Large Amplitude, Proton- and Cation-Activated Latch-Type Mechanical Switches: O-Protonated Amides Stabilized by Intramolecular, Low-Barrier Hydrogen Bonds within Macrocycles

Abstract

Large amplitude molecular switches have been developed using oxonium ions as the novel switching mechanism. Macrocycles that contain a polyether ring that are preorganized and of optimum geometry such that strong, linear Low-Barrier Hydrogen Bonds (LBHB, 2.4 to 2.6 A in length) are formed between a protonated amide oxygen and a cyclic ether, that lend significant iminol character to the amide. Deprotonation yields a large conformational change between closed and open forms, mindful of a new hinged, latch-type mechanical proton switch. Numerous open and closed forms have been characterized by X-ray crystallography, and the intramolecular hydrogen bond that forms between the protonated amide oxygen and the cyclic polyether oxygen accounts for the stability of these new acids. The open form of the deprotonated adducts persist in solution as indicated by the magnitude of coupling constants and other Nuclear Overhauser Effect experiments. Different saturated and unsaturated solid acids have been characterized including products derived from acetonitrile, propionitrile, caprylonitrile, acrylonitrile and adiponitrile, and also by reaction with primary amides in the case of phenyl and norbornene derivatives. We have also demonstrated that metal cations can replace the proton in the switching mechanism, characteristic of nascent synthetic pores.