ELECTRON-TRANSFER CATALYSIS IN THE ACTIVATION OF C-H BONDS BY IRIDIUM COMPLEXES

Iridium(III) dimethyl complexes, Cp*Ir(PR(3))Me(2) (Cp* = eta(5)-C(5)Me(5); R = Ph (1a), Me (1b), react slowly (1a), or not at all (Ib), with C-H bonds of aromatic hydrocarbons under severe conditions (110 degrees C, 2 weeks) to give methane and the new methyl aryl derivatives Cp*Ir(PPh(3))(Me)(Ar) (Ar = C6H5, C(6)H(4)Me, C6H4CF3, C(6)H(3)Me(2)). With benzene and trifluorotoluene, the diaryl complexes Cp*Ir(PPh(3))Ar-2 are also formed. In contrast, the reaction of both la and Ib with arenes, in the presence of catalytic amounts of an oxidant (Cp(2)Fe(+)PF(6)(-), AgBF4, or Ph(3)C(+)BF(4)(-)), proceeds rapidly at room temperature: the corresponding methyl aryl derivatives CP*Ir(PR(3))(Me)(AT) (R = Ph; Ar = C6H5, C6D5, C(6)H(4)Me, C6H4CF3, C(6)H(3)Me(2) R = Me; Ar = C6H5, C6D5, C6H4Cl, C6H4Br, C6H4F, C6H4CF3, C6H4NO2, C(6)H(4)Me, C(6)H(3)Me(2)) are produced. In the reaction with benzene-d(6), a mixture of CH4 and CH3D is obtained. When substituted arenes are used, only the meta and para C-H bonds react. In the case of la, the methyl aryl derivative is in equilibrium with the orthometalated compound [GRAPHICS] (12), which is the primary reaction product. Evidence is provided for a facile one-electron oxidation of la and Ib. The electrochemical oxidation of la, Ib, and related iridium(III) dialkyls in CH2Cl2 involves a 1-electron process, yielding the corresponding iridium(IV) paramagnetic cations, as shown by coupled electrochemical-ESR studies. AgBF4 oxidation of 1a and 1b in CH2Cl2 gives instead radical species, which have been shown by I I ESR spectroscopy to have the "tucked-in" structure [GRAPHICS] Such species are proved to be involved as intermediates in the above arene C-H activation reactions.