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[subscript 2]BPG[subscript 2]DEV
Author(s)Friedle, Simone; Kodanko, Jeremy J.; Morys, Anna J.; Hayashi, Takahiro; Lippard, Stephen J.; Moenne-Loccoz, Pierre; ... Show more Show less
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In order to model the syn disposition of histidine residues in carboxylate-bridged non-heme diiron enzymes, we prepared a new dinucleating ligand, H[subscript 2]BPG[subscript 2]DEV, 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[subscript 2](μ-O)(H[subscript 2]O)2(BPG[subscript 2]DEV)](ClO[subscript 4])[subscript 2] (6), [Fe[subscript 2](μ-O)(μ-O[subscript 2]CAr[superscript iPrO])(BPG[subscript 2]DEV)](ClO[subscript 4]) (7), and [Fe[subscript 2](μ-O)(μ-CO[subscript 3])(BPG[subscript 2]DEV)] (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[subscript 2] in organic solvents. A common maroon-colored intermediate (λ[subscript max] = 490 nm; ε = 1500 M[superscript −1] cm[superscript −1]) forms in reactions of 6, 7, or 8 with H[subscript 2]O[subscript 2] and NEt[subscript 3] in CH[subscript 3]CN/H[subscript 2]O solutions. Mass spectrometric analyses of this species, formed using [superscript 18]O-labeled H[subscript 2]O[subscript 2], 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[subscript 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 H[subscript 2]O[subscript 2], as observed spectroscopically and by mass spectrometry.
DepartmentMassachusetts Institute of Technology. Department of Chemistry
Journal of the American Chemical Society
American Chemical Society (ACS)
Friedle, Simone, Jeremy J. Kodanko, Anna J. Morys, Takahiro Hayashi, Pierre Moënne-Loccoz, and Stephen J. Lippard. “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.” Journal of the American Chemical Society 131, no. 40 (October 14, 2009): 14508-14520.
Author's final manuscript