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Laser Flash Photolysis Studies of Nitritomanganese(III) Tetraphenylporphyrin. Reactions of O2, NO, and Pyridine with Manganese(II) Tetraphenylporphyrin

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journal contribution
posted on 30.04.1998, 00:00 by Mikio Hoshino, Yutaka Nagashima, Hiroshi Seki, Malcolm De Leo, Peter C. Ford
Laser photolysis of nitritomanganese(III) tetraphenylporphyrin, MnIIITPP(ONO) (1), in degassed toluene gives manganese(II) tetraphenylporphyrin, MnIITPP (2), plus NO2. The quantum yield, Φ, for the formation of MnIITPP is dependent on the excitation wavelength:  Φ = 0.045 at 355 nm and Φ = 0.0064 at 532 nm. Continuous photolysis studies reveal that the quantum yield for the net photodecomposition of 1 is much smaller (10-4). Thus, MnIITPP produced by laser photolysis of MnIIITPP(ONO) mostly returns to 1 by recombination with NO2 according to second-order kinetics with a rate constant 2.2 × 109 M-1 s-1. The kinetics for the reactions of MnIITPP with O2, NO, and pyridine were investigated in detail. In aerated toluene, MnIITPP reversibly reacts with oxygen to yield the dioxygen adduct, MnTPP(O2) (3). The rate constants, kf(O2) and kb(O2), for the formation and dissociation of 3 at 300 K were determined to be 1.93 × 107 M-1 s-1 and 9.0 × 104 s-1, respectively, and the equilibrium constant, kf(O2)/kb(O2), is therefore 2.1 × 102 M-1. MnIITPP reacts with pyridine to give MnIITPP(Py) with a rate constant of 9.5 × 108 M-1 s-1. MnIITPP reacts with NO to yield nitrosylmanganese porphyrin, MnIITPP(NO), with the rate constant 5.3 × 108 M-1 s-1. The association reactions of MnIITPP with NO, pyridine, O2, and NO2 are discussed in comparison with those of other metalloporphyrins.