Macrocyclic Polyradicaloids with Unusual Super-ring Structure and Global Aromaticity

Abstract

Summary π-Conjugated molecules with an annulene-within-an-annulene (AWA) super-ring structure have been pursued by chemists for a long time but with limited success. Here, we report the synthesis of two stable cyclopenta-ring-fused oligo( m -phenylene) macrocycles, 8MC-M and 10MC-M , both showing moderate polyradical character. The octaradicaloid 8MC-M exhibits a [24]annulene-within-[32]annulene structure, which is globally aromatic despite the fact that it possesses 4 n π-electrons. Theoretical analyses reveal that the π-electrons in the inner and outer rings follow Baird's rule such that both are triplet biradicals, and overall the molecule is a ground-state spin singlet. The decaradicaloid 10MC-M also possesses a super-aromatic AWA structure with 30 and 40 π-electrons in the inner and outer rings, respectively. However, it has a triplet ground state with a singlet inner ring satisfying Huckel's rule and a triplet outer ring following Baird's rule. Therefore, molecules with a decoupled AWA super-ring structure are eventually attained and also show unusual global aromaticity.