Bis-palladium(II) complex of doubly N-confused octaphyrin(1.1.1.1.1.1.1.1): Möbius aromaticity and chiroptical properties

A novel bis-palladium(II) complex of doubly N-confused octaphyrin (Pd2-2) adopting a Möbius-twisted topological structure was synthesized and characterized. Due to the effective 36π-electronic delocalization over the Möbius-twisted octaphyrin scaffold, characteristic Soret and Q-like absorption features were observed in the near-infrared (NIR) region. However, the complex Pd2-2 exhibited weak aromatic character attributed to the distinct cross-conjugated resonance contribution as inferred from the 1H NMR chemical shifts as well as theoretical assessments (e.g. nucleus-independent chemical shift (NICS)). Since the bis-palladium(II) complexation of doubly N-confused octaphyrin 2 imparted significant conformational stability, topologically chiral enantiomers of Pd2-2 were successfully separated as (P)- and (M)-twisted forms. The resulting isomers revealed relatively large circular dichroism (CD) responses with an absorption anisotropy factor of gabs = 0.009 in the NIR region (λ = 823 nm). In addition, the cyclic voltammogram of Pd2-2 revealed redox-rich properties due to its large π-conjugated system.