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

Iridium(III) dimethyl complexes, CpIr(PR(3))Me(2) (Cp* = eta(5)-C(5)M e(5); R = Ph (1a), Me (1b), react slowly (1a), or not at all (Ib), wit h C-H bonds of aromatic hydrocarbons under severe conditions (110 degr ees 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)). W ith benzene and trifluorotoluene, the diaryl complexes CpIr(PPh(3))Ar -2 are also formed. In contrast, the reaction of both la and Ib with a renes, in the presence of catalytic amounts of an oxidant (Cp(2)Fe(+)P F(6)(-), AgBF4, or Ph(3)C(+)BF(4)(-)), proceeds rapidly at room temper ature: the corresponding methyl aryl derivatives CPIr(PR(3))(Me)(AT) (R = Ph; Ar = C6H5, C6D5, C(6)H(4)Me, C6H4CF3, C(6)H(3)Me(2) R = Me; A r = 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 mixtu re 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 GRAPHI CS (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 invol ves a 1-electron process, yielding the corresponding iridium(IV) param agnetic cations, as shown by coupled electrochemical-ESR studies. AgBF 4 oxidation of 1a and 1b in CH2Cl2 gives instead radical species, whic h have been shown by I I ESR spectroscopy to have the ''tucked-in'' st ructure GRAPHICS Such species are proved to be involved as intermedi ates in the above arene C-H activation reactions.