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

摘要

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 analogs. Three novel oxo-bridged diiron(III) complexes [Fe2(μ-O)(H2O)2-(BPG2DEV)](ClO4)2 (>6), [Fe2(μ-O)(μ-O CAr^iPrO)(BPG2DEV)](ClO4) (>7), and [Fe2(μ-O)(μ-CO3)(BPG2DEV)] (>8) were prepared. Single crystal X-ray structural characterization confirms that two pyridines 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 (λmax = 490 nm; ε = 1500 M₋₁ cm⁻¹) forms in reactions of >6, >7, or >8 with H₂O₂ and NEt₃ in CH₃CN/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 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 (ToMOH) 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 reformed in significant quantities after a second addition of H₂O₂, as observed spectroscopically and by mass spectrometry.