Ferrocene Ferrocenium

Fc + Fc + 
Fc + + Fc self-exchange reaction
Ferrocene
• Water insoluble
• Stable 18 econfiguration
• 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 ( 1 H NMR)
• Ferrocenium, the paramagnetic species, broadens peaks and changes chemical shifts
• Increase in line width, Δν, is proportional to the concentration of ferrocenium and is independent of ferrocene concentration and spectrometer frequency

Determination of k ex
, the rate of self-exchange, of the ferrocene/ferrocenium system through applying 1 H NMRline broadening technique
(Roth, Lovell, and Mayer. J. Am. Chem. Soc.
(2000) 122:5486-5498.)

• Purification of ferrocene (Acros Organics) by sublimation (above 100 °C)
• Synthesis of ferrocenium hexafluorophosphate under anaerobic conditions through Schlenk line apparatus
(Hendrickson, Sohn, and Gray. Inorganic Chem.
(1971)
10:1559-1563.)
• Preparation of ferrocenium solutions in stock solution of 14.997mM ferrocene in acetone-d6 solvent
• Kinetic analysis through 1 HNMR line broadening technique (200 MHz Varian)

conc.(mM)
0
2.0157
4.0314
6.1815
7.7940
chemical shift (ppm) Δshift (ppm)
4.11
0
4.94
0.83
7.33
6.06
7.33
6.22
7.33
7.324
1.95
2.11
3.214
10.213
9.952
5.842
Table 1 — 1 H NMR Chemical Shift Data
Pure ferrocene sample peak at 4.11 ppm
•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
6.1815mM ferrocenium in ferrocene peak at 6.06 ppm
Figure 1 —14.997mM Ferrocene Full
Spectrum
7.33 ppm peak represents protons involved in the rapid exchange in relation to the much slower NMR time scale
Figure 2 —6.1815 mM ferrocenium in ferrocene full spectrum

1H NMR Line Broadening Analysis conc.(mM)
0
2.0157
4.0314
6.1815
line width
(Hz)
0.919621
116.863
0.852412
256.352
479.394
278.01
10.8682
295.516
Δυ (Hz)
115.9434
255.4324
277.0904
πΔυ (Hz)
0
364.2469
802.4645
870.5051
7.794
10.213
294.5964
925.5018
Table 2 — 1 HNMR Line Width Data
Pure ferrocene —line width of 0.9196 Hz
Figure 3 —14.997mM 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 —Δν of
294.60 Hz
Figure 4 —Expanded 7.794mM ferrocenium spectrum

Figure 5 —πΔν vs. ferrocenium concentration plot
From slope of πΔν vs. concentration, k ex determined to be
3.1416x10
6 M -1 s -1 .
Linearity demonstrates direct relationship between concentration of paramagnetic ferrocenium and line width changes

• K ex of 3.1416x10
system
6 M -1 s -1 determined for experimental ferrocene/ferrocenium
• In comparison, literature value of K ex of 4.6 x 10 6 M -1 decamethylferrocene/decamethylferrocenium system s -1 found for
(Nielson, McManis, Safford, and Weaver. J. Phys. Chem . (1989)93:2152-2157)
• Discrepancies (31.70% error) are attributed to possible oxidation of NMR samples, which could introduce paramagnetic oxygen, contamination, and transfer losses.
• Temperature-dependent studies using dynamic NMR techniques
• Investigation of ferrocene derivatives, for example decamethylferrocene, which are more stable in aerobic conditions