DFT/ECP Study of C-H Activation by (PCP)Ir and (PCP)Ir(H)2 (PCP = η3-1,3-C6H3(CH2PR 2)2). Enthalpies and Free Energies of Associative and Dissociative Pathways

Karsten Krogh-Jespersen, Margaret Czerw, Mira Kanzelberger, Alan S Goldman

Research output: Contribution to journalArticle

43 Citations (Scopus)

Abstract

(PCP)Ir(H)2 (PCP = η3-1,3-C6H3(CH2PR 2)2) complexes are highly effective catalysts for the dehydrogenation of alkanes; in particular, they are the first efficient molecular catalysts for alkane dehydrogenation that do not require a sacrificial hydrogen acceptor. Using density functional theory/effective core potential methods, we have examined C - H bond cleavage in alkanes and arenes by both (PCP)Ir and (PCP)Ir(H)2. C - H addition to the dihydride is accompanied by loss of H2; both associative and dissociative pathways for this exchange reaction have been examined. The energetic barrier (ΔE) for associative displacement of H2 by benzene is much lower than the barrier for a dissociative pathway involving initial loss of H2; however, the pathways have very comparable free energy barriers (ΔG). Extrapolation to the higher temperatures, bulkier phosphine ligands, and the alkane substrates used experimentally leads to the conclusion that the pathway for the "acceptorless" dehydrogenation of alkanes is dissociative. For hydrocarbon/hydrocarbon exchanges, which are required for transfer - dehydrogenation, dissociative pathways are calculated to be much more favorable than associative pathways. We emphasize that it is the free energy, not just the internal energy or enthalpy, that must be considered for elementary steps that show changes in molecularity.

Original languageEnglish
Pages (from-to)56-63
Number of pages8
JournalJournal of Chemical Information and Computer Sciences
Volume41
Issue number1
DOIs
Publication statusPublished - Jan 2001

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Alkanes
Discrete Fourier transforms
Paraffins
activation
Free energy
Enthalpy
Dehydrogenation
Chemical activation
energy
phosphine
Hydrocarbons
Catalysts
Energy barriers
Benzene
Extrapolation
Density functional theory
Hydrogen
Ion exchange
Ligands
Substrates

ASJC Scopus subject areas

  • Chemistry(all)
  • Information Systems
  • Computer Science Applications
  • Computational Theory and Mathematics

Cite this

DFT/ECP Study of C-H Activation by (PCP)Ir and (PCP)Ir(H)2 (PCP = η3-1,3-C6H3(CH2PR 2)2). Enthalpies and Free Energies of Associative and Dissociative Pathways. / Krogh-Jespersen, Karsten; Czerw, Margaret; Kanzelberger, Mira; Goldman, Alan S.

In: Journal of Chemical Information and Computer Sciences, Vol. 41, No. 1, 01.2001, p. 56-63.

Research output: Contribution to journalArticle

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abstract = "(PCP)Ir(H)2 (PCP = η3-1,3-C6H3(CH2PR 2)2) complexes are highly effective catalysts for the dehydrogenation of alkanes; in particular, they are the first efficient molecular catalysts for alkane dehydrogenation that do not require a sacrificial hydrogen acceptor. Using density functional theory/effective core potential methods, we have examined C - H bond cleavage in alkanes and arenes by both (PCP)Ir and (PCP)Ir(H)2. C - H addition to the dihydride is accompanied by loss of H2; both associative and dissociative pathways for this exchange reaction have been examined. The energetic barrier (ΔE‡) for associative displacement of H2 by benzene is much lower than the barrier for a dissociative pathway involving initial loss of H2; however, the pathways have very comparable free energy barriers (ΔG‡). Extrapolation to the higher temperatures, bulkier phosphine ligands, and the alkane substrates used experimentally leads to the conclusion that the pathway for the {"}acceptorless{"} dehydrogenation of alkanes is dissociative. For hydrocarbon/hydrocarbon exchanges, which are required for transfer - dehydrogenation, dissociative pathways are calculated to be much more favorable than associative pathways. We emphasize that it is the free energy, not just the internal energy or enthalpy, that must be considered for elementary steps that show changes in molecularity.",
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AB - (PCP)Ir(H)2 (PCP = η3-1,3-C6H3(CH2PR 2)2) complexes are highly effective catalysts for the dehydrogenation of alkanes; in particular, they are the first efficient molecular catalysts for alkane dehydrogenation that do not require a sacrificial hydrogen acceptor. Using density functional theory/effective core potential methods, we have examined C - H bond cleavage in alkanes and arenes by both (PCP)Ir and (PCP)Ir(H)2. C - H addition to the dihydride is accompanied by loss of H2; both associative and dissociative pathways for this exchange reaction have been examined. The energetic barrier (ΔE‡) for associative displacement of H2 by benzene is much lower than the barrier for a dissociative pathway involving initial loss of H2; however, the pathways have very comparable free energy barriers (ΔG‡). Extrapolation to the higher temperatures, bulkier phosphine ligands, and the alkane substrates used experimentally leads to the conclusion that the pathway for the "acceptorless" dehydrogenation of alkanes is dissociative. For hydrocarbon/hydrocarbon exchanges, which are required for transfer - dehydrogenation, dissociative pathways are calculated to be much more favorable than associative pathways. We emphasize that it is the free energy, not just the internal energy or enthalpy, that must be considered for elementary steps that show changes in molecularity.

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