C60, C80, C0, Go!

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  • Published: May 15, 2009
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: C60, C80, C0, Go!

X-ray crystallography, nuclear magnetic resonance spectroscopy, and other techniques have allowed German chemists to demonstrate their synthesis of the first non-carbon analogue of the C80 fullerene molecule.

Manfred Scheer, Andrea Schindler, and Christian Gröger of the Institute for Inorganic Chemistry at the University of Regensburg, Germany, and Alexander Virovets and Eugenia Peresypkina of the Nikolaev Institute of Inorganic Chemistry, Siberian Division of Russian Academy of Sciences in Novosibirsk, Russia, provide details of their work in the current issue of Angewandte Chemie.

The discovery of an entirely novel allotrope of carbon in the form of the soccer-ball-shaped C60 fullerene molecule, nicknamed buckminsterfullerene, or the buckyballs was a minor revolution in chemistry. But, C60 was not the only fullerenes, there were several other highly symmetrical versions, such as C80.

There is really only one fullerene C60 but C80 offers seven different possible structural forms. Many of these, such as the icosahedral version with 20 sides are relatively unstable but represent an intriguing hollow target for synthetic chemists.

Scheer and colleagues, however, are not interested in building hollow spheres of carbon, their focus is the possibility of building inorganic counterparts for these compounds that would come with all the potential of functional ligands, active metal centres and still have the ability to act as hollow hosts for small guest molecules. As such, the team has used a template approach to aggregation to create an organometallic analogue of C80.

The researchers used pentaphosphaferrocene (a five-membered ring made of phosphorus atoms bound to an iron atom) and copper chloride for their synthesis. The template molecule was a carborane, a hybrid inorganic-organic compound made from carbon, boron, and hydrogen. This template has fivefold symmetry and is approximately 0.8 nanometres across.

In the reaction, the individual building blocks aggregate around the carborane to form a spherical supermolecule with fullerene-type geometry, the team explains. The resulting sphere encloses the carborane as its guess producing an icosahedral structure comprising twenty copper and sixty phosphorus atoms arranged into twelve rings containing five phosphorus atoms each and 30 six-membered rings containing two copper and four phosphorus atoms.

A quick comparison shows that the carbon [80]fullerene is a lot smaller than the analogue with its carbon-free Cu20P60 core. The external diameter of the complete structure is 2.3 nanometres, about twice the size of [80]fullerene at 1.1 nm, whereas the core itself is just 1.6 nm. The team explains that the inner cavity is almost spherical with an internal diameter of about 0.82 nm, which is much smaller than the inner diameter of previous analogues of [60]fullerene. Indeed, the inner cavity of the present inorganic structure is just big enough for the ortho-carborane molecule to be completely encapsulated.

"Template-controlled aggregation has been shown to be an efficient route to large, entirely spherical molecules of fullerene-type topology," explains Scheer. "The guest molecule determines the size and composition of the fullerene-type product."

The team adds that the compound crystallizes in a cubic space group and surprisingly packs with a primitive cubic packing motif in the crystal lattice, although the X-ray structure shows that there are large cavities between the structures filled with solvent molecules. They also used magic-angle spinning NMR spectroscopy and infra-red spectroscopy to investigate further the characteristics of their inorganic fullerene analogue. The team points out that the inorganic shell interacts electronically with the enclosed guest molecule.

Scheer's is not the only team creating fullerene analogues in Germany this month. At the time of writing, Achim Müller of the University of Bielefeld. His team are striving to find ways to use pentagonal building blocks in chemistry, something to which tetrahedral carbon atoms are obviously averse.

In the current issue of Chemical Communications, his team has used pentagonal metal oxide based units to construct spherical structures he refers to as Keplerates.

"The advantage of the present communication is that different types of pentagonal metal-oxide based units and linkers for them can be used for the syntheses of a variety of spherical molecules like the Keplerates while these building blocks can be obtained according to a logical method based on stimuli," Müller told SpectroscopyNOW, "this allows us to follow new routes in chemistry and materials science." Those stimuli could be magnetic centres, he explains, so this offers a molecular magnetism perspective to the research.

The primary motivation for this part of Müller's work is to learn how to generate and use pentagonal units in a general sense, he adds, "This will allow other groups in the future to follow this route with perspectives for several fields."



Hollow chemical (Angewandte/Scheer)

Inorganic [80]fullerene analogue, a hollow chemical promise

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