Relative Binding Affinity of Thiolate, Imidazolate, Phenoxide, and Nitrite Toward the {Fe(NO)2} Motif of Dinitrosyl Iron Complexes (DNICs): The Characteristic Pre-Edge Energy of {Fe(NO)2}9 DNICs

The synthesis, characterization, and transformation of the anionic {Fe(NO)(2)}(9) dinitrosyl iron complexes (DNICs) [(NO)(2)Fe(ONO)(2)](-) (1), [(NO)(2)Fe(OPh)(2)](-) (2), [(NO)(2)Fe(OPh)(C3H3N2)](-) (3) (C3H3N2=imidazolate), [(NO)(2)Fe(OPh)(-SC4H3S)](-) (4), [(NO)(2)Fe(p-OPhF)(2)](-) (5), and [(NO)(2)Fe(SPh)(ONO)](-) (6) were investigated. The binding affinity of ligands ([SPh](-), [-SC4H3S](-), [C3H3N2](-), [OPh](-), and [NO2](-)) toward the {Fe(NO)(2)}(9) motif follows the ligand-displacement series [SPh](-) similar to [-SC4H3S](-) > [C3H3N2](-) > [OPh](-) > [NO2](-). The findings, the pre-edge energy derived from the 1s -> 3d transition in a distorted T-d environment of the Fe center falling within the range of 7113.4-7113.8 eV for the anionic {Fe(NO)(2)}(9) DNICs, implicate that. the iron metal center of DNICs is tailored to minimize the electronic changes accompanying changes in coordinated ligands. Our results bridging the ligand-substitution reaction study and X-ray absorption spectroscopy study of the electronic richness of the {Fe(NO)(2)}(9) core may point the way to understanding the reasons for nature's choice of combinations of cysteine, histidine, and tyrosine in protein-bound DNICs and rationalize that most DNICs characterized/proposed nowadays are bound to the proteins almost through the thiolate groups of cysteinate/glutathione side chains in biological systems.