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

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.