New insights into the synthesis of porphyrin-corrole and biscorrole systems

Abstract

As in the case of porphyrins, the relatively easier synthesis of meso-substituted corroles compared to that of β-pyrrole substituted ones greatly limits the use of these latter derivatives. Here we show, for the synthesis of β-substituted biscorrole and porphyrin-corrole dyads, that a simple replacement of sodium hydrogen carbonate by sodium acetate in the final cyclisation step of the a,c-biladiene to the corrole ring, increases the yield by at least a factor of 3. This reaction can be extended to the synthesis of β-substituted monocorroles. Moreover, when cobalt acetate is added instead of the reoxidation step, a 34 to 63% yield in porphyrin-spacer- Co ( III ) corrole is obtained, thus avoiding isolation of the free-base and further metalation of this latter derivative. This “cyclisation-oxidation-metalation” reaction is of particular interest in the preparation of heterobimetallic species in porphyrin-corrole series. Indeed, it leads to the formation of the monocobalt derivative with the cobalt atom inside the corrole moiety allowing for the further insertion of a large variety of metals into the porphyrin macrocycle. Crystals of the monocobalt complex of a biphenylene bridged porphyrin-corrole derivative were obtained in the presence of pyridine and the structure was determined by X-ray diffraction analysis. The structure shows that only one pyridine molecule is bound to the cobalt centre in an exo position, thus indicating a strong steric hindrance from the porphyrin counterpart.