Reaction of a Co(III)-Peroxo Complex and NO: Formation of a Putative Peroxynitrite Intermediate

Mechanistic insights into nitric oxide oxygenation (NOO) reactions of {CrNO}5 and {CoNO}8

Here, we report the nitric oxide oxygenation (NOO) reactions of two distinct metal nitrosyls {Co-nitrosyl (S = 0) vs. Cr-nitrosyl (S = 1/2)}. In this regard, we synthesized and characterized [(BPMEN)Co(NO)]2+ ({CoNO}8, 1) to compare its NOO reaction with that of [(BPMEN)Cr(NO)(Cl-)]+ ({CrNO}5, 2), having a similar ligand framework. Kinetic measurements showed that {CrNO}5 is thermally more stable than {CoNO}8. Complexes 1 and 2, upon reaction with the superoxide anion (O2˙-), generate [(BPMEN)CoII(NO2-)2] (CoII-NO2-, 3) and [(BPMEN)CrIII(NO2-)Cl-]+ (CrIII-NO2-, 4), respectively, with O2 evolution. Furthermore, analysis of these NOO reactions and tracking of the N-atom using 15N-labeled NO (15NO) revealed that the N-atoms of 3 (CoII-15NO2-) and 4 (CrIII-15NO2-) derive from the nitrosyl (15NO) moieties of 1 and 2, respectively. This work represents a comparative study of oxidation reactions of {CoNO}8vs. {CrNO}5, showing different rates of the NOO reactions due to different thermal stability. To complete the NOM cycle, we reacted 3 and 4 with NO, and surprisingly, only 3 generated {CoNO}8 species, while 4 was unreactive towards NO. Furthermore, the phenol ring nitration test, performed using 2,4-di-tert-butylphenol (2,4-DTBP), suggested the presence of a proposed peroxynitrite (PN) intermediate in the NOO reactions of 1 and 2.

Reaction of a Co(III)-peroxo complex with nitric oxide: putative formation of a peroxynitrite intermediate

A Co(II) complex, [CoII(L)2(H2O)2](ClO4)2, 1, having a bidentate ligand L [L = bis(3,5-dimethylpyrazolyl)methane] has been synthesized. Complex 1 in acetonitrile solution at -40 °C, in the presence of H2O2 and NEt3, afforded the corresponding Co(III)-peroxo species, [CoIII(L)2(O22-)]+, as the transient intermediate 1a. Thermal instability precluded its isolation and further characterization. The addition of nitric oxide (NO) gas into the freshly prepared [CoIII(L)2(O22-)]+ in acetonitrile at -40 °C resulted in the corresponding Co(II)-nitrato complex, [CoII(L)2(NO3)](ClO4) (2). The reaction is proposed to proceed through a putative Co(II)-peroxynitrite intermediate 1b. It was evidenced by the characteristic phenol ring nitration reaction.

Ligand Control of Dinitrosyl Iron Complexes for Selective Superoxide-Mediated Nitric Oxide Monooxygenation and Superoxide-Dioxygen Interconversion

