A Survey on The Self-Assembly of N-Heterocyclic Carbenes: The Influence of Noble Metal Surfaces

Abstract

Monolayers formed by N-heterocyclic carbenes on the (111) faces of noble metal surfaces are characterized by means of scanning tunneling microscopy. Monolayer films were prepared through in vacuo deposition. A wide range of systems were surveyed, primarily concerned on combinations of carbenes with methyl, ethyl and isopropyl wingtip groups and an imidaloze or benzimidazole backbone structure on the Au(111), Ag(111) and Cu(111) surfaces. Scanning tunneling microscope measurements were used to indirectly determine the adsorption modes. The wingtip groups were found to largely determine the adsorption mode, in agreement with previous studies. Small wingtip groups form bis-M(NHC)2 complexes that lie nearly parallel to the surface, whereas the bulkier isopropyl wingtip carbenes tend to adsorb upright, atop metal adatoms. In both cases, carbenes on the surface demonstrate their ability to self-assemble, resulting in a number of novel self-assembled monolayers. Self-assembly patterns in complex based surface lattices appeared unaffected by changes in the surface's crystal structure. Adatom-bound assembly patterns appear sensitive to such changes. Furthermore, not all systems studied readily self-assemble. This is rationalized through discussion into the underlying mechanisms governing the carbene's behavior on the surface, including intracomplex interactions and adatom extraction processes. Results indicate the substrate's electronic structures play a role in both.