Modeling the syn disposition of nitrogen donors in non-heme diiron enzymes. Synthesis, characterization, and hydrogen peroxide reactivity of diiron(III) complexes with the syn N-donor ligand H₂BPG₂DEV

In order to model the syn disposition of histidine residues in carboxylate-bridged non-heme diiron enzymes, we prepared a new dinucleating ligand, H₂BPG₂DEv, that provides this geometric feature. The ligand incorporates biologically relevant carboxylate functionalities, which have not been explored as extensively as nitrogen-only analogues. Three novel oxo-bridged diiron(III) complexes, [Fe₂(μ-O)(H₂O) ₂(BPG₂DEV)](ClO₄)₂ (6), [Fe ₂(μ-O)(μ-O₂CAr^iPrO)(BPG ₂DEV)](ClO₄) (7), and [Fe₂(μ-O)(μ-CO ₃)(BPG₂DEV)] (8), were prepared. Single-crystal X-ray structural characterization confirms that two pyridyl groups are bound syn with respect to the Fe-Fe vector in these compounds. The carbonato-bridged complex 8 forms quantitatively from 6 in a rapid reaction with gaseous CO₂ in organic solvents. A common maroon-colored intermediate (λ_max = 490 nm; ε = 1500 M^-1 cm^-1) forms in reactions of 6, 7, or 8 with H₂O₂ and NEt₃ in CH ₃CN/H₂O solutions. Mass spectrometric analyses of this species, formed using ¹⁸O-labeled H₂O₂, indicate the presence of a peroxide ligand bound to the oxo-bridged diiron(III) center. The Mössbauer spectrum at 90 K of the EPR-silent intermediate exhibits a quadrupole doublet with δ = 0.58 mm/s and ΔE_Q = 0.58 mm/s. The isomer shift is typical for a peroxodiiron(III) species, but the quadrupole splitting parameter is unusually small compared to those of related complexes. These Mössbauer parameters are comparable to those observed for a peroxo intermediate formed in the reaction of reduced toluene/o-xylene monooxygenase hydroxylase with dioxygen. Resonance Raman studies reveal an unusually low-energy O-O stretching mode in the peroxo intermediate that is consistent with a short diiron distance. Although peroxodiiron(III) intermediates generated from 6, 7, and 8 are poor O-atom-transfer catalysts, they display highly efficient catalase activity, with turnover numbers up to 10 000. In contrast to hydrogen peroxide reactions of diiron(III) complexes that lack a dinucleating ligand, the intermediates generated here could be re-formed in significant quantities after a second addition of H₂O₂, as observed spectroscopically and by mass spectrometry.