Nitric oxide (NO) is an important signaling molecule that is involved in many physiological and pathological functions. Iron–sulfur proteins are one of the main reaction targets for NO, and the [Fe–S] clusters within these proteins are converted to various iron nitrosyl species upon reaction with NO, of which dinitrosyl iron complexes (DNICs) are the most prevalent. Much progress has been made in identifying the origin of cellular DNIC generation. However, it is not well-understood which other products besides DNICs may form during [Fe–S] cluster degradation nor what effects DNICs and other degradation products can have once they are generated in cells. Even more elusive is an understanding of the manner by which cells cope with unwanted [Fe–S] modifications by NO. This Account describes our synthetic modeling efforts to identify cluster degradation products derived from the [2Fe–2S]/NO reaction in order to establish their chemical reactivity and repair chemistry. Our intent is to use the chemical knowledge that we generate to provide insight into the unknown biological consequences of cluster modification.Our recent advances in three different areas are described. First, new reaction conditions that lead to the formation of previously unrecognized products during the reaction of [Fe–S] clusters with NO are identified. Hydrogen sulfide (H2S), a gaseous signaling molecule, can be generated from the reaction between [2Fe–2S] clusters and NO in the presence of acid or formal H• (e–/H+) donors. In the presence of acid, a mononitrosyl iron complex (MNIC) can be produced as the major iron-containing product. Second, cysteine analogues can efficiently convert MNICs back to [2Fe–2S] clusters without the need for any other reagents. This reaction is possible for cysteine analogues because of their ability to labilize NO from MNICs and their capacity to undergo C–S bond cleavage, providing the necessary sulfide for [2Fe–2S] cluster formation. Lastly, unique dioxygen reactivity of various types of DNICs has been established. N-bound neutral {Fe(NO)2}10 DNICs react with O2 to generate low-temperature stable peroxynitrite (ONOO–) species, which then carry out nitration chemistry in the presence of phenolic substrates, relevant to tyrosine nitration chemistry. The reaction between S-bound anionic {Fe(NO)2}9 DNICs and O2 results in the formation of Roussin’s red esters (RREs) and thiol oxidation products, chemistry that may be important in biological cysteine oxidation. The N-bound cationic {Fe(NO)2}9 DNICs can spontaneously release NO, and this property can be utilized in developing a new class of NO-donating agents with anti-inflammatory activity.

The nitrosylation of inorganic protein cofactors, specifically that of [Fe–S] clusters to form iron nitrosyls, plays a number of important roles in biological systems. In some of these cases, it is expected that a repair process reverts the nitrosylated iron species to intact [Fe–S] clusters. The repair of nitrosylated [2Fe–2S] cluster, primarily in the form of protein-bound dinitrosyl iron complexes (DNICs), has been observed in vitro and in vivo, but the mechanism of this process remains uncertain. The present work expands upon a previous observation (Fitzpatrick et al. J. Am. Chem. Soc. 2014, 136, 7229) of the ability of mononitrosyl iron complexes (MNICs) to be converted into [2Fe–2S] clusters by the addition of nothing other than a cysteine analogue. Herein, each of the critical elementary steps in the cluster repair has been dissected to elucidate the roles of the cysteine analogue. Systematic variations of a cysteine analogue employed in the repair reaction suggest that (i) the bidentate coordination of a cysteine analogue to MNIC promotes NO release from iron, and (ii) deprotonation of the α carbon of the ferric-bound cysteine analogue leads to the C–S cleavage en route to the formation of [2Fe–2S] cluster. The [2Fe–2S] cluster bearing a cysteine analogue has also been synthesized from thiolate-bridged iron dimers of the form [Fe2(μ-SR)2(SR)4]0/2–, which implies that such species may be present as intermediates in the cluster repair. In addition to MNICs, mononuclear tetrathiolate ferric or ferrous species have been established as another form of iron from which [2Fe–2S] clusters can be generated without need for any other reagent but a cysteine analogue. The results of these experiments bring to light new chemistry of classic coordination complexes and provides further insight into the repair of NO-modified [2Fe–2S] clusters.

Reversible modification of iron-sulfur clusters by nitric oxide acts as a genetic switch in a group of regulatory proteins. While the conversion of [Fe-S] clusters to iron-nitrosyls has been widely studied in the past, little is known about the reverse process, the repair of [Fe-S] clusters. Reported here is a system in which a mononitrosyl iron complex (MNIC), (PPN)[Fe(StBu)3(NO)] (1), is converted to a [2Fe-2S] cluster, (PPN)2[Fe2S2(SCH2CH2C(O)OMe)4] (2). This conversion requires only the addition of a cysteine analogue, 3-mercaptomethylpropionate (MMP), at room temperature without the need for any other reagents. The identity of 2 was confirmed spectroscopically, chemically, crystallographically, and analytically. Mass spectrometry and 34S labeling studies support that the bridging sulfides in 2 derive from the added MMP, the cysteine analogue. The NO lost during the conversion of 1 to 2 is trapped in a dinitrosyl iron side product, (PPN)[Fe(SCH2CH2C(O)OMe)2(NO)2] (4). The present system implies that MNICs are likely intermediates in the repair of NO-damaged [2Fe-2S] clusters and that cysteine is a viable molecule responsible for the destabilization of MINCs and the formation of [2Fe-2S] clusters.

