Pnictogen, chalcogen, halogen: NMR looks into the blue holes

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  • Published: Apr 1, 2018
  • Author: David Bradley
  • Channels: NMR Knowledge Base
thumbnail image: Pnictogen, chalcogen, halogen: NMR looks into the blue holes

Novel bonds

The origin of the pnictogen catalysis: The large empty space, the «blue hole», on antimony, inside the molecule. Example: molecular model of Sb(C6F5)3, empty space in the molecule in blue. Sb = antimony. Credit: UNIGE

New catalysts containing antimony could complement more well-known interactions such as hydrogen bonds or chalcogen-sulfur bonds in chemical reactions for a wide range of products. Nuclear magnetic resonance (NMR) spectroscopy provides the clues.

Chemists at the University of Geneva (UNIGE), Switzerland, have just discovered that chemical bonds based on antimony – an almost forgotten element - could be used to make innovative materials by opening up novel bond types in catalysts that were not though possible before. Many catalytic processes involve transient hydrogen bonds between substrate and catalyst. In recent years, chalcogen-sulfur connections have emerged as another catalytic form. Now, the introduction of antimony bonds up the creative possibilities for chemists still further. A different type of bonding simply means new ways to initiate contact between catalyst and substrate and provide alternative routes for molecular transformations to take place.

Catalysts in a rush

"My team is constantly looking for new bonds for catalysis,” explains organic chemist Stefan Matile of UNIGE. “After discovering the sulfur-based bond for catalysis, called the chalcogen, two years ago, we decided to look at another category in the periodic table, pnictogen elements, which are distinguished by their metallic components.” The elements nitrogen, phosphorus, arsenic, antimony, and bismuth all belong to this category.

Matile points out that, “Research usually focuses on studying the electrons of the elements. We took the opposite approach: we only examined the empty spaces left by the electrons, which are essential for molecular construction, so we could look for possible new interactions.” The heavier pnictogen elements, are in a sense more flexible and deformable than, for instance, the very compact hydrogen atom. The spaces in between leave room for reactions to grow.

The team's theoretical calculations on seven elements allowed them to visualize those spaces, adds Amalia Poblador-Bahamonde. "We first carried out computer modeling of the seven elements alone so we could visualise where both the electrons and the empty spaces were located," she explains. "We then did it again with the molecules to be tested in order to measure the strength of the new bond.” The more visible the molecule’s empty spaces became, the better the bond works, making more effective catalysts based on that element. The theoretical work revealed antimony to be the best of the seven and so the team turned their attention to this portion of the periodic table, something that organic chemists had little explored previously.

NMR revelations

They then turned to NMR spectroscopy to obtain the experimental evidence to support the theory. "Our results were perfectly consistent with the theoretical predictions,” postgraduate student Sebastian Benz explains. “Antimony proved to be ultra-efficient, up to 4000 times faster than the other elements tested in creating a new structure!”

Matile points out that antimony is not only ultra-fast in catalysis but it works from within, affect the material’s entire environment and allowing the chemists to be more precise when undertaking particular chemical transformations. This is not the last word in catalytic bonds, however. "We don’t intend to stop there, of course: we’re going to keep on looking for new bonds and ways in which we can make use of them,” adds Matile.

The team further explains the concept and its potential in the journal Angewandte Chemie: "These findings demonstrate that there is no reason to ignore pnictogen bonds," they say. "On the contrary, they suggest that pnictogens in general and antimony in particular will remain the most powerful donors for operational sigma-hole interactions by far." They hope that their findings will stimulate the integration of these elements into novel functional systems for catalysis . It is perhaps the configurational stability and the sigma holes that lie within rather than exposed on the surface as with halogens and chalcogens, that means chiral pnictogen bond donors could particularly useful in asymmetric catalysis.

Related Links

Angew Chem Int Edn Engl 2018, online: "Catalysis with Pnictogen, Chalcogen, and Halogen Bonds"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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