On the mechanism of (PCP)Ir-catalyzed acceptorless dehydrogenation of alkanes: A combined computational and experimental study

Karsten Krogh-Jespersen, Margaret Czerw, Nadine Summa, Kenton B. Renkema, Patrick D. Achord, Alan S Goldman

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Abstract

Pincer complexes of the type (RPCP)IrH2, where (RPCP)Ir is [η3-2,6-(R2PCH2)2 C6H3] Ir, are the most effective catalysts reported to date for the "acceptorless" dehydrogenation of alkanes to yield alkenes and free H2. We calculate (DFT/B3LYP) that associative (A) reactions of (MepCP)IrH2 with model linear (propane, n-PrH) and cyclic (cyclohexane, CyH) alkanes may proceed via classical Ir(V) and nonclassical Ir(III)(η2-H2) intermediates. A dissociative (D) pathway proceeds via initial loss of H2, followed by C-H addition to (MePCP)Ir. Although a slightly higher energy barrier (ΔE‡) is computed for the D pathway, the calculated free-energy barrier (ΔG‡) for the D pathway is significantly lower than that of the A pathway. Under standard thermodynamic conditions (STP), C-H addition via the D pathway has ΔG°‡ = 36.3 kcal/mol for CyH (35.1 kcal/mol for n-PrH). However, acceptorless dehydrogenation of alkanes is thermodynamically impossible at STP. At conditions under which acceptorless dehydrogenation is thermodynamically possible (for example, T = 150°C and PH2 = 10 x 10-7 atm), ΔG‡ for C-H addition to (MePCP)Ir (plus a molecule of free H2) is very low (17.5 kcal/mol for CyH, 16.7 kcal/mol for n-PrH). Under these conditions, the rate-determining step for the D pathway is the loss of H2 from (MePCP)IrH2 with ΔGD‡ ≈ ΔHD‡ = 27.2 kcal/mol. For CyH, the calculated ΔG°‡ for C-H addition to (MePCP)IrH2 on the A pathway is 35.2 kcal/mol (32.7 kcal/mol for n-PrH). At catalytic conditions, the calculated free energies of C-H addition are 31.3 and 33.7 kcal/mol for CyH and n-PrH addition, respectively. Elimination of H2 from the resulting "seven- coordinate" Ir-species must proceed with an activation enthalpy at least as large as the enthalpy change of the elimination step itself (AH ≈ 11-13 kcal/mol), and with a small entropy of activation. The free energy of activation for H2 elimination (ΔGA‡) is hence found to be greater than ca. 36 kcal/mol for both CyH and n-PrH under catalytic conditions. The overall free-energy barrier of the A pathway is calculated to be higher than that of the D pathway by ca. 9 kcal/mol. Reversible C-H(D) addition to (RPCP)IrH2 is predicted to lead to H/D exchange, because the barriers for hydride scrambling are extremely low in the "seven-coordinate" polyhydrides. In agreement with calculation, H/D exchange is observed experimentally for several deuteriohydrocarbons with the following order of rates: C6D6 > mesitylene-d12 <n-decane-d22 ≫ cyclohexane-d12. Because H/D exchange in cyclohexane-d12 solution is not observed even after 1 week at 180°C, we estimate that the experimental barrier to cyclohexane C-D addition is greater than 36.4 kcal/mol. This value is considerably greater than the experimental barrier for the full catalytic dehydrogenation cycle for cycloalkanes (ca. 31 kcal/mol). Thus, the experimental evidence, in agreement with calculation, strongly indicates that the A pathway is not kinetically viable as a segment of the "acceptorless" dehydrogenation cycle.

