THE REACTION OF 5,10,15,20-TETRAKIS(2,6-DICHLORO-3-SULFONATOPHENYL)PORPHINATOIRON(III) HYDRATE WITH ALKYL AND ACYL HYDROPEROXIDES - THE DYNAMICS OF REACTION OF WATER-SOLUBLE AND NON MU-OXO DIMER FORMING IRON(III) PORPHYRINS IN AQUEOUS-SOLUTION .7.

Kinetic and product studies (30 °C, μ = 0.20 with NaNO3, between pH 1.0 and 12.35) are reported for the reaction in water of an iron(III) octa-o-chloro-substituted tetraphenylporphyrin [5,10,15,20-tetrakis(2,6-dichloro-3-sulfonatophenyl) porphinatoiron(III) hydrate ((2)FeIII(H2O)2 ⇌ (2)FeIII(HO)(H2O) + H+ ⇌ (2)FeIII(OH)2 + 2H+) with a number of acyl and alkyl hydroperoxides (t-BuOOH, Ph(CH3)2COOH, Ph2(CO2CH3)COOH, Ph2(CN)COOH, m-ClC6H4CO3H, p-NO2C6H4CO3H, and PhCH2CO3H). All reactions are first-order in the hydroperoxide and in (2)FeIII(X)2 (where X = H2O, HO-). The rates are (1) insensitive to the presence or absence of O2 and (2) not subject to specific-acid, general-acid, nor general-base catalysis at any pH. For the reaction with r-BuOOH, characteristics of the pH dependence of the second-order rate constant (kly) are as follows; (1) < pH 3.5, kly is pH independent; (2) from pH 3.5 to 9.5, kly increases and then decreases (“bell-shape curve”); (3) above pH 10, kly again increases and then decreases (second “bell-shape curve”). These results are explained if one assumes that three steady-state intermediates [(2)FeIII(H2O)(t-BuOOH), (2)FeIII(HO-)(t-BuOOH) ⇌ (2)FeIII(H2O)(t-BuOO-), and (2)FeIII(HO-)(t-BuOO-)] break down in the commitment step and are in acid-base equilibria. From product analysis, reactions with t-BuOOH provide 90% yield of (CH3)2CO and >50% yield of CH3OH. Neither CH4, C2H6, O2, or (t-BuO)2 could be detected. In the presence of 2,2′-azinobis(3-ethylbenzthiazoline-6-sulfonate) (ABTS) trap, (CH3)2CO is formed in 10-20% yield, and the main product (80-90%) is t-BuOH. With Ph(CH3)2COOH, Ph(CH3)C=O was formed in ∼70% yield at pH 7.25. Neither PhOH nor (CH3)2C=0 could be detected. These findings uniquely identify R(CH3)2CO⋅ intermediates and allow the mechanism (2)FeIII(H2O)(X) + R(CH3)2COOH →/(2)FeIV(O)(X) + R(CH3)2(O)(X)+R(CH3)2CO⋅/→ (2)FeIV(O)(X) + R(CH3)2CO⋅ followed by fragmentation of R(CH3)2CO⋅ (within the/intimate pair/and after solvent separation) to provide R(CH3)C=o + CH3⋅. Reaction of CH3⋅ with (2)FeIII(H2O)(X) or (2)FeIV(O)(X) leads to CH3OH plus (2)FeII(H2O) and (2)FeIII(H2O)(X), respectively. The putative (2)FeII(H2O) species was trapped (20% yield) with CO as (2)FeII(CO). The difference in product yields in the presence and absence of ABTS may be explained by the inability of ABTS to trap the components of the initially formed solvent caged pair,/(2)FeIV(O)(X) + R(CH3)2CO⋅/, but essentially complete trapping of the solvent-separated (2)FeIV(O)(X) and R(CH3)2CO⋅ species. The reaction of (2)FeIII(X)2 with t-BuOOH, and the other alkyl and acyl hydroperoxides investigated, gives rise to an observable iron(IV)-oxo porphyrin intermediate between pH 2.0 and 12. The formation of (2)FeIV(O)(X) with t-BuOOH or Ph(CH3)2COOH requires the presence of O2. This is consistent with the formation of (2)FeIV(0)(X) from the reaction of (2)FeII(H2O) with O2. Once generated, the spectra of the (2)FeIV(O)(X) species persist for an extended time [70 h (pH 4.87), 100 h (pH 6.3), 80 h (pH 7.25), 50 h (pH 10.37), and 24 h (pH 11.68)] prior to reconversion (>95%) to (2)FeIII(H2O)(X). Thus octa-o-chloro substitution provides greater stabilization of the iron(IV)-oxo tetraphenylporphyrin as compared to octa-o-methyl substitution. (2)FeIV(O)(X) is formed even in the absence of O2 in the reaction of (2)FeIII(H2O)(X) with acyl hydroperoxides (p-NO2C6H4CO3H, m-ClC6H4CO3H and PhCH2CO3H) and with alkyl hydroperoxides (Ph2(CN)COOH and Ph2(CO2CH3)COOH). The reaction of H2O2 with (2)FeIII(H2O)(X) does not generate (2)FeIV(O)(X), and a rapid decomposition of (2)FeIII(H2O)(X) occurs. Hydrogen peroxide reacts rapidly with authentic (2)FeIV(O)(X). © 1990, American Chemical Society. All rights reserved.