Chemistry of a high-oxidation-level manganese porphyrin in aqueous solution

Journal Article · Wed Jul 13 00:00:00 EDT 1988 · Inorg. Chem.; (United States)

DOI:https://doi.org/10.1021/ic00287a008· OSTI ID:5976287

Spreer, L O; Leone, A; Maliyackel, A C; Otvos, J W; Calvin, M

Manganese(III) tetrakis(N-methyl-4-pyridiniumyl)porphyrin (Mn/sup III/P/sup +/) (chloride salt) and other water-soluble manganese(III) porphyrins undergo facile one-electron electrochemical or chemical oxidation in alkaline solution. Best available evidence indicates that the final oxidized species is a manganese(IV) /mu/-oxo dimer, PMn/sup IV/-O-Mn/sup IV/P/sup 2+/. This same species is also produced by the reaction of manganese(II) porphyrin and oxygen. The Mn(IV) /mu/-oxo dimer has limited stability in water returning to 90-94% of the original Mn(III) porphyrin. The rate of this reaction is pH dependent with faster rates at lower pH. Oxygen is not produced during this reduction process. Rather, the reaction involves an unusual disproportionation in which a small percentage of the porphyrin macrocycles supply multiple electrons to rescue the remainder of the oxidized dimer. It was also found that the manganese(IV) dimer reacts rapidly with water-soluble olefins as it also does in aprotic solvent. A mechanism for the disproportionation reaction is discussed with a rate-determining step involving rearrangement of charge within the symmetric dimer to one with both oxidation equivalents on one metalloporphyrin unit, viz., PMn/sup IV/-O-Mn/sup III/P/sup +/ or PMn/sup III/-O-Mn/sup III/P/sup 2+/. This species undergoes nucleophilic attach by water or hydroxide, producing an isoporphyrin or bilirubin type molecule that has many olefinic bonds capable of reaction with remaining Mn(IV) /mu/-oxo dimer. Since coordination by OH/sup /minus// to the manganese center favors the higher manganese(IV) oxidation level, the pH dependence of the disproportionation can be explained by rearrangement within the dimer to a porphyrin-centered oxidation site. 24 references, 3 figures.

• Research Organization: Lawrence Berkeley Lab., CA (USA)
• DOE Contract Number: AC03-76SF00098
• OSTI ID: 5976287
• Journal Information: Inorg. Chem.; (United States), Journal Name: Inorg. Chem.; (United States) Vol. 27:14; ISSN INOCA
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400201* -- Chemical & Physicochemical Properties
AQUEOUS SOLUTIONS
CARBOXYLIC ACIDS
CHEMICAL REACTIONS
DATA
DISPERSIONS
EXPERIMENTAL DATA
HETEROCYCLIC ACIDS
HETEROCYCLIC COMPOUNDS
INFORMATION
MANGANESE COMPOUNDS
MIXTURES
NUMERICAL DATA
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
PORPHYRINS
REDOX REACTIONS
SOLUTIONS
TRANSITION ELEMENT COMPOUNDS