The crosstalk between two biologically important signaling molecules, nitric oxide (NO) and hydrogen sulfide (H2S), proceeds via elusive mechanism(s). Herein we report the formation of H2S by the action of NO on synthetic [2Fe-2S] clusters when the reaction environment is capable of providing a formal H• (e–/H+). Nitrosylation of (NEt4)2[Fe2S2(SPh)4] (1) in the presence of PhSH or tBu3PhOH results in the formation of (NEt4)[Fe(NO)2(SPh)2] (2) and H2S with the concomitant generation of PhSSPh or tBu3PhO•. The amount of H2S generated is dependent on the electronic environment of the [2Fe-2S] cluster as well as the type of H• donor. Employment of clusters with electron-donating groups or H• donors from thiols leads to a larger amount of H2S evolution. The 1/NO reaction in the presence of PhSH exhibits biphasic decay kinetics with no deuterium kinetic isotope effect upon PhSD substitution. However, the rates of decay increase significantly with the use of 4-MeO-PhSH or 4-Me-PhSH in place of PhSH. These results provide the first chemical evidence to suggest that [Fe-S] clusters are likely to be a site for the crosstalk between NO and H2S in biology.

Dinitrosyl iron complexes (DNICs) are widely considered NO storage and donor molecules in cells. However, what induces an NO release from iron in DNICs and the subsequent biological consequences remain elusive. The chemistry and biology of the NO release activity of DNICs are reported here. Changes in redox status or coordination number of discrete N-bound DNICs, respectively [Fe(TMEDA)(NO)2] (1) and [Fe(TMEDA)(NO)2I] (2), can generate a metastable {Fe(NO)2}9 DNIC, [Fe(TMEDA)(NO)2]+, with νNO at 1769 and 1835 cm−1 and an EPR signal at g = 2.04, that spontaneously releases NO in solution. The NO release activity of 2 results in the up- and downregulation of heme oxygenase-1 (HO-1) and inducible nitric oxide synthase (iNOS), respectively, in murine RAW 264.7 macrophages. Furthermore, treatment with 2 leads to downregulation of pro-inflammatory cytokines, TNF-α and IL-6, and upregulation of the anti-inflammatory cytokine, IL-10. Taken together, these results demonstrate that the appropriate control of redox and coordination chemistry of DNICs could enable them to become anti-inflammatory agents, suggesting a potential new role for cellular DNICs.

The reactivity of a series of {Fe(NO)2}9 dinitrosyl iron complexes bearing thiolate ligands with molecular oxygen is reported. These reactions result in the formation of the corresponding Roussin's red esters along with thiolate oxidation. This reactivity is contrasted with that previously reported for {Fe(NO)2}10 complexes.

Protonation-assisted deoxygenation of a mono-oxo molybdenum center has been observed in many oxotransferases when the enzyme removes an oxo group to regenerate a substrate binding site. Such a reaction is reported here with discrete synthetic mono-oxo bis(dithiolene) molybdenum and tungsten complexes, the chemistry of which had been rarely studied because of the instability of the resulting deoxygenated products. An addition of tosylic acid to an acetonitrile solution of [MoIVO(S2C2Ph2)2]2– (1) and [WIVO(S2C2Ph2)2]2– (2) results in the loss of oxide with a concomitant formation of novel deoxygenated complexes, [M(MeCN)2(S2C2Ph2)2] (M = Mo (3), W (4)), that have been isolated and characterized. Whereas protonation of 1 exclusively produces 3, two different reaction products can be generated from 2; an oxidized product, [WO(S2C2Ph2)2]−, is produced with 1 equiv of acid while a deoxygenated product, [W(MeCN)2(S2C2Ph2)2] (4), is generated with an excess amount of proton. Alternatively, complexes 3 and 4 can be obtained from photolysis of [Mo(CO)2(S2C2Ph2)2] (5) and [W(CO)2(S2C2Ph2)2] (6) in acetonitrile. A di- and a monosubstituted adducts of 3, [Mo(CO)2(S2C2Ph2)2] (5) and [Mo(PPh3)(MeCN)(S2C2Ph2)2] (7) are also reported.

