Bismercaptoethanediazacyclooctane as a N₂S₂ Chelating Agent and Cys−X−Cys Mimic for Fe(NO) and Fe(NO)₂

The N-protonated bismercaptoethanediazacyclooctane serves as a bidentate dithiolate ligand to oxidized Fe(NO)2 of Enemark−Feltam notation, E−F {Fe(NO)2}, mimicking Cys−X−Cys binding of Fe(NO)2 to proteins or thio-biomolecules. The neutral compound is characterized by the well-known g = 2.03 EPR signal which is a hallmark of dinitrosyl iron complexes, DNIC's. The Fe(NO)2 unit can be removed from the chelate by excess PhS-, producing (PhS)2Fe(NO)2-. Transfer of NO from Fe(H+bme-daco)(NO)2 (ν(NO) = 1740, 1696 cm-1) to FeII of [(bme-daco)Fe]2 yields the five-coordinate, square-pyramidal N2S2Fe(NO) (ν(NO) = 1649 cm-1), where NO is in the apical position. Its isotropic EPR signal at g = 2.05 is consistent with E−F {Fe(NO)}7 formulation. In excess NO, Roussin's red ester-type molecules are formed as dinuclear or tetranuclear species, {(μ-SRS)[Fe2(NO)4]}n (n =1, 2). These well-characterized molecules furnish reference points for positions and patterns in ν(NO) vibrational spectroscopy expected to be useful for in vivo studies of NO degradation of iron−sulfur clusters in ferredoxins.