Outer-sphere redox reactions in sterically hindered pentaam(m)inecobalt(III) complexes. A temperature and pressure dependence kinetic study

Abstract

Outer-sphere redox reactions between [Co(RNH₂)₅(H₂O)]³⁺ and [Fe(CN)₆]^4– have been studied as a function of pH, temperature and pressure. The effect of the size of the alkyl substituent on the amine, RNH₂, has been investigated for both the aqua- and the hydroxo-species in order to establish possible correlations between size and ion-pair formation constant, electron-transfer rate constant, and thermal and pressure activation parameters. The values obtained (at 45 °C and ambient pressure) indicate that the ion-pair formation constant decreases with increasing size of R (75, R = H; 40, Me; 23 dm³ mol^–1, Et), whereas the electron-transfer rate constant increases in this direction (0.11, R = H; 9.3, Me; 35 s^–1 Et). The activation enthalpies do not change, either with decreasing charge on the cobalt complex or with the size of the amine (87, R = H; 79, Me; 84 kJ mol ^–1Et). As for the activation volume, although a slight increase is observed on increasing the size of R (26.5, R = H; 29.4, Me; 33.1 cm³ mol ^–1Et). it is clear that solvational changes during electron transfer are mainly responsible for the values obtained.