Direct observation of key catalytic intermediates in a photoinduced proton reduction cycle with a diiron carbonyl complex

Mohammad Mirmohades, Sonja Pullen, Matthias Stein, Somnath Maji, Sascha Ott, Leif Hammarström, Reiner Lomoth

Research output: Contribution to journalArticle

24 Citations (Scopus)

Abstract

The structure and reactivity of intermediates in the photocatalytic cycle of a proton reduction catalyst, [Fe2(bdt)(CO)6] (bdt = benzenedithiolate), were investigated by time-resolved spectroscopy. The singly reduced catalyst [Fe2(bdt)(CO)6]-, a key intermediate in photocatalytic H2 formation, was generated by reaction with one-electron reductants in laser flash-quench experiments and could be observed spectroscopically on the nanoseconds to microseconds time scale. From UV/vis and IR spectroscopy, [Fe2(bdt)(CO)6]- is readily distinguished from the two-electron reduced catalyst [Fe2(bdt)(CO)6]2- that is obtained inevitably in the electrochemical reduction of [Fe2(bdt)(CO)6]. For the disproportionation rate constant of [Fe2(bdt)(CO)6]-, an upper limit on the order of 107 M-1 s-1 was estimated, which precludes a major role of [Fe2(bdt)(CO)6]2- in photoinduced proton reduction cycles. Structurally [Fe2(bdt)(CO)6]- is characterized by a rather asymmetrically distorted geometry with one broken Fe-S bond and six terminal CO ligands. Acids with pKa ≥ 12.7 protonate [Fe2(bdt)(CO)6]- with bimolecular rate constants of 4 × 106, 7 × 106, and 2 × 108 M-1 s-1 (trichloroacetic, trifluoroacetic, and toluenesulfonic acids, respectively). The resulting hydride complex [Fe2(bdt)(CO)6H] is therefore likely to be an intermediate in photocatalytic cycles. This intermediate resembles structurally and electronically the parent complex [Fe2(bdt)(CO)6], with very similar carbonyl stretching frequencies.

Original languageEnglish
Pages (from-to)17366-17369
Number of pages4
JournalJournal of the American Chemical Society
Volume136
Issue number50
DOIs
Publication statusPublished - Dec 17 2014

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Carbon Monoxide
Protons
Observation
Catalysts
Rate constants
Acids
Electrons
Ultraviolet spectroscopy
Hydrides
Stretching
Infrared spectroscopy
Ligands
Spectroscopy
Geometry
Lasers
Spectrum Analysis
Experiments
Trifluoroacetic Acid
Reducing Agents

ASJC Scopus subject areas

  • Chemistry(all)
  • Catalysis
  • Biochemistry
  • Colloid and Surface Chemistry

Cite this

Direct observation of key catalytic intermediates in a photoinduced proton reduction cycle with a diiron carbonyl complex. / Mirmohades, Mohammad; Pullen, Sonja; Stein, Matthias; Maji, Somnath; Ott, Sascha; Hammarström, Leif; Lomoth, Reiner.

In: Journal of the American Chemical Society, Vol. 136, No. 50, 17.12.2014, p. 17366-17369.

Research output: Contribution to journalArticle

Mirmohades, Mohammad ; Pullen, Sonja ; Stein, Matthias ; Maji, Somnath ; Ott, Sascha ; Hammarström, Leif ; Lomoth, Reiner. / Direct observation of key catalytic intermediates in a photoinduced proton reduction cycle with a diiron carbonyl complex. In: Journal of the American Chemical Society. 2014 ; Vol. 136, No. 50. pp. 17366-17369.
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abstract = "The structure and reactivity of intermediates in the photocatalytic cycle of a proton reduction catalyst, [Fe2(bdt)(CO)6] (bdt = benzenedithiolate), were investigated by time-resolved spectroscopy. The singly reduced catalyst [Fe2(bdt)(CO)6]-, a key intermediate in photocatalytic H2 formation, was generated by reaction with one-electron reductants in laser flash-quench experiments and could be observed spectroscopically on the nanoseconds to microseconds time scale. From UV/vis and IR spectroscopy, [Fe2(bdt)(CO)6]- is readily distinguished from the two-electron reduced catalyst [Fe2(bdt)(CO)6]2- that is obtained inevitably in the electrochemical reduction of [Fe2(bdt)(CO)6]. For the disproportionation rate constant of [Fe2(bdt)(CO)6]-, an upper limit on the order of 107 M-1 s-1 was estimated, which precludes a major role of [Fe2(bdt)(CO)6]2- in photoinduced proton reduction cycles. Structurally [Fe2(bdt)(CO)6]- is characterized by a rather asymmetrically distorted geometry with one broken Fe-S bond and six terminal CO ligands. Acids with pKa ≥ 12.7 protonate [Fe2(bdt)(CO)6]- with bimolecular rate constants of 4 × 106, 7 × 106, and 2 × 108 M-1 s-1 (trichloroacetic, trifluoroacetic, and toluenesulfonic acids, respectively). The resulting hydride complex [Fe2(bdt)(CO)6H] is therefore likely to be an intermediate in photocatalytic cycles. This intermediate resembles structurally and electronically the parent complex [Fe2(bdt)(CO)6], with very similar carbonyl stretching frequencies.",
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