Kinetic studies of tris(2,2′-bipyridine)iron(III) perchlorate with cobaloxime, [Co(dmgBF2)2(H2O)2]

The kinetics of the reduction of Fe(bipy)33+ by Co(dmgBF2)2(H2O)2 in 0.041 M HNO3/NaNO3 was found to be first-order in both the oxidizing and reducing agents and the second-order rate constant is given by kobs = k1 + k2K[Cl−], with k1=1.59 × 106 M−1 s−1and k2K = 1.83 × 108 M−2 s−1, at 25 °C. The term that is first-order in [Cl−] is attributed to the formation of an ion-pair between Fe(bipy)33+ and Cl−. For k1, the activation parameters ΔH* and ΔS* are 2.22 ± 0.02 kcal mol−1 and −22.7 ± 0.8 cal mol−1 K−1, respectively. The self-exchange rate constant of k22 ≈ 8.7 × 10−3 M−1 s−1 for Co(dmgBF2)2(H2O)2+/0 was estimated using Marcus theory and the known self-exchange rate constant for Fe(bipy)33+/2+.

The data for kinetics reduction of Fe(bipy)33+ by Co(dmgBF2)2(H2O)2 in 3.0 × 10−3 M H+ proceeds following the mechanism:Co(dmgBF2)2(H2O)2+Fe(bipy)33+→kICo(dmgBF2)2(H2O)2++Fe(bipy)32+Fe(bipy)33++Cl-⇄K[Fe(bipy)33+·Cl-][Fe(bipy)33+·Cl-]+Co(dmgBF2)2(H2O)2→k2productsThis proposal suggests that the [Cl−] dependence arises from formation of an ion-pair between Fe(bipy)33+ and Cl−. If ion pairing is assumed to be a fast pre-equilibrium and the observed second-order rate constant calculated when assuming that K[Cl−] ≪ 1,kobs=k2K[X]+k1kobs=k2K[X]+k1This is consistent with the linearity of the plot in Fig. 1.Figure optionsDownload as PowerPoint slide