Determination of the dominant catalyst derived from the classic [RhCp∗Cl2]2 precatalyst system: Is it single-metal Rh1Cp∗-based, subnanometer Rh4 cluster-based, or Rh(0) n nanoparticle-based cyclohexene hydrogenation catalysis at room temperature and mild pressures?

Ercan Bayram, John C. Linehan, John L. Fulton, Nathaniel K. Szymczak, Richard G. Finke

Research output: Contribution to journalArticle

13 Citations (Scopus)

Abstract

Determining the kinetically dominant catalyst in a given catalytic system is a forefront topic in catalysis. The [RhCp∗Cl2]2 (Cp∗ = [η5-C5(CH3)5]) system pioneered by Maitlis and co-workers is a classic precatalyst system from which homogeneous mononuclear Rh1, subnanometer Rh4 cluster, and heterogeneous polymetallic Rh(0)n nanoparticle have all arisen as viable candidates for the true hydrogenation catalyst, depending on the precise substrate, H2 pressure, temperature, and catalyst concentration conditions. Addressed herein is the question of whether the prior assignment of homogeneous, mononuclear Rh1Cp∗-based catalysis is correct, or are trace Rh4 subnanometer clusters or possibly Rh(0)n nanoparticles the dominant, actual cyclohexene hydrogenation catalyst at 22 °C and 2.7 atm initial H2 pressure? The observation herein of Rh4 species by in operando-X-ray absorption fine structure (XAFS) spectroscopy, at the only slightly more vigorous conditions of 26 °C and 8.3 atm H2 pressure, and the confirmation of Rh4 clusters by ex situ mass spectroscopy raises the question of the dominant, room temperature, and mild pressure cyclohexene hydrogenation catalyst derived from the classic [RhCp∗Cl2]2 precatalyst pioneered by Maitlis and co-workers. Ten lines of evidence are provided herein to address the nature of the true room temperature and mild pressure cyclohexene hydrogenation catalyst derived from [RhCp∗Cl2]2. Especially significant among those experiments are quantitative catalyst poisoning experiments, in the present case using 1,10-phenanthroline. Those poisoning studies allow one to distinguish mononuclear Rh1, subnanometer Rh4 cluster, and Rh(0)n nanoparticle catalysis hypotheses. The evidence obtained provides a compelling case for a mononuclear, Rh1Cp∗-based cyclohexene hydrogenation catalyst at 22 °C and 2.7 atm H2 pressure. The resultant methodology, especially the quantitative catalyst poisoning experiments in combination with in operando spectroscopy, is expected to be more broadly applicable to the study of other systems and the what is the true catalyst? question.

Original languageEnglish
Pages (from-to)3876-3886
Number of pages11
JournalACS Catalysis
Volume5
Issue number6
DOIs
Publication statusPublished - Jun 5 2015

Keywords

  • XAFS
  • catalysis
  • catalyst poisoning studies
  • determination of the dominant catalyst
  • in operando spectroscopic studies
  • nanoparticle catalysis
  • organometallic complex catalysis
  • rhodium
  • subnanometer cluster catalysis

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)

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