Tren-Capped Hexaphyrin Zinc Complexes: Interplaying Molecular Recognition, Möbius Aromaticity, and Chirality

Area of Science:

• Supramolecular Chemistry
• Organic Chemistry
• Materials Science

Background:

• Hexaphyrin scaffolds offer unique topology, aromaticity, and metal coordination properties beyond porphyrins.
• Molecular recognition studies using hexaphyrins are limited due to a lack of versatile synthetic strategies for creating sophisticated hosts.

Purpose of the Study:

• To develop a general synthetic strategy for creating sophisticated hexaphyrin-based molecular hosts.
• To investigate the molecular recognition capabilities of novel hexaphyrin derivatives.

Main Methods:

• Post-synthetic modification of a meso-(2-aminophenyl) tris-substituted platform to cap the hexaphyrin frame with tripodal units like tris(2-aminoethyl)amine (tren).
• Characterization of the resulting tren-capped hexaphyrins, including their conformational changes upon metalation and ligand binding.
• Evaluation of chiral induction and chiroptical properties using chiral amino ligands.

Main Results:

• Tren-capped hexaphyrins were synthesized in three steps, exhibiting a 28π-conjugated system.
• Zn(II) metalation induced a planar-to-twisted conformational change, transforming the system from Hückel antiaromatic to Möbius aromatic while preserving π overlap.
• Selective and cooperative binding of acetato and amino ligands to zinc was observed, with allosteric tuning by tren unit protonation.
• High chiral induction (up to 77% diastereomeric excess) was achieved with chiral amino ligands, leading to strong circular dichroism signals.

Conclusions:

• The developed synthetic approach provides access to sophisticated hexaphyrin hosts for molecular recognition.
• Tren-capped hexaphyrins demonstrate tunable electronic properties and selective guest binding, mimicking metalloenzyme active sites.
• These Möbius aromatic compounds show significant potential for applications in chiral sensing and catalysis.