Oxidation of benzene to phenol with hydrogen peroxide catalyzed by α-pyrrolidonate-bridged mixed-valent tetranuclear platinum complexes

α-pyrrolidonate-bridged mixed-valent tetranuclear platinum complexes, with a general formula of [Pt₄(NH₃)₈(C₄H₆NO)₄]ⁿ⁺ (n = 4, 5, 6, 8; C₄H₆NO = bridging α-pyrrolidonate ligand), have been employed instead of iron (II) salts in Fenton's reagent, in oxidation of benzene to phenol with hydrogen peroxide. Phenol and other oxidation products formed in the present platinum catalyses were analyzed with reversed-phase liquid chromatography. Formation of hydroxyl radical in the reaction between the α-pyrrolidone complexes and hydrogen peroxide has been confirmed with ESR spectroscopy with the use of a spin trapping method in which 5,5-dimethyl-1-pyrroline-1-oxide (DMPO) is employed as the spin trap. The redox behaviors of the α-pyrrolidone complexes in the reaction with hydrogen peroxide have been monitored with UV-VIS spectroscopy. Although the phenol yield based on the consumed hydrogen peroxide is not so high, the turnover number for the catalyst is higher in the present system. Oxidation of the hydroxycyclohexadienyl radical into phenol seems to be effectively performed by the α-pyrrolidone complexes, and no biphenyl is formed in the present catalyses.