Self-organization leads to intertwined molecular
rings.
It has been a beloved symbol for centuries, prized
as an ornament found in engravings and embroidery, mosaics, and
tattoos - and now as a molecule: Solomon’s knot, a motif consisting of
two doubly intertwined rings. A team of researchers from the
University of California, Los Angeles (USA), and Nottingham Trent
University (UK) have now used a self-organization process to get
molecular building blocks to weave themselves into a Solomon-type
knot. "The secret of our success is the careful selection of metal
ions and solvents," revealed J. Fraser Stoddart in the journal
Angewandte Chemie. "Although various molecular species compete with
each other in solution, the Solomon’s knot wins out during the
crystallization process simply because it crystallizes better."
Systems consisting of individual molecular
components that are not chemically bound to each other, but rather are
tied together through purely mechanical means, are an enormous
challenge for scientists. Stoddart, one of the pioneers in the area of
supramolecular chemistry, has successfully produced a whole series of
such structures. For example, he and his team have produced a system
of molecules in the form of Borromean rings, whose name is derived
from an Italian family that used such interlocked rings in their crest.
Stoddart’s Borromean rings are formed from an 18-component
self-assembly process in which six organic pieces with two "teeth" and
another six with three "teeth" grip six zinc ions, producing the
mutually interlocked three ring system. Things get particularly
interesting when zinc and copper ions are mixed in a 1:1 ratio: a
12-component self-assembly process ensues to interlock two rings twice
over instead of three, resulting in the formation of a molecular
Solomon knot, isolated upon crystallization. The four loops of the
knot are stabilized by two copper and two zinc ions. In solution,
there is initially an equilibrium between the different types of knots.
During crystallization, the Solomon’s knot form is preferred over the
Borromean rings.
"In the making of these exotic compounds, chemical
bonds are being broken just as fast as they are being formed until the
compound that feels the most comfortable emerges as the final product,"
explains Stoddart.
Source / Further
information:
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Publishing date: 15-Dec-2006
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Cari D. Pentecost, Kelly S. Chichak, Andrea J.
Peters, Gareth W. V. Cave, Stuart J. Cantrill, J. Fraser Stoddart
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A Molecular Solomon Link - Angewandte Chemie
International Edition 2007, 46, No. 1-2, 218–222, doi:
10.1002/anie.200603521
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