Shaking up gold and palladium

Sometimes, chemistry can be a real grind. But for Tomislav Friščić at McGill University, that’s the whole point. His team has developed a mechanochemical method that turns noble metals like gold and palladium into useful salts and complexes without needing solvents or harsh reagents (Angew. Chem. Int. Ed. 2018, DOI: 10.1002/anie.201712602).

Noble metals are widely used in applications including as catalysts and in electronics, but generating soluble noble metal compounds requires some aggressive conditions. A common way to make their chloride salts, for example, involves reacting the metals with aqua regia, a mixture of concentrated nitric and hydrochloric acids.

Friščić’s team came up with a convenient and less hazardous alternative. The chemists mixed metal powder, pellets, or wire with potassium peroxymonosulfate, an oxidizing agent commonly known as Oxone, and simple halide salts such as potassium chloride or ammonium chloride. Then they put the mixture in a zirconia jar with a 1-cm-wide zirconia ball and gave it a ferocious mechanical shake at room temperature for up to 90 minutes.

This ball milling process created a range of noble metal halides in almost quantitative yields, which the researchers then dissolved in water and recrystallized to isolate salts such as potassium tetrachloropalladate (K₂PdCl₄). Bromide salts worked just as well, although the yield of iodides was lower. “It’s a remarkable result that illustrates the value of mechanochemistry,” says Carsten Bolm, a mechanochemistry researcher at RWTH Aachen University. “It’s a very benign protocol.”

By adding ligands to the dry mixture and milling it for another 30 minutes, the researchers also created various gold and palladium complexes, including grams of bis(triphenylphosphine)palladium chloride, which they successfully used as a catalyst in Suzuki coupling reactions to create biphenyl compounds. This one-pot approach to producing metal complexes means that “you’re doubling up the solvent savings,” by skipping a separate step of mixing the milled metal salts and ligands in solution, says Duncan L. Browne, who works on mechanochemistry at Cardiff University.

Friščić’s team could even use palladium catalyst waste as the starting material in this process to make fresh Suzuki catalyst, effectively recovering 89% of the palladium from the original spent catalyst. Friščić thinks this could help researchers to recycle valuable catalysts that are often discarded: “In a lab, there are a lot of noble metals that go to waste,” he says.

Friščić is now discussing the process with companies in the metal refining industry and hopes to explore whether it could be used for other kinds of noble metal recycling. “I’m looking forward to getting my hands on some electronic waste and seeing how good we are at extracting gold from it,” he says.

CORRECTION: This story was updated on Jan. 29, 2018, to correct the formulas in the image caption for the two gold compounds.