stable than other hydrido alkyl complexes. The methyl complex was produced by first creating a hydrido cyclohexyl complex and then allowing it to react with methane, m... 

The fact that iridium does not react with methane is easy to understand from the calculations. According to Figure 4 the spin-crossing point lies about 18 kcal mol above the reactant asymptote. This barrier is sufficiently high to prevent access to the low-spin C-H insertion well for ground state Ir at 300 K. Comparably high barriers are expected for the Ir reactions with larger alkanes. 

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). 

Following this observation, a general approach for the synthesis of pincer-type methylene arenium compounds was developed (Scheme 3.4). Upon reaction of the methyl rhodium (or iridium) complexes 5 with a slight excess of triflic acid, dihydrogen (not methane ) was evolved to form the methylene arenium complexes 4a.11 Thus, the methylene arenium form is clearly preferred over the benzylic M(III) form, in which the positive charge is localized at the metal center. 

The iridium complex [Ir(cod)(//2-,PrPCH2CH2OMe)]+BF4 (22) in dichloro-methane at 25 °C at 1 bar H2 is a particularly active catalyst for the hydrogenation of phenyl acetylene to styrene [29]. In a typical experiment, an average TOF of 50 mol mol-1 h-1 was obtained (calculated from a turnover number, TON, of 125) with a selectivity close to 100%. The mechanism of this reaction has been elucidated by a combination of kinetic, chemical and spectroscopic data (Scheme 14.10). 

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