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
workings.
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