The accelerating effect of Sc3+ on the electron-transfer (ET) reduction of the p-benzoquinone derivative 1-(p-tolylsulfinyl)-2,5-benzoquinone (TolSQ) by 10,10′-dimethyl-9,9′-biacridine ((AcrH)2) at 233 K changes to a decelerating effect with increasing reaction temperature; the observed second-order rate constant ket decreases with increasing Sc3+ concentration at high concentrations of Sc3+ at 298 K. At 263 K the ket value remains constant with increasing Sc 3+ concentration. Such a remarkable difference with regard to dependence of ket on [Sc3+] between low and high temperatures results from the difference in relative activity of two ET pathways that depend on temperature, one of which affords 1:1 complex TolSQ --Sc3+, and the other 1:2 complex TolSQ --(Sc3+)2 with additional binding of Sc 3+ to TolSQ--Sc3+. The formation of TolSQ --Sc3+ and TolSQ--(Sc3+)2 complexes was confirmed by EPR spectroscopy in the ET reduction of TolSQ in the presence of low and high concentrations of Sc3+, respectively. The effects of metal ions on other ET reactions of quinones to afford 1:1 and 1:2 complexes between semiquinone radical anions and metal ions are also reported. The ET pathway affording the 1:2 complexes has smaller activation enthalpies ΔH≠ and more negative activation entropies ΔS ≠ because of stronger binding of metal ions and more restricted geometries of the ET transition states as compared with the ET pathway to afford the 1:1 complexes.
- Electron transfer
- Radical ions