Oxidative addition of soluble iridium and rhodium complexes to carbon-hydrogen bonds in methane and higher alkanes (1)

Abstract

A new type of iridium complex has been synthesized which successfully converts alkanes into hydridoalkylmetal complexes (M + R-H →R-M-H). This material has the general formula Cp*(L)IrH2, where Cp* = n5 -C5Me5, and L = PMe3 (or, in a few cases, related phosphines). Upon irradiation with ultraviolet light, the dihydride loses H2, generating the reactive intermediate Cp*IrL, which reacts rapidly with C-H bonds in every molecule so far investigated (including alkanes), leading to hydridoalkyliridium complexes Cp* (L) Ir (R) (H) • Evidence has been obtained that this C-H insertion (oxidative addition) reaction proceeds through a simple three-center transition state and does not involve organic free radicals as intermediates. In accordance with this, the intermediate Cp*IrL reacts most rapidly with C-H bonds having relatively high bond energies, such as those at primary carbon centers, in small organic rings, and in aromatic rings. This contrasts directly with the type of hydrogen-abstraction selectivity characteristic of organic radicals. The hydridoalkyliridium products of the insertion reactions can be converted into functionalized organic molecules-alkyl halides-by treatment with mercuric chloride followed by halogens. Expulsion (reductive elimination) of the hydrocarbon from the hydridoalkyliridium complexes can be induced by Lewis acids or heat, regenerating the reactive intermediate Cp*IrL, which is then capable of attacking the C-H bond of other hydrocarbons. This property has been used to examine the interconversion of different hydridoalkyliridium complexes. By determining the equilibrium constants for these interconversions, one obtains a method of estimating relative iridium-carbon bond energies. The equilibrations have also been used to devise a thermal method for activating methane. In this case, heating the cyclohexyl-(hydrido)iridium complex in cyclooctane under 20 atm of CH4 produced a 58% yield of Cp*(L) Ir (CH3) (H), which is the thermodynamically most stable C-H insertion product in this system. Oxidative addition of the corresponding rhodium complexes Cp*RhL to alkane C-H bonds has also been observed, although the products formed in this case are much less stable, and undergo reductive elimination at -20°. These and other recent observations provide an incentive for reexamining the factors which have been assumed to control the rate of reaction of transition metal complexes with C-H bonds- notably the need for electron-rich metals and the close proximity of reacting centers. © 1984 IUPAC