Dynamics of Reaction of (meso-Tetrakis(2,6-dimethyl-3-sulfonatophenyl)porphinato)iron(III) Hydrate with tert-Butyl Hydroperoxide in Aqueous Solution. 2. Establishment of a Mechanism That Involves Homolytic O-O Bond Breaking and One-Electron Oxidation of the Iron(III) Porphyrin

Article abstract of DOI:10.1021/ja00186a050

The reaction of t-BuOOH with the water soluble and non-μ-oxo dimer forming (5,10,15,20-tetrakis(2,6-dimethyl-3-sulfonatophenyl)porphinato)iron(III) hydrate ((1)Fe^III(X)(H2O) where X=H2O or HO(1-)) was studied in aqueous solution between pH 2 and 13 in the absence of an agent for the trapping of reaction intermediates.Products of t-BuOOH decomposition are (CH3)2CO (90percent), CH3OH (90percent), and t-BuOH (15percent), while neither CH4, C2H6, O2, nor (t-BuO)2 could be detected.That (CH3)2CO and CH3OH are formed through fragmentation of t-BuO^. to (CH3)2CO and CH3^. is shown by theobservation that the reaction of Ph(CH3)2COOH with (1)Fe^III(X)(H2O) provides acetophenone but not phenol.This must be a consequence of homolytic O-O bond scission with the formation of Ph(CH3)2CO^..The reaction of (1)Fe^III(X)(H2O) with m-ClC6H4CO3H and the hydroperoxides t-BuOOH, Ph(CH3)2COOH, Ph2C(CO2CH3)OOH, and Ph2C(CN)OOH between pH 5 and 7 leads to the buildup of (1)Fe^IV(X)(H2O) species.The formation of an iron(IV)-oxo porphyrin species was established by titrimetric experiments as well as by carbon microelectrode voltommetry and 1e(1-) spectroelectrochemical generation of authentic (1)Fe^IV(X)(H2O) species.Formation of (1)Fe^IV(X)(H2O) species on oxidation of (1)Fe^III(X)(H2O) with m-ClC6H4CO3H occurs in the absence of O2, while formation of (1)Fe^IV(X)(H2O) species on oxidation of (1)Fe^III(X)(H2O) by the alkyl hydroperoxides, t-BuOOH and Ph(CH3)2COOH, requires the presence of O2.These observations require for the peracid a 2e(1-) oxidation with heterolytic O-O bond cleavage III(X)(H2O) --> m-ClC6H4CO2H + (^+.1)Fe^IV(X)(H2O)> followed by a comproportionation reaction <(1)Fe^III(X)(H2O)+ (^+.1)Fe^IV(X)(H2O) --> 2(1)Fe^IV(X)(H2O)>, while in the case of the reaction of the alkyl hydroperoxides a 1e(1-) oxidation occurs with homolytic O-O bond breaking.The fragmentation of the resultant t-BuO^. provides (CH3)2CO and CH3^., and reaction of the latter with (1)Fe^III(X)(H2O) yields CH3OH and (1)Fe^II(X)(H2O).The resultant iron(II) porphyrin species reacts with O2 <2(1)Fe^II(X)(H2O) + O2 --> 2(1)Fe^IV(X)(H2O)>.The putative (1)Fe^II(X)(H2O) intermediate was identified by its trapping with CO.Also, t-BuO^. and CH3^. intermediates were identified by their spin trapping with 5,5-dimethyl-1-pyrroline N-oxide.The rate constant for reaction of hydrogen peroxide with (1)Fe^IV(X)(H2O) exceeds that for other hydroperoxides investigated.Thus, (1)Fe^IV(X)(H2O) does not accumulate in the reaction of H2O2 with (1)Fe^III(X)(H2O).The rapid reaction of H2O2 with (1)Fe^IV(X)(H2O) provides (1)Fe^III(X)(H2O) and presumably O2^.- + H(1+).This marks the first observation of the reaction of H2O2 with a compound II species.The pH dependence of the kinetics for the reaction of (1)Fe^III(X)(H2O) with t-BuOOH has been determined and shown to be comparable to those of a previous study which...