Activation and Oxidation of Mesitylene C-H Bonds by (Phebox)Iridium(III) Complexes

Meng Zhou, Samantha I. Johnson, Yang Gao, Thomas J. Emge, Robert J. Nielsen, William A. Goddard, Alan S Goldman

Research output: Contribution to journalArticle

7 Citations (Scopus)

Abstract

A pincer iridium(III) complex, (Phebox)Ir(OAc)2OH2 (1) (Phebox = 3,5-dimethylphenyl-2,6-bis(oxazolinyl)), selectively cleaves the benzylic C-H bond of mesitylene to form an isolable iridium mesityl complex, (Phebox)Ir(mesityl)(OAc) (3), in >90% yield. The trifluoroacetate analogue, (Phebox)Ir(OCOCF3)2OH2 (2), was synthesized to compare with complex 1 for C-H activation, and (Phebox)Ir(mesityl)(OCOCF3) (4) was synthesized by ligand exchange of complex 3. Both complexes 1 and 2 catalyze H/D exchange between mesitylene and D2O at 180 °C, exclusively at the benzylic position; 2 gave a higher turnover number (11 TO) than 1 (6 TO) in 12 h. Using d4-acetic acid as the source of deuterium, up to 92 turnovers of benzylic H/D exchange of mesitylene were obtained with complex 1. (Phebox)Ir(OCOCF3)2OH2 catalyzed the benzylic C-H oxidation of mesitylene using Ag2O as a terminal oxidant at 130 C, to form 3,5-dimethylbenzaldehyde and 3,5-dimethylbenzoic acid in 35% ± 4% yield (5.1 ± 0.6 TO). DFT calculations were used to investigate two possible pathways for the catalytic oxidation of mesitylene: (1) C-H activation followed by oxy-functionalization and (2) Ir-oxo formation followed by outer-sphere C-H hydroxylation. Results of calculations of the C-H activation pathway appear to be the more consistent with the experimental observations. (Chemical Equation Presented).

Original languageEnglish
Pages (from-to)2879-2888
Number of pages10
JournalOrganometallics
Volume34
Issue number12
DOIs
Publication statusPublished - Jun 22 2015

Fingerprint

mesitylene
Iridium
iridium
Chemical activation
activation
Oxidation
oxidation
Trifluoroacetic Acid
Hydroxylation
Catalytic oxidation
Deuterium
acetic acid
Oxidants
Discrete Fourier transforms
Acetic Acid
deuterium
analogs
Ligands
ligands
acids

ASJC Scopus subject areas

  • Organic Chemistry
  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

Cite this

Zhou, M., Johnson, S. I., Gao, Y., Emge, T. J., Nielsen, R. J., Goddard, W. A., & Goldman, A. S. (2015). Activation and Oxidation of Mesitylene C-H Bonds by (Phebox)Iridium(III) Complexes. Organometallics, 34(12), 2879-2888. https://doi.org/10.1021/acs.organomet.5b00200

Activation and Oxidation of Mesitylene C-H Bonds by (Phebox)Iridium(III) Complexes. / Zhou, Meng; Johnson, Samantha I.; Gao, Yang; Emge, Thomas J.; Nielsen, Robert J.; Goddard, William A.; Goldman, Alan S.

In: Organometallics, Vol. 34, No. 12, 22.06.2015, p. 2879-2888.