Original languageEnglish
Pages (from-to)11404-11416
Number of pages13
JournalJournal of the American Chemical Society
Volume124
Issue number38
DOIs
Publication statusPublished - Sep 25 2002

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Alkanes
Dehydrogenation
Cyclohexane
Paraffins
Free energy
Energy barriers
Chemical activation
Enthalpy
Cycloparaffins
Propane
Alkenes
Entropy
Thermodynamics
Discrete Fourier transforms
Hydrides
Olefins
Linear Models
Ion exchange
Hydrogen
Catalysts

ASJC Scopus subject areas

  • Chemistry(all)

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On the mechanism of (PCP)Ir-catalyzed acceptorless dehydrogenation of alkanes : A combined computational and experimental study. / Krogh-Jespersen, Karsten; Czerw, Margaret; Summa, Nadine; Renkema, Kenton B.; Achord, Patrick D.; Goldman, Alan S.

In: Journal of the American Chemical Society, Vol. 124, No. 38, 25.09.2002, p. 11404-11416.

Research output: Contribution to journalArticle

Krogh-Jespersen, Karsten ; Czerw, Margaret ; Summa, Nadine ; Renkema, Kenton B. ; Achord, Patrick D. ; Goldman, Alan S. / On the mechanism of (PCP)Ir-catalyzed acceptorless dehydrogenation of alkanes : A combined computational and experimental study. In: Journal of the American Chemical Society. 2002 ; Vol. 124, No. 38. pp. 11404-11416.
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abstract = "Pincer complexes of the type (RPCP)IrH2, where (RPCP)Ir is [η3-2,6-(R2PCH2)2 C6H3] Ir, are the most effective catalysts reported to date for the {"}acceptorless{"} dehydrogenation of alkanes to yield alkenes and free H2. We calculate (DFT/B3LYP) that associative (A) reactions of (MepCP)IrH2 with model linear (propane, n-PrH) and cyclic (cyclohexane, CyH) alkanes may proceed via classical Ir(V) and nonclassical Ir(III)(η2-H2) intermediates. A dissociative (D) pathway proceeds via initial loss of H2, followed by C-H addition to (MePCP)Ir. Although a slightly higher energy barrier (ΔE‡) is computed for the D pathway, the calculated free-energy barrier (ΔG‡) for the D pathway is significantly lower than that of the A pathway. Under standard thermodynamic conditions (STP), C-H addition via the D pathway has ΔG°‡ = 36.3 kcal/mol for CyH (35.1 kcal/mol for n-PrH). However, acceptorless dehydrogenation of alkanes is thermodynamically impossible at STP. At conditions under which acceptorless dehydrogenation is thermodynamically possible (for example, T = 150°C and PH2 = 10 x 10-7 atm), ΔG‡ for C-H addition to (MePCP)Ir (plus a molecule of free H2) is very low (17.5 kcal/mol for CyH, 16.7 kcal/mol for n-PrH). Under these conditions, the rate-determining step for the D pathway is the loss of H2 from (MePCP)IrH2 with ΔGD‡ ≈ ΔHD‡ = 27.2 kcal/mol. For CyH, the calculated ΔG°‡ for C-H addition to (MePCP)IrH2 on the A pathway is 35.2 kcal/mol (32.7 kcal/mol for n-PrH). At catalytic conditions, the calculated free energies of C-H addition are 31.3 and 33.7 kcal/mol for CyH and n-PrH addition, respectively. Elimination of H2 from the resulting {"}seven- coordinate{"} Ir-species must proceed with an activation enthalpy at least as large as the enthalpy change of the elimination step itself (AH ≈ 11-13 kcal/mol), and with a small entropy of activation. The free energy of activation for H2 elimination (ΔGA‡) is hence found to be greater than ca. 36 kcal/mol for both CyH and n-PrH under catalytic conditions. The overall free-energy barrier of the A pathway is calculated to be higher than that of the D pathway by ca. 9 kcal/mol. Reversible C-H(D) addition to (RPCP)IrH2 is predicted to lead to H/D exchange, because the barriers for hydride scrambling are extremely low in the {"}seven-coordinate{"} polyhydrides. In agreement with calculation, H/D exchange is observed experimentally for several deuteriohydrocarbons with the following order of rates: C6D6 > mesitylene-d12 22 ≫ cyclohexane-d12. Because H/D exchange in cyclohexane-d12 solution is not observed even after 1 week at 180°C, we estimate that the experimental barrier to cyclohexane C-D addition is greater than 36.4 kcal/mol. This value is considerably greater than the experimental barrier for the full catalytic dehydrogenation cycle for cycloalkanes (ca. 31 kcal/mol). Thus, the experimental evidence, in agreement with calculation, strongly indicates that the A pathway is not kinetically viable as a segment of the {"}acceptorless{"} dehydrogenation cycle.",
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TY - JOUR

