One-Electron Reduction of Carbonium Ions. III. The Effect of Added Anions on the Rate of Reduction of the Tropylium Ion with Cr(II)

The one-electron reduction of tropylium fluoroborate with Cr(II) was carried out in aqueous solution containing various anions (Cl−, Br−, SO42−, HSO4−, BF4− and ClO4−) which were added as proton acids or as sodium salts, and the reaction rate at 25°C was measured by the use of the flow method. It was found that Cl−, Br−, and SO42− exhibit the characteristic rate-accelerating effect, whereas BF4− and ClO4− show a slight rate-retarding effect. From the extrapolation of the anion concentration to zero, it was indicated that this reaction can proceed even in the absence of added anions. From these findings, together with the results of chemical and spectrophotometrical analyses of the Cr(III) species, it was concluded that the reduction of the tropylium ion with Cr(II) proceeds through an electron-transfer step where the anion (Cl−, Br−, or SO42−) or the water molecule acts as a weakly interacting electron-transfer bridge. This mechanism is essentially analogous to the “inner-sphere mechanism” known in the fi...