Cyclo[6]pyridine[6]pyrrole: a dynamic, twisted macrocycle with no meso bridges

Area of Science:

• Supramolecular Chemistry
• Organic Synthesis
• Materials Science

Background:

• Porphyrin analogues are crucial in various scientific fields.
• Controlling macrocyclic conformation is key to tuning material properties.
• Large, non-bridged macrocycles present unique synthetic and structural challenges.

Purpose of the Study:

• To synthesize a novel large porphyrin analogue, cyclo[6]pyridine[6]pyrrole.
• To investigate the conformational dynamics of the synthesized macrocycle in different states.
• To explore methods for controlling and locking the macrocycle's conformation.

Main Methods:

• Suzuki coupling for macrocycle synthesis.
• NMR spectroscopy to study conformational exchange and protonation.
• X-ray crystallography to determine solid-state structures.
• Spectroscopic techniques (UV-Vis, fluorescence) to characterize different forms.

Main Results:

• Successful synthesis of cyclo[6]pyridine[6]pyrrole via Suzuki coupling.
• Neutral macrocycle exists as a mixture of rapidly interconverting figure-eight conformers.
• Protonation induces conformational changes to ruffled planar or figure-eight structures.
• Bisboron difluoride complexation locks the macrocycle into a stable figure-eight conformation.

Conclusions:

• Cyclo[6]pyridine[6]pyrrole is a versatile macrocycle with tunable conformations.
• Structural dynamics are influenced by protonation state and counterions.
• Boron difluoride complexation provides a method for conformational locking.
• Distinct structural forms exhibit unique spectroscopic and crystallographic signatures.