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 (^R4PCP)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 (^R4PCP)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 ΔG^- = 5.9 kcal/mol in the case of (^tBu4PCP)Ir(H)(o-phenanthrenyl). The computational results are fully consistent with, and facilitate explaining, the experimental observations. (^tBu4PCP)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 (^iPr4PCP)Ir, which is much less hindered than (^tBu4PCP)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. Intermolecular double C-H addition has also been explored computationally. Addition of benzene to the Ir(III) species (^R4PCP)Ir(H)Ph to afford (^R4PCP)Ir(H)₂Ph₂ is calculated to have a very low barrier for the sterically uncrowded (^Me4PCP)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.