Selective Ortho C−H Activation of Haloarenes by an Ir(I) System

The cationic PNP-Ir(I)(cyclooctene) complex 1 (PNP = 2,6-bis-(di-tert-butyl phosphino methyl)pyridine) reacts with benzene at 25 °C to quantitatively yield the crystallographically characterized, square pyramidal, iridium phenyl hydride complex cis-(PNP)Ir(Ph)(H), 2, in which the hydride is trans to the vacant coordination site. The cationic complex 2 is stable to heating at 100 °C, in sharp contrast to the previously reported unstable neutral, isoelectronic (PCP)Ir(H)(Ph) (PCP = η3-2,6-( t Bu2PCH2)2C6H3). Heating of 2 at 50 °C with other arenes results in arene exchange. Complex 1 activates C−H bonds of chloro- and bromobenzene with no C-halide oxidative addition being observed. Selective ortho C−H activation takes place, the process being directed by halogen coordination and being thermodynamically and kinetically favorable. The meta- and para-C−H activation products are formed at a slower rate than the ortho isomer and are converted to it. NMR data and an X-ray crystallographic study of the ortho-activated chlorobenzene complex, which was obtained as the only product upon heating of 1 with chlorobenzene at 60 °C, show that the chloro substituent is coordinated to the metal center.