Unusual Synthetic Pathway for an {Fe(NO)2}9 Dinitrosyl Iron Complex (DNIC) and Insight into DNIC Electronic Structure via Nuclear Resonance Vibrational Spectroscopy
详细信息 查看全文
- 作者：Amy L. Speelman ; Bo Zhang ; Alexey Silakov ; Kelsey M. Skodje ; E. Ercan Alp ; Jiyong Zhao ; Michael Y. Hu ; Eunsuk Kim ; Carsten Krebs ; Nicolai Lehnert
- 刊名：Inorganic Chemistry
- 出版年：2016
- 出版时间：June 6, 2016
- 年：2016
- 卷：55
- 期：11
- 页码：5485-5501
- 全文大小：841K
- 年卷期：0
- ISSN：1520-510X
文摘
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.