Di(p-dibenzi)[40]decaphyrin(1.0.0.0.0.1.0.0.0.0) Pd^II Complex: A Weakly Hückel 38π-Aromatic Macrocycle

Di(p-benzi)[40]decaphyrin(1.0.0.0.0.1.0.0.0.0) BF₂ complex and tris(p-benzi)[60]pentadecaphyrin(1.0.0.0.0.1.0.0.0.0.1.0.0.0.0) BF₂ complex were synthesized by Suzuki-Miyaura coupling of α,α'-diborylated tetrapyrrole BF₂ -complex with 1,4-diiodobenzene. Bis-BF₂ complex was converted to bis-Pd^II complex via its free base. Macrocycles bis-BF₂ and tris-BF₂ complex take Möbius topology but are nonaromatic, since the macrocyclic conjugation is disrupted by the locally aromatic 1,4-phenylene units. In contrast, bis-Pd^II complex is a weakly Hückel 38π-aromatic macrocycle as evinced by its red-shifted, enhanced, and structured Q-like bands and a small electrochemical HOMO-LUMO gap. Interestingly, one 1,4-pheylene part of bis-Pd^II complex takes a quinonoidal distorted structure and the other takes a usual benzene structure in a figure-eight conformation with Hückel topology.