Reversible Ring Oxidation of a β–β Linked 24π Antiaromatic Sapphyrin to a 22π Aromatic Dication Dimer

Abstract

Unlike aromatic porphyrins, conventional oxidative coupling reactions do not yield dimeric or oligomeric antiaromatic porphyrinoids. We describe here the first example of a β–β-linked antiaromatic expanded porphyrinoid through a novel synthetic strategy. It contains two 24π sapphyrin units, which undergo independent reversible two-electron oxidation to yield the aromatic tetracation. The neutral and oxidized states have both been validated by electronic absorption spectroscopy, mass spectrometry, and spectro-electrochemical studies. 1H NMR spectroscopy and quantum chemical calculations substantiated the contrasting ring current effect for both the parent 24π sapphyrin and its β–β dimer in their neutral and oxidized states. Single-crystal X-ray diffraction studies confirmed the planar conformation for the parent 24π sapphyrin, its 22π dication, and the neutral β–β-linked dimer.