Dodecahedrane
Dodecahedrane is a chemical compound, a hydrocarbon with the formula C20H20, whose carbon atoms are arranged as the vertices (corners) of a regular dodecahedron[1]. Each carbon is bound to three neighboring carbon atoms and to a hydrogen atom. This compound is one of the three possible Platonic hydrocarbons, the other two being cubane and tetrahedrane. Dodecahedrane does not occur in nature and has no significant uses. It was synthesized by Leo Paquette in 1982, primarily for the "aesthetically pleasing symmetry of the dodecahedral framework"[1].
The angle between the C-C bonds in each carbon atom is 108°, which is the angle between adjacent sides of a regular pentagon. This value is quite close to the 109.5° central angle of a regular tetrahedron—the ideal angle between the bonds on an atom with sp3 hybridisation. As a result, there is minimal angle strain. However, the molecule has significant levels of torsional strain as a result of the eclipsed conformation along each edge of the dodecahedron[1].
The molecule has perfect icosahedral (Ih) symmetry, as evidenced by its proton NMR spectrum in which all hydrogen atoms appear at a single chemical shift of 3.38 ppm. Unlike buckminsterfullerene, dodecahedrane has no delocalized electrons and hence has no aromaticity[1].
For over 30 years, several research groups actively pursued the total synthesis of dodecahedrane. A review article published in 1978 described the different strategies that existed up to then. The first attempt was initiated in 1964 by R.B. Woodward with the synthesis of the compound triquinacene which was thought to be able to simply dimerize to dodecahedrane. Other groups were also in the race, for example that of Philip Eaton and Paul von Ragué Schleyer. Leo Paquette's group at Ohio State University was the first to succeed, synthesizing the compound in 1982[1].
A variety of dodecahedrane derivatives have been synthesized and reported in the literature. Substitution of all 20 hydrogens by fluorine atoms yields the relatively unstable perfluorododecahedrane C20F20, which was obtained in milligram quantities. Trace amounts of the analogous perchlorododecahedrane C20Cl20 were obtained, among other partially chlorinated derivatives, by reacting C20H20 dissolved in liquid chlorine under pressure at about 140 °C and under intense light for five days. Complete replacement by heavier halogens seems increasingly difficult due to their larger size. Half or more of the hydrogen atoms can be substituted by hydroxyl groups to yield polyols, but the extreme compound C20(OH)20 remained elusive as of 2006. Amino-dodecahedranes comparable to amantadine have been prepared, but were more toxic and with weaker antiviral effects. Annulated dodecahedrane structures have been proposed[1].
Molecules whose framework forms a closed cage, like dodecahedrane and buckminsterfullerene, can encapsulate atoms and small molecules in the hollow space within. Those insertions are not chemically bonded to the caging compound, but merely mechanically trapped in it. Cross, Saunders and Prinzbach succeeded in encapsulating helium atoms in dodecahedrane by shooting He+ ions at a film of the compound. They obtained microgram quantities of He@C20H20 (the "@" being the standard notation for encapsulation), which they described as a quite stable substance[1].
Dodecahedrane is the most highly symmetric hydrocarbon possible, with icosahedral (Ih) symmetry. It is in the same symmetry group as the all-carbon buckminsterfullerene. Largely because of its high symmetry, it has an extremely high melting point for a hydrocarbon. Dodecahedrane crystals take the face-centered cubic structure[2].
Citations:
[1] https://en.wikipedia.org/wiki/Dodecahedrane
[2] https://www.acs.org/molecule-of-the-week/archive/d/dodecahedrane.html
[3] https://www.sciencedirect.com/topics/chemistry/dodecahedrane
[4] https://arxiv.org/pdf/2311.06614.pdf
[5] https://pubs.rsc.org/en/content/articlehtml/2014/ra/c3ra43628d