Porphyrin derivatives bearing fused aromatic moieties at the periphery have been revealed to show unique optical and magnetic properties on the basis of coplanarity between the porphyrin core and the fused aromatic moieties and the resultant expansion of π-conjugated circuits. Recently, we have reported an efficient procedure of a quadruply ring-fused porphyrin (QFP, 1), having four fused five-membered rings at the periphery. Based on the ring-fused structure and the resultant expansion of the π-conjugation, QFP exhibits a narrow HOMO-LUMO gap and positively shifted reduction potentials relative to the corresponding tetraphenylporphyrin derivatives. In this study, transition metal complexes of QFP (1-M; M = ZnII, FeII, or CoII) have been synthesized and characterized. Each the metal complex exhibits unique properties based on the ring-fused structures and expanded π-conjugation of the QFP ligand. The Zn complex, 1-Zn, shows strong Lewis acidity of the Zn center, and strong binding of a pyridine molecule on the Zn center causes dome-type distortion of the QFP ligand. The dome-distorted QFP molecule strongly captures fullerenes, C60 and C70, with a 2:1 binding mode even in solution (Figure). Owing to the low LUMO level of the QFP ligand, axial ligand substitution of the Co complex, 1-Co, with pyridine from THF causes intramolecular electron transfer (IET) from the CoII center to the QFP2– ligand, affording CoIII(QFP●3–). This IET is caused by lowering the redox potential of the CoII/CoIII couple over the first reduction potential of the QFP ligand due to the strong σ-donation of the pyridine ligands. The formed CoIII(QFP●3–) state is air-sensitive and gradually converts to CoIII(QFP2–) by auto-oxidation under air. In this presentation, detailed characteristics of the QFP-metal complexes will be reported. 

Transition metal complexes of a quadruply-fused porphyrin

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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