Electron transfer reactions between copper(II) porphyrin complexes and various oxidizing reagents in acetonitrile

Masahiko Inamo, Hideto Kumagai, Ushio Harada, Sumitaka Itoh, Satoshi Iwatsuki, Koji Ishihara, Hideo D. Takagi

    Research output: Contribution to journalArticle

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    Abstract

    Homogeneous electron transfer reactions of the Cu(II) complexes of 5,10,15,20-tetraphenylporphyrin (TPP) and 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP) with various oxidizing reagents were spectrophotometrically investigated in acetonitrile. The reaction products were confirmed to be the π-cation radicals of the corresponding Cu(II)-porphyrin complexes on the basis of the electronic spectra and the redox potentials of the complexes. The rate of the electron transfer reaction between the Cu(II)-porphyrin complex and solvated Cu2+ was determined as a function of the water concentration under the pseudo first-order conditions where Cu2+ is in large excess over the Cu(II)-porphyrin complex. The decrease in the pseudo first-order rate constant with increasing the water concentration was attributed to the stepwise displacement of acetonitrile in [Cu(AN)6]2+ (AN = acetonitrile) by water, and it was concluded that only the Cu2+ species fully solvated by acetonitrile, [Cu(AN)6]2+, possesses sufficiently high redox potential for the oxidation of Cu(II)-OEP and Cu(II)TPP. The reactions of the Cu(II)-porphyrin complexes with other oxidizing reagents such as [Ni(tacn)2]3+ (tacn = 1,4,7-triazacyclononane) and [Ru(bpy)3]3+ (bpy = 2,2′-bipyridine) were too fast to be followed by a conventional stopped-flow technique. Marcus cross relation for the outer-sphere electron transfer reaction was used to estimate the rate constants of the electron self-exchange reaction between Cu(II)-porphyrin and its π-cation radical: log(k/M-1 s-1) = 9.5 ± 0.5 for TPP and log(k/M -1 s-1) = 11.1 ±0.5 for OEP at 25.0 °C. Such large electron self-exchange rate constants are typical for the porphyrin-centered redox reactions for which very small inner- and outer-sphere reorganization energies are required.

    Original languageEnglish
    Pages (from-to)1703-1707
    Number of pages5
    JournalDalton Transactions
    Issue number11
    Publication statusPublished - 2004 Jun 7

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    Porphyrins
    Copper
    Electrons
    Rate constants
    Cations
    Water
    Redox reactions
    Reaction products
    acetonitrile
    Oxidation
    5,10,15,20-tetraphenylporphyrin
    octaethylporphyrin

    ASJC Scopus subject areas

    • Chemistry(all)

    Cite this

    Inamo, M., Kumagai, H., Harada, U., Itoh, S., Iwatsuki, S., Ishihara, K., & Takagi, H. D. (2004). Electron transfer reactions between copper(II) porphyrin complexes and various oxidizing reagents in acetonitrile. Dalton Transactions, (11), 1703-1707.

    Electron transfer reactions between copper(II) porphyrin complexes and various oxidizing reagents in acetonitrile. / Inamo, Masahiko; Kumagai, Hideto; Harada, Ushio; Itoh, Sumitaka; Iwatsuki, Satoshi; Ishihara, Koji; Takagi, Hideo D.

    In: Dalton Transactions, No. 11, 07.06.2004, p. 1703-1707.

