Cyclo[6]pyridine[6]pyrrole: A Dynamic, Twisted Macrocycle with No Meso Bridges

A large porphyrin analogue, cyclo[6]­pyridine[6]­pyrrole,
containing
no meso bridging atoms, has been synthesized through Suzuki coupling.
In its neutral form, this macrocycle exists as a mixture of two figure-eight
conformers that undergo fast exchange in less polar solvents. Upon
protonation, the dynamic twist can be transformed into species that
adopt a ruffled planar structure or a figure-eight shape depending
on the extent of protonation and counteranions. Conversion to a bisboron
difluoride complex via deprotonation with NaH and treatment with BF3 acts to lock the macrocycle into a figure-eight conformation.
The various forms of cyclo[6]­pyridine[6]­pyrrole are characterized
by distinct NMR, X-ray crystallographic, and spectroscopic features.