A tale of two ligands. Subtle differences in sterics exert major differences in the energetics of alkane dehydrogenation catalyzed by pincer complexes (tBuPCP)Ir vs. (tBuPOCOP)Ir

Soumik Biswas, Ritu Ahuja, Amlan Ray, Yuriy Choliy, Karsten Krogh-Jespersen, Maurice Brookhart, Alan S Goldman

Research output: Chapter in Book/Report/Conference proceedingConference contribution

Abstract

Complexes of pincer-ligated iridium fragments (tBuPCP)Ir and (tBuPOCOP)Ir are effective catalysts for the transfer-dehydrogenation of alkanes, a reaction with great potential value. Precursors of both species are effective co-catalysts as components in alkane metathesis systems based on tandem alkane-dehydrogenation/olefin-metathesis. However, of this pair, only (tBuPCP)Ir has been reported to effect metathesis of Cn alkanes with the desirable selectivity for the C2n-2 product. Although these ligands might seem quite similar, especially in terms of steric parameters, we find profound differences in their reactivity. For example, (tBuPOCOP)Ir(C2H4) is approximately 6 times faster than (tBuPCP)IrH2 for cyclooctane/t-butylethylene transfer dehydrogenation. But for n-octane/1-hexene transfer dehydrogenation, the order is markedly reversed: (tBuPCP)IrH2 is > ca. 100 times faster than (tBuPOCOP)Ir(C2H4). Preliminary results indicate that the rate-determining step in (tBuPCP)Ir-co-catalyzed n-alkane metathesis is hydrogenation of the internal olefin intermediate, while n-alkane dehydrogenation is rate-determining with (tBuPOCOP)Ir. DFT calculations are in accord with these results, and with the surprising conclusion that the differences are largely due to steric effects. The smaller O linkage in the POCOP ligand (vs. CH2 in PCP) "holds back" the PR2 groups to significantly "open up" the ligand, allowing easier access for alkanes (especially non-linear) but also much stronger binding of olefins which inhibits reactivity. The selectivity between branched and n-alkanes has also been studied. (tBuPCP)Ir-based catalysts show remarkably high selectivity (>1000:1) for the dehydrogenation of a terminal alkyl group vs. dehydrogenation of a primary-tertiary C-C linkage.

Original languageEnglish
Title of host publicationACS National Meeting Book of Abstracts
Publication statusPublished - 2008
Event236th National Meeting and Exposition of the American Chemical Society, ACS 2008 - Philadelpia, PA, United States
Duration: Aug 17 2008Aug 21 2008

Other

Other236th National Meeting and Exposition of the American Chemical Society, ACS 2008
CountryUnited States
CityPhiladelpia, PA
Period8/17/088/21/08

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Alkanes
Dehydrogenation
Paraffins
Ligands
Alkenes
Olefins
Catalysts
Iridium
Catalyst selectivity
Discrete Fourier transforms
Hydrogenation

ASJC Scopus subject areas

  • Chemistry(all)
  • Chemical Engineering(all)

Cite this

Biswas, S., Ahuja, R., Ray, A., Choliy, Y., Krogh-Jespersen, K., Brookhart, M., & Goldman, A. S. (2008). A tale of two ligands. Subtle differences in sterics exert major differences in the energetics of alkane dehydrogenation catalyzed by pincer complexes (tBuPCP)Ir vs. (tBuPOCOP)Ir. In ACS National Meeting Book of Abstracts

A tale of two ligands. Subtle differences in sterics exert major differences in the energetics of alkane dehydrogenation catalyzed by pincer complexes (tBuPCP)Ir vs. (tBuPOCOP)Ir. / Biswas, Soumik; Ahuja, Ritu; Ray, Amlan; Choliy, Yuriy; Krogh-Jespersen, Karsten; Brookhart, Maurice; Goldman, Alan S.

ACS National Meeting Book of Abstracts. 2008.

Research output: Chapter in Book/Report/Conference proceedingConference contribution

