Molecular Meccano. 1. [2]Rotaxanes and a [2]Catenane Made to Order

A new synthetic strategy for the elaboration of supramolecular species and molecular compounds containing noncovalently interacting components is described, with the long-term objective of constructing highly ordered, wholly synthetic assemblies from readily available starting materials. These could serve as a basis for the future development of mechanoelectrical and photoelectrical communication systems and devices capable of storing and processing information. The approach was conceived against a background of a quarter of a century's experience in supramolecular, alias host‒guest, chemistry. It is based on the use of irreversibly interlocked molecular systems that take the form of catenanes and rotaxanes. Such compounds are seen to be the ideal vehicles through which to transfer from supramolecular and host‒guest chemistry the knowledge and experience gained from studying complexes between small chemical entities to very much larger molecular assemblies. Once we know how to interlock molecular components irreversibly and efficiently, we shall have a very much clearer idea on how to intertwine related polymer chains reversibly. A number of template-directed syntheses of [2]rotaxanes and a [2]catenane is discussed. They illustrate that there are inherently simple ways of making apparently complex unnatural products from appropriate substrates without the need for reagent control or catalysis. The noncovalent bonding interactions that are used to self-assemble the 1:1 complexes, which serve as precursors to the rotaxanes and the catenane, as well as to the [2]rotaxanes and the [2]catenane themselves, “live on” in their structures and superstructures after the self-assembly process is complete. A variety of methods, including X-ray crystallography, fast atom bombardment mass spectrometry, ultra violet-visible, luminescence, nuclear magnetic resonance, and electron spin resonance spectroscopies, and electrochemistry, demonstrate the high structural order that is incorporated into these new molecular assemblies in both the solid and solution states.