Reactions of Nitrogen Oxides with Heme Models. Characterization of NO and NO₂ Dissociation from Fe(TPP)(NO₂)(NO) by Flash Photolysis and Rapid Dilution Techniques:  Fe(TPP)(NO₂) as an Unstable Intermediate

Described are studies directed toward elucidating the controversial chemistry relating to the solution phase reactions of nitric oxide with the iron(II) porphyrin complex Fe(TPP)(NO) (1, TPP = meso-tetraphenylporphinato^2-). The only reaction observable with clean NO is the formation of the diamagnetic dinitrosyl species Fe(TPP)(NO)₂ (2), and this is seen only at low temperatures (K₁ < 3 M^-1 at ambient temperature). However, 1 does readily react reversibly with N₂O₃ in the presence of excess NO to give the nitro nitrosyl complex Fe(TPP)(NO₂)(NO) (3), suggesting that previous claims that 1 promotes NO disproportionation to give 3 may have been compromised by traces of air in the nitric oxide sources. It is also noted that 3 undergoes reversible loss of NO to give the elusive nitro species Fe(TPP)(NO₂) (4), which has been implicated as a powerful oxygen atom transfer agent in reactions with various substrates. Furthermore, in the presence of excess NO₂, the latter undergoes oxidation to the stable nitrato analogue Fe(TPP)(NO₃) (5). Owing to such reactivity of Fe(TPP)(NO₂), flash photolysis and stopped-flow kinetics rather than static techniques were necessary for the accurate measurement of dissociation equilibria characteristic of Fe(TPP)(NO₂)(NO) in 298 K toluene solution. Flash photolysis of 3 resulted in competitive NO₂ and NO dissociation to give Fe(TPP)(NO) and Fe(TPP)(NO₂), respectively. The rate constant for the reaction of 1 with N₂O₃ to generate Fe(TPP)(NO₂)(NO) was determined to be 1.8 × 10⁶ M^-1 s^-1, and that for the NO reaction with 4 was similarly determined to be 4.2 × 10⁵ M^-1 s^-1. Stopped-flow rapid dilution techniques were used to determine the rate constant for NO dissociation from 3 as 2.6 s^-1. The rapid dilution experiments also demonstrated that Fe(TPP)(NO₂) readily undergoes further oxidation to give Fe(TPP)(NO₃). The mechanistic implications of these observations are discussed, and it is suggested that NO₂ liberated spontaneously from Fe(P)(NO₂) may play a role in an important oxidative process involving this elusive species.