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)

• Published in: Organometallics Vol. 35; no. 11; pp. 1613 - 1623
• Main Authors: Laviska, David A, Zhou, Tian, Kumar, Akshai, Emge, Thomas J, Krogh-Jespersen, Karsten, Goldman, Alan S
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 13.06.2016; Amer Chemical Soc
• Subjects: Chemistry; Chemistry, Inorganic & Nuclear; Chemistry, Organic; Physical Sciences; Science & Technology; CATALYZED ASYMMETRIC HYDROGENATION; DEHYDROGENATION; MECHANISM; ALKENE HYDROGENATION; IRIDIUM COMPLEXES; BOND ACTIVATION; OXIDATIVE ADDITION; ENERGY BARRIERS; CLEAVAGE; METAL-COMPLEXES
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/acs.organomet.6b00055

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.