Chemical reactions usually proceed when we heat the reagents or when we shine light on them. Others happen when an acid or an alkali is added. Rare is the reaction that occurs when we squeeze or stretch the ingredients, but such mechanochemical reactions could be useful in making novel compounds that are wholly inaccessible to conventional chemistry. One approach to mechanochemistry that is yet to come to the fore is the idea of compression leading to chemical changes. Now, just such reactions have been followed using high-pressure x-ray absorption spectroscopy (XAS) and x-ray diffraction (XRD) experiments performed at the U.S. Department of Energy’s Advanced Photon Source (APS) and Advanced Light Source (ALS), respectively. This research reveals the secrets of what might be referred to as a "molecular anvil" in action.
Chemists have tested some of the possibilities of mechanochemistry. By tugging on polymer strands they have deformed and even managed to break bonds within the molecules that make up the chain-like strands. With these long molecules under tensile stress, they have seen reorganization of the atoms within the chains, unzipping the polymers. They have even seen so-called “ring-opening” reactions, where groups of carbon atoms joined in a circle, and those circles hooked into the polymer chain have been pulled open. They have also observed disulfide reduction reactions taking place because of mechanical forces. All of these reactions offer a model for what might be possible when mechanical stresses and strains are applied to molecules.
The present research by a multi-institute, multi-nation collaboration demonstrates how a molecular-level stress can be converted into a strain in a material that comprises a compressible, soft component (the mechanophore), and incompressible, hard ligands (Fig. 1). Ligands are simply chemical groups, while the term mechanophore refers to the part of the system that responds to mechanical stress just as a chromophore in a molecule responds to color. In parallel with this, the researchers in this study also discuss the converse wherein rigid ligands that are touching each other impede relative motion and so block any reaction that might otherwise be induced by compression.
Experiments with crystals of copper(I) m-carborane-9-thiolate (Cu-S-M9) demonstrate the molecular anvil concept. In this reaction system, pressure drives the redox reactions in materials known generally as metal–organic chalcogenides of which Cu-S-M9 is an example. These materials incorporate molecular elements that have different degrees of compressibility depending on how they are compressed, they are heterogeneous.
The m-carborane-9-thiolate (M9) ligands are structurally rigid because of their cage-like nature. As such, bending of bond angles or shearing of adjacent chains of atoms in the material activates the metal–chalcogen bonds and this actually releases the metal as its elemental form. This is a very different reaction route to that would otherwise be taken by the same chemicals if they were simply heated rather than being exposed to mechanical stress. Heating the material produces copper(I) sulfide rather than metallic copper. The XAS data obtained at the High Pressure Collaborative Access Team beamline 16-BM-D at the Argonne National Laboratory APS, and the XRD data from the Lawrence Berkeley National Laboratory ALS helps to explain this difference in terms of the positions of the atoms within the material and how mechanochemistry, rather than heat, leads them to rearrange in two very different ways. (The APS and ALS are Office of Science user facilities.)
The important outcome from this current approach to mechanochemistry is that, whereas polymer mechanochemistry requires long chain-like molecules that are difficult to handle, the new anvil approach involves different types of chemical ligands that have been well studied in conventional chemistry and might be adapted easily to expand their repertoire as agents for this kind of chemistry under pressure.
See: Hao Yan1,2*, Fan Yang1,2*, Ding Pan3,4*, Yu Lin1, J. Nathan Hohman5, Diego Solis-Ibarra6, Fei Hua Li1,2, Jeremy E.P. Dahl1, Robert M.K. Carlson1, Boryslav A. Tkachenko7, Andrey A. Fokin7,8, Peter R. Schreiner7, Giulia Galli9, Wendy L. Mao1,2, Zhi-Xun Shen1,2, and Nicholas A. Melosh1,2*, "Sterically controlled mechanochemistry under hydrostatic pressure,” Nature 554, 505 (22 February 2018). DOI: 10.1038/nature25765
Author affiliations: 1SLAC National Accelerator Laboratory, 2Stanford University, 3Hong Kong University of Science and Technology, 4HKUST Fok Ying Tung Research Institute, 5Lawrence Berkeley National Laboratory, 6Universidad Nacional Autónoma de México, 7Justus-Liebig University, 8Igor Sikorsky Kiev Polytechnic Institute, 9The University of Chicago
Correspondence: * nmelosh@stanford.edu
We thank C. Park and D. Popov from the Advanced Photon Source for XAS measurements, and C. Beavers, J. Yan, and S. Teat from the Advanced Light Source for help with XRD measurements. This work was supported by the U.S. Department of Energy (DOE) Office of Science-Basic Energy Sciences, Division of Materials Science and Engineering, under contracts DE-AC02-76SF00515 and DE-FG02-06ER46262. D.P. acknowledges support from Hong Kong Research Grants Council (project number ECS-26305017), the National Natural Science Foundation of China (project number 11774072), and the Alfred P. Sloan Foundation through the Deep Carbon Observatory. D.S.-I. acknowledges support from PAPIIT IA203116/27 and CONACYT FC-2015-2/829. This research used resources of the Advanced Light Source, which is a U.S. DOE Office of Science User Facility under contract DE-AC02-05CH11231. Portions of this work were performed at the Stanford Nano Shared Facilities, supported by the National Science Foundation under award ECCS-1542152. This research also used resources of the Advanced Photon Source, a DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract DE-AC02-06CH11357.
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