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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ASJC Scopus subject areas

  • Chemistry(all)

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