Through nitrosylation of [Fe-S] proteins, or the chelatable iron pool, a dinitrosyl iron unit (DNIU) [Fe(NO)2] embedded in the form of low-molecular-weight/protein-bound dinitrosyl iron complexes (DNICs) was discovered as a metallocofactor assembled under inflammatory conditions with elevated levels of nitric oxide (NO) and superoxide (O2-). In an attempt to gain biomimetic insights into the unexplored transformations of the DNIU under inflammation, we investigated the reactivity toward O2- by a series of DNICs [(NO)2Fe(μ-MePyr)2Fe(NO)2] (1) and [(NO)2Fe(μ-SEt)2Fe(NO)2] (3). During the superoxide-induced conversion of DNIC 1 into DNIC [(K-18-crown-6-ether)2(NO2)][Fe(μ-MePyr)4(μ-O)2(Fe(NO)2)4] (2-K-crown) and a [Fe3+(MePyr)x(NO2)y(O)z]n adduct, stoichiometric NO monooxygenation yielding NO2- occurs without the transient formation of peroxynitrite-derived •OH/•NO2 species. To study the isoelectronic reaction of O2(g) and one-electron-reduced DNIC 1, a DNIC featuring an electronically localized {Fe(NO)2}9-{Fe(NO)2}10 electronic structure, [K-18-crown-6-ether][(NO)2Fe(μ-MePyr)2Fe(NO)2] (1-red), was successfully synthesized and characterized. Oxygenation of DNIC 1-red leads to the similar assembly of DNIC 2-K-crown, of which the electronic structure is best described as paramagnetic with weak antiferromagnetic coupling among the four S = 1/2 {FeIII(NO-)2}9 units and S = 5/2 Fe3+ center. In contrast to DNICs 1 and 1-red, DNICs 3 and [K-18-crown-6-ether][(NO)2Fe(μ-SEt)2Fe(NO)2] (3-red) display a reversible equilibrium of "3 + O2- ⇋ 3-red + O2(g)", which is ascribed to the covalent [Fe(μ-SEt)2Fe] core and redox-active [Fe(NO)2] unit. Based on this study, the supporting/bridging ligands in dinuclear DNIC 1/3 (or 1-red/3-red) control the selective monooxygenation of NO and redox interconversion between O2- and O2 during reaction with O2- (or O2).

Nitric Oxide Oxygenation Reactions of Cobalt-Peroxo and Cobalt-Nitrosyl Complexes

Here, we report a comparative study of nitric oxide oxidation (NOO) reactions of Co^III-peroxo (Co^III-O₂^2-) and Co-nitrosyl ({CoNO}⁸) complexes bearing the same N₄-donor ligand (HMTETA) framework. In this regard, we prepared and characterized two new [(HMTETA)Co^III(O₂^2-)]⁺ (2, S = 2) and [(HMTETA)Co(NO)]²⁺ (3, S = 1) complexes from [(HMTETA)Co^II(CH₃CN)₂]²⁺ (1). Both complexes (2 and 3) are characterized by different spectroscopic measurements, including their DFT-optimized structures. Complex 2 produces Co^II-nitrato [(HMTETA)Co^II(NO₃^-)]⁺ (Co^II-NO₃^-, 4) complex in the presence of NO. In contrast, when 3 reacted with a superoxide (O₂^•-) anion, it generated Co^II-nitrito [(HMTETA)Co^II(NO₂^-)]⁺ (Co^II-NO₂^-, 5) with O₂ evolution. Experiments performed using ^18/16O-labeled superoxide (¹⁸O₂^•-/¹⁶O₂^•-) showed that O₂ originated from the O₂^•- anion. Both the NOO reactions are believed to proceed via a presumed peroxynitrite (PN) intermediate. Although we did not get direct spectral evidence for the proposed PN species, the mechanistic investigation using 2,4-di-tert-butylphenol indirectly suggests the formation of a PN intermediate. Furthermore, tracking the source of the N-atom in the above NOO reactions using ¹⁵N-labeled nitrogen (¹⁵NO) revealed N-atoms in 4 (Co^II-¹⁵NO₃^-) and 5 (Co^II-¹⁵NO₂^-) derived from the ¹⁵NO moiety.

Reaction of a {Co(NO)}8 complex with superoxide: Formation of a six coordinated [CoII(NO)(O2-)] species followed by peroxynitrite intermediate

A nitrosyl complex of cobalt(II) porphyrinate, [Co(F₂₀TPP^2-)(NO)], (F₂₀TPPH₂ = 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin) having {Co(NO)}⁸ configuration was synthesized and characterized by means of spectroscopic and structural analyses. Single crystal X-ray structure of the complex revealed the square pyramidal geometry around the cobalt center with a bent nitrosyl group. It reacts with superoxide (O₂^-) ion in CH₂Cl₂ at -40 °C to result in the corresponding nitrite (NO₂^-) complex. Involvement of a cobalt(II)-peroxynitrite intermediate is proposed in the course of the reaction. Moreover, spectroscopic studies suggested the formation of a transient six-coordinated [Co^II(NO)(O₂^-)] species.