T1 - On the mechanism of (PCP)Ir-catalyzed acceptorless dehydrogenation of alkanes

T2 - A combined computational and experimental study

AU - Krogh-Jespersen, Karsten

AU - Czerw, Margaret

AU - Summa, Nadine

AU - Renkema, Kenton B.

AU - Achord, Patrick D.

AU - Goldman, Alan S

PY - 2002/9/25

Y1 - 2002/9/25

N2 - Pincer complexes of the type (RPCP)IrH2, where (RPCP)Ir is [η3-2,6-(R2PCH2)2 C6H3] Ir, are the most effective catalysts reported to date for the "acceptorless" dehydrogenation of alkanes to yield alkenes and free H2. We calculate (DFT/B3LYP) that associative (A) reactions of (MepCP)IrH2 with model linear (propane, n-PrH) and cyclic (cyclohexane, CyH) alkanes may proceed via classical Ir(V) and nonclassical Ir(III)(η2-H2) intermediates. A dissociative (D) pathway proceeds via initial loss of H2, followed by C-H addition to (MePCP)Ir. Although a slightly higher energy barrier (ΔE‡) is computed for the D pathway, the calculated free-energy barrier (ΔG‡) for the D pathway is significantly lower than that of the A pathway. Under standard thermodynamic conditions (STP), C-H addition via the D pathway has ΔG°‡ = 36.3 kcal/mol for CyH (35.1 kcal/mol for n-PrH). However, acceptorless dehydrogenation of alkanes is thermodynamically impossible at STP. At conditions under which acceptorless dehydrogenation is thermodynamically possible (for example, T = 150°C and PH2 = 10 x 10-7 atm), ΔG‡ for C-H addition to (MePCP)Ir (plus a molecule of free H2) is very low (17.5 kcal/mol for CyH, 16.7 kcal/mol for n-PrH). Under these conditions, the rate-determining step for the D pathway is the loss of H2 from (MePCP)IrH2 with ΔGD‡ ≈ ΔHD‡ = 27.2 kcal/mol. For CyH, the calculated ΔG°‡ for C-H addition to (MePCP)IrH2 on the A pathway is 35.2 kcal/mol (32.7 kcal/mol for n-PrH). At catalytic conditions, the calculated free energies of C-H addition are 31.3 and 33.7 kcal/mol for CyH and n-PrH addition, respectively. Elimination of H2 from the resulting "seven- coordinate" Ir-species must proceed with an activation enthalpy at least as large as the enthalpy change of the elimination step itself (AH ≈ 11-13 kcal/mol), and with a small entropy of activation. The free energy of activation for H2 elimination (ΔGA‡) is hence found to be greater than ca. 36 kcal/mol for both CyH and n-PrH under catalytic conditions. The overall free-energy barrier of the A pathway is calculated to be higher than that of the D pathway by ca. 9 kcal/mol. Reversible C-H(D) addition to (RPCP)IrH2 is predicted to lead to H/D exchange, because the barriers for hydride scrambling are extremely low in the "seven-coordinate" polyhydrides. In agreement with calculation, H/D exchange is observed experimentally for several deuteriohydrocarbons with the following order of rates: C6D6 > mesitylene-d12 22 ≫ cyclohexane-d12. Because H/D exchange in cyclohexane-d12 solution is not observed even after 1 week at 180°C, we estimate that the experimental barrier to cyclohexane C-D addition is greater than 36.4 kcal/mol. This value is considerably greater than the experimental barrier for the full catalytic dehydrogenation cycle for cycloalkanes (ca. 31 kcal/mol). Thus, the experimental evidence, in agreement with calculation, strongly indicates that the A pathway is not kinetically viable as a segment of the "acceptorless" dehydrogenation cycle.

