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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.
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Ferrocene/Ferrocenium Self-Exchange Reaction A Kinetics Study Ferrocene Ferrocenium Esther PakJohns Hopkins University Chemistry DepartmentWednesday section
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, Δν, is proportional to the concentration of ferrocenium and is independent of ferrocene concentration and spectrometer frequency Ferrocene • Water insoluble • Stable 18 e- configuration
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 • 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 1—1H 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 2—6.1815 mM ferrocenium in ferrocene full spectrum Figure 1—14.997mM Ferrocene Full Spectrum
Results 1H NMR Line Broadening Analysis Pure ferrocene—line width of 0.9196 Hz Table 2—1HNMR Line Width Data 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
Results From slope of πΔν vs. concentration, kex determined to be 3.1416x106 M-1s-1. Figure 5—πΔν vs. ferrocenium concentration plot Linearity demonstrates direct relationship between concentration of paramagnetic ferrocenium and line width changes
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
