Accelerating and decelerating effects of metal ions on electron-transfer reduction of quinones as a function of temperature and binding modes of metal ions to semiquinone radical anions

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 k_et decreases with increasing Sc³⁺ concentration at high concentrations of Sc³⁺ at 298 K. At 263 K the k_et value remains constant with increasing Sc ³⁺ concentration. Such a remarkable difference with regard to dependence of k_et 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.