ferrocene/ferrocenium self-exchange reaction a kinetics study
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Ferrocene/Ferrocenium Self-Exchange Reaction A Kinetics Study. Ferrocene. Ferrocenium. Esther Pak Johns Hopkins University Chemistry Department Wednesday section. Background Information. Fc + Fc + Fc + + Fc self-exchange reaction. - PowerPoint PPT PresentationTRANSCRIPT
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