Generation of H2S from Thiol-Dependent NO Reactivity of Model [4Fe-4S] Cluster and Roussin's Black Anion.

Iron-sulfur clusters (Fe-S) have been well established as a target for nitric oxide (NO) in biological systems. Complementary to protein-bound studies, synthetic models have provided a platform to study what iron nitrosylated products and byproducts are produced depending on a controlled reaction environment. We have previously shown a model [2Fe-2S] system that produced a dinitrosyl iron complex (DNIC) upon nitrosylation along with hydrogen sulfide (H2S), another important gasotransmitter, in the presence of thiol, and hypothesized a similar reactivity pattern with [4Fe-4S] clusters which have largely produced inconsistent reaction products across biological and synthetic systems. Roussin's black anion (RBA), [Fe4(µ3-S)3(NO)7]-, is a previously established reaction product from synthetic [4Fe-4S] clusters with NO. Here, we present a new reactivity for the nitrosylation of a synthetic [4Fe-4S] cluster in the presence of thiol and thiolate. [Et4N]2[Fe4S4(SPh)4] (1) was nitrosylated in the presence of excess PhSH to generate H2S and an "RBA-like" intermediate that when further reacted with [NEt4][SPh] produced a {Fe(NO)2}9 DNIC, [Et4N][Fe(NO)2(SPh)2] (2). This "RBA-like" intermediate proved difficult to isolate but shares striking similarities to RBA in the presence of thiol based on IR υ(NO) stretching frequencies. Surprisingly, the same reaction products were produced when the reaction started with RBA and thiol. Similar to 1/NO, RBA in the presence of thiol and thiolate generates stoichiometric amounts of DNIC while releasing its bridging sulfides as H2S. These results suggest not only that RBA may not be the final product of [4Fe-4S] + NO but also that RBA has unprecedented reactivity with thiols and thiolates which may explain current challenges around identifying biological nitrosylated Fe-S clusters.

A Peroxynitrite Dicopper Complex: Formation via Cu-NO and Cu-O2 Intermediates and Reactivity via O-O Cleavage Chemistry.

A mixed-valent Cu(I)Cu(II) complex, [CuI,II2(UN-O-)]2+ (1), reacts with NO(g) at -80 °C to form [CuI,II2(UN-O-)(NO)]2+ (2), best described as a mixed-valent nitrosyl complex that has a ν(N-O) band at 1670 cm-1 in its infrared (IR) spectrum. Complex 2 undertakes a one-electron oxidation via the addition of O2(g) to generate a new intermediate, best described as a superoxide and nitrosyl adduct, [CuII2(UN-O-)(NO)(O2-)]2+ (3), based on its distinctively blue-shifted ν(N-O) band at 1853 cm-1. Over the course of 20 min at -80 °C, 3 is converted to the peroxynitrite (PN) complex [CuII2(UN-O-)(-OON═O)]2+ (4), which was characterized by low-temperature electrospray ionization mass spectrometry (ESI-MS) and IR spectroscopy; ν(N-O) absorptions at 1520 and 1640 cm-1 have been assigned as cis- and trans-conformers of the PN ligand in 4. Alternatively, the superoxide complex [CuII2(UN-O-)(O2•-)]2+ (5) is found to react with NO(g) to generate the same intermediate superoxide and nitrosyl adduct 3 (based on IR criteria), which likewise converts to the same PN complex 4. The O-O bond in 4 undergoes heterolysis in dichloromethane solvent and is postulated to produce nitronium ion, leading to ortho-nitration of 2,4-di-tert-butylphenol (DTBP). However, in 2-methyltetrahydrofuran as solvent, the O-O bond undergoes homolysis to generate •NO2 (detected spectrophotometrically) and a putative higher-valent complex, [CuII,III2(UN-O-)(O2-)]2+, that abstracts a H-atom from DTBP to give [CuII2(UN-O-)(OH)]2+ and a phenoxyl radical. The latter may dimerize to form the bis-phenol observed experimentally or couple with the •NO2 present, leading to o-phenol nitration.

Dimethyl 2,2'-di-nitro-biphenyl-4,4'-di-carboxyl-ate.

The title compound, C16H12N2O8, exhibits two near-planar aromatic ester groups with ar-yl-ester dihedral angles of 2.1 (2) and 4.2 (3)°. The dihedral angle between the aromatic rings is 58.0 (1)°. The two nitro groups are tilted slightly from the plane of the aromatic rings, making dihedral angles of 14.1 (1) and 8.2 (2)°. In the crystal, mol-ecules are connected by weak C-H⋯O inter-actions, forming a three-dimensional network.

Dimethyl 2-amino-biphenyl-4,4'-di-carboxyl-ate.

The title compound, C16H15NO4, exhibits two near-planar aromatic ester groups with a maximum aryl-ester torsion angle of 1.9 (2)°. The dihedral angle between the benzene rings is 44.7 (1)°. In the crystal, N-H⋯O hydrogen bonding is observed along with C-H⋯O contacts, forming chanins along [101]. No π-π inter-actions were noted between the benzene rings.

Dioxygen reactivity of a biomimetic [4Fe-4S] compound exhibits [4Fe-4S] to [2Fe-2S] cluster conversion.

Fumarate and nitrate reductase (FNR) is a gene regulatory protein that controls anaerobic to aerobic respiration in Escherichia coli, for which O2 serves as a control switch to induce a protein structural change by converting [4Fe-4S] cofactors to [2Fe-2S] clusters. Although biomimetic models can aid in understanding the complex functions of their protein counterparts, the inherent sensitivity of discrete [Fe-S] molecules to aerobic conditions poses a unique challenge to mimic the O2-sensing capability of FNR. Herein, we report unprecedented biomimetic O2 reactivity of a discrete [4Fe-4S] complex, [Fe4S4(SPhF)4]2- (1) where SPhF is 4-fluorothiophenolate, in which the reaction of 1 with O2(g) in the presence of thiolate produces its [2Fe-2S] analogue, [Fe2S2(SPhF)4]2- (2), at room temperature. The cluster conversion of 1 to 2 can also be achieved by employing disulfide as an oxidant under the same reaction conditions. The [4Fe-4S] to [2Fe-2S] cluster conversion by O2 was found to be significantly faster than that by disulfide, while the reaction with disulfide produced higher yields of 2.

Crystal structure of dimethyl 9H-carbazole-2,7-di-carb-oxy-late.

In the title compound, C16H13NO4, the carbazole ring system is almost planar with non-H atoms possessing a mean deviation from planarity of 0.037 Å. The two ester groups are orientated trans to one another and tilted slightly from the mean plane of the carbazole ring system, making dihedral angles of 8.12 (6) and 8.21 (5)°. In the crystal, mol-ecules are linked by pairs of N-H⋯O hydrogen bonds forming inversion dimers. The dimers are linked by parallel slipped π-π inter-actions, forming slabs propagating along the b-axis direction [inter-centroid distance = 3.6042 (8) Å, inter-planar distance = 3.3437 (5) Å, slippage = 1.345 Å].