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A
Round-up of Carbon chemistry
(Reprinted by permission
from
Chemistry in Britain, 1996, 32(9) 34-35)
MUSICMATCH 9 REGISTRATION KEY
The discovery of C60, and
shortly afterwards a whole series of other fullerenes,
was greeted with near disbelief. Not until Huffman and
Kr„tschmer's paper was published did chemists
finally agree to rewrite the textbooks. From then on,
they have had to rethink radically their ideas about
carbon chemistry.
The first step was to try and understand the chemistry of
these molecules: how do they react; what compounds can be
made from them; and, equally, important, what
applications might they have? To begin with, chemists
tried to draw analogies with conventional carbon
chemistry. For C60 the obvious comparison was with the
archetypal ring molecule benzene. It was a poor fit.
Unlike the aromatic, stabilising, ring current of
benzene, C60, was found to contain two opposing ring
currents that cancel each other out. Rather, C60, is an
electron deficient electron acceptor, which rapidly
decomposes in the presence of light and trace amounts of
ozone in the air.
On the face of it, this was good news. As a 'super
alkene', chemists reckoned, C60 should be a precursor for
a wealth of potentially useful compounds. The problem of
targeting addition across a 60-atom structure was
challenge they couldn't resist.
Several groups set about the task of taming these
reactions. The Sussex group focused on selective
halogenations. Independently, researchers at DuPont also
made significant advances in this area and the resulting
halides became important intermediates in a variety of
chemical reactions, paving the way for a whole series of
fullerene derivatives based on substitution of bromine by
a variety of other groups such as phenyl and methoxy.
Work by John Holloway at the University of Leicester,
along with several other groups, to produce selectively
the potentially more readily substituted, low fluorine
content, fluorofullerenes proved more difficult. Only
recently have researchers begun to overcome some of the
problems inherent in such reactions, with the publication
in 1996 of a paper by Roger Taylor at Sussex and Olga
Boltalina at Moscow State University, describing the
preparation of C60F36-38.
Polymer chemists were quick to get in on the game. In the
early 1990s chemists at Sandia in New Mexico successfully
prepared the first C60 copolymer, by reacting the
diradical xylylene with C60. Although there are no
foreseeable applications for such fullerene polymers in
the short term, one polymer comprising C60 and Pd atoms
is showing promise for catalysing hydrogenation
reactions.
Organometallic chemists
have also had a field day with the fullerenes, producing
osmium, platinum and iridium derivatives among others.
And while conventional chemistry might have limited them
to attaching metals on the outside, the unique spherical
shape of the fullerenes has also enabled them to prepare
a series of metal-encapsulating derivatives, including
La@C60, La@C82 and Sc2@C84. Work is also under way to try
and encapsulate small molecules such as carbon monoxide
and there is speculation about the possibility of using
such cages as delivery vehicles for drugs and radioactive
nucleides, or as miniature sensors.
There is an interesting reverse scenario. Researchers at
the University of California at Santa Barbara led by Fred
Wudl have discovered that the soluble fullerene
derivative di(phenylethylamino-succinate) fulleroid
inhibits the enzyme HIV-1 protease, a key enzyme in the
human immuno-deficiency virus, by sitting snugly in the
enzyme's active site. Even more startling, researchers at
Emory University in Atlanta have since found that the
same derivative is also active against another HIV
enzyme, reverse transcriptase. Unlike the anti-AIDS drug
AZT, which is only effective against acutely infected
cells, the C60 derivative also inhibits reproduction of
HIV in cells chronically infected with HIV.
But perhaps the biggest surprise of the newly emerging
field of fullerene chemistry was the finding by Robert
Haddon and colleagues at AT&T Bell Laboratories
(Chem.Br. September 1994, 746), that alkali metal salts
of C60 are superconductors. In particular, the group
reported in a March 1991 issue of Nature, that the
potassium fulleride salt K3C60 has the high
superconducting temperature (Tc) of 18K. Soon afterwards,
Katsumi Tanigaki in Japan raised the Tc for
superconducting fullerides even higher, with Cs2RbC60
having a Tc of 33k. The current record, set by Otto
Zhou and colleagues of Bell Laboratories in the US,
stands at 40K for Cs3C60 - ever nearer to the 77K target
desirable for commercial applications. Theoretically the
potential of such superconductors is enormous - with
possible uses from Maglev trains to MRI magnets. Before
fullerene superconductors become commercially viable,
however, chemists must also overcome problems of
reactivity in air; K3C60 is pyrophoric and the thin films
quickly lose their conductivity. Nevertheless, there is
growing confidence that chemical modification will lead
to more robust materials that may find some novel
applications as lightweight electric motors and
electromagnets.
