Acid catalysis in thermal and photoinduced electron-transfer reactions

Shunichi Fukuzumi, Kunio Ishikawa, Katsuhiko Hironaka, Toshio Tanaka

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Abstract

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 ket 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 ket 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.

Original languageEnglish
Pages (from-to)751-760
Number of pages10
JournalJournal of the Chemical Society, Perkin Transactions 2
Issue number6
Publication statusPublished - Dec 1 1987
Externally publishedYes

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All Science Journal Classification (ASJC) codes

  • Chemistry(all)

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