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 language | English |
|---|---|
| Pages (from-to) | 13044-13053 |
| Number of pages | 10 |
| Journal | Journal of the American Chemical Society |
| Volume | 126 |
| Issue number | 40 |
| Publication status | Published - Oct 13 2004 |
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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 journal › Article
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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.
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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 -