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 SbCl 6 , SbCl 6 , and SbCl 6 , respectively, which could be assigned to relatively localized phenoxyl radical species. The cyclic and differential pulse voltammograms of SbCl 6 showed two redox waves with a large separation between the first and second redox potentials compared with the separations observed for SbCl 6 and SbCl 6 . This suggests that SbCl 6 is more stabilized than SbCl 6 and SbCl 6 . The NIR band of SbCl 6 showed a larger blue shift than that of SbCl 6 and SbCl 6 . The EPR spectrum of 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+ , while the EPR intensity of SbCl 6 was much weaker than that of SbCl 6 . These results indicate that the pendent indole moiety stabilizes the Cu(II)-phenoxyl radical in SbCl 6 most probably by stacking with the phenoxyl moiety, which is further supported by DFT calculations.
All Science Journal Classification (ASJC) codes
- Inorganic Chemistry