Reduction of Copper(II) Complexes of Tripodal Ligands by Nitric Oxide and Trinitrosation of the Ligands

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.

Elucidation of the Electrocatalytic Nitrite Reduction Mechanism by Bio-Inspired Copper Complexes

Mononuclear copper complexes relevant to the active site of copper nitrite reductases (CuNiRs) are known to be catalytically active for the reduction of nitrite. Yet, their catalytic mechanism has thus far not been resolved. Here, we provide a complete description of the electrocatalytic nitrite reduction mechanism of a bio-inspired CuNiR catalyst Cu(tmpa) (tmpa = tris(2-pyridylmethyl)amine) in aqueous solution. Through a combination of electrochemical studies, reaction kinetics, and density functional theory (DFT) computations, we show that the protonation steps take place in a stepwise manner and are decoupled from electron transfer. The rate-determining step is a general acid-catalyzed protonation of a copper-ligated nitrous acid (HNO2) species. In view of the growing urge to convert nitrogen-containing compounds, this work provides principal reaction parameters for efficient electrochemical nitrite reduction. This contributes to the investigation and development of nitrite reduction catalysts, which is crucial to restore the biogeochemical nitrogen cycle.

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”.

Dansyl-appended CuII-complex-based nitroxyl (HNO) sensing with living cell imaging application and DFT studies

We introduce herein, a novel copper complex-based fluorescent probe[Cu^II(DQ₄₆₈)Cl]⁺that exhibits a significant fluorescence turn-on response towards nitroxyl with high selectivity over other biological reactive oxygen, nitrogen and sulfur species, including nitric oxide.

Nitric Oxide Reactivity of a Cu(II) Complex of an Imidazole-Based Ligand: Aromatic C-Nitrosation Followed by the Formation of N-Nitrosohydroxylaminato Complex

A binuclear Cu(II) complex, 1, [Cu₂(L^-)₂(OAc)](OAc) of imidazole-based ligand LH {LH = 2-(bis(2-ethyl-5-methyl-1H-imidazol-4-yl)methyl)phenol} was synthesized and characterized spectroscopically and structurally. Addition of an equivalent amount of nitric oxide (NO) by a gastight syringe to the acetonitrile:methanol (5:1, v/v) solution of complex 1 at room temperature resulted in the reduction of Cu(II) center to Cu(I) with concomitant C-nitrosation of the ligand. Spectroscopic characterization of the resulting Cu(I) complex (1a) of the C-nitrosylated ligand, L' {L' = 2-(bis(2-ethyl-5-methyl-1H-imidazol-4-yl)methyl)-4-nitroso-phenol} has been done. The Cu(I) complex, 1a, further reacted with NO to result in the corresponding N-nitrosohydroxylaminato complex, 2, [Cu₂(L-ONNO)₂](OAc)₂ through the formation of a Cu(I)-nitrosyl intermediate. A small fraction of the nitrosyl intermediate decomposed to the corresponding Cu(II) complex 3, [Cu(L')₂], and N₂O in a parallel reaction.

Effect of ligand denticity on the nitric oxide reactivity of cobalt(ii) complexes

NO reactivity of three Co(ii) complexes, 1, 2 and 3 have been studied in degassed methanol solution. The complexes differ from each other in terms of denticity and flexibility of the ligand fameworks. Complex 1 undergoes reductive nitrosylation of the metal ion; 2 results in corresponding [Co^III(NO⁻)] complex; whereas 3 does not react with NO.

Nitrite reduction by copper through ligand-mediated proton and electron transfer

Nitrite reduction by a copper complex featuring a proton-responsive tripodal ligand is demonstrated. Gaseous nitric oxide was confirmed as the sole NO X by-product in quantitative yield. DFT calculations predict that nitrite reduction occurs via a proton and electron transfer process mediated by the ligand. The reported mechanism parallels nitrite reduction by copper nitrite reductase.

