Oxidation of the zinc complex of [capped] 5,15-[2,5-bis(2`-phenyleneoxymethyl)-1,4-dimethoxybenzene]-2, 8,12,18-tetramethyl-3,7,13,17-tetrabutylporphyrin with organic peroxides in the presence of imidazole at 298 K results in complete breakdown of the complex. The nitrogen base exerts a significant effect on the rate of metal porphyrin oxidation. The kinetic parameters of the reaction were determined. The structures of the sterically strained metal porphyrin and intermediate products of its oxidation were optimized by PM3 calculations. A geometry analysis shows that the deformations of the macroring increase upon formation of the extra complex and its reaction with peroxides.It is known that the peroxide oxidation of metal porphyrins and phthalocyanines under aerobic conditions, resulting in their breakdown, is similar to processes leading to the discoloration of chlorophyll and its derivatives. Therefore, the results of physicochemical studies of oxidative degradation of metal porphyrins can be used for simulating the related processes in natural systems [1,2].The choice of [capped] porphyrins in which the meso positions of the macroring are covalently linked with various bridging groups shielding the reaction center is governed by diversity of their structures and properties. Xylene and toluene are often used as solvents for studying the spectral, coordination, and catalytic properties of porphyrins. However, these solvents can contain appreciable amounts of organic peroxides capable of redox reactions with metal porphyrins (MP) [33 6]. As a part of systematic studies of the behavior of sterically strained zinc porphyrins in reactions with compounds containing available oxygen, we examined in this work the peroxide oxidation of [capped] zinc 5,15-[2,5-bis(2`-phenyleneoxymethyl)-1,4-dimethoxybenzene]-2, 8,12,7,13, in the presence of a nitrogen base (imidazole, Im) in o-xylene.The possible oxidation sites in metal porphyrins are both the central atom and the macroring [339]. In the case of zinc, apparently, only the second pathway is possible. According to the successively recorded electronic absorption spectra of ZnP and (Im)ZnP in o-xylene containing peroxides (CH 3 C 6 H 4 CH 2 OOH and CH 3 C 6 H 4 CH 2 OOCH 2 C 6 H 4 CH 3 ) (Fig. 1), the chromophore system of the porphyrin is broken down without formation of stable intermediate chromophores. The coordination of the peroxide with the zinc ion is impossible because of the increased basicity of the sterically strained macrocyclic ligand compared to unstrained porphyrins. The enhancement of the base properties of the [capped] zinc porphyrin is also due to the presence of alkyl groups at the b-positions and to the decreased aromaticity of the sterically strained macroring.Two principally different mechanisms of oxidation of the [capped] zinc porphyrin are possible under the experimental conditions. It is known that hydrogen peroxide can hydroxylate unsaturated organic compounds [8]. Our system contains two differtent peroxides (CH 3 C 6 H 4 CH 2 OOH and CH 3 C...
