Highly effective pincer-ligated iridium catalysts for alkane dehydrogenation. DFT calculations of relevant thermodynamic, kinetic, and spectroscopic properties

Keming Zhu, Patrick D. Achord, Xiawei Zhang, Karsten Krogh-Jespersen, Alan S. Goldman

Research output: Contribution to journalArticle

204 Citations (Scopus)

Abstract

The p-methoxy-substituted pincer-ligated iridium complexes, (MeO- tBuPCP)IrH4 (RPCP = κ3-C 6H3-2,6-(CH2PR2)2) and (MeO-iPrPCP)IrH4, are found to be highly effective catalysts for the dehydrogenation of alkanes (both with and without the use of sacrificial hydrogen acceptors). These complexes offer an interesting comparison with the recently reported bis-phosphinite "POCOP" (RPOCOP = κ3-C6H3-2,6-(OPR2) 2) pincer-ligated catalysts, which also show catalytic activity higher than unsubstituted PCP analogues (Göttker-Schnetmann, I.; White, P.; Brookhart, M. J. Am. Chem. Soc. 2004, 126, 1804). On the basis of νCO values of the respective CO adducts, the MeO-PCP complexes appear to be more electron-rich than the parent PCP complexes, whereas the POCOP complexes appear to be more electron-poor. However, the MeO-PCP and POCOP ligands are calculated (DFT) to show effects in the same directions, relative to the parent PCP ligand, for the kinetics and thermodynamics of a broad range of reactions including the addition of C-H and H-H bonds and CO. In general, both ligands favor (relative to unsubstituted PCP) addition to the 14e (pincer)Ir fragments but disfavor addition to the 16e complexes (pincer)IrH2 or (pincer)-Ir(CO). These kinetic and thermodynamic effects are all largely attributable to the same electronic feature: O → C(aryl) π-donation, from the methoxy or phosphinito groups of the respective ligands. DFT calculations also indicate that the kinetics (but not the thermodynamics) of C-H addition to (pincer)Ir are favored by σ-withdrawal from the phosphorus atoms. The high νCO value of (POCOP)Ir(CO) is attributable to electrostatic effects, rather than decreased Ir-CO π-donation or increased OC-Ir σ-donation.

Original languageEnglish
Pages (from-to)13044-13053
Number of pages10
JournalJournal of the American Chemical Society
Volume126
Issue number40
Publication statusPublished - Oct 13 2004

Fingerprint

Iridium
Alkanes
Dehydrogenation
Carbon Monoxide
Thermodynamics
Discrete Fourier transforms
Paraffins
Ligands
Catalysts
Kinetics
Electrons
Phosphorus
Electrostatics
Catalyst activity
Atoms
Hydrogen
Static Electricity

ASJC Scopus subject areas

  • Chemistry(all)

Cite this

Highly effective pincer-ligated iridium catalysts for alkane dehydrogenation. DFT calculations of relevant thermodynamic, kinetic, and spectroscopic properties. / Zhu, Keming; Achord, Patrick D.; Zhang, Xiawei; Krogh-Jespersen, Karsten; Goldman, Alan S.

In: Journal of the American Chemical Society, Vol. 126, No. 40, 13.10.2004, p. 13044-13053.

Research output: Contribution to journalArticle

@article{6867d3cf166840d38ee33f4f68c56694,
title = "Highly effective pincer-ligated iridium catalysts for alkane dehydrogenation. DFT calculations of relevant thermodynamic, kinetic, and spectroscopic properties",
abstract = "The p-methoxy-substituted pincer-ligated iridium complexes, (MeO- tBuPCP)IrH4 (RPCP = κ3-C 6H3-2,6-(CH2PR2)2) and (MeO-iPrPCP)IrH4, are found to be highly effective catalysts for the dehydrogenation of alkanes (both with and without the use of sacrificial hydrogen acceptors). These complexes offer an interesting comparison with the recently reported bis-phosphinite {"}POCOP{"} (RPOCOP = κ3-C6H3-2,6-(OPR2) 2) pincer-ligated catalysts, which also show catalytic activity higher than unsubstituted PCP analogues (G{\"o}ttker-Schnetmann, I.; White, P.; Brookhart, M. J. Am. Chem. Soc. 2004, 126, 1804). On the basis of νCO values of the respective CO adducts, the MeO-PCP complexes appear to be more electron-rich than the parent PCP complexes, whereas the POCOP complexes appear to be more electron-poor. However, the MeO-PCP and POCOP ligands are calculated (DFT) to show effects in the same directions, relative to the parent PCP ligand, for the kinetics and thermodynamics of a broad range of reactions including the addition of C-H and H-H bonds and CO. In general, both ligands favor (relative to unsubstituted PCP) addition to the 14e (pincer)Ir fragments but disfavor addition to the 16e complexes (pincer)IrH2 or (pincer)-Ir(CO). These kinetic and thermodynamic effects are all largely attributable to the same electronic feature: O → C(aryl) π-donation, from the methoxy or phosphinito groups of the respective ligands. DFT calculations also indicate that the kinetics (but not the thermodynamics) of C-H addition to (pincer)Ir are favored by σ-withdrawal from the phosphorus atoms. The high νCO value of (POCOP)Ir(CO) is attributable to electrostatic effects, rather than decreased Ir-CO π-donation or increased OC-Ir σ-donation.",
author = "Keming Zhu and Achord, {Patrick D.} and Xiawei Zhang and Karsten Krogh-Jespersen and Goldman, {Alan S.}",
year = "2004",
month = "10",
day = "13",
language = "English",
volume = "126",
pages = "13044--13053",
journal = "Journal of the American Chemical Society",
issn = "0002-7863",
publisher = "American Chemical Society",
number = "40",

