| Abstract
| - The compounds [Fe2(μ-OH)(μ-Ph4DBA)(TMEDA)2(OTf)] (4) and [Fe2(μ-OH)(μ-Ph4DBA)(DPE)2(OTf)] (7) (Ph4DBA2- = dibenzofuran-4,6-bis(diphenylacetate)) were prepared as synthetic models for the non-heme diiron protein hemerythrin. In the presence of a small nitrogen donor, both complexes react with dioxygen at low temperature to yield a (μ-oxo)diiron(III) species bearing a terminally coordinated hydroperoxide ligand, characterized by Mössbauer, EPR, and EXAFS spectroscopic methods.
- 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 beenprepared as synthetic models for the dioxygen-binding non-heme diiron protein hemerythrin (Hr). X-raycrystallography reveals that, in the solid state, these compounds contain the asymmetric coordination environmentfound 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 = 2.81mm s-1) are also in good agreement. Oxygenation of the diiron(II) complexes dissolved in CH2Cl2 containing 3equiv 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-1cm-1) for 7. These spectra are remarkably similar to that of oxyHr, 330 nm (6800 M-1 cm-1) and 500 nm (2200M-1 cm-1). The electron paramagnetic resonance (EPR) spectrum of the cryoreduced, mixed-valence dioxygenadduct 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 oxidizedspecies 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.
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