Unusual Synthetic Pathway for an {Fe(NO)2}9 Dinitrosyl 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)}(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 Mossbauer 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 pi-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.