    Research output: Contribution to journalArticle

    Inamo, M, Kumagai, H, Harada, U, Itoh, S, Iwatsuki, S, Ishihara, K & Takagi, HD 2004, 'Electron transfer reactions between copper(II) porphyrin complexes and various oxidizing reagents in acetonitrile', Dalton Transactions, no. 11, pp. 1703-1707.
    Inamo, Masahiko ; Kumagai, Hideto ; Harada, Ushio ; Itoh, Sumitaka ; Iwatsuki, Satoshi ; Ishihara, Koji ; Takagi, Hideo D. / Electron transfer reactions between copper(II) porphyrin complexes and various oxidizing reagents in acetonitrile. In: Dalton Transactions. 2004 ; No. 11. pp. 1703-1707.
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    abstract = "Homogeneous electron transfer reactions of the Cu(II) complexes of 5,10,15,20-tetraphenylporphyrin (TPP) and 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP) with various oxidizing reagents were spectrophotometrically investigated in acetonitrile. The reaction products were confirmed to be the π-cation radicals of the corresponding Cu(II)-porphyrin complexes on the basis of the electronic spectra and the redox potentials of the complexes. The rate of the electron transfer reaction between the Cu(II)-porphyrin complex and solvated Cu2+ was determined as a function of the water concentration under the pseudo first-order conditions where Cu2+ is in large excess over the Cu(II)-porphyrin complex. The decrease in the pseudo first-order rate constant with increasing the water concentration was attributed to the stepwise displacement of acetonitrile in [Cu(AN)6]2+ (AN = acetonitrile) by water, and it was concluded that only the Cu2+ species fully solvated by acetonitrile, [Cu(AN)6]2+, possesses sufficiently high redox potential for the oxidation of Cu(II)-OEP and Cu(II)TPP. The reactions of the Cu(II)-porphyrin complexes with other oxidizing reagents such as [Ni(tacn)2]3+ (tacn = 1,4,7-triazacyclononane) and [Ru(bpy)3]3+ (bpy = 2,2′-bipyridine) were too fast to be followed by a conventional stopped-flow technique. Marcus cross relation for the outer-sphere electron transfer reaction was used to estimate the rate constants of the electron self-exchange reaction between Cu(II)-porphyrin and its π-cation radical: log(k/M-1 s-1) = 9.5 ± 0.5 for TPP and log(k/M -1 s-1) = 11.1 ±0.5 for OEP at 25.0 °C. Such large electron self-exchange rate constants are typical for the porphyrin-centered redox reactions for which very small inner- and outer-sphere reorganization energies are required.",
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    AU - Inamo, Masahiko

    AU - Kumagai, Hideto

    AU - Harada, Ushio

    AU - Itoh, Sumitaka

    AU - Iwatsuki, Satoshi

    AU - Ishihara, Koji

    AU - Takagi, Hideo D.

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    N2 - Homogeneous electron transfer reactions of the Cu(II) complexes of 5,10,15,20-tetraphenylporphyrin (TPP) and 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP) with various oxidizing reagents were spectrophotometrically investigated in acetonitrile. The reaction products were confirmed to be the π-cation radicals of the corresponding Cu(II)-porphyrin complexes on the basis of the electronic spectra and the redox potentials of the complexes. The rate of the electron transfer reaction between the Cu(II)-porphyrin complex and solvated Cu2+ was determined as a function of the water concentration under the pseudo first-order conditions where Cu2+ is in large excess over the Cu(II)-porphyrin complex. The decrease in the pseudo first-order rate constant with increasing the water concentration was attributed to the stepwise displacement of acetonitrile in [Cu(AN)6]2+ (AN = acetonitrile) by water, and it was concluded that only the Cu2+ species fully solvated by acetonitrile, [Cu(AN)6]2+, possesses sufficiently high redox potential for the oxidation of Cu(II)-OEP and Cu(II)TPP. The reactions of the Cu(II)-porphyrin complexes with other oxidizing reagents such as [Ni(tacn)2]3+ (tacn = 1,4,7-triazacyclononane) and [Ru(bpy)3]3+ (bpy = 2,2′-bipyridine) were too fast to be followed by a conventional stopped-flow technique. Marcus cross relation for the outer-sphere electron transfer reaction was used to estimate the rate constants of the electron self-exchange reaction between Cu(II)-porphyrin and its π-cation radical: log(k/M-1 s-1) = 9.5 ± 0.5 for TPP and log(k/M -1 s-1) = 11.1 ±0.5 for OEP at 25.0 °C. Such large electron self-exchange rate constants are typical for the porphyrin-centered redox reactions for which very small inner- and outer-sphere reorganization energies are required.

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