A Switchable Hybrid [2]-Catenane Based on Transition Metal Complexation and π-Electron Donor-Acceptor Interactions

A bimodal [2]-catenane has been synthesized via a copper(I) templated synthesis. The compound contains both a transition metal coordination site and a set of π-electron rich and π-electron deficient aromatic units suitable for the formation of acceptor-donor complexes. Each constituent ring is thus different from the other, and the organic backbone can adopt two favored contrasting orientations by circumrotation of one ring within the other:  (i) in the metal complex mode, each dpp unit (dpp = 2,9-diphenyl-1,10-phenanthroline) is entwined about the other, while a cationic species is complexed in the coordination site thus created; (ii) in the organic π-electron acceptor-donor complex mode, the dpp fragments are remote from one another, and the π-electron rich and π-electron deficient units stack to form a complex. The conversion of one binding mode to the other implies complete topographical rearrangement of the molecule. It can be triggered by adding or removing the cation center (Cu⁺, Li⁺, or H⁺), bonded to the dpp-containing complexing site. Interestingly, this switching process can be easily monitored by ¹H NMR, since it involves drastic relative orientational changes. It can also be evidenced by electronic spectroscopy. In particular, the proton-driven rearrangement reactions lead to significant changes in the absorption spectrum, which correspond to the appearance (by deprotonation) and disappearance (by protonation of the dpp) of a charge transfer band (around 470 nm) resulting from the π-electron donor-acceptor noncovalent interaction.