From Pentagon to Nonagon: Conformation-, Size-, and Charge-Dependent Global (Anti)aromaticity in All-Benzenoid Macrocycles

Abstract The global (anti)aromaticity of all-benzenoid macrocycles, particularly its modulation by odd-membered size-dependent effects of phenyl units, remains largely unexplored. Herein, we report the synthesis and comprehensive characterization of four macrocycles spanning ring sizes from pentagon to nonagon, incorporating methine and amine units. Single-crystal X-ray analysis reveals that [5]C4N1-BCP adopts a nearly planar π-conjugated structure with significant strain. At low temperatures, [5]C4N1-BCP and its dication [5]C4N1-BCP2+ exhibit 26π-electron global aromatic and 24π-electron global antiaromatic pathways, respectively, in accordance with Hückel’s 4N+2 and 4N rules. In contrast, [7]C4N3-BCP, [8]C4N4-BCP, and [9]C6N3-BCP display predominantly localized aromaticity arising from their highly distorted macrocyclic frameworks, which adopt half-chair, chair, and boat-like conformations, respectively. The tetracation [7]C4N3-BCP4+ exhibits localized aromaticity due to the severe conformational distortions inherent to its odd-membered architecture, as supported by VT NMR spectroscopy. Notably, despite its boat-like conformation, [8]C4N4-BCP4+ displays 40π-electron global antiaromaticity at low temperatures, whereas [9]C6N3-BCP2+ shows 46π-electron global aromaticity under similar conditions. Furthermore, aromatic resonance energy analysis indicates that charge enhances π-electron delocalization across the macrocyclic framework, thereby promoting global ring currents, while the overall degree of global (anti)aromaticity diminishes with increasing macrocyclic size. The systematic pentagon-to-nonagon comparison elucidates the roles of ring size, geometry, and charge state in governing global (anti)aromaticity in all-benzenoid macrocycles, providing deeper insight into the aromaticity and electronic properties of extended π-conjugated macrocyclic molecules and materials.