Insight into the Electronic Structure of Biomimetic Dinitrosyliron Complexes (DNICs): Toward the Syntheses of Amido-Bridging Dinuclear DNICs

The ubiquitous function of nitric oxide (NO) guided the biological discovery of the natural dinitrosyliron unit (DNIU) [Fe­(NO)₂] as an intermediate/end product after Fe nitrosylation of nonheme cofactors. Because of the natural utilization of this cofactor for the biological storage and delivery of NO, a bioinorganic study of synthetic dinitrosyliron complexes (DNICs) has been extensively explored in the last 2 decades. The bioinorganic study of DNICs involved the development of synthetic methodology, spectroscopic discrimination, biological application of NO-delivery reactivity, and translational application to the (catalytic) transformation of small molecules. In this Forum Article, we aim to provide a systematic review of spectroscopic and computational insights into the bonding nature within the DNIU [Fe­(NO)₂] and the electronic structure of different types of DNICs, which highlights the synchronized advance in synthetic methodology and spectroscopic tools. With regard to the noninnocent nature of a NO ligand, spectroscopic and computational tools were utilized to provide qualitative/quantitative assignment of oxidation states of Fe and NO in DNICs with different redox levels and ligation modes as well as to probe the Fe–NO bonding interaction modulated by supporting ligands. Besides the strong antiferromagnetic coupling between high-spin Fe and paramagnetic NO ligands within the covalent DNIU [Fe­(NO)₂], in polynuclear DNICs, the effects of the Fe···Fe distance, nature of the bridging ligands, and type of bridging modes on the regulation of the magnetic coupling among paramagnetic DNIU [Fe­(NO)₂] are further reviewed. In the last part of this Forum Article, the sequential reaction of {Fe­(NO)₂}¹⁰ DNIC [(NO)₂Fe­(AMP)] (1-red) with NO_(g), HBF₄, and KC₈ establishes a synthetic cycle, {Fe­(NO)₂}⁹-{Fe­(NO)₂}⁹ DNIC [(NO)₂Fe­(μ-dAMP)₂Fe­(NO)₂] (1) → {Fe­(NO)₂}⁹ DNIC [(NO₂)­Fe­(AMP)]­[BF₄] (1-H) → {Fe­(NO)₂}¹⁰ DNIC 1-red → DNIC 1, for the transformation of NO into HNO/N₂O. Of importance, the NO-induced transformation of {Fe­(NO)₂}¹⁰ DNIC 1-red and [(NO)₂Fe­(DTA)] (2-red; DTA = diethylenetriamine) unravels a synthetic strategy for preparation of the {Fe­(NO)₂}⁹-{Fe­(NO)₂}⁹ DNICs [(NO)₂Fe­(μ-NHR)₂Fe­(NO)₂] containing amido-bridging ligands, which hold the potential to feature distinctive physical properties, chemical reactivities, and biological applications.