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-Feltarn notation, E-F {Fe(NO)(2)},(9) 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)(2)Fe(NO)(2)(-). Transfer of NO from Fe(H(+)bme-daco)(NO)(2) (v(NO) = 1740, 1696 cm(-1)) to Fell of [(bme-daco)Fe](2) yields the five-coordinate, square-pyramidal N2S2-Fe(NO) (v(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, {(mu-SRS)[Fe-2(NO)(4)]}(n) (n =1, 2). These well-characterized molecules furnish reference points for positions and patterns in v(NO) vibrational spectroscopy expected to be useful for in vivo studies of NO degradation of iron-sulfur clusters in ferredoxins.