Biswas, S, Ahuja, R, Ray, A, Choliy, Y, Krogh-Jespersen, K, Brookhart, M & Goldman, AS 2008, A tale of two ligands. Subtle differences in sterics exert major differences in the energetics of alkane dehydrogenation catalyzed by pincer complexes (tBuPCP)Ir vs. (tBuPOCOP)Ir. in ACS National Meeting Book of Abstracts. 236th National Meeting and Exposition of the American Chemical Society, ACS 2008, Philadelpia, PA, United States, 8/17/08.
Biswas S, Ahuja R, Ray A, Choliy Y, Krogh-Jespersen K, Brookhart M et al. A tale of two ligands. Subtle differences in sterics exert major differences in the energetics of alkane dehydrogenation catalyzed by pincer complexes (tBuPCP)Ir vs. (tBuPOCOP)Ir. In ACS National Meeting Book of Abstracts. 2008
Biswas, Soumik ; Ahuja, Ritu ; Ray, Amlan ; Choliy, Yuriy ; Krogh-Jespersen, Karsten ; Brookhart, Maurice ; Goldman, Alan S. / A tale of two ligands. Subtle differences in sterics exert major differences in the energetics of alkane dehydrogenation catalyzed by pincer complexes (tBuPCP)Ir vs. (tBuPOCOP)Ir. ACS National Meeting Book of Abstracts. 2008.
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abstract = "Complexes of pincer-ligated iridium fragments (tBuPCP)Ir and (tBuPOCOP)Ir are effective catalysts for the transfer-dehydrogenation of alkanes, a reaction with great potential value. Precursors of both species are effective co-catalysts as components in alkane metathesis systems based on tandem alkane-dehydrogenation/olefin-metathesis. However, of this pair, only (tBuPCP)Ir has been reported to effect metathesis of Cn alkanes with the desirable selectivity for the C2n-2 product. Although these ligands might seem quite similar, especially in terms of steric parameters, we find profound differences in their reactivity. For example, (tBuPOCOP)Ir(C2H4) is approximately 6 times faster than (tBuPCP)IrH2 for cyclooctane/t-butylethylene transfer dehydrogenation. But for n-octane/1-hexene transfer dehydrogenation, the order is markedly reversed: (tBuPCP)IrH2 is > ca. 100 times faster than (tBuPOCOP)Ir(C2H4). Preliminary results indicate that the rate-determining step in (tBuPCP)Ir-co-catalyzed n-alkane metathesis is hydrogenation of the internal olefin intermediate, while n-alkane dehydrogenation is rate-determining with (tBuPOCOP)Ir. DFT calculations are in accord with these results, and with the surprising conclusion that the differences are largely due to steric effects. The smaller O linkage in the POCOP ligand (vs. CH2 in PCP) {"}holds back{"} the PR2 groups to significantly {"}open up{"} the ligand, allowing easier access for alkanes (especially non-linear) but also much stronger binding of olefins which inhibits reactivity. The selectivity between branched and n-alkanes has also been studied. (tBuPCP)Ir-based catalysts show remarkably high selectivity (>1000:1) for the dehydrogenation of a terminal alkyl group vs. dehydrogenation of a primary-tertiary C-C linkage.",
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AU - Biswas, Soumik

AU - Ahuja, Ritu

AU - Ray, Amlan

AU - Choliy, Yuriy

AU - Krogh-Jespersen, Karsten

AU - Brookhart, Maurice

AU - Goldman, Alan S

PY - 2008

Y1 - 2008

N2 - Complexes of pincer-ligated iridium fragments (tBuPCP)Ir and (tBuPOCOP)Ir are effective catalysts for the transfer-dehydrogenation of alkanes, a reaction with great potential value. Precursors of both species are effective co-catalysts as components in alkane metathesis systems based on tandem alkane-dehydrogenation/olefin-metathesis. However, of this pair, only (tBuPCP)Ir has been reported to effect metathesis of Cn alkanes with the desirable selectivity for the C2n-2 product. Although these ligands might seem quite similar, especially in terms of steric parameters, we find profound differences in their reactivity. For example, (tBuPOCOP)Ir(C2H4) is approximately 6 times faster than (tBuPCP)IrH2 for cyclooctane/t-butylethylene transfer dehydrogenation. But for n-octane/1-hexene transfer dehydrogenation, the order is markedly reversed: (tBuPCP)IrH2 is > ca. 100 times faster than (tBuPOCOP)Ir(C2H4). Preliminary results indicate that the rate-determining step in (tBuPCP)Ir-co-catalyzed n-alkane metathesis is hydrogenation of the internal olefin intermediate, while n-alkane dehydrogenation is rate-determining with (tBuPOCOP)Ir. DFT calculations are in accord with these results, and with the surprising conclusion that the differences are largely due to steric effects. The smaller O linkage in the POCOP ligand (vs. CH2 in PCP) "holds back" the PR2 groups to significantly "open up" the ligand, allowing easier access for alkanes (especially non-linear) but also much stronger binding of olefins which inhibits reactivity. The selectivity between branched and n-alkanes has also been studied. (tBuPCP)Ir-based catalysts show remarkably high selectivity (>1000:1) for the dehydrogenation of a terminal alkyl group vs. dehydrogenation of a primary-tertiary C-C linkage.

AB - Complexes of pincer-ligated iridium fragments (tBuPCP)Ir and (tBuPOCOP)Ir are effective catalysts for the transfer-dehydrogenation of alkanes, a reaction with great potential value. Precursors of both species are effective co-catalysts as components in alkane metathesis systems based on tandem alkane-dehydrogenation/olefin-metathesis. However, of this pair, only (tBuPCP)Ir has been reported to effect metathesis of Cn alkanes with the desirable selectivity for the C2n-2 product. Although these ligands might seem quite similar, especially in terms of steric parameters, we find profound differences in their reactivity. For example, (tBuPOCOP)Ir(C2H4) is approximately 6 times faster than (tBuPCP)IrH2 for cyclooctane/t-butylethylene transfer dehydrogenation. But for n-octane/1-hexene transfer dehydrogenation, the order is markedly reversed: (tBuPCP)IrH2 is > ca. 100 times faster than (tBuPOCOP)Ir(C2H4). Preliminary results indicate that the rate-determining step in (tBuPCP)Ir-co-catalyzed n-alkane metathesis is hydrogenation of the internal olefin intermediate, while n-alkane dehydrogenation is rate-determining with (tBuPOCOP)Ir. DFT calculations are in accord with these results, and with the surprising conclusion that the differences are largely due to steric effects. The smaller O linkage in the POCOP ligand (vs. CH2 in PCP) "holds back" the PR2 groups to significantly "open up" the ligand, allowing easier access for alkanes (especially non-linear) but also much stronger binding of olefins which inhibits reactivity. The selectivity between branched and n-alkanes has also been studied. (tBuPCP)Ir-based catalysts show remarkably high selectivity (>1000:1) for the dehydrogenation of a terminal alkyl group vs. dehydrogenation of a primary-tertiary C-C linkage.

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