Molecular Meccano. 3. Constitutional and Translational Isomerism in [2]Catenanes and [n]Pseudorotaxanes

The mutual molecular recognition expressed between two classes of compounds has led to the self-assembly of a range of [2]catenanes, composed of cyclic polyethers intercepted by π-electron donors, and a range of [n]-pseudorotaxanes, composed of similar acyclic polyethers, and various tetracationic cyclophanes. These molecular self-assembly processes rely upon the recognition between (i) π-electron rich and π-electron deficient aromatic units and (ii) hydrogen bond donors and acceptors, within the different components. The constitution of the π-electron rich and the π-electron deficient structural components in these molecular and supramolecular structures has a profound effect on the organization of the various assemblies and on their dynamic properties with respect to each other both in solution and in the solid state. The techniques of X-ray crystallography, fast-atom bombardment mass spectrometry, ¹H, ¹³C, and dynamic nuclear magnetic resonance, ultraviolet/visible spectroscopies, and electrochemistry have been used in the solid and solution states to assess the nature of the structures of the catenanes and the superstructures of the pseudorotaxanes. The successful assembly of these catenanes and pseudorotaxanes, through the transcription of programmed molecular information, in the form of noncovalent bonding interactions, lends support to the contention that self-assembly is a viable paradigm for the construction of nanometer-scale molecular and supramolecular structures incorporating a selection of simple building blocks.