Transient formation of the oxo–iron(IV) porphyrin radical cation during the reaction of iron(III) tetrakis‐5,10,15,20‐(N‐methyl‐4‐pyridyl)porphyrin with hydrogen peroxide in aqueous solution

Abstract

The reaction of iron(III) tetrakis-5,10,15,20-(N-methyl-4-pyridyl)porphyrin (Fe(III)TMPyP) with hydrogen peroxide (H(2)O(2)) and the catalytic activity of the reaction intermediates on the luminescent peroxidation of luminol in aqueous solution were studied by using a double-mixing stopped-flow system. The observed luminescence intensities showed biphasic decay depending on the conditions. The initial flashlight decayed within <1 s followed by a sustained emission for more than 30 s. Computer deconvolution of the time-resolved absorption spectra under the same conditions revealed that the initial flashlight appeared during the formation of the oxo-iron(IV) porphyrin, TMPyPFe(IV) = O, which is responsible for the sustained emission. The absorption spectra 0.0-0.5 s did not reproduce well by a simple combination of the two spectra of Fe(III)TMPyP and TMPyPFe(IV) = O, indicating that transient species was formed at the initial stage. Addition of uric acid (UA) caused a significant delay in the initiation of the luminol emission as well as in the formation of the TMPyPFe(IV) = O. Both of them were completely diminished in the presence of UA equimolar with H(2)O(2), while mannitol had no effect at all. The delay of the light emission as well as the appearance of TMPyPFe(IV) = O was directly proportional to the [UA](0) but other kinetic profiles were not changed significantly. Based on these observations and the kinetic analysis, we confirmed the involvement of the oxo-iron(IV) porphyrin radical cation, (TMPyP)(.+)Fe(IV) = O, as an obligatory intermediate in the rate-determining step of the overall reaction, Fe(III)TMPyP + H(2)O(2) --> TMPyPFe(IV) = O, with a rate constant of k = 4.3 x 10(4)/mol/L/s. The rate constants for the reaction between the (TMPyP)(.+)Fe(IV) = O and luminol, and between the TMPyPFe(IV) = O and luminol were estimated to be 3.6 x 10(6)/mol/L/s and 1.31 x 10(4)/mol/L/s, respectively.

Kinetic simulation studies on the transient formation of the oxo‐iron(IV) porphyrin radical cation during the reaction of iron(III) tetrakis‐5,10,15,20‐(N‐methyl‐4‐pyridyl)‐porphyrin with hydrogen peroxide in aqueous solution

“…In addition, a complete reaction scheme is presented to account for all of the absorption and emission data. Using the kinetic parameters and the reaction schemes, all observed data have been well simulated and the conclusion of the previous paper (9), an existence of (TMPyP) · + Fe(IV)=O, has also been quantitatively confirmed.…”

“…Recent studies have been demonstrated that the metalloporphyrin Fe(III)TMPyP is an efficient catalyst for the electrochemical reduction of molecular oxygen (1,2) and for the oxidation of organic and inorganic compounds (3)(4)(5)(6)(7)(8)(9)(10)(11). It is soluble in aqueous solvents and has negligible tendencies to aggregate than do negatively charged porphyrins such as tetracarboxyphynylporphyrin (TCPP) and tetraphynylporphyrin-sulphonate (TPPS) (12).…”

“…It is soluble in aqueous solvents and has negligible tendencies to aggregate than do negatively charged porphyrins such as tetracarboxyphynylporphyrin (TCPP) and tetraphynylporphyrin-sulphonate (TPPS) (12). This catalyst is, therefore, one of the most popular peroxidase mimic for oxidation (electron removal) (3,4,9), oxygenation (5,6), hydroxylation (7) and radical formation (4,8,13,14). It was reported that the compound I and the compound II-like intermediates, (TSMP) · + Fe(IV)=O and (TSMP)Fe(IV)=O, have been produced from Fe(III)TSMP (TSMP = meso-tetrakis-3-sulphonatomesityl porphyrin) which is regarded as an anionic porphyrin (15).…”

“…However, our earlier confirmation for the formation of this intermediate in the reaction of Fe(III)TMPyP with H 2 O 2 have opened up the possibility to estimate the amount and the reactivity of this ambiguous intermediate (9). For this purpose, it seemed desirable to undertake a kinetic investigation of the Fe(III)TMPyP-H 2 O 2 reaction and, in particular, to apply a computer simulation of the reaction, in which the rate equations are integrated numerically and the results are compared with experimental data.…”

Electrochemical investigation of a novel metalloporphyrin intercalated layered niobate modified electrode and its electrocatalysis on ascorbic acid

“…Interactions of water-soluble metalloporphyrin Fe III TMPyP with various compounds relating to biological functions have been intensively studied. For example, the mechanism of its electrochemical reduction of molecular oxygen has been discussed since the late 1970s [13][14][15][16][17][18][19]; its interaction with deoxyribonucleic acid (DNA) has been reported, and immobilization with DNA as modified electrode in detecting nitrite was investigated [20]; the electrocatalytic reactions of iron porphyrin on sulfur oxoanions [21] and hydrogen dioxide (H 2 O 2 ) [16,[22][23][24] have also been studied. However, the direct utilization of iron porhyrin in electrochemical analysis is not very convenient because it can dissolve in water and is poor in stability and reproducibility as electrode modified material.…”