Tamed Germanium

BY ENTOMBING a germanium dication within a cryptand molecule, chemists in Canada have managed to catch this highly reactive species. The work, published in Science (2008, 322, 1360), represents the first time that a doubly charged nonmetallic ion without any covalent bonds has been isolated. The discovery could provide an avenue for creating other so-called naked nonmetallic cations, such as Si²⁺ and P³⁺, the researchers say.

Isolating this novel reactive intermediate is "a remarkable and exciting achievement," notes Northwestern University chemistry professor Joseph B. Lambert in a commentary that accompanies the paper. "Under particular circumstances, reactive intermediates have been isolated and characterized, but establishing the existence of an entirely new class of intermediates is rare indeed," he adds.

After trying unsuccessfully to crystallize a crown ether complex of the elusive Ge²⁺, Paul A. Rupar, Kim M. Baines, and Viktor N. Staroverov of the University of Western Ontario turned to cryptand molecules. These bicyclic macromolecular polyether cages have been used to imprison metallic cations. Because a cryptand has the ability to completely surround an ion, the chemists reasoned that it could protect the remarkably reactive Ge²⁺ from nucleophilic counterions and solvent molecules.

Trapping the dication with the cryptand turned out to be surprisingly easy, Baines says. It worked on the first try, rapidly producing a white precipitate when the researchers mixed a germanium carbene complex with a cryptand. "It is just a beautiful reaction," Baines adds. "We hope it will inspire others to think of using cryptands in this area of chemistry."

Crystallographic studies indicate the Ge²⁺ sits halfway between the cryptand's nitrogen atoms and is equidistant from its oxygen atoms. Although the ion is too far away to form covalent bonds with any of these atoms, Lambert believes it is likely that each atom provides a "small amount of bonding." Consequently, he describes the germanium as "nearly naked," even though the germanium has been stripped of any bonding partners. "Without a doubt," he adds, it is "a species for which there is no precedent in nonmetallic inorganic chemistry."