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(mu -carboxylato) (mu -1,8-naphthyridine)diiron(II) complex. [Fe-2(BPMAN)(mu -O2CPhCy)(2)](OTf2) (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 (DeltaE(p) = 80 mV) and +781 mV (Delta Ep = 74 mV) vs Cp2Fe+/ Cp2Fe, corresponding to the (FeFeII)-Fe-III/(FeFeII)-Fe-II and (FeFeIII)-Fe-III/(FeFeII)-Fe-III couples, respectively. This result is unprecedented for diiron complexes having no single atom bridge. Dinuclear complexes [Fe-2(BPMAN) (mu -OH)(mu -O2CPhCy)]-(OTf)(2) (2) and [Mn-2(BPMAN)(u-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 [Fe-2(BEAN)(mu -O2CPhCy)(3)](OTf) (4) and [Fe-2(BBBAN)(mu -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) benzimidazolylethyr]-N-benzylaminomethyl}-1,8-naphthyridine. The cyclic voltammograms of these complexes were recorded. The Mossbauer properties of the diiron compounds were studied.