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