Ferrocene/Ferrocenium Self-Exchange Reaction

A Kinetics Study

Esther PakJohns Hopkins University Chemistry Department

Wednesday section

Ferrocene Ferrocenium

Background Information

Ferrocene

Fc + Fc+ Fc + + Fcself-exchange reaction

•Water insoluble

•Stable 18 e- configuration

•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, Δν, is proportional to the concentration of ferrocenium and is independent of ferrocene concentration and spectrometer frequency

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) 122:5486-5498.)

Experimental Methods

•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 1HNMR line broadening technique (200 MHz Varian)

Results

Table 1—1H NMR Chemical Shift Data

•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

Figure 2—6.1815 mM ferrocenium in ferrocene

full spectrum

Figure 1—14.997mM Ferrocene Full Spectrum

conc.(mM)chemical shift

(ppm) Δshift (ppm)

0 4.11 0

2.0157 4.94 0.83

  7.33  

4.0314 6.06 1.95

  7.33  

6.1815 6.22 2.11

  7.33  

7.7940 7.324 3.214

     

10.213 9.952 5.842

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

Results

Figure 3—14.997mM ferrocene in acetone-d6 spectrum

Figure 4—Expanded 7.794mM ferrocenium spectrum

1H NMR Line Broadening Analysis

conc.(mM)line width (Hz) Δυ (Hz) πΔυ (Hz)

0 0.919621   0

2.0157 116.863 115.9434 364.2469

  0.852412    

4.0314 256.352 255.4324 802.4645

  479.394    

6.1815 278.01 277.0904 870.5051

  10.8682    

7.794 295.516 294.5964 925.5018

10.213      

Table 2—1HNMR Line Width Data

Pure ferrocene—line width of 0.9196 Hz

7.794 ferrocenium in ferrocene—Δν of 294.60 Hz

Introduction of paramagnetic ferrocene increases line width due to fast electron transfer among the aromatic cyclopentadiene protons

Results

Figure 5—πΔν vs. ferrocenium concentration plot

Linearity demonstrates direct relationship between concentration of paramagnetic ferrocenium and line width changes

From slope of πΔν vs. concentration, kex determined to be 3.1416x106 M-1s-1.

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/decamethylferrocenium system (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.

Future Studies•Temperature-dependent studies using dynamic NMR techniques

•Investigation of ferrocene derivatives, for example decamethylferrocene, which are more stable in aerobic conditions