Characterization of the Bridged Hyponitrite Complex {[Fe(OEP)]2(μ-N2O2)}: Reactivity of Hyponitrite Complexes and Biological Relevance

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Timothy C. Berto †, Nan Xu ‡, Se Ryeon Lee †,Anne J. McNeil †, E. Ercan Alp §, Jiyong Zhao §,George B. Richter-Addo *‡, and Nicolai Lehnert *†

† Department of Chemistry and Department of Biophysics, University of Michigan, 930 N. University Avenue, Ann Arbor, Michigan 48109, United States

‡ Department of Chemistry and Biochemistry, University of Oklahoma, 101 Stephenson Parkway, Norman, Oklahoma 73019, United States

§ Argonne National Laboratory, APS/XFD, 431/D003, Argonne, Illinois 60439, United States

Inorg. Chem., Article ASAP

DOI: 10.1021/ic5002573

Publication Date (Web): June 27, 2014

Copyright © 2014 American Chemical Society

*E-mail: grichteraddo@ou.edu., *E-mail: lehnertn@umich.edu.

Synopsis

The model complex {[Fe(OEP)]2(μ-N2O2)} offers a unique opportunity to study the electronic structure and reactivity of hyponitrite-bridged diiron complexes. The ground state of this complex is best described as having two intermediate-spin (S = 3/2) iron centers that are weakly antiferromagnetically coupled. The complex decomposes over time in solution to yield [Fe(OEP)(NO)]. The biological significance of this reaction for nitric oxide reductases is discussed.

Section:

Inorganic Chemicals and Reactions

Abstract

The detoxification of nitric oxide (NO) by bacterial NO reductase (NorBC) represents a paradigm of how NO can be detoxified anaerobically in cells. In order to elucidate the mechanism of this enzyme, model complexes provide a convenient means to assess potential reaction intermediates. In particular, there have been many proposed mechanisms that invoke the formation of a hyponitrite bridge between the heme b3 and nonheme iron (FeB) centers within the NorBC active site. However, the reactivity of bridged iron hyponitrite complexes has not been investigated much in the literature. The model complex {[Fe(OEP)]2(μ-N2O2)} offers a unique opportunity to study the electronic structure and reactivity of such a hyponitrite-bridged complex. Here we report the detailed characterization of {[Fe(OEP)]2(μ-N2O2)} using a combination of IR, nuclear resonance vibrational spectroscopy, electron paramagnetic resonance, and magnetic circular dichroism spectroscopy along with SQUID magnetometry. These results show that the ground-state electronic structure of this complex is best described as having two intermediate-spin (S = 3/2) iron centers that are weakly antiferromagnetically coupled across the N2O22– bridge. The analogous complex {[Fe(PPDME)]2(μ-N2O2)} shows overall similar properties. Finally, we report the unexpected reaction of {[Fe(OEP)]2(μ-N2O2)} in the presence and absence of 1-methylimidizole to yield [Fe(OEP)(NO)]. Density functional theory calculations are used to rationalize why {[Fe(OEP)]2(μ-N2O2)} cannot be formed directly by dimerization of [Fe(OEP)(NO)] and why only the reverse reaction is observed experimentally. These results thus provide insight into the general reactivity of hyponitrite-bridged iron complexes with general relevance for the N–N bond-forming step in NorBC.

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