Helical Inversion of Peptide-based Supramolecular Co2+ Complexes: Co2+-Triggered Helicity Inversion in a Supramolecular Polymer

Herein, we report the helical inversion of supramolecular polymeric complexes of Co2+ containing a peptide-based ligand comprising one alanine and three glycine moieties and an achiral terpyridine group. The helicity of the peptide-based supramolecular polymer is controlled via strain-induced chirality at different stoichiometric ligand/Co2+ ratios. The supramolecular polymer S-1 adopts octahedral geometry with right-handed helicity (P-type) in the presence of <0.7 equiv of Co2+. In contrast, it adopts coexisting octahedral and square-pyramidal geometries in the presence of 1.0 equiv of Co2+ and presents left-handed helicity (M-type). The helicities of the supramolecular polymer R-1 with Co2+, prepared using the chirally opposite ligand, were completely opposite to those of S-1. Furthermore, the circular dichroism intensities of supramolecular polymers S-1 and R-1 in the presence of Co2+ were 900-1500 times higher than those of free S-1 and R-1. In addition, the helical inversion was completely reversible and controllable by the addition of more Co2+ or ligand.