Mechanism of the oxidation of 1-(ferrocenyl)-ethanone/ethanol by dicyanobis(phenanthroline)iron(III)

Abstract

This study highlights that dicyanobis(phenanthroline)iron(III); [FeIII(phen)2(CN)2]+ oxidises 1-ferrocenylethanone; [CpFeIICpCOMe], and 1-ferrocenylethanol; [CpFeIICpCHOHMe] through a complex mechanism. The kinetic data suggest that the reaction(s) whilst undergoing completion passed through three stages. The first stage was noted soon after starting the reaction(s) and that the reaction(s) followed an overall zeroth order in the first stage that continued to a ∼30% and ∼51% proceeding of the reaction in case of 1-ferrocenylethanone and 1-ferrocenylethanol, respectively. The reaction(s) was found to follow overall second order in the second phase. The second phase culminated into a third phase near the end of the reaction(s) when the rate of redox reaction(s) competed the rate of decrease in solubility of one of the product species; [FeII(phen)2(CN)2]. The third phase may be termed as the competition phase. We identified these phases because the reactants and the intermediates participated in the reaction(s). Protonation of the reducing agents generated intermediates. The intermediates reduced [FeIII(phen)2(CN)2]+ to [FeII(phen)2(CN)2] with a different order of reaction as compared to the initial reactants. The effect of variation in the concentration of protons (0.2–5.2 mM) showed that the conjugate acid of 1-ferrocenylethanone; [CpFeIICpC+OHMe] catalyses the oxidation of [CpFeIICpCOMe]. The protonation of 1-ferrocenylethanol to form [CpFeIICpCHO+H2Me] reduced the rate of oxidation of [CpFeIICpCHOHMe]. These opposing results were verified by observing that the rate of oxidation of [CpFeIICpCOMe] was increased by an elevation in the ionic strength (primary salt effect), and deaccelerated by reducing the dielectric constant or increasing the proportion of dioxane, 10–50 (% v/v). Together, these results support the pivotal role of [CpFeIICpC+OHMe] in the rate-determining step. Neutral [CpFeIICpCHOHMe] takes part in the rate-determining step of the redox reaction between [CpFeIICpCHOHMe] and [FeIII(phen)2(CN)2]+, where the studies pertaining to influence of ionic strength and dielectric constant corroborate these findings. These outcomes of the study helped to postulate a mechanism for each reaction, and to propose the rate law(s). Keywords: Iron coordination complexes, Dicyanobis(phenanthroline)iron(III), 1-Ferrocenylethanone, 1-Ferrocenylethanol, 1,4-Dioxane, Visible absorption spectrophotometry