It usually takes a great deal of effort to get molecular components to join up with one another and form discrete structures such as rings. Now a group of supramolecular chemists has demonstrated a new approach to convincing the molecules themselves to form such structures spontaneously.
“The interactions of hydrogen bonding and coordination of transition metal ions have dominated the field,” says McMaster University chemistry professor Ignacio Vargas-Baca. Since the early 1970s, Vargas-Baca adds, crystallographers have noticed that atoms of heavy main-group elements prefer centres rich in electron density. However, it has been only recently that this behaviour has been systematically exploited to create intermolecular bonds. In 2013 the International Union of Pure and Applied Chemistry recommended a definition for halogen bonding, highlighting those cases involving atoms such as bromine or iodine.
Vargas-Baca’s own interest was with other heavy elements, specifically those in Group 16 of the periodic table known as the chalcogens. His work revealed examples of cyclic molecules that formed automatically when single tellurium-oxygen chalcogen bonds took the place of hydrogen bonds, a process unlike anything seen in halogen bonding. The resulting macrocyclic molecules are stable in solution, can bind transition metals ions and host small molecules. When mineral acids such as hydrochloric acid or hydrobromic acid are added, hydrogen ions protonate the individual components and the structure falls apart. However, removing the acid is enough to reconstitute the rings. “The beauty of this is that not only are the molecules tolerant of water but we also have a means of turning off and on the self-assembly,” Vargas-Baca says. “This is very exciting.”
These findings by Vargas-Baca and his colleagues have been published in Nature Communications as well as a follow-up paper in the Canadian Journal of Chemistry. While there may ultimately be some interesting applications of this phenomenon in areas such as gas storage or catalysis, for now he is focused on simplifying the synthesis of the building blocks, which currently requires inert atmosphere techniques, and turn it into something readily accomplished on a bench top. “Indeed, we have recently found an easier method to make these types of compounds,” he says.