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

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)Mn-v(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 x 105 I Lambda(-1) s(-1) for cis-stilbene and k = 1.4 x 10(5) M-1 s(-1) for diphenylmethane, and the kinetic isotope effect in oxidation of ethylbenzene and ethyl benzene- d(10) is k(H)/k(D) = 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)Mn-v(O) derivatives. The apparent rate constants for reactions of (porphyrin)Mn-IV(O) species show inverted reactivity order with (TPFPP) < (TMPyP) < (TPP) in reactions with cis-stilbene, triphenylamine, and triphenylphosphine. The inverted reactivity results because (porphyrin)Mn-IV(O) disproportionates to (porphyrin)(MnX)-X-III and (porphyrin)Mn-v(O), which is the primary oxidant, and the equilibrium constants for disproportionation of (porphyrin)Mn-IV(O) are in the order (TPFPP) < (TMPyP) < (TPP). The fast comproportionation reaction of (TPFPP)Mn-v(O) with (TPFPP)(MnCl)-Cl-III to give (TPFPP)Mn-IV-(0) (k = 5 x 108 M-1 s(-1)) and disproportionation reaction of (TPP)Mn-IV(O) to give (TPP)Mnv(O) and (TPP)-I eta Lambda X-III (k approximate to 2.5 x 10(9) M-1 s(-1)) were observed. The relative populations of (porphyrin)Mn-V(O) and (porphyrin)Mn-IV(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.