STRUCTURAL STUDIES OF METALLOPORPHYRINS .9. LOOPING-OVER COBALT PORPHYRINS - COORDINATING PROPERTIES AND APPLICATION TO DIOXYGEN FIXATION AND ACTIVATION

“Looping-over” cobalt tetraarylporphyrins bearing an imidazole or a primary amino group attached by alkyl chains varying from three to five methylene groups to the meta position of one of the phenyls have been shown by ESR spectroscopy to form intermolecular dimers in toluene. No evidence for dimer formation was found for the corresponding ortho substituted compounds. Both porphyrins bind dioxygen, but ESR spectroscopy indicates that meta-substituted cobalt porphyrins bind O2 only at higher concentrations, which suggests the existence of dimeric species, whereas some of the ortho-substituted ones form dioxygen adducts whatever their concentration, in agreement with the presence of monomeric dioxygen adducts. Cobalt “looping-over” porphyrins however bind O2 less strongly than the n-butylamine or N-methylimidazole complexes of CoTTP. The oxidation of 2,6-di-tert-butylphenol by molecular oxygen was used to test the catalytic efficiency of the cobalt porphyrins: the rate of oxidation is largely dependent on the position and the length of the tether, the ortho-substituted porphyrins being more efficient than the metasubstituted ones. Addition of triethylamine to the reaction mixture increases the rate of oxidation by factors varying between 4 and 30. This effect can be assigned in part to a partial conversion of the phenol to the more oxidizable phenolate anion. © 1990, American Chemical Society. All rights reserved.