Dinitrosyl Iron Complexes (DNICs): From Biomimetic Synthesis and Spectroscopic Characterization toward Unveiling the Biological and Catalytic Roles of DNICs

Dinitrosyl iron complexes (DNICs) have been recognized as storage and transport agents of nitric oxide capable of selectively modifying crucial biological targets via its distinct redox forms (NO+, NO• and NO–) to initiate the signaling transduction pathways associated with versatile physiological and pathological responses. For decades, the molecular geometry and spectroscopic identification of {Fe­(NO)2}9 DNICs ({Fe­(NO) x } n where n is the sum of electrons in the Fe 3d orbitals and NO π* orbitals based on Enemark–Feltham notation) in biology were limited to tetrahedral (CN = 4) and EPR g-value ∼2.03, respectively, due to the inadequacy of structurally well-defined biomimetic DNICs as well as the corresponding spectroscopic library accessible in biological environments. The developed synthetic methodologies expand the scope of DNICs into nonclassical square pyramidal and trigonal bipyramidal (CN = 5) and octahedral (CN = 6) {Fe­(NO)2}9 DNICs, as well as two/three accessible redox couples for mononuclear {Fe­(NO)2}9/10 and dinuclear [{Fe­(NO)2}9/10–{Fe­(NO)2}9/10] DNICs with biologically relevant S/O/N ligation modes. The unprecedented molecular geometries and electronic states of structurally well-defined DNIC models provide the foundation to construct a spectroscopic library for uncovering the identity of DNICs in biological environments as well as to determine the electronic structures of the {Fe­(NO)2} core in qualitative and quantitative fashions by a wide range of spectroscopic methods. On the basis of 15N NMR, electron paramagnetic resonance (EPR), IR, cyclic voltammetry (CV), superconducting quantum interference device (SQUID) magnetometry, UV–vis, single-crystal X-ray crystallography, and Fe/S K-edge X-ray absorption and Fe Kβ X-ray emission spectroscopies, the molecular geometry, ligation modes, nuclearity, and electronic states of the mononuclear {Fe­(NO)2}9/10 and dinuclear [{Fe­(NO)2}9/10–{Fe­(NO)2}9/10] DNICs could be characterized and differentiated. In addition, Fe/S K-edge X-ray absorption spectroscopy of tetrahedral DNICs deduced the qualitative assignment of Fe/NO oxidation states of {Fe­(NO)2}9 DNICs as a resonance hybrid of {FeII(•NO)­(NO–)}9 and {FeIII(NO–)2}9 electronic states; the quantitative NO oxidation states of [(PhS)3Fe­(NO)]−, [(PhS)2Fe­(NO)2]−, and [(PhO)2Fe­(NO)2]− were further achieved by newly developed valence to core Fe Kβ X-ray emission spectroscopy as −0.58 ± 0.18, −0.77 ± 0.18, and −0.95 ± 0.18, respectively. The i