Binding modes in metal ion complexes of quinones and semiquinone radical anions: Electron-transfer reactivity

Junpei Yuasa, Tomoyoshi Suenobu, Shunichi Fukuzumi

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52 Scopus citations


9,10-Phenanthrenequinone (PQ) and 1,10-phenanthroline-5,6-dione (PTQ) form 1:1 and 2:1 complexes with metal ions (Mn+ = Sc3+, Y 3+, Mg2+, and Ca2+) in acetonitrile (MeCN), respectively. The binding constants of PQ-Mn+ complexes vary depending on either the Lewis acidity or ion radius of metal ions. The one-electron reduced species (PTQ.-) forms 1:1 complexes with M n+, and PQ.- also forms 1:1 complexes with Sc 3+, Mg2+, and Ca2+, whereas PQ.- forms 1:2 complexes with Y3+ and La3+, as indicated by electron spin resonance (ESR) measurements. On the other hand, semiquinone radical anions (Q.- and NQ.-) derived from p-benzoquinone (Q) and 1,4-naphthoquinone (NQ) form Sc3+-bridged π-dimer radical anion complexes, Q.--(Sc3+)n-Q and NQ .--(Sc3+)n-NQ (n = 2 and 3), respectively. The one-electron reduction potentials of quinones (PQ, PTQ, and Q) are largely positively shifted in the presence of Mn+. The rate constant of electron transfer from CoTPP (TPP2- = dianion of tetraphenylporphyrin) to PQ increases with increasing the concentration of Sc3+ to reach a constant value, when all PQ molecules form the 1:1 complex with Sc3+. Rates of electron transfer from 10,10′-dimethyl-9,9′-biacridine [(AcrH)2] to PTQ are also accelerated significantly by the presence of Sc3+, Y3+, and Mg2+, exhibiting a first-order dependence with respect to concentrations of metal ions. In contrast to the case of o-quinones, unusually high kinetic orders are observed for rates of Sc3+-promoted electron transfer from tris(2-phenylpyridine)iridium(III) [Ir(ppy)3] to p-quinones (Q): second-order dependence on concentration of Q, and second- and third-order dependence on concentration of Sc3+ due to formation of highly ordered radical anion complexes, Q.-(Sc3+) n-Q (n = 2 and 3).

Original languageEnglish
Pages (from-to)942-954
Number of pages13
Issue number4
StatePublished - 10 Apr 2006


  • EPR spectroscopy
  • Electron transfer
  • Kinetics
  • Metal-ion complexes
  • Quinones


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