Linear free-energy relationships and inverted Marcus region in the horseradish peroxidase-catalyzed oxidation of ferrocenes by hydrogen peroxide

Second-order rate constants (k(3)) for the steady-state horseradish peroxid ase (HRP, isoenzyme C)-catalyzed oxidation of a variety of mono-, di (1,1') -, and pentamethyl ferrocenes by hydrogen peroxide into the corresponding f erricenium cations at 25 degrees C, pH 7 have been used to investigate ster ic and electronic effects on the reactivity in terms of the linear free-ene rgy relationships (LFER) and the Marcus formalism for an outer-sphere elect ron transfer. There is a linear correlation with a negative slope between I n k(3) and the formal redox potentials of ferrocenes (E degrees') for the m ono- and di-substituted molecules suggesting that, in this series, k(3) inc reases with increasing the reaction driving force. The pentamethylated deri vatives Cp*FeC5H4X, the E degrees' values of which are significantly lower compared with other ferrocenes, display markedly lower reactivity as to be anticipated based on LFER. This observation is rationalized by assuming tha t the great increase in the driving force for Cp*FeC5H4X is neutralized by the steric retardation imposed by the pentamethlyated cyclopentadienyl ring . The steric hindrance is believed to increase in the electron transfer dis tance from the organometallic donor to the heme of HRP which takes place on going from C5H4YFeC5H4X to Cp*FeC5H4X species. Alternatively, the reactivi ty trends reported can be accounted for in terms of the Marcus theory for t he outer-sphere electron transfer, the Cp*FeC5H4X species being the spectat ors of the inverted Marcus region. (C) 1999 Elsevier Science S.A. All right s reserved.