Equally promising is the potential of so-called bucky
tubes - elongated pipelines of hexagonal carbon faces
capped at both ends with the requisite 12 pentagons
needed for curvature. Physicist Roger Bacon first
observed these unusual 'carbon whiskers' in the 1960s,
but it was Sumio Iijima in Japan who first appreciated
their significance in terms of fullerene structure
in1991. Others have also been quick to appreciate their
potential. Carbon nanotubes are predicted to be stronger
than any known material (even diamond), with potential
applications in numerous nanoscale architectures such as
microelectronics. In 1992 Iijima's NEC colleagues Thomas
Ebbesen and Pulickel Ajayan brought this prospect a step
closer by perfecting a way of making bulk quantities of
single-walled nanotubes. Using this material, Iijima and
Ajayan later found a way of filling the tubes with molten
lead through a mechanism not unlike that of capillary
action. The possibility of using such filled tubes as
molecular scale wires is attracting considerable
interest, fuelled by the observation that at below about
7K lead becomes superconducting.
And the explosion of interest in fullerene chemistry does
not stop there. Chemists at DuPont, for example, have
recently produced even more novel forms of carbon: carbon
'sea urchins' comprising central cores of gadolinium
carbide from which radiate carbon nanotubes; and
'nanoworms' made of a palladium crystal head and a
segmented tail of carbon tubes.
Like their counterparts in the world of conventional
carbon chemistry, some of these fullerene compounds have
also been found to occur as isomers. The first example of
a chiral fullerene, C76, was reported in 1991 - a joint
effort between Patrick Fowler and David Manolopoulos at
the University of Nottingham and Francoise Diederich and
Robert Whetten at the University of California in Los
Angeles. Shortly after Manolopoulos had predicted the
chirality of C76, Diederich produced the corresponding
19-line 13C NMR spectrum. Since then chemists have
discovered one chiral form of C78 and have found out that
C84 occurs as two different forms, though these have yet
to be isolated.
But perhaps even more surprising is the revelation that
C60 has been around on the Earth since the end of the
Mesozoic era - formed among the black soot that enveloped
the Earth as a result of whatever global catastrophe
wiped out the dinosaurs 65 million years ago. The
question is. of course, how it got there.
Recent investigations of meteorites are providing us with
some clues. Since 1994, researchers claim to have been
observing the characteristic signatures of C60+ and C70+
in meteorites, and early in 1996 researchers at the
Scripps Institute of Oceanography in California reported
finding C60-entrapped He in a 2000m year-old meteor
impact crater at Sudbury in Ontario. The ratios of He-3
to He-4 in these samples match the interstellar ratios
found in meteorites and interplanetary dust particles,
lending support to the extraterrestrial origins of these
organic molecules.
The other explanation (though the isotopic ratios are
inconsistent) is that the fullerenes may have been formed
during the intense heat resulting from impact. Whatever
the answer, making fullerenes is a lot simpler than we
first thought. Forget the expensive lasers, or even the
bell jar apparatus. More recent research shows that we
have probably all made C60 in the soot of a Bunsen burner
flame at some time or another - only its reactivity with
air has prevented us from finding it any sooner...
The story continues ...
It is hard to keep up with the chemistry of fullerenes,
as the papers keep on flooding out. There is also a
considerable presence of the Internet and Chemistry &
Industry produced a useful digest of resources -
Buckyballs - lining up for the Net - in November 1996 at
their web site (http://ci.mond.org).
This provides links into many bucky ball sites.
Diamond, graphite and the fullerenes are all forms of
carbon - carbon allotropes - and are quite different in
their structures, and in their physical and chemical
properties. Diamond has a 3-dimensional network structure
(sp3), graphite has a 2 dimensional layer structure (sp2)
and C60 and the other fullerenes are molecules (sp2) and
form molecular solids: in solid C60 the spherical
molecules form a FCC lattice. Is there a missing
allotrope? There have been several reports of a polymeric
form of carbon based on sp hybridisation with -CðC-Cð
chains.
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