Self-Assembled, Helically Stacked Anionic Aggregates of 2,5,8,11-Tetra-tert-butylcycloocta[1,2,3,4-def;5,6,7,8-d‘e‘f‘]bisbiphenylene, Stabilized by Electrostatic Interactions

Abstract

Tetraanions of alkyl-substituted derivatives of cycloocta[1,2,3,4-def;5,6,7,8-d'e'f']bisbiphenylene (BPD) and their counter lithium cations self-assemble to form helically stacked assemblies, including a dimer, a trimer, and a tetramer. NMR self-diffusion measurements and unprecedented magnetic shielding effects for the sandwiched lithium cations support their aggregated nature. The D(2)-tetramer assembly is fully characterized by NMR spectroscopy, providing unequivocal evidence for a helix of four tetraanionic BPD layers with an estimated relative twist angle of about 45 degrees and interlayer spacing of ca. 4 A. The barrier for racemization through the in-plane inter-deck rotation is DeltaG(200)= 9.5 +/- 0.2 kcal mol(-1) in the dimer compared to >15 kcal mol(-1) in the tetramer.