Rational design and synthesis of highly active pincer-iridium catalysts for alkane dehydrogenation

Sabuj Kundu, Yuriy Choliy, Gao Zhuo, Ritu Ahuja, Thomas J. Emge, Ralf Warmuth, Maurice Brookhart, Karsten Krogh-Jespersen, Alan S Goldman

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Abstract

"PCP" -pincer-ligated iridium complexes have been found to be highly effective catalysts for the dehydrogenation of alkanes. We report a computational and experimental study of the effect on catalytic activity resulting from systematically varying steric crowding by the substitution of methyl groups for the phosphino tert-butyl groups of ( R4PCP)Ir ( R4PCP = κ 3-C 6H 3-2,6-(CH 2PR 2) 2; R = 1Bu or Me). DFT calculations for ( R4PCP)Ir species (R 4 = tBu 4 or tBu 3Me) indicate that the ratedetermining step in the n-alkane/1-alkene transfer dehydrogenation cycle is β-H elimination by ( R4PCP)Ir(n-alkyl)(H). It is calculated that the transition state for this step is ca. 10 kcal/mol lower for ( tBu3MePCP)Ir than for ( tBu4PCP)Ir (relative to the corresponding free ( R4PCP)Ir). However, this catalytically favorable effect is calculated to be partially offset by the strong binding of 1-alkene to ( tBu3MePCP)Ir in the resting state, so the overall barrier is thus lower by only ca. 4 kcal/mol. Further Me-for- tBu substitutions have a smaller effect on the transition states, and the calculated energy of the olefin-bound resting states is lowered by comparable amounts; therefore these additional substitutions are predicted to have little overall favorable effect on catalytic rates. ( tBu3MePCP)IrH 4 has been synthesized and isolated, and its catalytic activity has been investigated. It is indeed found to be a more active catalyst precursor than ( tBu4PCP)IrH 4 for alkane transfer dehydrogenation. ( tBu2Me2PCP) IrH 4 was also synthesized and as a catalyst precursor is found to afford somewhat lower activity than ( tBu3MePCP)IrH 4. However, synthetic precursors of ( tBu2Me2PCP)IrH 4 tended to yield dinuclear clusters, while complex mixtures were observed during catalysis that were not amenable to characterization. It is therefore not clear if the lesser catalytic activity of ( tBu2Me2PCP)Ir vs ( tBu3MePCP)Ir derivatives is due to the energetics of the actual catalytic cycle or due to deactivation of this catalyst via the facile formation of clusters.

Original languageEnglish
Pages (from-to)5432-5444
Number of pages13
JournalOrganometallics
Volume28
Issue number18
DOIs
Publication statusPublished - Sep 28 2009

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Iridium
Alkanes
Dehydrogenation
dehydrogenation
iridium
alkanes
Alkenes
alkenes
catalytic activity
catalysts
Catalyst activity
Catalysts
Substitution reactions
substitutes
synthesis
cycles
crowding
Complex Mixtures
Discrete Fourier transforms
deactivation

ASJC Scopus subject areas

  • Organic Chemistry
  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

Cite this

Rational design and synthesis of highly active pincer-iridium catalysts for alkane dehydrogenation. / Kundu, Sabuj; Choliy, Yuriy; Zhuo, Gao; Ahuja, Ritu; Emge, Thomas J.; Warmuth, Ralf; Brookhart, Maurice; Krogh-Jespersen, Karsten; Goldman, Alan S.

In: Organometallics, Vol. 28, No. 18, 28.09.2009, p. 5432-5444.

Research output: Contribution to journalArticle

Kundu, S, Choliy, Y, Zhuo, G, Ahuja, R, Emge, TJ, Warmuth, R, Brookhart, M, Krogh-Jespersen, K & Goldman, AS 2009, 'Rational design and synthesis of highly active pincer-iridium catalysts for alkane dehydrogenation', Organometallics, vol. 28, no. 18, pp. 5432-5444. https://doi.org/10.1021/om900568f
Kundu, Sabuj ; Choliy, Yuriy ; Zhuo, Gao ; Ahuja, Ritu ; Emge, Thomas J. ; Warmuth, Ralf ; Brookhart, Maurice ; Krogh-Jespersen, Karsten ; Goldman, Alan S. / Rational design and synthesis of highly active pincer-iridium catalysts for alkane dehydrogenation. In: Organometallics. 2009 ; Vol. 28, No. 18. pp. 5432-5444.
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abstract = "{"}PCP{"} -pincer-ligated iridium complexes have been found to be highly effective catalysts for the dehydrogenation of alkanes. We report a computational and experimental study of the effect on catalytic activity resulting from systematically varying steric crowding by the substitution of methyl groups for the phosphino tert-butyl groups of ( R4PCP)Ir ( R4PCP = κ 3-C 6H 3-2,6-(CH 2PR 2) 2; R = 1Bu or Me). DFT calculations for ( R4PCP)Ir species (R 4 = tBu 4 or tBu 3Me) indicate that the ratedetermining step in the n-alkane/1-alkene transfer dehydrogenation cycle is β-H elimination by ( R4PCP)Ir(n-alkyl)(H). It is calculated that the transition state for this step is ca. 10 kcal/mol lower for ( tBu3MePCP)Ir than for ( tBu4PCP)Ir (relative to the corresponding free ( R4PCP)Ir). However, this catalytically favorable effect is calculated to be partially offset by the strong binding of 1-alkene to ( tBu3MePCP)Ir in the resting state, so the overall barrier is thus lower by only ca. 4 kcal/mol. Further Me-for- tBu substitutions have a smaller effect on the transition states, and the calculated energy of the olefin-bound resting states is lowered by comparable amounts; therefore these additional substitutions are predicted to have little overall favorable effect on catalytic rates. ( tBu3MePCP)IrH 4 has been synthesized and isolated, and its catalytic activity has been investigated. It is indeed found to be a more active catalyst precursor than ( tBu4PCP)IrH 4 for alkane transfer dehydrogenation. ( tBu2Me2PCP) IrH 4 was also synthesized and as a catalyst precursor is found to afford somewhat lower activity than ( tBu3MePCP)IrH 4. However, synthetic precursors of ( tBu2Me2PCP)IrH 4 tended to yield dinuclear clusters, while complex mixtures were observed during catalysis that were not amenable to characterization. It is therefore not clear if the lesser catalytic activity of ( tBu2Me2PCP)Ir vs ( tBu3MePCP)Ir derivatives is due to the energetics of the actual catalytic cycle or due to deactivation of this catalyst via the facile formation of clusters.",
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T1 - Rational design and synthesis of highly active pincer-iridium catalysts for alkane dehydrogenation

