Oxidation of ferrocene with peroxides.

摘要

Oxidation of ferrocene was accomplished by reacting Fc with benzoyl peroxide (C6H5CO2OCOC6H5), hydrogen peroxide (H2O2), and tetranitromethane (TNM) to produce the ferricenium cation (Fc+). Ferrocene reacts with TNM instantaneously to produce Fc+, whereas the peroxides oxidize Fc to produce two moles of Fc+ per mole of peroxide. The blue ferricenium cation was stable to air, oxygen, and water in acidic aqueous acetonitrile solvent. The time dependence of oxidation of Fc to Fc+ depended linearly on the concentration of Fc and the concentration of the oxidants, but was independent of acid concentration, affording a rate expression d[Fc +]/dt = k[Fc] [peroxide]. Activation parameters obtained from the temperature dependence of the rate constants k were determined using the Eyring equation, d[ln(k/T)] = ln(kappakB/h) - DeltaH ‡/RT + DeltaS‡/R. For hydrogen peroxide, DeltaH ‡ = 11.0 +/- 1.9 kcal/mol and DeltaS‡ = -27.5 +/- 0.5 eu, and for benzoyl peroxide, DeltaH ‡ = 9.8 + 0.9 kcal/mol and DeltaS‡ = -28.1 + 0.2 eu. The energy DeltaG estimated for thermodynamic electron transfer from reported values of redox potentials (Eo ox = 0.56-0.62 V vs. NHE for oxidation of Fc and Eo red = -(0.63-0.68) V for reduction of hydrogen peroxide) and the Nerst equation is 13-14 kcal/mol, as compared to the observed activation energy DeltaG ‡ = 19.1 kcal/mol at 22°C.