Characterization of the one-electron oxidized Cu(II)-salen complexes with a side chain aromatic ring: the effect of the indole ring on the Cu(II)-phenoxyl radical species

  • Hiromi Oshita
  • Takayoshi Yoshimura
  • Seiji Mori
  • Fumito Tani
  • Yuichi ShimazakiEmail author
  • Osamu YamauchiEmail author
Original Paper
Part of the following topical collections:
  1. Special issue to celebrate the 80th birthday of Helmut Sigel


To gain insights into the role of the proximal indole ring in the redox-active metal center as seen in galactose oxidase, we prepared the Cu(II)-salen-type complexes having a pendent indol-3-ylmethyl (1), methyl (2) or benzyl (3) group substituted on the ethylenediamine moiety and investigated the structures and redox properties by various physicochemical methods and theoretical calculations. Neutral complexes 1, 2, and 3 showed no significant difference in the UV–Vis–NIR and EPR spectra. One-electron oxidation of 1, 2, and 3 by addition of 1 equiv. of thianthrenyl radical gave [1]SbCl 6 , [2]SbCl 6 , and [3]SbCl 6 , respectively, which could be assigned to relatively localized phenoxyl radical species. The cyclic and differential pulse voltammograms of [1]SbCl 6 showed two redox waves with a large separation between the first and second redox potentials compared with the separations observed for [2]SbCl 6 and [3]SbCl 6 . This suggests that [1]SbCl 6 is more stabilized than [2]SbCl 6 and [3]SbCl 6 . The NIR band of [1]SbCl 6 showed a larger blue shift than that of [2]SbCl 6 and [3]SbCl 6 . The EPR spectrum of [2]SbCl 6 exhibited an intense signal at the g value of 2 due to partial disproportionation to form the EPR active two-electron oxidized complex [2] 2+ , while the EPR intensity of [1]SbCl 6 was much weaker than that of [2]SbCl 6 . These results indicate that the pendent indole moiety stabilizes the Cu(II)-phenoxyl radical in [1]SbCl 6 most probably by stacking with the phenoxyl moiety, which is further supported by DFT calculations.


Cu(II)-salen complexes Phenoxyl radical Indole ring ππ Stacking Galactose oxidase 



We gratefully acknowledge the helpful discussions and suggestions by Prof. Dr. Hitoshi Abe, High Energy Accelerator Research Organization (KEK) and SOKENDAI (the Graduate University for Advanced Studies), Japan and Prof. Dr. Tatsuo Yajima, Kansai University, Japan. This work was supported in part by Grants-in-Aid for Scientific Research (Nos. 25410060 and 16K05716 to Y. S., and No. 15K05411 to S. M.) from the Ministry of +Education, Culture, Sports, Science, and Technology of Japan, and Cooperative Research Program of “Network Joint Research Center for Materials and Devices” (Institute for Materials Chemistry and Engineering, Kyushu University). This work was also supported by a Grant-in-Aid for Scientific Research on Innovative Areas “Precise Formation of a Catalyst Having a Specified Field for Use in Extremely Difficult Substrate Conversion Reactions” from MEXT, Japan (16H01001) to S. M. The generous allotment of computation time from the Research Center for Computational Science (RCCS), the National Institutes of Natural Sciences, Japan, is also gratefully acknowledged.

Supplementary material

775_2017_1508_MOESM1_ESM.pdf (652 kb)
Supplementary material 1 (PDF 651 kb)


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Copyright information

© SBIC 2017

Authors and Affiliations

  • Hiromi Oshita
    • 1
  • Takayoshi Yoshimura
    • 1
  • Seiji Mori
    • 1
    • 2
  • Fumito Tani
    • 3
  • Yuichi Shimazaki
    • 2
    Email author
  • Osamu Yamauchi
    • 4
    • 5
    Email author
  1. 1.Graduate School of Science and EngineeringIbaraki UniversityMitoJapan
  2. 2.College of ScienceIbaraki UniversityMitoJapan
  3. 3.Institute for Materials Chemistry and EngineeringKyushu UniversityFukuokaJapan
  4. 4.Faculty of Chemistry, Materials and BioengineeringKansai UniversitySuitaJapan
  5. 5.Department of Chemistry, Graduate School of ScienceNagoya UniversityNagoyaJapan

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