Synthetic chemists are always keen to push their discipline to the very limits, and making ever-larger molecules that have well-defined shapes and precise elemental compositions is one way to highlight just what modern chemical synthesis can achieve. Many chemists are also fascinated with cyclic molecules — an obsession that can almost certainly be traced back to benzene — and so the synthesis of giant macrocycles is a particularly appealing challenge. Beyond a certain size, even macrocycles made up of rigid hydrocarbon building blocks such as phenyl rings and alkyne groups show some degree of flexibility, but one way to make stiffer structures is to introduce spokes that connect the macrocyclic rim to a central hub.
Now, a team of researchers in Germany led by Stefan-Sven Jester and Sigurd Höger have made a large molecular hexagon — with a corner-to-corner distance of almost 12 nm — in which the outer rim is linked to a hexaphenylbenzene hub through six spokes. The synthesis, which relies heavily on Pd-catalysed couplings, begins by constructing corner units of the hexagon in which half of each adjacent edge of the hexagon are located meta to each other on a benzene ring. In the next step a spoke is added to the corner units between the two edge pieces and, after some selective deprotection chemistry, six corners — with free alkynes at the end of their spokes — are coupled to the central hexaphenylbenzene hub. Following deprotection of the two half-edges attached to each corner, the hexagonal framework is stitched together in yet another Pd-catalysed coupling reaction, this time between two alkynes on each edge.
The resulting molecular wheel is made up of 75 benzene rings and 96 alkyne linkers connected in various arrangements, and comprises a total of 1,878 carbon atoms and 2,682 hydrogen atoms. Although the NMR spectroscopic and mass spectrometric data are consistent with the proposed structure, the signals are quite broad in each case. More definitive proof of the structure comes from scanning tunnelling microscopy images, which show well-defined hexagonal wheels with features that correspond to the expected molecular structure.
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Cantrill, S. Huge hexagons. Nature Chem 7, 95 (2015). https://doi.org/10.1038/nchem.2179