Aromatic C-nitrosation by a copper(II)-nitrosyl complex

Copper(II) complex of 4-amino-3-hydroxy-1-sulphonic acid was synthesized and characterized. Upon addition of nitric oxide, the copper(II) center of the complex in methanol was found to undergo reduction through an unstable copper(II)-nitrosyl intermediate. The formation of the intermediate was confirmed by UV-visible and FT-IR spectroscopy. The reduction of the copper(II) center was accompanied with a simultaneous C-nitrosation of the aromatic ring of the ligand. The C-nitrosation product was isolated and characterized by various spectroscopic analyses.

Phenol ring nitration induced by the unprecedented reduction of the Cu(II) centre by nitrogen dioxide

Nitrogen dioxide (˙NO2) induces tyrosine nitration through a radical mechanism in biological systems. Two copper(II) complexes, 1 and 2, with ligands L₁ and L₂ [L₁ = 2,4-di-tert-butyl-6-(((2-(dimethylamino)ethyl)(isopropyl)amino)methyl)phenol; L₂ = 6,6'-(((2-(dimethylamino)ethyl)azanediyl)bis(methylene))bis(2,4-di-tert-butylphenol)], respectively, have been made to react with ˙NO2. In both cases, the reduction of the copper(II) center was observed in the presence of ˙NO2 which induces phenol ring nitration through nitronium ion (NO2(+)) formation.

Efficient synthesis of new asymmetric tripodal ligands using microwave irradiation, and their crystal structures

An efficient method for synthesis of asymmetric tripodal ligands was described by using microwave irradiation.

A novel copper(II) complex as a nitric oxide turn-on fluorosensor: intracellular applications and DFT calculation

We report, herein, the development of an easily synthesizable novel dansyl-based turn-on NO sensor L2. The UV-Vis titration data of L2 with Cu(2+) display a gradual increase in absorbance at 418 nm with [Cu(2+)], which were analyzed by using a non-linear least-squares computer-fit program yielding K = (1.16 ± 0.36) × 10(6) M(-1) and n = (1.28 ± 0.03) indicating a 1 : 1 complexation. The ground state geometries of L2 as well as its complex [Cu(L2)Cl](+) (1) were optimized by DFT calculations which showed that in complex 1 the central metal ion is in distorted tetrahedral geometry with bond distances very close to those found in analogous Cu(2+) complexes. The fluorescence of L2 was dramatically quenched (∼60-fold) through complexation with paramagnetic Cu(2+) to form [Cu(L2)Cl](+) in MeCN-H2O (9 : 1, v/v) at pH 7.2 in HEPES buffer, which on further treatment with Angeli's salt (Na2N2O3) restores its fluorescence property by ∼15-fold due to the reduction of Cu(2+) to Cu(+) by NO generated in solution from Na2N2O3. The lifetime measurements displayed a substantial decrease in the lifetime of free ligand L2 (τ0 = 12 ns) on complexation with Cu(2+) (τ0 = 2.1 ns). The detection limit of NO calculated by the 3σ method gives a value of 1.6 nM. The NO induced fluorescence enhancement of [Cu(II)(L2)Cl](+) was due to the reduction of [Cu(II)(L2)Cl](+) (1) to [Cu(I)(L2)](+) (2) and is supported by the disappearance of the d-d transition band at 850 nm as well as the X-band EPR signal of 1. The selective "turn on" fluorogenic behavior of L2 was examined on HeLa cells of human cervical cancer origin by fluorescence microscopy which showed very intense intracellular fluorescence that was strongly suppressed by the addition of Cu(2+) but it regains its fluorescence property on further incubation with Angeli's salt (Na2N2O3). The existence of [Cu(II)(L2)Cl](+) and [Cu(I)(L2)](+) in solution was confirmed by ESI-MS(+) (m/z) analysis. The effect of different biologically relevant cations and anions on the fluorescence property of L2 indicates that it was only the [Cu(II)(L2)Cl](+) which displayed high selectivity for NO, indicating its suitability for intracellular application without much worry about its cytotoxicity in a specified dose.

