Intramolecular Electron Transfers Thwart Bistability in a Pentanuclear Iron Complex

With the intention
to investigate the redox properties of polynuclear
complexes as previously reported for the pentamanganese complex [{MnII(μ-bpp)3}2­MnIIIMnII2(μ3-O)]3+ (23+), we focused on the analogous
pentairon complex that was previously isolated as all-ferrous. As
Masaoka and co-workers recently published, aerobic synthesis leads
to the [{FeII(μ-bpp)3}2­FeIIIFeII2(μ3-O)]3+ complex (13+). This
species exhibits in acetonitrile solution four reversible one-electron
oxidation waves. Accordingly, the three oxidized species 14+, 15+, and 16+ with a 3FeII2FeIII, 2FeII3FeIII, and 1FeII4FeIII composition, respectively, were generated
by bulk electrolysis and isolated. Mössbauer spectroscopy allowed
us to determine the spin states of all the iron ions and to unambiguously
locate the sites of the successive oxidations. They all occur in the
μ3-oxo core except for the 14+ to 15+ process
that presents a striking electronic rearrangement, with both metals
in axial position being oxidized while the core is reduced to the
[FeIIIFeII2(μ3-O)]5+ oxidation level. This strongly differs from the redox behavior
of the Mn5 system. The origin of this electronic switch
is discussed.