The unprecedented strong paratropic ring current of a bis-PdII complex of 5,10,23-trimesityl [28]heptaphyrin(1.1.0.0.1.0.0)

Abstract

Acid-catalyzed Friedel–Crafts-type cyclization of tetrapyrrolic BF₂ complex 1 and α,α′-dibromotripyrrin 2 gave 5,10,23-trimesityl [28]heptaphyrin(1.1.0.0.1.0.0) BF₂ complex 3BF₂ as a stable and moderate antiaromatic macrocycle. Demetalation of 3BF₂ with methanesulfonic acid followed by treatment with HCl gave free-base salt 3HCl that holds a chloride anion at the core. This salt displays a planar structure with an inverted pyrrole and a stronger paratropic ring current. Metalation of neutral free-base 3 with PdCl₂ gave bis-Pd^II complex 3Pd₂ as a stable antiaromatic molecule. The ¹H NMR spectrum of 3Pd₂ displays signals due to pyrrolic β-protons in the range of −1.06 ∼ −1.90 ppm, indicating the unprecedented strong paratropic ring current.