Angewandte Chemie International Edition
Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 2001, 40 (4), 711-715
A Gallium cluster
has an unusual structure
In the field of molecular electronics scientists are searching for molecules that can be used as tiny building blocks to replace classical electronic components (e.g. transistors). High hopes have been pinned on the class of molecules known as metalloid clusters. Metalloid clusters consist of a "clump" of metal atoms that is surrounded by a nonmetallic protective covering (ligands). The deciding feature: the metal-metal contacts outnumber the metal-ligand bonds. To date, the largest such cluster that has been structurally characterized is made of 77 aluminum atoms. Now Hansgeorg Schnöckel and Andreas Schnepf have bested their own result with the synthesis and structural characterization of an 84-atom gallium (Ga) cluster.
Macroscopically, this new cluster compound appears as black, shiny metallic crystals. On the atomic level, it has an unexpected structure. 64 of the Ga atoms are "naked", having no interactions with ligands. The researchers have determined the arrangement of these "naked" Ga atoms to be as follows: 32 Ga atoms are arranged in an elongated form resembling an American football. On the outside they are surrounded by another 30 "naked" gallium atoms in a meandering fashion. Within the otherwise hollow football is a pair of gallium atoms with an extremely short bond. Says Schnöckel, "among molecular compounds, this arrangement is unique."
The topology and bonding relationships within the football are more reminiscent of the class of compounds called fullerenes than of the relationships that dominate in pure gallium metal (Fullerenes are cage-like molecules made of carbon atoms. The best known of these consists of 60 carbon atoms and looks like a soccer ball.). The Ga84 cluster can thus be considered a link between metalloid clusters and fullerenes.
Within the crystal, the Ga84 units are arranged so that the points of the footballs point at each other. This forms a tube bundle made of gallium clusters. "We expect these unusual cluster structures to have some quirks, possibly in their conductivity - maybe they are superconductors," speculates Schnöckel. Intensive work is now being carried out to measure the conductivity of these highly sensitive crystals.