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 language | English |
|---|---|
| Title of host publication | ACS National Meeting Book of Abstracts |
| Publication status | Published - 2008 |
| Event | 236th National Meeting and Exposition of the American Chemical Society, ACS 2008 - Philadelpia, PA, United States Duration: Aug 17 2008 → Aug 21 2008 |
Other
| Other | 236th National Meeting and Exposition of the American Chemical Society, ACS 2008 |
|---|---|
| Country | United States |
| City | Philadelpia, PA |
| Period | 8/17/08 → 8/21/08 |
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ASJC Scopus subject areas
- Chemistry(all)
- Chemical Engineering(all)
Cite this
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 proceeding › Conference contribution
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TY - GEN
T1 - 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
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.
UR - http://www.scopus.com/inward/record.url?scp=77955914573&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=77955914573&partnerID=8YFLogxK
M3 - Conference contribution
AN - SCOPUS:77955914573
SN - 9780841269941
BT - ACS National Meeting Book of Abstracts
ER -