Diiron Complexes of 1,8-Naphthyridine-Based Dinucleating Ligands as Models for Hemerythrin

Abstract

The hydroxo-bridged diiron(II) compounds [Fe2(BPEAN)(μ-OH)(OTf)](OTf)2 (1) and [Fe2(BEPEAN)(μ-OH)](OTf)3 (2) were prepared by using 1,8-naphthyridine-based dinucleating ligands BPEAN and BEPEAN, where BPEAN = 2,7-bis{bis[2-(2-pyridyl)ethyl]aminomethyl}-1,8-naphthyridine and BEPEAN = 2,7-bis{bis[2-(2-(5-ethyl)pyridyl)ethyl]aminomethyl}-1,8-naphthyridine. When compound 2 was treated with excess 30% aqueous H2O2 in acetonitrile at −40 °C, a red-brown species (3) was produced. The UV−vis spectrum of 3 exhibited an absorption maximum at 505 nm (ε = 1500 M-1 cm-1), close to that observed for oxyHr. Resonance Raman experiments revealed an isotope-sensitive O−O stretching band at 868 cm-1. When a mixture of 1:1 H2O2/D2O2 (25% in 1:1 H2O/D2O) was used to generate 3, a broader Raman band centered at 870 cm-1 appeared, indicating the peroxide group to be protonated. The 1H ENDOR spectrum of 3, cryoreduced to the diiron(II,III) state, showed a signal with A ≈ 12 MHz that disappeared when D2O2 in D2O was used to generate 3, providing further evidence for the presence of a hydroperoxide ligand bound to iron. The EPR spectrum of the cryoreduced sample revealed that 3 has a (μ-oxo)diiron(III) core, a conclusion supported by Mössbauer spectroscopy. The Mössbauer spectrum exhibited the unusual quadrupole splitting values that are characteristic of the diiron(III) center of oxyHr. Thus, all spectroscopic properties of 3 are consistent with it being a hydroperoxo-bound (μ-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 room temperature, acetonitrile/water solutions of 3 decayed to iron(II) species, releasing the iron-bound hydroperoxide group to form O2.