Chem. Commun. 48, 7562–7564 (2012)

Org. Lett. 14, 3424–3426 (2012)

Credit: © 2012 RSC

Polyynes are compounds in which linear chains of sp-hybridized carbon atoms are held together with alternating single and triple bonds. Not only have the somewhat unusual composition and structure of these carbon-rich materials attracted the interest of chemists for many years, but they also represent discrete examples of the proposed one-dimensional allotrope of carbon known as carbyne.

As long ago as the 1970s, synthetic chemists were able to make polyynes containing up to 16 acetylenic units, but these reactive species would readily polymerize if not kept in dilute solutions. More recently, it has been found that placing bulky groups at each end of the carbon chain results in stable compounds that can be isolated.

The calming influence of the end groups on the reactivity of polyyne chains only extends so far, however, and as the chains get longer, alternative approaches to modulate their behaviour are going to be required. Now, two research teams have independently shown how polyyne chains can be 'insulated' by encircling them with a macrocycle to produce interlocked molecules known as rotaxanes. John Gladysz and co-workers from Texas A&M University coupled together pairs of butadiyne building blocks — each with a bulky platinum-based group at one end — in the presence of a 33-membered macrocyclic ring to give a tetrayne-based rotaxane. In a process that is referred to as 'active templating', a copper ion bound to the macrocycle mediates the coupling of the two terminal alkynes.

Using the same macrocycle and a similar copper-mediated coupling approach, a team of researchers led by Harry Anderson at the University of Oxford and Rik Tykwinski at the University of Erlangen-Nuremberg, were able to make a series of polyyne rotaxanes comprising 4, 6 or 10 acetylenic units. In this case, however, the polyyne chains were capped with 'super-trityl' groups rather than metal-based complexes. Anderson, Tykwinski and co-workers then went on to demonstrate that the environment around the polyyne chain could be influenced by binding a metal ion to the phenanthroline group of the macrocycle.