AB - Pincer complexes of the type (RPCP)IrH2, where (RPCP)Ir is [η3-2,6-(R2PCH2)2 C6H3] Ir, are the most effective catalysts reported to date for the "acceptorless" dehydrogenation of alkanes to yield alkenes and free H2. We calculate (DFT/B3LYP) that associative (A) reactions of (MepCP)IrH2 with model linear (propane, n-PrH) and cyclic (cyclohexane, CyH) alkanes may proceed via classical Ir(V) and nonclassical Ir(III)(η2-H2) intermediates. A dissociative (D) pathway proceeds via initial loss of H2, followed by C-H addition to (MePCP)Ir. Although a slightly higher energy barrier (ΔE‡) is computed for the D pathway, the calculated free-energy barrier (ΔG‡) for the D pathway is significantly lower than that of the A pathway. Under standard thermodynamic conditions (STP), C-H addition via the D pathway has ΔG°‡ = 36.3 kcal/mol for CyH (35.1 kcal/mol for n-PrH). However, acceptorless dehydrogenation of alkanes is thermodynamically impossible at STP. At conditions under which acceptorless dehydrogenation is thermodynamically possible (for example, T = 150°C and PH2 = 10 x 10-7 atm), ΔG‡ for C-H addition to (MePCP)Ir (plus a molecule of free H2) is very low (17.5 kcal/mol for CyH, 16.7 kcal/mol for n-PrH). Under these conditions, the rate-determining step for the D pathway is the loss of H2 from (MePCP)IrH2 with ΔGD‡ ≈ ΔHD‡ = 27.2 kcal/mol. For CyH, the calculated ΔG°‡ for C-H addition to (MePCP)IrH2 on the A pathway is 35.2 kcal/mol (32.7 kcal/mol for n-PrH). At catalytic conditions, the calculated free energies of C-H addition are 31.3 and 33.7 kcal/mol for CyH and n-PrH addition, respectively. Elimination of H2 from the resulting "seven- coordinate" Ir-species must proceed with an activation enthalpy at least as large as the enthalpy change of the elimination step itself (AH ≈ 11-13 kcal/mol), and with a small entropy of activation. The free energy of activation for H2 elimination (ΔGA‡) is hence found to be greater than ca. 36 kcal/mol for both CyH and n-PrH under catalytic conditions. The overall free-energy barrier of the A pathway is calculated to be higher than that of the D pathway by ca. 9 kcal/mol. Reversible C-H(D) addition to (RPCP)IrH2 is predicted to lead to H/D exchange, because the barriers for hydride scrambling are extremely low in the "seven-coordinate" polyhydrides. In agreement with calculation, H/D exchange is observed experimentally for several deuteriohydrocarbons with the following order of rates: C6D6 > mesitylene-d12 22 ≫ cyclohexane-d12. Because H/D exchange in cyclohexane-d12 solution is not observed even after 1 week at 180°C, we estimate that the experimental barrier to cyclohexane C-D addition is greater than 36.4 kcal/mol. This value is considerably greater than the experimental barrier for the full catalytic dehydrogenation cycle for cycloalkanes (ca. 31 kcal/mol). Thus, the experimental evidence, in agreement with calculation, strongly indicates that the A pathway is not kinetically viable as a segment of the "acceptorless" dehydrogenation cycle.

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