Pressure squeezes reduction reactions out of crystals

For the first time, chemists have broken specific bonds by squeezing a crystal.

Chemists typically drive reactions with heat and light, but mechanochemical synthesis has proved its utility in some circumstances. Grinding reagents through ball milling can reduce the need for solvents, while stretching some linear polymers can induce ring opening and other transformations. Nicholas A. Melosh at Stanford University and SLAC National Accelerator Laboratory and colleagues have shown that compressing crystals can selectively initiate chemical reactions with the help of ligands that control which bonds break (Nature 2018, DOI: 10.1038/nature25765).

The researchers squeezed crystals of copper(I) m-carborane-9-thiolate (Cu-S-M9) in a diamond anvil cell. At 12 gigapascals of pressure, a copper-sulfur bond broke and copper(I) got reduced to copper(0), according to measurements with transmission electron microscopy and energy dispersive X-ray spectroscopy. At pressures below 8 GPa, the compound’s structure deformed but returned to its initial state, while some bonds broke between 8 and 12 GPa. The group notes that squeezing the crystals produced unique transformations. For example, heating Cu-S-M9 yields cuprous sulfate, not copper(0).

Melosh explains that carborane’s cubic structure resists compression, transferring force instead to the copper-sulfur bonds. The geometry of Cu-S-M9 directs that force asymmetrically within the crystal, deforming the Cu₄S₄ core. Melosh says density functional theory simulations confirmed that the pressure causes electron density in molecular orbitals to move away from the strained copper-sulfur bond before it breaks and an electron moves from sulfur to copper.

Ligands can also block mechanochemical transformations. The researchers squeezed copper(I) adamantane-1-thiolate at 20 GPa and nothing happened. In this case, the adamantane ligands pushed against one another, preventing any strain on the copper-sulfur bonds.

That type of ligand-based control could make the technique more broadly useful. If the relationship between ligand choice and reactivity extends to other metal complexes, this work will “constitute a major step towards establishing the molecular mechanisms of small-molecule mechanochemistry,” says Roman Boulatov, who studies polymers at the University of Liverpool.

To show that the technique worked beyond their initial model system, Melosh’s group demonstrated similar results in silver thiolate complexes. Melosh sees potential for the method in selective addition and substitution reactions because chemists could use applied pressure to control the direction at which reagents attack each other in a crystal. “That’s difficult to do in solution,” Melosh says. He’s also interested in some reactions that are currently energy intensive, like reduction of carbon dioxide and nitrogen.