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.

Sixth Ligand Induced HNO/NO- Release by a Five-Coordinated Cobalt(II) Nitrosyl Complex Having a {CoNO}8 Configuration.

Nitroxyl (HNO) and nitroxide (NO-) anion, the one-electron-reduced form of nitric oxide (NO), have been shown to have distinct advantages over NO from pharmacological and therapeutic points of view. However, the role of nitroxyl in chemical biology has not yet been studied as extensively as that of NO. Consequently, only a few examples of HNO donors such as Angeli's salt, Piloty's acid, or acyl- and acyloxynitroso derivatives are known. However, the intrinsic limitations of all of these hinder their widespread utility. Metal nitrosyl complexes, although few examples, could serve as an efficient HNO donor. Here, a cobalt nitrosyl complex of the {CoNO}8 (1) configuration has been reported. This complex in the presence of a sixth ligand [BF4-, DTC- (diethyldithiocarbamate anion), or imidazole] releases/donates HNO/NO-. This has been confirmed using well-known HNO/NO- acceptors like [Fe(TPP)Cl] and [Fe(DTC)3]. The HNO release has been authenticated further by the detection and estimation of N2O using gas chromatography-mass spectroscopy as well as its reaction with PPh3.

Reaction of a nitrosyl complex of Mn(II)-porphyrinate with superoxide: NOD activity is favoured over SOD activity.

A five-coordinated {Mn(NO)}6 complex of Mn(II)-porphyrinate, [Mn(TMPP2-)(NO)], 1 {TMPPH2 = 5,10,15,20-tetrakis(4-methoxyphenyl)porphyrin}, upon reaction with two equivalents of superoxide (O2-) in THF at -40 °C results in the corresponding MnIII-OH complex [MnIII(TMPP2-)(OH)], 2, via the formation of a putative MnIII-peroxynitrite intermediate. Spectral studies and chemical analysis suggest that one equivalent of superoxide ion is consumed to oxidize the metal center of complex 1 leading to [MnIII(TMPP2-)(NO)]+, while the subsequent equivalent reacts with [MnIII(TMPP2-)(NO)]+ to form the corresponding peroxynitrite intermediate. UV-visible and X-band EPR spectroscopic studies suggest the involvement of a MnIV-oxo species in the reaction, which forms through the O-O bond cleavage of the peroxynitrite moiety with concomitant release of NO2. The formation of MnIII-peroxynitrite is further supported by the well-established phenol ring nitration experiment. The released NO2 has been trapped using TEMPO. It should be noted that in cases of MnII-porphyrin complexes, the reaction with superoxide generally proceeds through a SOD-like pathway where the first equivalent of superoxide ion oxidizes the MnII center and itself is reduced to peroxide (O22-), while the subsequent equivalent of superoxide reduces the MnIII center with the release of O2. In contrast, here the second equivalent of superoxide reacts with the MnIII-nitrosyl complex and follows a NOD-like pathway.

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(F20TPP2-)(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 [CoII(NO)(O2-)] species.

Can a Nitrosyl of a Mn(II)-Porphyrin Complex Release Nitroxyl/HNO?

In general, the nitrosyl complexes of Mn(II)-porphyrinate having the {Mn(NO)}6 configuration are not considered as HNO or nitroxyl (NO-) donors because of [MnI-NO+] nature. A nitrosyl complex of Mn(II)-porphyrin, [Mn(TMPP2-)(NO)], 1 [TMPPH2 = 5,10,15,20-tetrakis-4-methoxyphenylporphyrin], is shown to release HNO in the presence of HBF4. It is evidenced from the characteristic reaction of HNO with triphenylphosphine and isolation of the [(TMPP2-)MnIII(H2O)2](BF4), 2. This is attributed to the fact that H+ from HBF4 polarizes the NO group whereas the BF4- interacts with metal ion to stabilize the Mn(III) form. These two effects cooperatively result in the release of HNO from complex 1. In addition, complex 1 behaves as a nitroxyl (NO-) donor in the presence of [Fe(dtc)3] (dtc = diethyldithiocarbamate anion) and [Fe(TPP)(Cl)] (TPP = 5,10,15,20-tetraphenylporphyrinate) to result in [Fe(dtc)2(NO)] and [Fe(TPP)(NO)], respectively.