Copper(I) nitrosyls from reaction of copper(II) thiolates with S-nitrosothiols: mechanism of NO release from RSNOs at Cu

S-nitrosothiols (RSNOs) serve as ready sources of biological nitric oxide activity, especially in conjunction with copper centers. We report a novel pathway for the generation of NO within the coordination sphere of copper model complexes from reaction of copper(II) thiolates with S-nitrosothiols. Reaction of tris(pyrazolyl)borate copper(II) thiolates (iPr2)TpCu-SR (R = C6F5 or CPh3) with (t)BuSNO leads to formation of (iPr2)TpCu(NO) and the unsymmetrical disulfide RS-S(t)Bu. Quantum mechanical investigations with B3LYP-D3/6-311G(d) suggest formation of a κ(1)-N-RSNO adduct (iPr2)TpCu(SR)(R'SNO) that precedes release of RSSR' to deliver (iPr2)TpCu(NO). This process is reversible; reaction of (iPr2)TpCu(NO) (but not (iPr2)TpCu(NCMe)) with C6F5S-SC6F5 forms (iPr2)TpCu-SC6F5. Coupled with the facile, reversible reaction between (iPr2)TpCu(NO) and C6F5SNO to give (iPr2)TpCu-SC6F5 and 2 equiv NO, we outline a new, detailed catalytic cycle for NO generation from RSNOs at Cu.

Reaction of a copper(II)-nitrosyl complex with hydrogen peroxide: putative formation of a copper(I)-peroxynitrite intermediate

The reaction of a Cu(II)-nitrosyl complex (1) with hydrogen peroxide at -20 °C in acetonitrile results in the formation of the corresponding Cu(I)-peroxynitrite intermediate. The reduction of the Cu(II) center was monitored by UV-visible spectroscopic studies. Formation of the peroxynitrite intermediate has been confirmed by its characteristic phenol ring nitration reaction as well as isolation of corresponding Cu(I)-nitrate (2). On air oxidation, 2 resulted in the corresponding Cu(II)-nitrate (3). Thus, these results demonstrate a possible decomposition pathway for H(2)O(2) and NO through the formation of a peroxynitrite intermediate in biological systems.

Nitric oxide reactivity of copper(II) complexes of bidentate amine ligands: effect of substitution on ligand nitrosation

Three copper(ii) complexes with bidentate ligands L(1), L(2) and L(3) [L(1), N,N(/)-dimethylethylenediamine; L(2), N,N(/)-diethylethylenediamine and L(3), N,N(/)-diisobutylethylenediamine], respectively, were synthesized as their perchlorate salts. The single crystal structures for all the complexes were determined. The nitric oxide reactivity of the complexes was studied in acetonitrile solvent. The formation of thermally unstable [Cu(II)-NO] intermediate on reaction of the complexes with nitric oxide in acetonitrile solution was observed prior to the reduction of copper(II) centres to copper(I). The reduction was found to result with a simultaneous mono- and di-nitrosation at the secondary amine sites of the ligand. All the nitrosation products were isolated and characterized. The ratio of the yield of mono- and di-nitrosation product was found to be dependent on the N-substitution present in the ligand framework.

Introducing a 2-His-1-Glu nonheme iron center into myoglobin confers nitric oxide reductase activity

A conserved 2-His-1-Glu metal center, as found in natural nonheme iron-containing enzymes, was engineered into sperm whale myoglobin by replacing Leu29 and Phe43 with Glu and His, respectively (swMb L29E, F43H, H64, called Fe(B)Mb(-His)). A high resolution (1.65 A) crystal structure of Cu(II)-CN(-)-Fe(B)Mb(-His) was determined, demonstrating that the unique 2-His-1-Glu metal center was successfully created within swMb. The Fe(B)Mb(-His) can bind Cu, Fe, or Zn ions, with both Cu(I)-Fe(B)Mb(-His) and Fe(II)-Fe(B)Mb(-His) exhibiting nitric oxide reductase (NOR) activities. Cu dependent NOR activity was significantly higher than that of Fe in the same metal binding site. EPR studies showed that the reduction of NO to N(2)O catalyzed by these two enzymes resulted in different intermediates; a five-coordinate heme-NO species was observed for Cu(I)-Fe(B)Mb(-His) due to the cleavage of the proximal heme Fe-His bond, while Fe(II)-Fe(B)Mb(-His) remained six-coordinate. Therefore, both the metal ligand, Glu29, and the metal itself, Cu or Fe, play crucial roles in NOR activity. This study presents a novel protein model of NOR and provides insights into a newly discovered member of the NOR family, gNOR.