Figure-Eight Macrocyclic Salen Dimers Synthesized from <i>ortho</i> -Phenylene-Bridged Bissalicylaldehyde

The development of novel nonplanar, twisted macrocyclic ligands remains an important challenge. Schiff-base condensation of bissalicylaldehyde derivatives and diamines typically affords cyclic salen oligomers, but many of the reported examples have planar structures. We now report figure-eight-shaped twisted macrocyclic salen dimers and its boron complex. An <i>o</i>-phenylene-bridged bissalicylaldehyde designed for this purpose adopts a nonplanar structure due to the steric hindrance between its hydroxy groups and the <i>o</i>-phenylene linker. Furthermore, its intramolecular dipole moments stabilize a conformation in which the two hydroxy groups are positioned on opposite sides. Due to these structural characteristics, the reaction with diamines led to the successful formation of the figure-eight-shaped frameworks. Furthermore, by leveraging the twisted nature of the ligand, which orients the NO salicylaldimine moieties outward, a chiral macrocyclic tetranuclear boron-difluoride complex has been successfully synthesized.