Diiron complexes of 1,8-naphthyridine-based dinucleating ligands as modelsfor hemerythrin

The hydroxo-bridged diiron(II) compounds [Fe-2(BPEAN)(mu -OH)(OTf)](OTf)(2) (1) and [Fe-2(BEPEAN)(mu -OH)](OTf)(3) (2) were prepared by using 1,8-naph thyridine-based dinucleating ligands BPEAN and BEPEAN, where BPEAN = 2,7-bi s{bis[2-(2-pyridyl)ethyl]aminomethyl}-1,8-naphthyridine and BEPEAN 2,7-bis( bis[2-(2-(5-ethyl)pyridyl)ethyl] When compound 2 was treated with excess 30 % aqueous H2O2 in acetonitrile at -40 degreesC, a red-brown species (3) was produced. The UV-vis spectrum of 3 exhibited an absorption maximum at 505 nm (epsilon = 1500 M-1 cm(-1)), close to that observed for oxyHr. Resonance Raman experiments revealed an isotope-sensitive O-O stretching band at 858 cm(-1). When a mixture of I:1 H2O2/D2O2 (25% in 1:1 H2O/D2O) was used to g enerate 3, a broader Raman band centered at 870 cm(-1) appeared, indicating the peroxide group to be protonated. The H-1 ENDOR spectrum of 3, cryoredu ced to the diiron(II,III) state, showed a signal with A approximate to 12 M Hz that disappeared when D2O2 in D2O was used to generate 3, providing furt her evidence fur the presence of a hydroperoxide ligand bound to iron. The EPR spectrum of the cryoreduced sample revealed that 3 has a (mu -oxo)diiro n(III) core, a conclusion supported by Mossbauer spectroscopy. The Mossbaue r spectrum exhibited the unusual quadrupole splitting values that are chara cteristic of the diiron(III) center of oxyHr. Thus, all spectroscopic prope rties of 3 are consistent with it being a hydroperoxo-bound (mu -oxo)diiron (III) complex. The hydroperoxide ligand is more resistant, to deprotonation than in mononuclear iron(III) analogues, which may reflect the presence of a hydrogen bond between the hydroperoxide and bridging oxide groups. At ro om temperature, acetonitrile/water solutions of 3 decayed to iron(II) speci es, releasing the iron-bound hydroperoxide group to form O-2.