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

Dinitrosyl iron complexes (DNICs) are among the most abundant NO-derived cellular species. Monomeric DNICs can exist in the {Fe(NO)2}9or {Fe(NO)2}10oxidation 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}9DNIC [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}7complex to yield the {Fe(NO)2}9DNIC 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}9DNICs with neutral N-donor ligands, 1exhibits high solution stability and can be characterized structurally and spectroscopically. Reduction of 1yields the corresponding {Fe(NO)2}10DNIC [Fe(dmp)(NO)2] (2). The Mössbauer isomer shift of 2is 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 1and 2are 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–1in 2in 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)2unit upon reduction of 1to 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}10DNICs in comparison to the corresponding {Fe(NO)2}9complexes, 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.