TY - JOUR
T1 - Unusual Synthetic Pathway for an {Fe(NO)2}9 Dinitrosyl Iron Complex (DNIC) and Insight into DNIC Electronic Structure via Nuclear Resonance Vibrational Spectroscopy
AU - Speelman, Amy L.
AU - Zhang, Bo
AU - Silakov, Alexey
AU - Skodje, Kelsey M.
AU - Alp, E. Ercan
AU - Zhao, Jiyong
AU - Hu, Michael Y.
AU - Kim, Eunsuk
AU - Krebs, Carsten
AU - Lehnert, Nicolai
N1 - Funding Information:
This work was supported by the National Science Foundation (CHE-1305777 to N.L. and CHE-1254733 to E.K.). We acknowledge Dr. Jeff Kampf (University of Michigan) for X-ray crystallographic analysis of 1 and 3, and funding from NSF grant CHE-0840456 for X-ray instrumentation. A.L.S. acknowledges support from an NSF Graduate Research Fellowship and the Rackham Graduate School (University of Michigan) in the form of a Rackham Predoctoral Fellowship. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.
Publisher Copyright:
© 2016 American Chemical Society.
PY - 2016/6/6
Y1 - 2016/6/6
N2 - 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.
AB - 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.
UR - http://www.scopus.com/inward/record.url?scp=84973345894&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=84973345894&partnerID=8YFLogxK
U2 - 10.1021/acs.inorgchem.6b00510
DO - 10.1021/acs.inorgchem.6b00510
M3 - Article
AN - SCOPUS:84973345894
SN - 0020-1669
VL - 55
SP - 5485
EP - 5501
JO - Inorganic chemistry
JF - Inorganic chemistry
IS - 11
ER -