Ammonium Binding

Background

The concept of threading a molecule containing an NH₂⁺ center through the cavity of the crown ether dibenzo[24]crown-8 (DB24C8) was first recorded in the literature by Busch in 1995 in which a rotaxane was assembled using this type of recognition motif. As an extension to our work on intertwined and interlocked systems, we were also independently pursuing investigations on the potential for R₂NH₂⁺ ions to thread through the cavity of DB24C8 in order to generate [2]pseudorotaxanes. We demonstrated that the hexafluorophosphate (PF₆) salts of R₂NH₂⁺ ions — such as the dibenzylammonium ion (DBA⁺) — can indeed pierce the macroring of DB24C8, leading to the formation of threaded complexes in solution. Furthermore, the strength of the interaction was shown to depend markedly upon the nature of the solvent in which the two components were mixed. This observation confirms that the primary driving force responsible for the threading interaction is the potential for strong hydrogen bonds to be formed between the acidic NH²⁺ protons and the ring of oxygen atoms located in the DB24C8 framework.

This relatively simple concept — where one R₂NH₂⁺ ion is threaded through one monotopic crown ether (dibenzo[24]crown-8) — became the building blocks for our molecular meccano kit and has led to more elaborate multiply encircled and/or multiply threaded superstructures. We are currently continuing our extensive examination of the solid-state behavior of these interwoven complexes by varying substitutions on the crown ether and R₂NH₂⁺ ion, as well as varying the constitution and ring size of crown ethers.

A plethora of rotaxanes, catenanes, and interwoven bundle-like supermolecules has been successfully synthesized using the dialkylammonium ion / crown ether recognition motif in our laboratories. We have developed methodology using stoppering, slippage, or clipping approach for assembling such supermolecules. We have also shown that overall structure can be differentiated kinetically or thermodynamically.

The idea of threading a rod-shaped molecule through the macrocyclic cavity of another ring-shaped one can be applied to the propagation step of a supramolecular polymerization. We are currently investigating this area by covalently coupling two mutually recognizing components to one another, thus affording a self-complementary monomer, which has the capacity to self-assemble into either linear or cyclic daisy chain arrays.