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.
ASJC Scopus subject areas
- Organic Chemistry
- Physical and Theoretical Chemistry
- Inorganic Chemistry