New types of degradation products of iron–sulfur clusters by nitric oxide (NO) have been identified in the acidic environment. In the absence of acid, NO reacts with (Et4N)2[Fe2S2Cl4] (1) to form a {Fe(NO)2}9 dinitrosyliron complex, (Et4N)[Fe(NO)2Cl2] (2), wherein the bridging sulfides are oxidized to elemental sulfur by four electrons (2S2– → 2S0 + 4e–). In contrast, the successive additions of NO and HCl to 1 result in the formation of a {Fe(NO)}7 mononitrosyliron complex, (Et4N)[Fe(NO)Cl3] (3), along with elemental sulfur and hydrogen sulfide (H2S), which are the two-electron-oxidized products of the bridging sulfides (2S2– + 2H+ → H2S + S0 + 2e–). The results demonstrate that the acidic environment plays a significant role in controlling the chemistry of an iron–sulfur cluster with NO and imply how two important gaseous molecules, NO and H2S, can be interconnected through iron–sulfur clusters.

Inspired by the CO2-reductatse activity of tungsten-dependent formate dehydrogenases (W-FDHs), a reduced W-FDH model, [WIV(OH)(S2C2Ph2)2]−, was prepared in situ through hydrolysis of [WIV(OPh)(S2C2Ph2)2]− (1) and its reactivity with CO2 was investigated. The reaction between [WIV(OH)(S2C2Ph2)2]− and CO2 at room temperature leads to the formation of [WIV(O)(S2C2Ph2)2]2– (2), which slowly oxidizes to [WV(O)(S2C2Ph2)2]− (3). Isotopic labeling experiments reveal that the O atom in CO2 incorporates into 3. This implies that there is carbonic anhydrase like activity, in which carbonation and decarboxylation are mediated by a bis(dithiolene)tungsten complex.

A new {Fe(NO)2}10 dinitrosyl iron complex possessing a 2,9-dimethyl-1,10-phenanthroline ligand has been prepared. This complex exhibits dioxygenase activity, converting NO to nitrate (NO3−) anions. During the oxygenation reaction, formation of reactive nitrating species is implicated, as shown in the effective o-nitration with a phenolic substrate.

Cellular dinitrosyl iron complexes (DNICs) have long been considered NO carriers. Although other physiological roles of DNICs have been postulated, their chemical functionality outside of NO transfer has not been demonstrated thus far. Here we report the unprecedented dioxygen reactivity of a N-bound {Fe(NO)2}10 DNIC, [Fe(TMEDA)(NO)2] (1). In the presence of O2, 1 becomes a nitrating agent that converts 2,4,-di-tert-butylphenol to 2,4-di-tert-butyl-6-nitrophenol via formation of a putative iron-peroxynitrite [Fe(TMEDA)(NO)(ONOO)] (2) that is stable below −80 °C. Iron K-edge X-ray absorption spectroscopy on 2 supports a five-coordinated metal center with a bound peroxynitrite in a cyclic bidentate fashion. The peroxynitrite ligand of 2 readily decays at increased temperature or under illumination. These results suggest that DNICs could have multiple physiological or deleterious roles, including that of cellular nitrating agents.

The reactivity of a series of {Fe(NO)2}9 dinitrosyl iron complexes bearing thiolate ligands with molecular oxygen is reported. These reactions result in the formation of the corresponding Roussin's red esters along with thiolate oxidation. This reactivity is contrasted with that previously reported for {Fe(NO)2}10 complexes.

Dinitrosyl iron complexes (DNICs) are widely considered NO storage and donor molecules in cells. However, what induces an NO release from iron in DNICs and the subsequent biological consequences remain elusive. The chemistry and biology of the NO release activity of DNICs are reported here. Changes in redox status or coordination number of discrete N-bound DNICs, respectively [Fe(TMEDA)(NO)2] (1) and [Fe(TMEDA)(NO)2I] (2), can generate a metastable {Fe(NO)2}9 DNIC, [Fe(TMEDA)(NO)2]+, with νNO at 1769 and 1835 cm−1 and an EPR signal at g = 2.04, that spontaneously releases NO in solution. The NO release activity of 2 results in the up- and downregulation of heme oxygenase-1 (HO-1) and inducible nitric oxide synthase (iNOS), respectively, in murine RAW 264.7 macrophages. Furthermore, treatment with 2 leads to downregulation of pro-inflammatory cytokines, TNF-α and IL-6, and upregulation of the anti-inflammatory cytokine, IL-10. Taken together, these results demonstrate that the appropriate control of redox and coordination chemistry of DNICs could enable them to become anti-inflammatory agents, suggesting a potential new role for cellular DNICs.

A new {Fe(NO)2}10 dinitrosyl iron complex possessing a 2,9-dimethyl-1,10-phenanthroline ligand has been prepared. This complex exhibits dioxygenase activity, converting NO to nitrate (NO3−) anions. During the oxygenation reaction, formation of reactive nitrating species is implicated, as shown in the effective o-nitration with a phenolic substrate.