The first ever molecular catalyst specifically designed for mechanochemical reaction conditions enables highly efficient transformations at near room temperature.
Chemists at Hokkaido University and the Institute for Chemical Reaction Design and Discovery (WPI-ICReDD) have developed the first custom-designed high-performance catalyst optimized for solid-state synthesis and mechanochemistry. The team found that by attaching long polymer molecules to a metal catalyst, they could trap the catalyst in a liquid phase, enabling the efficient reaction at near room temperature. This approach, stated in Journal of the American Chemical Societyit can achieve cost and energy savings if it is adapted for wide application in chemical research and industry.
Synthetic chemical reactions usually take place in a solution, where dissolved molecules can mix and interact freely. But in recent years, chemists have developed a process called mechanochemical synthesis, in which solid-state crystals and powders are ground together. This approach is advantageous because it reduces the use of hazardous solvents and can allow reactions to proceed faster and at lower temperatures, saving energy costs. It can also be used for reactions between compounds that are difficult to dissolve in available solvents.
However, solid-state reactions occur in a completely different environment than solution-based reactions. Previous studies have found that palladium complex catalysts originally designed for use in solution often do not function adequately in solid-state chemical-mechanical reactions, and that high reaction temperatures are required. The use of an unmodified palladium catalyst for solid-state reactions resulted in limited efficiencies due to the tendency of palladium to aggregate in an inactive state. The team chose to embark on a new direction, catalyst design to get around the problem of mechanical assembly.
“We developed an innovative solution, which binds palladium through a specially designed phosphine ligand to a large polymer molecule called polyethylene glycol,” Ito explains.
Polyethylene glycol molecules form a region between solids that behaves as a liquid phase at the molecular level, where the Suzuki Miyura Chemical-Mechanical cross-coupling reactions proceed more efficiently and without the accumulation of the palladium problem. In addition to achieving a much higher yield of product, the reaction proceeded effectively near room temperature—a previously best-performing alternative required heating up to 120°C. Similar cross coupling reactions are widely used in research and the chemical industry.
“This is the first demonstration of a system specifically modified to harness the potential of palladium composite catalysts in the unique environment of a mechanochemical reaction,” says Kubota.
They believe it could be adapted to many other reactions, as well as to catalysts that use other transition metals in the periodic table.
Widespread adoption of this process, and others like it, could eventually lead to significant savings in cost and energy consumption in commercial chemical processes while allowing for more environmentally friendly large-scale production of many useful chemicals.
