Reaction of a nitrosyl complex of Mn(ii)–porphyrinate with superoxide: NOD activity is favoured over SOD activity

Abstract

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 Mn^III–OH complex [Mn^III(TMPP²⁻)(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(TMPP²⁻)(NO)]⁺, while the subsequent equivalent reacts with [Mn^III(TMPP²⁻)(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 NO₂. The formation of Mn^III–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 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 (O₂²⁻), while the subsequent equivalent of superoxide reduces the Mn^III center with the release of O₂. In contrast, here the second equivalent of superoxide reacts with the Mn^III–nitrosyl complex and follows a NOD-like pathway.