Halogenated Metalloporphyrin Complexes as Catalysts for Selective Reactions of Acyclic Alkanes with Molecular Oxygen

Abstract

We have shown that halogenation of the porphyrin ring of a metalloporphyrin complex can convert a catalytically inactive material into an exceptionally active catalyst for the selective reaction of an alkane with molecular oxygen. The greater the degree of halogenation of the ring, the greater is the catalytic activity of the metal complex. The product profile, while characteristic of radical reactions, is sensitive to the nature of the metal center. Iron complexes are generally more active than those of cobalt, manganese, or chromium. The activity of iron complexes is directly related to the Fe(III)/(II) reduction potential of the porphyrin complex. There is also a similar correlation between the Fe(III)/Fe(II) reduction potential and the rate at which iron haloporphyrin complexes decompose alkyl hydroperoxides. These iron perhaloporphyrin complexes are not only the most active known liquid phase alkane air-oxidation catalysts, they are also the most active hydroperoxide decomposition catalysts known to date. The nature of the products formed is dependent on the structure of the aliphatic substrate that is oxidized and can be rationalized by a catalytic pathway that very efficiently generates alkyl and alkoxy radicals at low temperatures. The relationship between the electrochemical properties of these complexes and the rates of alkane oxidation and hydroperoxide decomposition lends insight into possible mechanisms of catalytic activity.