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, H(2)BPG(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(2)(mu-O)(H(2)O)(2)(BPG(2)DEV)](ClO(4))(2) (6), [Fe(2)(mu-O)(mu-O(2)CAr(iPrO))(BPG(2)DEV)](ClO(4)) (7), and [Fe(2)(mu-O)(mu-CO(3))(BPG(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(2) in organic solvents. A common maroon-colored intermediate (lambda(max) = 490 nm; epsilon = 1500 M(-1) cm(-1)) forms in reactions of 6, 7, or 8 with H(2)O(2) and NEt(3) in CH(3)CN/H(2)O solutions. Mass spectrometric analyses of this species, formed using (18)O-labeled H(2)O(2), indicate the presence of a peroxide ligand bound to the oxo-bridged diiron(III) center. The Mossbauer spectrum at 90 K of the EPR-silent intermediate exhibits a quadrupole doublet with delta = 0.58 mm/s and DeltaE(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 Mossbauer 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(2)O(2), as observed spectroscopically and by mass spectrometry.
Friedle
Simone
S
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
Kodanko
Jeremy J
JJ
Morys
Anna J
AJ
Hayashi
Takahiro
T
Moënne-Loccoz
Pierre
P
Lippard
Stephen J
SJ
D018360:Crystallography, X-Ray; D004798:Enzymes / Q000737:chemistry* / Q000378:metabolism; D005290:Ferric Compounds / Q000138:chemical synthesis / Q000737:chemistry* / Q000378:metabolism; D006861:Hydrogen Peroxide / Q000737:chemistry* / Q000378:metabolism; D008024:Ligands; D009584:Nitrogen / Q000737:chemistry* / Q000378:metabolism; D019159:Nonheme Iron Proteins / Q000737:chemistry* / Q000378:metabolism; D010084:Oxidation-Reduction; D021241:Spectrometry, Mass, Electrospray Ionization; D015204:Spectroscopy, Mossbauer; D013059:Spectrum Analysis, Raman
D004798:Enzymes; D005290:Ferric Compounds; D008024:Ligands; D019159:Nonheme Iron Proteins; D006861:Hydrogen Peroxide; D009584:Nitrogen