CATALYZED OXIDATION REACTIONS OF 1,2-DIHYDROXYBENZENE (CATECHOL) IN AERATED AQUEOUS-SOLUTIONS OF AL-3+

The slow oxidation of catechol by O2 in the pH range of 4.5-7.0 generated intensely-colored oligomeric and polymeric products when Al3+ (as well as Ga3+ or Sc3+) cations were present in solution. A soluble blue polymeric cation with an intense visible absorption at 620-650 nm and a structureless electron spin resonance (ESR) signal near the free electron position was formed in acidic solution. This product is believed to be a partially oxidized quinone formed by oxidative polymerization of aromatic rings via metal-promoted CC bonding. ESR identified an apparent radical intermediate of this process, a ring-coupled dimer complexed to two metal ions. ESR also identified Al3+-complexed o-benzosemiquinone monomers in aqueous solution near pH 7, an indication that the radical is stabilized in the metal-chelated form. At the highest pH and catechol concentrations studied, air oxidation produced intensely green solutions in which ESR could identify a radical monomer bonded to one Al3+ ion, and possibly a radical trimer bonded to two Al3+ ions. A mole excess of Al3+ appeared to suppress the formation of CO bond-coupled products relative to the more darkly colored CC bond-coupled products. The possible involvement of intermolecular charge transfer processes in these products is discussed here. © 1990.