Hexaphyrin-cyclodextrin hybrids: Getting larger, getting confused

The diversity of hexaphyrin-cyclodextrin hybrids (HCD), previously formed from the covalent assembly of regular hexaphyrin and alpha-cyclodextrin subunits, has been increased following two main directions: enlarging the confined space provided by the cyclodextrin and tuning the N-core of the hexaphyrin aiming at bimetallic complexes. The larger beta-cyclodextrin unit was selectively functionalized with aldehyde linkers on its primary rim which were further reacted with 2 eq. of pentafluorophenyl tripyrrane in acidic conditions, followed by DDQ oxidation. Doubly-linked HCD hybrids were obtained with structural variations in the hexaphyrin macrocycles depending on the MSA concentration (2 vs. 200 mM) producing either regular ([26] rectangular/[28] Mobius) or doubly N-confused dioxo ([26] rectangular) scaffolds. For the latter, two isomers were isolated featuring mirror transoid skeletons and long-side rneso-linking patterns, giving rise to planar chirality at the origin of intense and opposite electronic circular dichroism (ECD) spectra. Zn(II) complexation was then investigated leading to two main findings. (i) The regular [28] Matins HCD afforded a mixture of monometallic Mobius Zn(II) complexes in the presence of acac ligand. ECD spectroscopy indicated a chirality transfer from the beta-cyclodextrin favoring a P Mobius twist, but opposite to that arising from the narrower alpha-cyclodextrin congener (M twist favored). (ii) Owing to N3O coordination boxes, dissymmetric bimetallic Zn(II) complexes were readily formed with the doubly N-confused dioxo HCDs through a positive cooperative process. Both metal centers bind a DMAP ligand in axial positions, which is of interest for the further engineering of novel ditopic chiral metalloreceptors.