FerroceneFerrocenium SelfExchange Reaction A Kinetics Study

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Ferrocene/Ferrocenium Self-Exchange Reaction A
Kinetics Study
Ferrocene
Ferrocenium
Esther PakJohns Hopkins University Chemistry
DepartmentWednesday section
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Background Information
Fc Fc ? Fc Fcself-exchange reaction

• Understanding electron transfer reactions is
  critical for elucidation of biological and
  industrial processes
• Ferrocene, the diamagnetic species, exhibits a
  sharp peak at 4.2 ppm (1H NMR)
• Ferrocenium, the paramagnetic species, broadens
  peaks and changes chemical shifts
• Increase in line width, ??, is proportional to
  the concentration of ferrocenium and is
  independent of ferrocene concentration and
  spectrometer frequency

Ferrocene

• Water insoluble
• Stable 18 e- configuration

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Aim of study
Determination of kex, the rate of self-exchange,
of the ferrocene/ferrocenium system through
applying 1H NMR-line broadening technique (Roth,
Lovell, and Mayer. J. Am. Chem. Soc. (2000)
1225486-5498.)
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Experimental Methods

• Purification of ferrocene (Acros Organics) by
  sublimation (above 100C)
• Synthesis of ferrocenium hexafluorophosphate
  under anaerobic conditions through Schlenk line
  apparatus(Hendrickson, Sohn, and Gray. Inorganic
  Chem. (1971) 101559-1563.)
• Preparation of ferrocenium solutions in stock
  solution of 14.997mM ferrocene in acetone-d6
  solvent
• Kinetic analysis through 1HNMR line broadening
  technique (200 MHz Varian)

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Results

• Proton peaks attributed to superaromatic
  hydrogens from cyclopentadiene ligands of the
  ferrous cation
• Acetone-d6 reference peak at 2.05 ppm
• Increase in in the concentration of ferrocenium,
  the paramagnetic species, correlates in
  increasing downshield chemical shift

Table 11H NMR Chemical Shift Data
Pure ferrocene sample peak at 4.11 ppm
6.1815mM ferrocenium in ferrocene peak at 6.06 ppm
7.33 ppm peak represents protons involved in the
rapid exchange in relation to the much slower NMR
time scale
Figure 26.1815 mM ferrocenium in ferrocene full
spectrum
Figure 114.997mM Ferrocene Full Spectrum
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Results
1H NMR Line Broadening Analysis
Pure ferroceneline width of 0.9196 Hz
Table 21HNMR Line Width Data
Figure 314.997mM ferrocene in acetone-d6 spectrum
Introduction of paramagnetic ferrocene increases
line width due to fast electron transfer among
the aromatic cyclopentadiene protons
7.794 ferrocenium in ferrocene?? of 294.60 Hz
Figure 4Expanded 7.794mM ferrocenium spectrum
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Results
From slope of p?? vs. concentration, kex
determined to be 3.1416x106 M-1s-1.
Figure 5p?? vs. ferrocenium concentration plot
Linearity demonstrates direct relationship
between concentration of paramagnetic ferrocenium
and line width changes
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Conclusions

• Kex of 3.1416x106 M-1s-1 determined for
  experimental ferrocene/ferrocenium system
• In comparison, literature value of Kex of 4.6 x
  106 M-1s-1 found for decamethylferrocene/decamethy
  lferrocenium system (Nielson, McManis, Safford,
  and Weaver. J. Phys. Chem. (1989)932152-2157)
• Discrepancies (31.70 error) are attributed to
  possible oxidation of NMR samples, which could
  introduce paramagnetic oxygen, contamination, and
  transfer losses.
• Future Studies
• Temperature-dependent studies using dynamic NMR
  techniques
• Investigation of ferrocene derivatives, for
  example decamethylferrocene, which are more
  stable in aerobic conditions