Modeling the Diiron Centers of Non-Heme Iron Enzymes. Preparation of Sterically Hindered Diiron(II) Tetracarboxylate Complexes and Their Reactions with Dioxygen

A series of diiron(II) complexes, [Fe 2 (μ-L)(μ-O 2 CR)(O 2 CR)(N) 2 ], where L is a dinucleating bis(carboxylate) ligand based on m -xylylenediamine bis(Kemp's triacid imide) and N is a pyridine- or imidazole-derived ligand, were prepared as models for the carboxylate-bridged non-heme diiron cores of the O 2 -activating enzymes, soluble methane monooxygenase hydroxylase (MMOH), and the R2 subunit of ribonucleotide reductase (RNR-R2). X-ray crystallographic studies revealed differences in the coordination geometry of the bridging monocarboxylate ligand, which shifts from monodentate to syn, syn -bidentate bonding modes. The extent of this carboxylate shift depends on both the steric bulk of the monocarboxylate and the basicity of the ancillary N-donor ligands. Exposure of these diiron(II) complexes to O 2 at −77 °C in nonpolar solvents (CH 2 Cl 2, THF, toluene) yielded deep blue solutions (λ max ≈ 580 nm, ε ≈ 1200 M - 1 cm - 1 ), consistent with the generation of diiron(III) peroxo species. This reaction was irreversible, and its stoichiometry was determined by manometry to be 1:1 in diiron(II) complex and O 2 . The diiron(III) peroxo complexes exhibited oxygen isotope-sensitive resonance Raman bands at ∼860 cm - 1, which are assigned to the O−O stretching frequency of a μ-1,2-peroxodiiron(III) core. 57 Fe Mössbauer spectroscopy confirmed the assignment of the diiron(III) oxidation level and indicated that the two iron sites have inequivalent environments (δ 1 ≈ 0.47 mm s - 1, Δ E Q1 ≈ 0.88 mm s - 1; δ 2 ≈ 0.63 mm s - 1, Δ E Q2 ≈ 1.20 mm s - 1 ). Kinetics experiments provided rate constants for the reaction and revealed it to be first order in both diiron(II) complex and O 2 . The factors controlling the rate of formation of the blue species and its stability are discussed.