Add time:08/15/2019         Source:sciencedirect.com

The electrochemical behaviour of the μ-oxo complexes [Fe2III,IIIO(L)4(H2O)2]4+ (L=2,2′-bipyridine (bpy, 1) and (−)4,5-pinene-2,2′-bipyridine (pb, 2)) has been investigated in CH3CN. Since some aqua substitution by CH3CN can occur (although, it is not demonstrated clearly) [Fe2III,IIIO(L)4(S)2]4+ (S=H2O and/or CH3CN) is a more accurate notation of complexes 1 and 2 species. Complexes 1 and 2 are reduced irreversibly and on the basis of the electrochemical and spectroscopic data, the reduction process implies an ECC mechanism. The controlled-potential reduction yields an equal amount of the corresponding mononuclear complexes [FeII(L)3]2+ and an ill-defined ‘Fe(III) oxide’. It is suggested that the primary one-electron reduced species [Fe2II,IIIO(L)4(S)2]3+ (S=H2O or CH3CN) decomposes into the mononuclear complex [FeII(L)2(S)2]2+ by cleavage of the dimeric structure. This mononuclear complex is further transformed into [Fe(L)3]2+ by the substitution of the S ligands by one L released from the second iron center, remaining at the +III oxidation state. The transformation of [FeII(L)2(S)2]2+ into [Fe(L)3]2+ is slower for 2 than for 1 due to larger steric hindrance of the pb ligand versus bpy. Addition of 2 H+ to solutions of 1 and 2 in CH3CN leads to the formation of new dinuclear [Fe2III,IIIO(L)2(S)6]4+ complexes. Under these conditions the reduction process implies two electrons with quasi-quantitative formation of the mononuclear bis-L complexes [FeII(L)2(S)2]2+ by a controlled-potential reduction. The implication of the redox behaviour of complexes 1 and 2 reported here with regard to their catalytic activity is discussed. It appears that the better stability and efficiency of 2 versus 1 as a catalyst for cyclohexane oxidation with tert-butyl hydroperoxide (TBHP) can be related to the slower rate of conversion of [FeII(L)2(S)2]2+ into the inactive species [Fe(L)3]2+ for 2.

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