Revisiting indeno[1,2-b]fluorene by steric promoted synthesis while isolating the second stable 4nπ indeno[2,1-a]fluorene

Abstract

Described herein are the steric promoted synthesis and characterization of symmetric, unsymmetric, and benzo-extended indeno[1,2-b]fluorenes 1–5, including the isolation of stable 4nπ indeno[2,1-a]fluorene 6. Single-crystal XRD analyses of 5 and 6 gave the unambiguous confirmation of unsymmetrical [1,2-b]IF and [2,1-a]IF motifs, respectively. The different ground state antiaromaticity of 5 and 6 was explained by Gimarc's approach for topological charge destabilization. The electronic properties of unsymmetrical IFs 2–5 mostly lie midway between 1 and its other known symmetrical counterparts.