}

TY - JOUR

T1 - Highly effective pincer-ligated iridium catalysts for alkane dehydrogenation. DFT calculations of relevant thermodynamic, kinetic, and spectroscopic properties

AU - Zhu, Keming

AU - Achord, Patrick D.

AU - Zhang, Xiawei

AU - Krogh-Jespersen, Karsten

AU - Goldman, Alan S.

PY - 2004/10/13

Y1 - 2004/10/13

N2 - The p-methoxy-substituted pincer-ligated iridium complexes, (MeO- tBuPCP)IrH4 (RPCP = κ3-C 6H3-2,6-(CH2PR2)2) and (MeO-iPrPCP)IrH4, are found to be highly effective catalysts for the dehydrogenation of alkanes (both with and without the use of sacrificial hydrogen acceptors). These complexes offer an interesting comparison with the recently reported bis-phosphinite "POCOP" (RPOCOP = κ3-C6H3-2,6-(OPR2) 2) pincer-ligated catalysts, which also show catalytic activity higher than unsubstituted PCP analogues (Göttker-Schnetmann, I.; White, P.; Brookhart, M. J. Am. Chem. Soc. 2004, 126, 1804). On the basis of νCO values of the respective CO adducts, the MeO-PCP complexes appear to be more electron-rich than the parent PCP complexes, whereas the POCOP complexes appear to be more electron-poor. However, the MeO-PCP and POCOP ligands are calculated (DFT) to show effects in the same directions, relative to the parent PCP ligand, for the kinetics and thermodynamics of a broad range of reactions including the addition of C-H and H-H bonds and CO. In general, both ligands favor (relative to unsubstituted PCP) addition to the 14e (pincer)Ir fragments but disfavor addition to the 16e complexes (pincer)IrH2 or (pincer)-Ir(CO). These kinetic and thermodynamic effects are all largely attributable to the same electronic feature: O → C(aryl) π-donation, from the methoxy or phosphinito groups of the respective ligands. DFT calculations also indicate that the kinetics (but not the thermodynamics) of C-H addition to (pincer)Ir are favored by σ-withdrawal from the phosphorus atoms. The high νCO value of (POCOP)Ir(CO) is attributable to electrostatic effects, rather than decreased Ir-CO π-donation or increased OC-Ir σ-donation.

AB - The p-methoxy-substituted pincer-ligated iridium complexes, (MeO- tBuPCP)IrH4 (RPCP = κ3-C 6H3-2,6-(CH2PR2)2) and (MeO-iPrPCP)IrH4, are found to be highly effective catalysts for the dehydrogenation of alkanes (both with and without the use of sacrificial hydrogen acceptors). These complexes offer an interesting comparison with the recently reported bis-phosphinite "POCOP" (RPOCOP = κ3-C6H3-2,6-(OPR2) 2) pincer-ligated catalysts, which also show catalytic activity higher than unsubstituted PCP analogues (Göttker-Schnetmann, I.; White, P.; Brookhart, M. J. Am. Chem. Soc. 2004, 126, 1804). On the basis of νCO values of the respective CO adducts, the MeO-PCP complexes appear to be more electron-rich than the parent PCP complexes, whereas the POCOP complexes appear to be more electron-poor. However, the MeO-PCP and POCOP ligands are calculated (DFT) to show effects in the same directions, relative to the parent PCP ligand, for the kinetics and thermodynamics of a broad range of reactions including the addition of C-H and H-H bonds and CO. In general, both ligands favor (relative to unsubstituted PCP) addition to the 14e (pincer)Ir fragments but disfavor addition to the 16e complexes (pincer)IrH2 or (pincer)-Ir(CO). These kinetic and thermodynamic effects are all largely attributable to the same electronic feature: O → C(aryl) π-donation, from the methoxy or phosphinito groups of the respective ligands. DFT calculations also indicate that the kinetics (but not the thermodynamics) of C-H addition to (pincer)Ir are favored by σ-withdrawal from the phosphorus atoms. The high νCO value of (POCOP)Ir(CO) is attributable to electrostatic effects, rather than decreased Ir-CO π-donation or increased OC-Ir σ-donation.

UR - http://www.scopus.com/inward/record.url?scp=5644254179&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=5644254179&partnerID=8YFLogxK

M3 - Article

VL - 126

SP - 13044

EP - 13053

JO - Journal of the American Chemical Society

JF - Journal of the American Chemical Society

SN - 0002-7863

IS - 40

ER -