Catalysts that can be activated using mechanical force are reported in a study online in Nature Chemistry this week. The ability to initiate catalytic activity in this fashion could lead to new self-healing materials that respond to ultrasound rather than heat or light.
Rint Sijbesma and co-workers made compounds in which either a silver or a ruthenium metal ion is sandwiched between two ligands that each contain a long polymer chain. When these compounds are dissolved in solution and exposed to ultrasound, collapsing bubbles cause the polymer chains to stretch. The mechanical force generated by this movement splits each molecule into two separate fragments one that remains bound to the metal ion and one that is not by breaking one of the weak metal ligand bonds.
Importantly, neither of the parent compounds is catalytically active until it has been torn into two by ultrasound. In the case of the silver compounds, the metal-free fragment can catalyse transesterification reactions, a type of reaction used to synthesize biofuels. For the ruthenium-based compounds, it is the metal-containing ligand that is catalytically active, and this can mediate olefin metathesis reactions to make both small molecules and polymers, which are used in making various products such add-ins to enhance fuels.
In an accompanying News & Views article, Jitendra Rathore and Alshakim Nelson say that the work by Sijbesma and co-workers "offers a new method for catalyst activation and is likely to stimulate the creation of other new catalysts that provide catalytic activity on demand."