Outer-sphere redox reactions of [CoIII{N5}(HnPO4)]n+[{N5}=(NH3)5, (NH2Me)5 or 10-amino-10-methyl-1,4,8,12-tetraazacyclopentadecane] complexes. A temperature- and pressure-dependence kinetic study on the effects of the different {N5} groups

Abstract

Outer-sphere redox reactions between [Co{N₅}(H_nPO₄)]ⁿ⁺[{N₅}=(NH₃)₅, (NH₂Me)₅ or 10-amino-10-methyl-1,4,8,12-tetraazacyclopentadecane (L)] and [Fe(CN)₆]^4– have been studied as a function of pH, {N₅}, temperature and pressure. The effect of the size of the {N₅} skeleton, has been investigated for the n= 0, 1, 2 systems in order to establish possible correlations between the size and charge of the cobalt(III) complex and the ion-pair formation constant, the electron-transfer rate constant, and the thermal and baric activation parameters. The values obtained indicate that the ionpair formation constants are the same, within experimental error, for all the systems studied. The electron-transfer rate constant for a given degree (n= 1) of protonation of the [Co{N₅}(H_nPO₄)]ⁿ⁺ complex increases on increasing the size of the monodentate amines, while an important decrease is observed when they are substituted by the N₅ macrocycle (L)[2.6 × 10^–3 s^–1, {N₅}=(NH₃)₅; 59 × 10^–3 s^–1, {N₅}=(NH₂Me)₅; 0.73 × 10^–3 s^–1, {N₅}= L; respectively at 35 °C]. The activation enthalpies do not show any significant change, neither with decreasing charge on the cobalt complex nor with the size of the amine. The values of ΔS^‡ and ΔV^‡ vary considerably with the degree of protonation of the phosphate ligands and the size of the Co^III cavity of the complexes. The opposite trends observed for the values of ΔS^‡[8 J K^–1 mol^–1 for {N₅}=(NH₂Me)₅(n= 1); –61 J K^–1 mol^–1 for {N₅}=(NH₃)₅(n= 1); 32 J K^–1 mol^–1 for {N₅}=(NH₃)₅(n= 0)] and ΔV^‡[44 cm³ mol^–1 for {N₅}=(NH₂Me)₅(n= 1); 37 cm³ mol^–1 for {N₅}=(NH₃)₅(n= 1); 17 cm³ mol^–1 for {N₅}=(NH₃)₅(n= 0)] are related to the existence of an important increase in hydrogen bond formation in the cobalt(III) complex on going to the transition state.