Global Aromaticity in Macrocyclic Cyclopenta-Fused Tetraphenanthrenylene Tetraradicaloid and Its Charged Species

摘要

A stable cyclopenta-fused tetraphenanthrenylene macrocycle, CPTP-M, was synthesized, and the structure was confirmed by X-ray crystallographic analysis. It exhibits a large radical character (number of unpaired electron, N-U = 3.52) and a small singlet-triplet energy gap (Delta ES-T = -2.8 kcal mol(-1) by SQUID). Its backbone contains 60 ([4n]) conjugated electrons and is globally antiaromatic. NMR measurements and theoretical calculations revealed that its dication/dianion is globally aromatic owing to the existence of [4n-2]/[4n+2]pi-conjugated electrons. Remarkably, the ring-current map of the tetraanion shows a unique ring-in-ring structure, with a diamagnetic outer ring and a paramagnetic inner ring. Accordingly, both the inner-rim and outer-rim protons are deshielded in its H-1 NMR spectrum. The tetraanion can be regarded as an isoelectronic structure of the known octulene, which shows similar electronic properties.