Acid catalysis in thermal and photoinduced electron-transfer reactions

The kinetics of acid catalysis of electron-transfer reactions from cis-dialkylcobalt(III) complexes, cis-[R₂Co(bpy)₂] ⁺ (R = Me, Et, and PhCH₂; bpy = 2,2′-bipyridine) to a series of p-benzoquinone derivatives (Q) in aqueous solution (H ₂O-EtOH; 5:1 v/v) or acetonitrile (MeCN) and from ferrocene derivatives (Fc) to the same series Q and of photoinduced electron-transfer reactions from [Ru(bpy)₃]²⁺* to acetophenone derivatives in MeCN have been studied at various concentrations of HClO ₄ as a catalyst at 298 K. The rate constant k_et of electron transfer from cis-[Et₂Co(bpy)]⁺ to Q in aqueous solution is independent of pH in the high pH region, pH > pK₁ where K₁ is the acid dissociation constant of the semiquinone radicals QH^.; the log k_et value increases linearly with decreasing pH with slope of -1 in the region pK₁ > pH > pK ₂ where K₂ is the acid dissociation constant of hydroquinone radical cation QH₂^+.. The slope changes to -2 in more acidic media, pK₂ > pH. This dependence of log k _et on pH in aqueous solution has been analysed quantitatively, based on the positive shift of the one-electron reduction potentials of Q in the presence of HClO₄; the shift is caused by protonation of reduced species of Q. The positive shift of one-electron reduction potentials of carbonyl compounds (p-benzoquinone and acetophenone derivatives) in the presence of HClO₄ has also been determined in MeCN by analysing the acid-catalysed electron-transfer reactions from cis-[Et₂Co(bpy) ₂]⁺ and Fc to p-benzoquinone derivatives and the photoinduced electron-transfer reactions from [Ru(bpy)₃] ^2+• to acetophenone derivatives in MeCN. The correlations between the redox and acid-base properties of carbonyl compounds and the factors determining the catalytic effect of acid on the electron-transfer reactions are discussed.