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.