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Non-heme High-Spin {FeNO}6–8 Complexes: One Ligand Platform Can Do It All
journal contribution
posted on 2018-08-14, 00:00 authored by Amy L. Speelman, Corey J. White, Bo Zhang, E. Ercan Alp, Jiyong Zhao, Michael Hu, Carsten Krebs, James Penner-Hahn, Nicolai LehnertHeme and non-heme iron–nitrosyl
complexes are important
intermediates in biology. While there are numerous examples of low-spin
heme iron–nitrosyl complexes in different oxidation states,
much less is known about high-spin (hs) non-heme iron–nitrosyls
in oxidation states other than the formally ferrous NO adducts ({FeNO}7 in the Enemark–Feltham notation). In this study, we
present a complete series of hs-{FeNO}6–8 complexes
using the TMG3tren coligand. Redox transformations from
the hs-{FeNO}7 complex [Fe(TMG3tren)(NO)]2+ to its {FeNO}6 and {FeNO}8 analogs
do not alter the coordination environment of the iron center, allowing
for detailed comparisons between these species. Here, we present new
MCD, NRVS, XANES/EXAFS, and Mössbauer data, demonstrating that
these redox transformations are metal based, which allows us to access
hs-Fe(II)–NO–, Fe(III)–NO–, and Fe(IV)–NO– complexes. Vibrational
data, analyzed by NCA, directly quantify changes in Fe–NO bonding
along this series. Optical data allow for the identification of a
“spectator” charge-transfer transition that, together
with Mössbauer and XAS data, directly monitors the electronic
changes of the Fe center. Using EXAFS, we are also able to provide
structural data for all complexes. The magnetic properties of the
complexes are further analyzed (from magnetic Mössbauer). The
properties of our hs-{FeNO}6–8 complexes are then
contrasted to corresponding, low-spin iron–nitrosyl complexes
where redox transformations are generally NO centered. The hs-{FeNO}8 complex can further be protonated by weak acids, and the
product of this reaction is characterized. Taken together, these results
provide unprecedented insight into the properties of biologically
relevant non-heme iron–nitrosyl complexes in three relevant
oxidation states.