626253-00-7Relevant academic research and scientific papers
Laser flash photolysis generation and kinetic studies of porphyrin-manganese-oxo intermediates. Rate constants for oxidations effected by porphyrin-MnV-oxo species and apparent disproportionation equilibrium constants for porphyrin-MnIV-oxo species
Zhang, Rui,Horner, John H.,Newcomb, Martin
, p. 6573 - 6582 (2005)
Porphyrin-manganese(V)-oxo and porphyrin-manganese(IV)-oxo species were produced in organic solvents by laser flash photolysis (LFP) of the corresponding porphyrin-manganese(III) perchlorate and chlorate complexes, respectively, permitting direct kinetic studies. The porphyrin systems studied were 5,10,15,20-tetraphenylporphyrin (TPP), 5,10,15,20- tetrakis(pentafluorophenyl)porphyrin (TPFPP), and 5,10,15,20-tetrakis(4- methylpyridinium)porphyrin (TMPyP). The order of reactivity for (porphyrin)MnV(O) derivatives in self-decay reactions in acetonitrile and in oxidations of substrates was (TPFPP) > (TMPyP) > (TPP). Representative rate constants for reaction of (TPFPP)MnV(O) in acetonitrile are k = 6.1 ×105 M-1 s-1 for cis-stilbene and k = 1.4 × 105 M-1 s-1 for diphenylmethane, and the kinetic isotope effect in oxidation of ethylbenzene and ethylbenzene-d10 is kH/kD = 2.3. Competitive oxidation reactions conducted under catalytic conditions display approximately the same relative rate constants as were found in the LFP studies of (porphyrin)MnV(O) derivatives. The apparent rate constants for reactions of (porphyrin)MnIV(O) species show inverted reactivity order with (TPFPP) IV(O) disproportionates to (porphyrin)MnIIIX and (porphyrin)MnV(O), which is the primary oxidant, and the equilibrium constants for disproportionation of (porphyrin)MnIV(O) are in the order (TPFPP) V(O) with (TPFPP)MnIIICl to give (TPFPP)MnIV-(O) (k = 5 × 108 M-1 s -1) and disproportionation reaction of (TPP)MnIV(O) to give (TPP)MnV(O) and (TPP)-MnIIIX (k ≈ 2.5 × 109 M-1 s-1) were observed. The relative populations of (porphyrin)MnV(O) and (porphyrin)MnIV(O) were determined from the ratios of observed rate constants for self-decay reactions in acetonitrile and oxidation reactions of cis-stilbene by the two oxo derivatives, and apparent disproportionation equilibrium constants for the three systems in acetonitrile were estimated. A model for oxidations under catalytic conditions is presented.
