Dinitrosyl iron complexes [E5Fe(NO)2]− (E = S, Se): A precursor of Roussin’s black salt [Fe4E3(NO)7]−

[PPN][Se5Fe(NO)2] (1) and [K-18-crown-6-ether][S5Fe(NO)2] (2′) were synthesized and characterized by IR, UV–Vis, EPR spectroscopy, magnetic susceptibility, and X-ray structure. [PPN][Se5Fe(NO)2] easily undergoes ligand exchange with S8 and (RS)2 (R = C7H4SN (5), o-C6H4NHCOCH3 (6), C4H3S (7)) to form [PPN][S5Fe(NO)2] and [PPN][(SR)2Fe(NO)2]. The reaction displays that [E5Fe(NO)2]− (E = Se (3), S (4)) facilely converts to [Fe4E3(NO)7]− by adding acid HBF4 or oxidant [Cp2Fe][BF4] in THF, respectively. Obviously, complexes 1 and 2′ serve as the precursors of the Roussin’s black salts 3 and 4. The electronic structure of {Fe(NO)2}9 core of [Se5Fe(NO)2]− is best described as a dynamic resonance hybrid of {Fe+1(radical dotNO)2}9 and {Fe−1(NO+)2}9 modulated by the coordinated ligands. The findings, EPR signal of g = 2.064 for 1 at 298 K, implicate that the low-molecular-weight DNICs and protein-bound DNICs may not exist with selenocysteine residues of proteins as ligands, since the existence of protein-bound DNICs and low-molecular-weight DNICs in vitro has been characterized with a characteristic EPR signal at g = 2.03. In addition, complex 2′ treated human erythroleukemia K562 cancer cells exposed to UV-A light greatly decreased the percentage survival of the cell cultures.