Ballbot-type motion of N-heterocyclic carbenes on gold surfaces

Abstract

Recently, N-heterocyclic carbenes (NHCs) were introduced as alternative anchors for surface modifications and so offered many attractive features, which might render them superior to thiol-based systems. However, little effort has been made to investigate the self-organization process of NHCs on surfaces, an important aspect for the formation of self-assembled monolayers (SAMs), which requires molecular mobility. Based on investigations with scanning tunnelling microscopy and first-principles calculations, we provide an understanding of the microscopic mechanism behind the high mobility observed for NHCs. These NHCs extract a gold atom from the surface, which leads to the formation of an NHC-gold adatom complex that displays a high surface mobility by a ballbot-type motion. Together with their high desorption barrier this enables the formation of ordered and strongly bound SAMs. In addition, this mechanism allows a complementary surface-assisted synthesis of dimeric and hitherto unknown trimeric NHC gold complexes on the surface.

N‐Heterocyclic Carbene Self‐assembled Monolayers on Copper and Gold: Dramatic Effect of Wingtip Groups on Binding, Orientation and Assembly

“…However, the 730 cm −1 peak was not suppressed; therefore, the spectrum reflects the coexistence of upright and flat‐lying species. Such behavior is not uncommon ‐a similar interpretation was proposed for the coverage‐dependence of benzoic acid adsorption on Cu(110) …”

“…Recently, N ‐heterocyclic carbenes (NHCs) have emerged as promising alternatives to thiols for the formation of robust, ordered SAMs on Au . Seminal work by Siemeling and Johnson showed that NHCs bind to planar Au surfaces.…”

“…These conditions would destroy typical thiol‐based SAMs . Other studies of NHC films on Au have shown remarkable effects on work function, and highly ordered structures can be imaged by low temperature scanning tunneling microscopy (STM) . In addition, the formation of strong C−Si bonds has recently been reported following NHC adsorption on Si(111) …”

“…Extraction of Cu atoms from steps and incorporation into molecular assemblies is a thermally activated process . Lifting of one atom from the (111) surface plane has precedent in the Au chemistry of isonitriles and thiols, with recent studies reporting similar effects for NHCs . Rodríguez‐Castillo et al.…”

N‐Heterocyclic Carbene‐Modified Au–Pd Alloy Nanoparticles and Their Application as Biomimetic and Heterogeneous Catalysts

“…The surface energy of Pd is higher compared to Au, so that the rearrangement would be thermodynamically favored . The trigger for the reconstruction process could be the binding of the NHC, which is known to form adatoms on Au surfaces . A similar behavior on the alloy NPs would lead to a pulling of Au atoms to the surface and would explain the Au enrichment determined by surface sensitive TXRF.…”