Entropy- and Hydrolytic-Driven Positional Switching of Macrocycle between Imine- and Hydrogen-Bonding Stations in Rotaxane-Based Molecular Shuttles

Abstract

The construction and switching properties of a novel class of molecular shuttles 1 with imine-bonding stations for macrocyclic diamine parts are reported. Studies on dithioacetalized [2]rotaxane 4 with two hydrogen-bonding stations and a masked imine-bonding station showed that protonation of a macrocycle increases the shuttling barrier due to hydrogen-bond formation between NH 3 (+) groups and the TEG-stations. Hydrolysis of the imine-bonds of the imine-bridged molecular shuttles 1b, c with TEG-stations could exclusively give the [2]rotaxane 2b, c.2H (2+), with the macrocycle hydrogen-bonded with the TEG-station. In contrast, 1a without TEG-stations gave an equilibrated mixture of 1a, monoimine 3a.H (+), and 2a.2H (2+) under similar acidic hydrolytic conditions. The equilibrium between 1b, c and 2b, c.2H (2+) to control the position of the macrocycle could be successfully switched to either side by applying acidic hydrolytic or dehydrating conditions. Furthermore, the equilibrium was largely biased to [2]rotaxane 2b, c.2H (2+) under acidic hydrolytic conditions and could be reversed in favor of bis-imine 1b, c just by heating. This is a successful example of a molecular shuttle exhibiting entropy-driven translational isomerism with remarkable positional discrimination. An examination of thermodynamic parameters showed that imine-bond hydrolyses and the formation of hydrogen bonds between the macrocycle and the station are thermodynamically matched processes, because both processes are enthalpically favored and accompanied by a loss of entropy. The combination of imine-bonding and hydrogen-bonding station in a rotaxane system is the key to realizing the clear entropy-driven positional switching of the macrocycle observed.