Unusual Synthetic Pathway for an {Fe(NO)2}9 Dinitrosyl Iron Complex (DNIC) and Insight into DNIC Electronic Structure via Nuclear Resonance Vibrational Spectroscopy

Amy L. Speelman, Bo Zhang, Alexey Silakov, Kelsey M. Skodje, E. Ercan Alp, Jiyong Zhao, Michael Y. Hu, Eunsuk Kim, Carsten Krebs, Nicolai Lehnert

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Abstract

Dinitrosyl iron complexes (DNICs) are among the most abundant NO-derived cellular species. Monomeric DNICs can exist in the {Fe(NO)2}9 or {Fe(NO)2}10 oxidation state (in the Enemark-Feltham notation). However, experimental studies of analogous DNICs in both oxidation states are rare, which prevents a thorough understanding of the differences in the electronic structures of these species. Here, the {Fe(NO)2}9 DNIC [Fe(dmp)(NO)2](OTf) (1; dmp = 2,9-dimethyl-1,10-phenanthroline) is synthesized from a ferrous precursor via an unusual pathway, involving disproportionation of an {FeNO}7 complex to yield the {Fe(NO)2}9 DNIC and a ferric species, which is subsequently reduced by NO gas to generate a ferrous complex that re-enters the reaction cycle. In contrast to most {Fe(NO)2}9 DNICs with neutral N-donor ligands, 1 exhibits high solution stability and can be characterized structurally and spectroscopically. Reduction of 1 yields the corresponding {Fe(NO)2}10 DNIC [Fe(dmp)(NO)2] (2). The Mössbauer isomer shift of 2 is 0.08 mm/s smaller than that of 1, which indicates that the iron center is slightly more oxidized in the reduced complex. The nuclear resonance vibrational spectra (NRVS) of 1 and 2 are distinct and provide direct experimental insight into differences in bonding in these complexes. In particular, the symmetric out-of-plane Fe-N-O bending mode is shifted to higher energy by 188 cm-1 in 2 in comparison to 1. Using quantum chemistry centered normal coordinate analysis (QCC-NCA), this is shown to arise from an increase in Fe-NO bond order and a stiffening of the Fe(NO)2 unit upon reduction of 1 to 2. DFT calculations demonstrate that the changes in bonding arise from an iron-centered reduction which leads to a distinct increase in Fe-NO π-back-bonding in {Fe(NO)2}10 DNICs in comparison to the corresponding {Fe(NO)2}9 complexes, in agreement with all experimental findings. Finally, the implications of the electronic structure of DNICs for their reactivity are discussed, especially with respect to N-N bond formation in NO reductases.

Original languageEnglish (US)
Pages (from-to)5485-5501
Number of pages17
JournalInorganic Chemistry
Volume55
Issue number11
DOIs
StatePublished - Jun 6 2016

All Science Journal Classification (ASJC) codes

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

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