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 H_2BPG_2DEV

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, H_2BPG_2DEV, 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, [Fe_2(μ-O)(H_2O)_2(BPG_2DEV)](ClO_4)_2 (6), [Fe_2(μ-O)(μ-O_2CAr~(PrO))(BPG_2DEV)](ClO_4) (7), and [Fe_2(μ-O)(μ-CO_3)(BPG_2DEV)] (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 CO_2 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 H_2O_2 and NEt_3 in CH_3CN/H_2O solutions. Mass spectrometric analyses of this species, formed using ~(18)O-labeled H_2O_2, indicate the presence of a peroxide ligand bound to the oxo-bridged diiron(III) center. The Moessbauer spectrum at 90 K of the EPR-silent intermediate exhibits a quadrupole doublet with <5 = 0.58 mm/s and ΔE_o = 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 Mossbauer parameters are comparable to those observed for a peroxo intermediate formed in the reaction of reduced toluene/oxylene 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 H_2O_2, as observed spectroscopically and by mass spectrometry.