Single and Double C–H Activation of Biphenyl or Phenanthrene. An Example of C–H Addition to Ir(III) More Facile than Addition to Ir(I)

The species ( R4 PCP)Ir are found to effect a double C–H activation addition of biphenyl or phenanthrene to give the corresponding cyclometalated complexes (biphenyl-2,2′-diyl and phenanthrene-4,5-diyl, respectively), which have been characterized spectroscopically and crystallographically. The rate-determining step of the overall reactions is calculated to be the 14-electron ( R4 PCP)Ir(I) fragment undergoing addition of the sterically hindered C–H bond positioned ortho to the interaryl ring C–C bond. The resulting Ir(III) aryl hydride undergoes a subsequent second C–H addition to give a cyclometalated Ir(V) dihydride complex. This C–H addition to Ir(III) is calculated to be very facile: e.g., a barrier as low as Δ G ⧧ = 5.9 kcal/mol in the case of ( tBu4 PCP)Ir(H)( o -phenanthrenyl). The computational results are fully consistent with, and facilitate explaining, the experimental observations. ( tBu4 PCP)Ir(NBE) adds an unhindered ( m or p ) C–H bond of biphenyl or phenanthrene (following loss of NBE) to give an observable Ir(III) aryl hydride. At ambient temperature these species slowly (ca. 24 h) convert to the cyclometalated complexes; the presumed o -C–H addition intermediate is never present at concentrations sufficiently high to be observed. In contrast, in the case of ( iPr4 PCP)Ir, which is much less hindered than ( tBu4 PCP)Ir, the reaction with biphenyl does not lead to any observable mono-C–H addition intermediate; this is consistent with a relatively rapid addition of the o -C–H bond followed by an even faster second C–H addition (cyclometalation) by the resulting Ir(III) aryl hydride. Inter molecular double C–H addition has also been explored computationally. Addition of benzene to the Ir(III) species ( R4 PCP)Ir(H)Ph to afford ( R4 PCP)Ir(H) 2 Ph 2 is calculated to have a very low barrier for the sterically uncrowded ( Me4 PCP)Ir species. We propose that the very facile kinetics of Ir(III)/Ir(V) C–H additions/eliminations has significant implications for C–C coupling and other catalytic reactions.