Electron-transfer catalysis in the activation of arene C-H bonds by Iridium complexes and the importance of steric factors in determining the nature of the activation products

The outcome of the activation reactions of arene C-H bonds with the complexes [IrMe(2)Cp*L] (Cp*=eta(5)-C(5)Me(5); L=PPh(3), 1a; PMePh(2), 1b; PMe(2)Ph, 1c; PMe(3), 1d) in the presence of catalytic amounts of [FeCp(2)](+) is strongly dependent on the nature of the phosphine. While la gives a mixture of intra- and inter-molecular reaction products, 1b and 1c give only the corresponding methyl phenyl derivatives [IrMePhCp*L] 3b and 3c. The intramolecular reaction product [Ir(C(6)H(4)PMePh)Cp*Me], 2b, is obtained by treatment of 1b with [FeCp(2)](+) in the absence of benzene, as a 1:1 mixture of the two diastereomers, whose configurations, together with that of the related (RR:SS) [Ir(C(6)H(4)PMePh)Cp*(CH(2)CMe(3))] obtained by a different route, have been established by H-1 NMR techniques. 2b, if reacted with benzene in the presence ol: the ferrocenium cation, produces quantitatively the methyl phenyl derivative 3b. The oxidatively induced activation of benzene-d(6) proceeds with different rates depending on the phosphine: k(rel)=1 (1a), 2 (1b), 2.5 (Ic), 7.5 (Id). Id is able to activate C-H bonds of biphenyl and naphthalene. Electrochemical techniques and ESR spectroscopy have been used for a mechanistic investigation of these reactions