Template-Directed Synthesis of Cyclocarbon Catenanes

This project falls within the EPSRC Physical Sciences research area, in the theme of Synthetic Supramolecular Chemistry.

Carbon forms more molecular compounds than any other element except hydrogen. During the last 25 years, the discovery of new carbon allotropes has led to new technologies with a multitude of applications in chemistry and physics. However, the only molecules consisting entirely of carbon that have been properly characterised are the fullerenes (C60, C70, etc.). This project aims to create a new class of all-carbon ring molecules (cyclocarbons), an elusive allotrope of carbon based entirely on sp-hybridised carbon atoms. In general, polyyne stability decreases with increasing number of adjacent alkyne units. Two strategies are available for increasing the stability of linear polyynes: (i) the use of bulky terminal substituents and; (ii) supramolecular encapsulation by threading the chain through macrocycles. Previous attempts towards the synthesis of cyclocarbons involved the use of carefully designed building blocks, known as masked alkyne equivalents (MAEs), that increase the stability of the chain during its synthesis, and can be 'unmasked' to yield the final polyyne upon application of a suitable stimulus. Recently, the Anderson group reported polyyne rotaxanes formed through an active metal template strategy, where a Cu(I) complex of a phenanthroline-based macrocycle directs the coupling of two terminal oligoynes through the cavity of the macrocycle. The rotaxanes were found to exhibit enhanced thermal stability due to the protective effect of the threaded macrocycle. The principal aims of this project are to develop a new phenanthroline-based MAE, and to use it to synthesise polyyne catenanes via a passive metal templated synthetic strategy. One macrocycle will be based on the phenanthroline moiety, while the other will contain a phenanthroline-based MAE. Using a MAE based on phenanthroline not only masks the alkyne backbone, but can also simultaneously be used as a cleavable supramolecular scaffold. This affords a traceless synthetic route to the interlocked compound involving a passive, rather than active, metal templates. Williamson ether synthesis should provide a reliable route to coupling the building blocks and is well known to be compatible with similar Cu-complexes. Alternatively, the use of Cu(I)-mediated oxidative alkyne homocoupling may be more elegant and the chemistry should be compatible. Owing to the different photochemistry, compared to phenanthrene-based MAEs, these novel phenanthroline-based MAEs may possess superior unmasking properties, thus yielding more efficient pathways to long chain polyynes than previously reported. The target cyclocarbon catenanes will be studied in solution using diverse spectroscopic techniques and characterised in the solid state by X-ray crystallography. Physical studies will provide detailed understanding of the electronic structures of cyclocarbons. These will include UV-vis-NIR absorption, fluorescence, IR and Raman spectroscopy and electrochemistry. Time-resolved IR and Raman studies will also be carried out at the Central Laser Facility, Rutherford-Appleton Laboratory, to gain information on the vibrational behaviour and geometry of electronic excited states. In summary, a new MAE is proposed that can be used as a scaffold for the synthesis of mechanically interlocked molecules. This methodology has many potential applications: completely traceless syntheses of interlocked compounds (when using active metal template synthesis in combination with cleavable binding sites); tighter encapsulation of polyynes; interlocked compounds based on polyynes as well as macrocycles with cleavable backbones.