Reactivity pathways for nitric oxide and nitrosonium with iron complexes in biologically relevant sulfur coordination spheres

Abstract

The interaction of nitric oxide (NO) with iron–sulfur cluster proteins results in the formation of dinitrosyl iron complexes (DNICs) coordinated by cysteine residues from the peptide backbone or with low molecular weight sulfur-containing molecules like glutathione. Such DNICs are among the modes available in biology to store, transport, and deliver NO to its relevant targets. In order to elucidate the fundamental chemistry underlying the formation of DNICs and to characterize possible intermediates in the process, we have investigated the interaction of NO (g) and NO+ with iron–sulfur complexes having the formula [Fe(SR)4]2−, where R=tBu, Ph, or benzyl, chosen to mimic sulfur-rich iron sites in biology. The reaction of NO (g) with [Fe(StBu)4]2− or [Fe(SBz)4]2− cleanly affords the mononitrosyl complexes (MNICs), [Fe(StBu)3(NO)]− (1) and [Fe(SBz)3(NO)]− (3), respectively, by ligand displacement. Mononitrosyl species of this kind were previously unknown. These complexes further react with NO (g) to generate the corresponding DNICs, [Fe(SPh)2(NO)2]− (4) and [Fe(SBz)2(NO)2]− (5), with concomitant reductive elimination of the coordinated thiolate donors. Reaction of [Fe(SR)4]2− complexes with NO+ proceeds by a different pathway to yield the corresponding dinitrosyl S-bridged Roussin red ester complexes, [Fe2(μ-StBu)2(NO)4] (2), [Fe2(μ-SPh)2(NO)4] (7) and [Fe2(μ-SBz)2(NO)4] (8). The NO/NO+ reactivity of an FeII complex with a mixed nitrogen/sulfur coordination sphere was also investigated. The DNIC and red ester species, [Fe(S-o-NH2C6H4)2(NO)2]− (6) and [Fe2(μ-S-o-NH2C6H4)2(NO)4] (9), were generated. The structures of 8 and 9 were verified by X-ray crystallography. The MNIC complex 1 can efficiently deliver NO to iron–porphyrin complexes like [Fe(TPP)Cl], a reaction that is aided by light. Removal of the coordinated NO ligand of 1 by photolysis and addition of elemental sulfur generates higher nuclearity Fe/S clusters.