Water-soluble meso-imidazolyl manganese porphyrins: Biomimetics and oxidation catalysis

The cationic manganese porphyrin complex Mn(III)[TDMImP] has been investigated as a biomimic of heme enzymes and an oxidation catalyst in aqueous solutions. This complex has the unique feature of being more acidic than previously studied cationic pyridyl porphyrins Mn(III)[2-TMPyP] and Mn(III)[4-TMPyP]. This effect is due to the higher electron-deficiency created by the meso-imidazolyl substituents.; Mn(III)[TDMImP] reacted readily with various peroxide-type oxidants. The oxidation with oxone generated an unusually stable oxo-Mn(V) complex, allowing its characterization by 1H NMR spectroscopy. The addition of t-butyl hydroperoxide and cumene hydroperoxide led to the formation of an oxo-Mn(IV) complex which was characterized by EPR spectroscopy. Following the addition of hydrogen peroxide, an oxo-Mn(V) complex was detected. This oxidation proceeds via formation of an intermediate, a Mn(III)-hydroperoxo species, which can be spectrally observed at high hydrogen peroxide concentrations. The kinetics of hydrogen peroxide activation was investigated and a mechanism is proposed. However, an oxo-Mn(IV) complex was observed when Mn(III)[TDMImP] was reacted with hydrogen peroxide in a carbonate buffer. The oxidant in this case is believed to be peroxymonocarbonate. A rationale based on thermodynamic considerations explaining the nature of the high-valent species observed and thus the nature of the intermediate O-O bond cleavage is developed.; The oxo-Mn(V) complex oxidizes bromide by an oxo-transfer mechanism forming Mn(III)[TDMImP] and OBr-. This transformation was shown to be reversible. Kinetic investigations allowed the calculation of the equilibrium constant of this interconversion. This led to the determination of the oxidation potential of the oxo-Mn(V)/Mn(III) couple (E = 1.014 V at pH 8), allowing a placement of the oxo-Mn(V) species on a thermochemical energy scale. Mn(III)[TDMImP] was investigated as an oxidation catalyst for electron transfer, hydrogen atom abstraction, epoxidation and hydroxylation reactions. These results are compared with the data previously obtained with Mn(III)[2-TMPyP] and Mn(III)[4-TMPyP]. The reactivity of these catalysts is strongly dependent on the structure of the porphyrin ligand. Interestingly, the more electron-withdrawing the porphyrin, the less reactive it is toward substrates.