Research output: Contribution to journalArticle

Zhou, M, Johnson, SI, Gao, Y, Emge, TJ, Nielsen, RJ, Goddard, WA & Goldman, AS 2015, 'Activation and Oxidation of Mesitylene C-H Bonds by (Phebox)Iridium(III) Complexes', Organometallics, vol. 34, no. 12, pp. 2879-2888. https://doi.org/10.1021/acs.organomet.5b00200
Zhou M, Johnson SI, Gao Y, Emge TJ, Nielsen RJ, Goddard WA et al. Activation and Oxidation of Mesitylene C-H Bonds by (Phebox)Iridium(III) Complexes. Organometallics. 2015 Jun 22;34(12):2879-2888. https://doi.org/10.1021/acs.organomet.5b00200
Zhou, Meng ; Johnson, Samantha I. ; Gao, Yang ; Emge, Thomas J. ; Nielsen, Robert J. ; Goddard, William A. ; Goldman, Alan S. / Activation and Oxidation of Mesitylene C-H Bonds by (Phebox)Iridium(III) Complexes. In: Organometallics. 2015 ; Vol. 34, No. 12. pp. 2879-2888.
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abstract = "A pincer iridium(III) complex, (Phebox)Ir(OAc)2OH2 (1) (Phebox = 3,5-dimethylphenyl-2,6-bis(oxazolinyl)), selectively cleaves the benzylic C-H bond of mesitylene to form an isolable iridium mesityl complex, (Phebox)Ir(mesityl)(OAc) (3), in >90{\%} yield. The trifluoroacetate analogue, (Phebox)Ir(OCOCF3)2OH2 (2), was synthesized to compare with complex 1 for C-H activation, and (Phebox)Ir(mesityl)(OCOCF3) (4) was synthesized by ligand exchange of complex 3. Both complexes 1 and 2 catalyze H/D exchange between mesitylene and D2O at 180 °C, exclusively at the benzylic position; 2 gave a higher turnover number (11 TO) than 1 (6 TO) in 12 h. Using d4-acetic acid as the source of deuterium, up to 92 turnovers of benzylic H/D exchange of mesitylene were obtained with complex 1. (Phebox)Ir(OCOCF3)2OH2 catalyzed the benzylic C-H oxidation of mesitylene using Ag2O as a terminal oxidant at 130 C, to form 3,5-dimethylbenzaldehyde and 3,5-dimethylbenzoic acid in 35{\%} ± 4{\%} yield (5.1 ± 0.6 TO). DFT calculations were used to investigate two possible pathways for the catalytic oxidation of mesitylene: (1) C-H activation followed by oxy-functionalization and (2) Ir-oxo formation followed by outer-sphere C-H hydroxylation. Results of calculations of the C-H activation pathway appear to be the more consistent with the experimental observations. (Chemical Equation Presented).",
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AB - A pincer iridium(III) complex, (Phebox)Ir(OAc)2OH2 (1) (Phebox = 3,5-dimethylphenyl-2,6-bis(oxazolinyl)), selectively cleaves the benzylic C-H bond of mesitylene to form an isolable iridium mesityl complex, (Phebox)Ir(mesityl)(OAc) (3), in >90% yield. The trifluoroacetate analogue, (Phebox)Ir(OCOCF3)2OH2 (2), was synthesized to compare with complex 1 for C-H activation, and (Phebox)Ir(mesityl)(OCOCF3) (4) was synthesized by ligand exchange of complex 3. Both complexes 1 and 2 catalyze H/D exchange between mesitylene and D2O at 180 °C, exclusively at the benzylic position; 2 gave a higher turnover number (11 TO) than 1 (6 TO) in 12 h. Using d4-acetic acid as the source of deuterium, up to 92 turnovers of benzylic H/D exchange of mesitylene were obtained with complex 1. (Phebox)Ir(OCOCF3)2OH2 catalyzed the benzylic C-H oxidation of mesitylene using Ag2O as a terminal oxidant at 130 C, to form 3,5-dimethylbenzaldehyde and 3,5-dimethylbenzoic acid in 35% ± 4% yield (5.1 ± 0.6 TO). DFT calculations were used to investigate two possible pathways for the catalytic oxidation of mesitylene: (1) C-H activation followed by oxy-functionalization and (2) Ir-oxo formation followed by outer-sphere C-H hydroxylation. Results of calculations of the C-H activation pathway appear to be the more consistent with the experimental observations. (Chemical Equation Presented).

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