Ligand Displacement Reaction Paths in a Diiron Hydrogenase Active Site Model Complex

Jan H. Blank, Salvador Moncho Escriva, Allen M. Lunsford, Edward Brothers, Marcetta Y. Darensbourg, Ashfaq A. Bengali

Research output: Contribution to journalArticle

Abstract

The mechanism and energetics of CO, 1-hexene, and 1-hexyne substitution from the complexes (SBenz)2[Fe2(CO)6] (SBenz=SCH2Ph) (1-CO), (SBenz)2[Fe2(CO)52-1-hexene)] (1-(η2-1-hexene)), and (SBenz)2[Fe2(CO)52-1-hexyne)] (1-(η2-1-hexyne)) were studied by using time-resolved infrared spectroscopy. Exchange of both CO and 1-hexyne by P(OEt)3and pyridine, respectively, proceeds by a bimolecular mechanism. As similar activation enthalpies are obtained for both reactions, the rate-determining step in both cases is assumed to be the rotation of the Fe(CO)2L (L=CO or 1-hexyne) unit to accommodate the incoming ligand. The kinetic profile for the displacement of 1-hexene is quite different than that for the alkyne and, in this case, both reaction channels, that is, dissociative (SN1) and associative (SN2), were found to be competitive. Because DFT calculations predict similar binding enthalpies of alkene and alkyne to the iron center, the results indicate that the bimolecular pathway in the case of the alkyne is lower in free energy than that of the alkene. In complexes of this type, subtle changes in the departing ligand characteristics and the nature of the mercapto bridge can influence the exchange mechanism, such that more than one reaction pathway is available for ligand substitution. The difference between this and the analogous study of (μ-pdt)[Fe(CO)3]2(pdt=S(CH2)3S) underscores the unique characteristics of a three-atom S−S linker in the active site of diiron hydrogenases.

Original languageEnglish
Pages (from-to)12752-12760
Number of pages9
JournalChemistry - A European Journal
Volume22
Issue number36
DOIs
Publication statusPublished - 2016

Fingerprint

Hydrogenase
Carbon Monoxide
Catalytic Domain
Ligands
Alkynes
Alkenes
Enthalpy
Substitution reactions
Discrete Fourier transforms
Free energy
Infrared spectroscopy
Spectrum Analysis
Iron
Chemical activation
Atoms
Kinetics

Keywords

  • density functional calculations
  • enzyme models
  • hydrogenases
  • iron complexes
  • reaction mechanisms

ASJC Scopus subject areas

  • Chemistry(all)

Cite this

Ligand Displacement Reaction Paths in a Diiron Hydrogenase Active Site Model Complex. / Blank, Jan H.; Moncho Escriva, Salvador; Lunsford, Allen M.; Brothers, Edward; Darensbourg, Marcetta Y.; Bengali, Ashfaq A.

In: Chemistry - A European Journal, Vol. 22, No. 36, 2016, p. 12752-12760.

Research output: Contribution to journalArticle

Blank, Jan H. ; Moncho Escriva, Salvador ; Lunsford, Allen M. ; Brothers, Edward ; Darensbourg, Marcetta Y. ; Bengali, Ashfaq A. / Ligand Displacement Reaction Paths in a Diiron Hydrogenase Active Site Model Complex. In: Chemistry - A European Journal. 2016 ; Vol. 22, No. 36. pp. 12752-12760.
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AU - Blank, Jan H.

AU - Moncho Escriva, Salvador

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AU - Darensbourg, Marcetta Y.

AU - Bengali, Ashfaq A.

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AB - The mechanism and energetics of CO, 1-hexene, and 1-hexyne substitution from the complexes (SBenz)2[Fe2(CO)6] (SBenz=SCH2Ph) (1-CO), (SBenz)2[Fe2(CO)5(η2-1-hexene)] (1-(η2-1-hexene)), and (SBenz)2[Fe2(CO)5(η2-1-hexyne)] (1-(η2-1-hexyne)) were studied by using time-resolved infrared spectroscopy. Exchange of both CO and 1-hexyne by P(OEt)3and pyridine, respectively, proceeds by a bimolecular mechanism. As similar activation enthalpies are obtained for both reactions, the rate-determining step in both cases is assumed to be the rotation of the Fe(CO)2L (L=CO or 1-hexyne) unit to accommodate the incoming ligand. The kinetic profile for the displacement of 1-hexene is quite different than that for the alkyne and, in this case, both reaction channels, that is, dissociative (SN1) and associative (SN2), were found to be competitive. Because DFT calculations predict similar binding enthalpies of alkene and alkyne to the iron center, the results indicate that the bimolecular pathway in the case of the alkyne is lower in free energy than that of the alkene. In complexes of this type, subtle changes in the departing ligand characteristics and the nature of the mercapto bridge can influence the exchange mechanism, such that more than one reaction pathway is available for ligand substitution. The difference between this and the analogous study of (μ-pdt)[Fe(CO)3]2(pdt=S(CH2)3S) underscores the unique characteristics of a three-atom S−S linker in the active site of diiron hydrogenases.

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