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)

David A. Laviska, Tian Zhou, Akshai Kumar, Thomas J. Emge, Karsten Krogh-Jespersen, Alan S Goldman

Research output: Contribution to journalArticle

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Abstract

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.

Original languageEnglish
Pages (from-to)1613-1623
Number of pages11
JournalOrganometallics
Volume35
Issue number11
DOIs
Publication statusPublished - Jun 13 2016

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phenanthrene
Chemical activation
Hydrides
activation
Hydrogen
Benzene
hydrides
Kinetics
diphenyl
Electrons
dihydrides
ambient temperature
Temperature
elimination
benzene
fragments

ASJC Scopus subject areas

  • Organic Chemistry
  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

Cite this

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). / Laviska, David A.; Zhou, Tian; Kumar, Akshai; Emge, Thomas J.; Krogh-Jespersen, Karsten; Goldman, Alan S.

In: Organometallics, Vol. 35, No. 11, 13.06.2016, p. 1613-1623.

Research output: Contribution to journalArticle

Laviska, David A. ; Zhou, Tian ; Kumar, Akshai ; Emge, Thomas J. ; Krogh-Jespersen, Karsten ; Goldman, Alan S. / 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). In: Organometallics. 2016 ; Vol. 35, No. 11. pp. 1613-1623.
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abstract = "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.",
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T1 - 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)

AU - Laviska, David A.

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AU - Kumar, Akshai

AU - Emge, Thomas J.

AU - Krogh-Jespersen, Karsten

AU - Goldman, Alan S

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N2 - 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.

AB - 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.

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