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

Over the past decade,
the hexaphyrin skeleton has emerged as a
multifaceted frame exhibiting strong interplay between topology, aromaticity,
and metal coordination, opening new research areas beyond porphyrins.
However, molecular recognition with hexaphyrins has been underexplored,
mainly because of the lack of general synthetic strategies leading
to sophisticated molecular hosts. Here we have developed a straightforward
approach for capping the heteroannulene frame with tripodal units
(e.g., tris­(2-aminoethyl)­amine [tren]) through postsynthetic modification
of a readily accessible meso-(2-aminophenyl) tris-substituted
platform. The resulting tren-capped hexaphyrins, obtained in three
steps from a 5-(aryl)­dipyrromethane precursor, display remarkable
features: (i) Considering the 28π-conjugated system, instantaneous
and site-selective Zn­(II) metalation at the level of a dipyrrin versus
tren unit triggers a planar-to-singly twisted conformational change
and hence a Hückel antiaromatic-to-Möbius aromatic transformation.
In spite of the tripodal linkage, a smooth twist and efficient π
overlap are preserved. (ii) Selective and cooperative binding of both
an acetato ligand and an amino ligand to zinc occurs in distinct confined
environments, reminiscent of substrate discrimination at the buried
metal centers of metalloenzymes. The ligand binding pockets are allosterically
tuned by monoprotonation of the tren unit. (iii) Substantial chiral
induction of the molecular twist is achieved using chiral amino ligands
(diastereomeric excess up to 77%, the highest reported to date for
a Möbius compound), to which is associated a strong chiroptical
signature in circular dichroism. These results provide unprecedented
insights into molecular recognition with hexaphyrins, paving the way
to innovative Möbius-type molecular hosts for sensing and catalysis.