Synthesis and Spectroscopic Studies of Non-Heme Diiron(III) Species with a Terminal Hydroperoxide Ligand: Models for Hemerythrin

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Abstract
Two compounds, [Fe2(μ-OH)(μ-Ph4DBA)(TMEDA)2(OTf)] (4) and [Fe2(μ-OH)(μ-Ph4DBA)(DPE)2(OTf)] (7), where Ph4DBA2- is the dinucleating bis(carboxylate) ligand dibenzofuran-4,6-bis(diphenylacetate), have been prepared as synthetic models for the dioxygen-binding non-heme diiron protein hemerythrin (Hr). X-ray crystallography reveals that, in the solid state, these compounds contain the asymmetric coordination environment found at the diiron center in the reduced form of the protein, deoxyHr. Mössbauer spectra of the models (4, δ = 1.21(2), ΔEQ = 2.87(2) mm s-1; 7, δav = 1.23(1), ΔEQav = 2.79(1) mm s-1) and deoxyHr (δ = 1.19, ΔEQ = 2.81 mm s-1) are also in good agreement. Oxygenation of the diiron(II) complexes dissolved in CH2Cl2 containing 3 equiv of N-MeIm (4) or neat EtCN (7) at −78 °C affords a red-orange solution with optical bands at 336 nm (7300 M-1 cm-1) and 470 nm (2600 M-1 cm-1) for 4 and at 334 nm (6400 M-1 cm-1) and 484 nm (2350 M-1 cm-1) for 7. These spectra are remarkably similar to that of oxyHr, 330 nm (6800 M-1 cm-1) and 500 nm (2200 M-1 cm-1). The electron paramagnetic resonance (EPR) spectrum of the cryoreduced, mixed-valence dioxygen adduct of 7 displays properties consistent with a (μ-oxo)diiron(II,III) core. An investigation of 7 and its dioxygen-bound adduct by extended X-ray absorption fine structure (EXAFS) spectroscopy indicates that the oxidized species contains a (μ-oxo)diiron(III) core with iron−ligand distances in agreement with those expected for oxide, carboxylate, and amine/hydroperoxide donor atoms. The analogous cobalt complex [Co2(μ-OH)(μ-Ph4DBA)(TMEDA)2(OTf)] (6) was synthesized and structurally characterized, but it was unreactive toward dioxygen.