Mercury in a Guilded Cage
by David Bradley
A US team has found a way to trap a mercury atom inside a gold metallocryptand
cage, which the team believes could open up the study of the metal's inner
Elemental mercury has been widely used in chemistry, but the elemental metal itself, unlike most other metals, has been found to have a rather limited coordination chemistry. Now, Vincent Catalano and Mark Malwitz of the University of Nevada in Reno and Bruce C. Noll of the University of Colorado in Boulder, say it is possible to encapsulate a mercury atom through strong, metallophilic interactions in a gold(I)-based metallocryptand. They use the multidentate ligand P2phen [2,9-bis(diphenylphosphino)-1,10-phenanthroline] to assist in building the mercury trap.
The researchers previously found that they could trap a thallium(I) ion in a gold-based metallocryptand to obtain a strongly luminescent complex. They fancied that mercury too might be a willing prisoner in such a cage and set about finding a way to capture it.
They synthesised the yellow gold cage simply by reacting a chloroform solution containing three equivalents of the colourless P2phen with two equivalents of gold tetrathiophene chloride and a single drop of elemental mercury. The chloride salt that is formed can then be reacted with a strong hexafluorophosphorus salt to release the cryptand as a free species. An initial analysis of the complex revealed it to be a very low coordination complex.
X-ray crystallography revealed that the mercury atom is held tightly on either side by a gold atom forming a three-metal unit within the cage (see diagram), and once incarcerated the atom cannot easily escape. The team is now working to alter the size and shape of the cage hoping to force the mercury atom into a traditional metal-ligand coordination environment, which has never been observed for this closed-shell species says Catalano.
Chem Commun 2001, 581-582; DOI: 10.1039/b100432h