Thermodynamic aspect: kinetics of the reduction of dicyanobis(phen)iron(III) by acetylferrocene and methylferrocenemethanol
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Protonation plays an important role in the redox reactions. We observed this leading role during the reduction of [FeIII(phen)2(CN)2]+ by FcCOMe and FcCHOHMe. The kinetic data showed that the reaction(s) followed a complex kinetics due to the formation of protonated acetylferrocene (FcC+OHMe), and or, protonated α-methylferrocenemethanol (FcCHO+H2Me) in aqueous dioxane (80% v/v). Our results helped us to conclude that the reactions were completed in three phases. An overall zeroth order was observed in the first phase of the reactions. In the second phase, the kinetic data showed an overall second order reaction. The third phase was a complex phase where the rate of redox reactions and the insolubility of the neutral product ([FeII(phen)2(CN)2]) competed with each other. We studied the effect of different factors to identify the reacting entities, which take part in the rate-determining step of each reaction in the second phase. Consequently, we determined the effects of selected factors upon the observed pseudo-first order rate constant(s) (k′ obs) of each reaction. The value of k′ obs increased upon addition of protons in the reaction mixture in case of FcCOMe, and it decreased during the oxidation of FcCHOHMe. Meanwhile, upon enhancing the ionic strength, we observed an increase in k′ obs for FcCOMe, and no change in the value of k′ obs during the reaction of FcCHOHMe. However, a decrease in k′ obs was noticed upon increasing the dielectric constant of the reaction mixture when the reductant was FcCOMe, and no effect was observed in case of FcCHOHMe. Together, these results suggested oxidation of FcC+OHMe and FcCHOHMe in the slow-step, and FcCOMe and FcCHO+H2Me during the fast-step. We refined our results by estimating the thermodynamic parameters of activation. The low values of activation energy and enthalpy of activation confirmed that the reduction of [FeIII(phen)2(CN)2]+ hardly depends upon temperature when the reducing agent is FcCOMe. The outcomes justified that the rate of reaction depends upon the unsaturated FcC+OHMe. This intermediate species contain a ‘carbonium ion’, which is very reactive and energetic. We obtained comparatively high values of the activation energy and enthalpy of activation for the reaction between [FeIII(phen)2(CN)2]+ and FcCHOHMe. The results show that FcCHOHMe is a saturated and stable compound that leads the slow-step and controls the rate of reaction.
KeywordsAcetylferrocene α-Methylferrocenemethanol Dicyanobis(phen)iron(III) Outer-sphere mechanism
The authors are indebted to the HEJ Research Institute of Chemistry, University of Karachi, Pakistan, for providing analytical services to carry out IR and microanalytical characterization of the synthesized compound(s).
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