Nitric Oxide Dioxygenation by O2 Adducts of Manganese Porphyrins

We describe here nitric oxide dioxygenation (NOD) by the dioxygen manganese porphyrin adducts Mn(Por)(η²-O₂) (Por^2- = the meso-tetra-phenyl or meso-tetra-p-tolylporphyrinato dianions, TPP^2- and TTP^2-). The Mn(Por)(η²-O₂) was assembled by adding O₂ to sublimed layers of Mn^II(Por). When NO was introduced and the temperature was slowly raised from 80 to 120 K, new IR bands with correlated intensities grew concomitant with depletion of the υ(O₂) band. Isotope labeling experiments with ¹⁸O₂, ¹⁵NO, and N¹⁸O combined with DFT calculations provide the basis for identifying the initial intermediates as the six-coordinate peroxynitrito complexes (ON)Mn(Por)(η¹-OONO). Further warming to room temperature led to formation of the nitrato complexes Mn(Por)(η¹-ONO₂), thereby demonstrating the ability of these metal centers to promote NOD. However, comparable quantities of the nitrito complexes Mn(Por)(η¹-ONO) are also formed. In contrast, when the analogous reactions were initiated with the weak σ-donor ligand tetrahydrofuran or dimethyl sulfide present in the layers, formation of Mn(Por)(η¹-ONO₂) is strongly favored (∼90%). The latter are formed via a 6-coordinate intermediate (L)Mn(Por)(η¹-ONO₂) (L = THF or DMS) that loses L upon warming. These reaction patterns are compared to those observed previously with analogous iron and cobalt porphyrin complexes.

Single-step benzene hydroxylation by cobalt(ii) catalysts via a cobalt(iii)-hydroperoxo intermediate

Cobalt(ii) complexes reported as efficient and selective catalysts for single-step phenol formation from benzene using H₂O₂. The catalysis proceeds likely via cobalt(iii)-hydroperoxo species.

Nitric oxide dioxygenation (NOD) reactions of CoIII-peroxo and NiIII-peroxo complexes: NODversusNO activation

A comparative study of “nitric oxide dioxygenationversusdioxygen or nitric oxide activation”.

Nitric Oxide Dioxygenase Activity of a Nitrosyl Complex of Mn(II)-Porphyrinate in the Presence of Superoxide: Formation of a Mn(IV)-oxo Species through a Putative Peroxynitrite Intermediate

A nitrosyl complex of Mn^II-porphyrinate, [(F₂₀TPP)Mn^II(NO)], 1 (F₂₀TPPH₂ = 5,10,15,20 tetrakis(pentafluorophenyl)porphyrin), was synthesized and characterized. Spectroscopic and structural characterization revealed complex 1 as a penta-coordinated Mn^II-nitrosyl with a linear Mn-N-O (180.0°) moiety. Complex 1 does not react with O₂. However, it reacts with superoxide (O₂^-) in THF at -80 °C to result in the corresponding nitrate (NO₃^-) complex, 2, via the formation of a presumed Mn^III-peroxynitrite intermediate. ESI-mass spectrometry and UV-visible and X-band EPR spectroscopic studies suggest the generation of Mn^IV-oxo species in the reaction through homolytic cleavage of the O-O bond of the peroxynitrite ligand as proposed in NOD activity. The intermediate formation of the Mn^III-peroxynitrite was further supported by the well accepted phenol ring nitration which resembles the biologically well-established tyrosine nitration.

Finding a new pathway for acid-induced nitrite reduction reaction: formation of nitric oxide with hydrogen peroxide

Here, we report a new pathway for nitrite reduction chemistry, formation of cobalt-nitrosyl ({Co^II-NO}⁸) with H₂O₂ in the reaction of a Co^II-nitrito complex with a one-fold acid (H⁺) via the formation of a Co^II-nitrous acid intermediate ({Co^II-ONOH}). Mechanistic investigations using ¹⁵N-labeled-¹⁵NO₂^- revealed that the N-atom in the {Co^II-NO}⁸ complex is derived from the nitrito ligand, and H₂O₂ came from the homolysis of the ON-OH moiety. Spectral evidence supporting the formation of the Co^II-ONOH intermediate and the generation of H₂O₂ is also presented.