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 Sc 3+ 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 Sc 3+ concentration at high concentrations of Sc 3+ 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 [Sc 3+ ] 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 .− –Sc 3+ , and the other 1:2 complex TolSQ .− –(Sc 3+ ) 2 with additional binding of Sc 3+ to TolSQ .− –Sc 3+ . The formation of TolSQ .− –Sc 3+ and TolSQ .− –(Sc 3+ ) 2 complexes was confirmed by EPR spectroscopy in the ET reduction of TolSQ in the presence of low and high concentrations of Sc 3+ , 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 Δ S ≠ 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.