AU - Kundu, Sabuj

AU - Choliy, Yuriy

AU - Zhuo, Gao

AU - Ahuja, Ritu

AU - Emge, Thomas J.

AU - Warmuth, Ralf

AU - Brookhart, Maurice

AU - Krogh-Jespersen, Karsten

AU - Goldman, Alan S

PY - 2009/9/28

Y1 - 2009/9/28

N2 - "PCP" -pincer-ligated iridium complexes have been found to be highly effective catalysts for the dehydrogenation of alkanes. We report a computational and experimental study of the effect on catalytic activity resulting from systematically varying steric crowding by the substitution of methyl groups for the phosphino tert-butyl groups of ( R4PCP)Ir ( R4PCP = κ 3-C 6H 3-2,6-(CH 2PR 2) 2; R = 1Bu or Me). DFT calculations for ( R4PCP)Ir species (R 4 = tBu 4 or tBu 3Me) indicate that the ratedetermining step in the n-alkane/1-alkene transfer dehydrogenation cycle is β-H elimination by ( R4PCP)Ir(n-alkyl)(H). It is calculated that the transition state for this step is ca. 10 kcal/mol lower for ( tBu3MePCP)Ir than for ( tBu4PCP)Ir (relative to the corresponding free ( R4PCP)Ir). However, this catalytically favorable effect is calculated to be partially offset by the strong binding of 1-alkene to ( tBu3MePCP)Ir in the resting state, so the overall barrier is thus lower by only ca. 4 kcal/mol. Further Me-for- tBu substitutions have a smaller effect on the transition states, and the calculated energy of the olefin-bound resting states is lowered by comparable amounts; therefore these additional substitutions are predicted to have little overall favorable effect on catalytic rates. ( tBu3MePCP)IrH 4 has been synthesized and isolated, and its catalytic activity has been investigated. It is indeed found to be a more active catalyst precursor than ( tBu4PCP)IrH 4 for alkane transfer dehydrogenation. ( tBu2Me2PCP) IrH 4 was also synthesized and as a catalyst precursor is found to afford somewhat lower activity than ( tBu3MePCP)IrH 4. However, synthetic precursors of ( tBu2Me2PCP)IrH 4 tended to yield dinuclear clusters, while complex mixtures were observed during catalysis that were not amenable to characterization. It is therefore not clear if the lesser catalytic activity of ( tBu2Me2PCP)Ir vs ( tBu3MePCP)Ir derivatives is due to the energetics of the actual catalytic cycle or due to deactivation of this catalyst via the facile formation of clusters.

AB - "PCP" -pincer-ligated iridium complexes have been found to be highly effective catalysts for the dehydrogenation of alkanes. We report a computational and experimental study of the effect on catalytic activity resulting from systematically varying steric crowding by the substitution of methyl groups for the phosphino tert-butyl groups of ( R4PCP)Ir ( R4PCP = κ 3-C 6H 3-2,6-(CH 2PR 2) 2; R = 1Bu or Me). DFT calculations for ( R4PCP)Ir species (R 4 = tBu 4 or tBu 3Me) indicate that the ratedetermining step in the n-alkane/1-alkene transfer dehydrogenation cycle is β-H elimination by ( R4PCP)Ir(n-alkyl)(H). It is calculated that the transition state for this step is ca. 10 kcal/mol lower for ( tBu3MePCP)Ir than for ( tBu4PCP)Ir (relative to the corresponding free ( R4PCP)Ir). However, this catalytically favorable effect is calculated to be partially offset by the strong binding of 1-alkene to ( tBu3MePCP)Ir in the resting state, so the overall barrier is thus lower by only ca. 4 kcal/mol. Further Me-for- tBu substitutions have a smaller effect on the transition states, and the calculated energy of the olefin-bound resting states is lowered by comparable amounts; therefore these additional substitutions are predicted to have little overall favorable effect on catalytic rates. ( tBu3MePCP)IrH 4 has been synthesized and isolated, and its catalytic activity has been investigated. It is indeed found to be a more active catalyst precursor than ( tBu4PCP)IrH 4 for alkane transfer dehydrogenation. ( tBu2Me2PCP) IrH 4 was also synthesized and as a catalyst precursor is found to afford somewhat lower activity than ( tBu3MePCP)IrH 4. However, synthetic precursors of ( tBu2Me2PCP)IrH 4 tended to yield dinuclear clusters, while complex mixtures were observed during catalysis that were not amenable to characterization. It is therefore not clear if the lesser catalytic activity of ( tBu2Me2PCP)Ir vs ( tBu3MePCP)Ir derivatives is due to the energetics of the actual catalytic cycle or due to deactivation of this catalyst via the facile formation of clusters.

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