Formation Pathway of Roussin’s Red Ester (RRE) via the Reaction of a {Fe(NO)2}10 Dinitrosyliron Complex (DNIC) and Thiol: Facile Synthetic Route for Synthesizing Cysteine-Containing DNIC

Transformation of {Fe­(NO)2}10 dinitrosyliron complex (DNIC) Fe­(CO)2(NO)2 into [{Fe­(NO)2}9]2 Roussin’s red ester (RRE) [(μ-S­(CH2)2NH2)­Fe­(NO)2]2 (3) triggered by cysteamine via the reaction pathway (intermediates) [{Fe­(NO)2}10]2­[(NO)2­Fe­(μ-CO)­(μ-S­(CH2)2­NH3)­Fe­(NO)2] (1) → {Fe­(NO)2}9­{Fe­(NO)2}10­[(NO)2Fe­(μ-S­(CH2)2­NH2)­(μ-S­(CH2)2­NH3)­Fe­(NO)2] (2) → RRE 3 is demonstrated. The 1-to-2-to-3 conversion is promoted by proton transfer followed by O2 oxidation and deprotonation. Additionally, a study on facile conversion of complex 3 to complexes [(SR)­(S­(CH2)2NH3)­Fe­(NO)2] [SR = 2-aminoethanethiolate (4), benzenethiolate (5)] and [(CysS))­(S­(CH2)2NH3)­Fe­(NO)2] (6) via reaction with thiols and the further utility of complex 5 as a template for synthesizing mixed-thiolate-containing reduced RRE (rRRE) [(μ-SC6H5)­(μ-S­(CH2)2NH3)­Fe2(NO)4] (7) provide the methodology for the synthesis and isolation of neutral, pure cysteine-/mixed-thiolate-containing DNIC/RRE. Compared to the conversion of complex 2 to complex 3 via reaction with O2, diphenyl disulfide triggers oxidation of complex 2 to lead to formation of the neutral {Fe­(NO)2}9 DNIC 5 and RRE 3. S–S bond activation of diphenyl disulfide by rRRE 2 may support the decay (oxidation) of rRRE species in ToMOC via the reduction of adjacent protein residues such as cystins, proposed by Lippard.