Activating catalysts with mechanical force

Abstract

Homogeneously catalysed reactions can be 'switched on' by activating latent catalysts. Usually, activation is brought about by heat or an external chemical agent. However, activation of homogeneous catalysts with a mechanical trigger has not been demonstrated. Here, we introduce a general method to activate latent catalysts by mechanically breaking bonds between a metal and one of its ligands. We have found that silver(I) complexes of polymer-functionalized N-heterocyclic carbenes, which are latent organocatalysts, catalyse a transesterification reaction when exposed to ultrasound in solution. Furthermore, ultrasonic activation of a ruthenium biscarbene complex with appended polymer chains results in catalysis of olefin metathesis reactions. In each case, the catalytic activity results from ligand dissociation, brought about by transfer of mechanical forces from the polymeric substituents to the coordination bond. Mechanochemical catalyst activation has potential applications in transduction and amplification of mechanical signals, and mechanically initiated polymerizations hold promise as a novel repair mechanism in self-healing materials.

Cephalopod-inspired design of electro-mechano-chemically responsive elastomers for on-demand fluorescent patterning

“…In the context of mechanoresponsive polymers, these systems represent a first generation of mechanochemical devices, in which localized covalent chemical changes (here, the conversion of spiropyran to merocyanine) and their association function can be turned on and off repeatedly in response to an external signal remotely controlled. Numerous mechanophores with chemical functionality including catalysis 56,57 and small molecule release 33,35 have been reported, and mechanochemical coupling offers the potential to add chemical response to existing soft device functionality. To that end, the development of material platforms that can trigger covalent mechanochemistry while recovering their initial shape 33,35 motivates the need for new mechanophores that, like the materials, are reversibly and repeatably activated.…”

Plasma Formation during Acoustic Cavitation: Toward a New Paradigm for Sonochemistry

“…Ultrasonic chain scission of polymers in solutions under the effect of these forces is known for a long time [56]. Recently the ultrasonic activation of homogeneous latent catalysts has been reported [57,58]. It was shown that the mechanochemical scission of a metal-ligand bond triggered with low-frequency ultrasound may be used to release the innate catalytic activity of either the ligand or the metal.…”

Temperature Induced Solubility Transitions of Various Poly(2-oxazoline)s in Ethanol-Water Solvent Mixtures

“…A wide range of response factors, i.e., triggers, have been developed such as chemical or biological agents for sensors [9][10][11], mechanical forces [12,13], UV-vis or near-IR irradiation [14,15], magnetic and electrical fields [16,17] as well as temperature [18]. The response of the "smart" material to the external triggering event can be manifold too, ranging from surface energy switching [10,19], a shape change [20], variation in absorption or emission [12], or the material can undergo a phase transition [15,18].…”