Mechanochemical and thermal cleavage of polymer linked copper (I)-biscarbene complexes

Abstract

The force-dependent activation of a latent mechanocatalyst based on polymeric Cu(I)-biscarbene complexes is demonstrated in solution by applied ultrasound, underscoring a mechanochemical activation pathway via an external acoustic field. Systematic experiments via ultrasound mediated activation of the Cu(I)-complex prove a chain length dependent cleavage, favored when longer polymer chains (Mn = 4750; 8900; 17200 g mol−1) are attached to the Cu(I)-biscarbene-complex, displaying an subsequent reaction/deactivation pathway with increased ultrasound energy. A different decomposition pathway is observed via purely thermal activation, based on a direct scission of the polymeric chain from the N-heterocyclic carbene, thus prohibiting the formation of the desired catalytically active species. Quantum chemical calculations together with experimental investigations support that splitting one carbene residue from a biscarbene-Cu(I)-center is favored mechanochemically at a force of around 900 pN, in turn lowering the activation energy significantly in comparison to the purely thermal activation pathway.