Synthesis and Electrochemical Studies of Diiron Complexes of 1,8-Naphthyridine-Based Dinucleating Ligands to Model Features of the Active Sites of Non-Heme Diiron Enzymes

A bis(μ-carboxylato)(μ-1,8-naphthyridine)diiron(II) complex, [Fe2(BPMAN)(μ-O2CPhCy)2](OTf)2 (1), was prepared
by using the 1,8-naphthyridine-based dinucleating ligand BPMAN, where BPMAN = 2,7-bis[bis(2-pyridylmethyl)aminomethyl]-1,8-naphthyridine. The cyclic voltammogram (CV) of this complex in CH2Cl2 exhibited two
reversible one-electron redox waves at +296 mV (ΔEp = 80 mV) and +781 mV (ΔEp = 74 mV) vs Cp2Fe+/Cp2Fe, corresponding to the FeIIIFeII/FeIIFeII and FeIIIFeIII/FeIIIFeII couples, respectively. This result is unprecedented
for diiron complexes having no single atom bridge. Dinuclear complexes [Fe2(BPMAN)(μ-OH)(μ-O2CPhCy)](OTf)2 (2) and [Mn2(BPMAN)(μ-O2CPhCy)2](OTf)2 (3) were also synthesized and structurally characterized.
The cyclic voltammogram of 2 in CH2Cl2 exhibited one reversible redox wave at −22 mV only when the potential
was kept below +400 mV. The CV of 3 showed irreversible oxidation at potentials above +900 mV. Diiron(II)
complexes [Fe2(BEAN)(μ-O2CPhCy)3](OTf) (4) and [Fe2(BBBAN)(μ-OAc)2(OTf)](OTf) (6) were also prepared
and characterized, where BEAN = 2,7-bis(N,N-diethylaminomethyl)-1,8-naphthyridine and BBBAN = 2,7-bis{2-[2-(1-methyl)benzimidazolylethyl]-N-benzylaminomethyl}-1,8-naphthyridine. The cyclic voltammograms of
these complexes were recorded. The Mössbauer properties of the diiron compounds were studied.