UCLA Chemists Make Structure of Breathtaking Beauty,
Molecular Counterpart of Interlocked Borromean Rings,
Whose Origins Date Back to Renaissance Italy
chemists have devised an elegant solution to an intricate
problem at the nanoscale that stumped scientists for
many years: They have made a mechanically interlocked
compound whose molecules have the topology of the beloved
interlocked Borromean rings. In the May 28 issue of
the journal Science, the team reports nanoscience that
could be described as art.
The UCLA group is the first to achieve this goal in
total chemical synthesis, which research groups worldwide
have been pursuing.
Named for a noble Italian family, the Borromean rings
first appeared on the family's coat of arms in the 15th
century. Examples of the rings can be seen in buildings
on three islands in northern Italy's Lake Maggiore,
which are still owned by the Borromeo family. The Borromean
link comprises three interlocked rings that form one
inseparable union such that cutting any one ring results
in the other two falling apart.
"This is nanoscience, but also much more,"
said Fraser Stoddart, UCLA's Fred Kavli Professor of
Nanosystems Sciences and director of the California
NanoSystems Institute at UCLA. "The Borromean Rings
pervade art, theology, mythology and heraldry, as well
as mathematics, physics and chemistry. Go to the Google
search engine and you are confronted with more than
"The realization of the Borromean link in a wholly
synthetic molecular form has long been regarded as the
most ambitious and challenging target in topological
chemistry — a Gordian knot," Stoddart said. "The
near-quantitative assembly of this topological link
from 18 components by templation around six metals of
six organic pieces with two 'teeth' and another six
with three 'teeth' to grip the metals, resulting in
the intermittent opening and closing of 12 carbon-nitrogen
double bonds, cuts this Gordian knot once and for all."
(An ancient Greek oracle foretold that whoever untied
the intricate Gordian knot, a knot with no ends exposed,
would rule all of Asia. The problem resisted all attempted
solutions until 333 B.C., when Alexander the Great is
said to have cut through the knot with his sword.)
The "high-risk, all-in-one, mix-the-pieces together,
and shake-them-all-about" approach was the brain-child
in November 1999 of graduate student Stuart Cantrill
in Stoddart's research group. Cantrill is now a lecturer
and research associate in UCLA's department of chemistry
Aided by the computational wizardry of fellow graduate
student Anthony Pease, Cantrill conceived a topology
that was modeled to vindicate the perfect matching of
the three identical, mutually interlocking rings around
the six metal templates. "The three rings slotted
into place perfectly, encompassing the six metals effortlessly
in three-dimensional space," Cantrill said.
"We both stared at the screen and agreed there
and then that it just had to work," Cantrill said.
"It looked so perfect, so beautiful."
"Putting the caboodle together in the computer
was one thing; translating it into a chemical reality
in the laboratory was quite another," Stoddart
said. Two of the three sets of six pieces could be bought,
but the remaining one had to be made in a complex seven-step
The first tentative steps were taken by Pease, who said,
"As a computational chemist, I would normally avoid
getting my hands wet in the laboratory, but this molecule
was so irresistible, I decided to give it a try."
Cantrill and Pease graduated from UCLA in 2001 and left
the completion of the synthesis of the all-important
third piece to postdoctoral fellow Shien-Hsien Chiu,
now an assistant professor of chemistry at the National
"With all three pieces in place, the most challenging
part of the puzzle still lay ahead of us," Stoddart
said. "I was then blessed with the arrival in 2002
of postdoctoral researcher extraordinaire Kelly Chichak.
He brought with him a knowledge base and expertise in
coordination and supramolecular chemistry that made
him a natural when it came to doing chemical synthesis
in a proofreading, error-checking fashion. It would
not have happened without Kelly's nous."
Chichak said, "I just happened to land in the right
place at the right time. I was immediately sucked into
the quest for the molecular Borromean rings because
of their rich history and appealing symmetry."
His challenge was to unearth just the right set of conditions
to coax 18 components to click together in one way and
give "a beautifully crafted molecule which literally
made itself in my hands," according to Chichak.
Stoddart views the near-quantitative assembly as one
of the finest that dynamic chemistry has delivered in
his laboratory to date. "It doesn't happen all
that often: it is good old thermodynamics to the rescue,
with a real vengeance at that."
Or as Chichak puts it, "Simply mix and heat and
a single product emerges out of the thousands of possibilities:
That's all I needed to do."
More than 30 years ago, Robert Woodward at Harvard and
Albert Eschenmoser at the Swiss Federal Institute of
Technology (ETH) in Zürich created Vitamin B12
chemically in a laboratory, a triumph of chemical synthesis,
Stoddart noted. "Similarly, during the past decade,
a total synthesis of Borromean rings in a molecular
form has become the Herculean challenge in contemporary
synthesis, where Darwinian selection operates in a chemically
evolving system," he said.
Chichak obtained X-ray-quality single crystals from
which postdoctoral fellow Gareth Cave solved the structure
in the laboratory of Jerry Atwood, chemistry professor
at the University of Missouri, Columbia.
Each molecule of the Borromean ring compound is 2.5
nanometers across and contains an inner chamber that
is a quarter of a cubic nanometer in volume and is lined
by 12 oxygen atoms.
"When your mind turns to potential applications,
the molecule has so much going for it," Stoddart
"Now that we are addressing what they might do
for us, the list becomes endless," Chichak said.
The ability to produce gram quantities of highly soluble
hosts that can locate a range of different transition
metals in an insulated octahedral array around an inner
oxygen atom-lined chamber, which can provide a welcoming
home for many different guests, suggests numerous ways
in which these molecular Borromean rings could be explored
as highly organized nano clusters in a materials setting
such as spintronics or in a biological context such
as medical imaging, Stoddart said.
"When all is said and done, the molecular Borromean
rings should be judged by their magnificent looks at
this early stage in their existence," Stoddart
said. "As one of the reviewers of the original
manuscript wrote, 'The beauty of the molecular structure
is really breathtaking.'"
Contact: Stuart Wolpert ( email@example.com )