Metallophthalocyanines as possible lignin peroxidase models

Several metalloporphyrins, particularly highly chlorinated water soluble meso-tetraphenylporphyrins, have been shown to be good biomimetics of the lignin peroxidases which degrade lignin in vivo. Metal complexes of the water soluble phthalocyaninetetrasulfonic acid have been examined as catalysts for the oxidation of lignin since the phthalocyanines are readily available and inexpensive. The copper(II), nickel(II) and cobalt(II) complexes showed little catalytic activity towards the oxidation of veratryl alcohol (a substrate of the lignin peroxidases). The iron(III) and manganese(III) complexes on the other hand were able to catalyze the oxidation of veratryl alcohol, 4-ethoxy-3-methoxyphenyl-glycerol-beta-guaiacyl ether (a beta-O-4-dimer) and 1-(4-ethoxy-3-methoxy)-2-(4-methoxyphenyl)-1,3-propanediol (a beta-1 dimer). These catalysts are, however, much less stable than the halogenated meso-tetraphenylporphyrins and this lower stability, which is dependent upon pH and the oxidant, limits their use as catalysts.