ABSTRACT
By their conformational flexibility, Möbius aromatic hexaphyrins provide a dynamic chirality attractive to develop stimuli responsive systems such as chiroptical switches. A regular [28]hexaphyrin has been equipped with a chiral coordinating arm to achieve transfer of chirality from a fix stereogenic element to the dynamic Möbius one. The arm allows straightforward formation of labile monometallic ZnII complexes with an exogenous ligand, either a carboxylato or an amino with opposite inwards/outwards orientations relative to the Möbius ring. As a corollary, the chiral coordinating arm is constrained over the ring or laterally, inducing opposite P and M Möbius configurations with unprecedented high stereoselectivity (diast. excess greater than 95 %). By tuning the transfer of chirality, these achiral effectors generate electronic circular dichroism spectra with bisignate Cotton effect of opposite signs. Switching between distinct chiroptical states was ultimately achieved in mild conditions owing to ligand exchange, with high robustness (10 cycles).
ABSTRACT
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.
ABSTRACT
Coordination of two 2,2'-bisdipyrrin ligands, bearing methyl ester or methylthioether peripheral groups, with Zn(II) cations leads not only to the formation of the expected linear helicates but also concomitantly to novel tri- and tetra-nuclear circular species that have been isolated and fully characterized in solution and by X-ray diffraction.
ABSTRACT
An unprecedented mode of assembly of helical motives and Ag(I) â ions in the crystalline state is described. The combination of a Zn(II) helicate based on a 2,2'-bisdpm bearing peripheral benzonitrile moieties with AgX salts, leads to the formation of a tetranuclear core containing Ag-π interactions. Depending on the coordinating ability of the X(-) anion and the solvents used, the tetranuclear complex self-assembles into coordination polymers of varying dimensionality. From the sequence of coordination events (Ag-π or Ag-peripheral site), one may envisage two possible construction scenarios. However, the Ag-π as primary event seems reasonable owing to the rather weak binding propensity of the nitrile group and the chelating nature of the π-clefts.
