Effect of Metal Surfaces in On-Surface Glaser Coupling

The homocoupling of alkynes at metal surfaces, which was disclosed recently, is a promising reaction for efficient construction of conjugated nanostructures at metal surfaces. However, the role of the metal substrate as well as the mechanistic course of this process have not been investigated. The metal surface could act cooperatively (a) for two-dimensional confinement to properly orient the organic reactant and (b) also as an active mediator in the C-C bond-forming reaction. Herein we report covalent coupling of the dimers of 1,4-diethynylbenzene at various metal surfaces. The model reaction was investigated experimentally by STM and also by theoretical DFT calculations. Detailed statistical analysis and the theoretical results strongly support the involvement of the metal surface in the C-C bond-forming process. On the basis of these investigations, a model with two possible reaction pathways is suggested to describe the process: C-C coupling via direct CH activation and C-C coupling via alkynyl activation by pi-complex formation.