Credit: © 2008 AAAS

Non-metals such as boron, carbon, silicon and germanium prefer to bond covalently and tend not to form cations. Singly charged non-metal cations have been observed previously but only in stabilized mono-coordinated states; dications have only been seen in highly coordinated forms. Now Kim Baines and colleagues at the University of Western Ontario have captured1 the 'naked' and highly reactive germanium dication by encapsulating it within a cryptand cage. Cryptands — bicyclic polyether ligands — have been used extensively to trap metallic cations such as lithium and calcium but have never been used to cage non-metals.

The germanium cation was trapped by reacting an N-heterocyclic carbene complex of GeCl(O3SCF3) with cryptand [2.2.2]. X-ray crystallography revealed that the germanium cation resides in the centre of the cryptand cage, which remains symmetric around the cation, providing three-dimensional protection from interactions with both solvent molecules and triflate counterions. The germanium dication cannot be considered completely free, due to interactions with the nitrogen and oxygen atoms of the cryptand, but electronic structure calculations reveal that these interactions are weak and non-covalent, with Ge–N and Ge–O bond orders calculated to be 0.11 and 0.1 respectively.

The encapsulated complex is able to react with anionic nucleophiles, suggesting that it could be a potentially useful precursor for other germanium compounds, but further study is needed to understand its reactivity. More importantly, this first example of an encapsulated non-metal cation should lead to the preparation of other 'naked' non-metals such as silicon and phosphorus by using other appropriately sized cryptands.