Adatom mediated adsorption of N‐heterocyclic carbenes on Cu(111) and Au(111)

Abstract

The adsorption of N-heterocyclic carbenes (NHCs) on Cu(111) and Au(111) surfaces is studied with density-functional theory. The role of the molecular side groups as well as the surface morphology in determining the adsorption geometry are explored in detail.Flat-laying NHCs, as observed experimentally for NHC with relatively small side groups, result from the adsorption at adatoms and give rise to the so-called ballbot configurations, which are more stable than adsorption on flat surfaces and provide an efficient precursor for the formation of bis(NHC) dimers. On Au(111), the resulting (NHC) 2 Au complexes are purely physisorbed and thus mobile. On the more reactive Cu(111), in contrast, the central Cu atom in the (NHC) 2 Cu dimer is still covalently bound to the surface, resulting in a mobility, which has to be thermally activated.

Thermopower of Molecular Junction in Harsh Thermal Environments

“…To avoid the instability issue of the thiol anchor, a handful of studies have recently demonstrated the potential of new functional groups for thiol-free anchors in constructing robust SAMs and electronic devices with them. , Among others, N-heterocyclic carbene (NHC) is an attractive candidate as an anchor for generating chemically resistant and thermally stable SAMs on coinage metal substrates. − The NHC is a strong σ-donor with weak π-bonding over a metal, and thus the NHC–metal bond is usually represented by a single bond . The covalent bond between carbenic C atom in NHC and metal is stronger than the analogous bond with S atom of thiol. − ,− The homolytic Au–C bond dissociation energy of NHC anchor is known to be 67 kcal/mol, which is ∼20 kcal/mol higher than the typical Au–S bond (45 kcal/mol). , The Au–C bond length is 2.0 Å, which is shorter than that of Au–S (2.2–2.6 Å). , The electrical property of NHC-based SAMs has been recently reported. , Tunneling decay coefficient for oligophenylene SAMs with benzimidazole-based NHC anchor is similar to the analogous SAMs with thiol anchor; current density at |0.5 V| for the NHC SAM is lower by ∼10 4 than that for the SAM of alkanethiolate of similar length . The objective of this work is to evaluate the Seebeck effect in NHC-based molecular junctions in the context of consistency and reliability under harsh thermal conditions.…”

Highly Ordered N-Heterocyclic Carbene Monolayers on Cu(111)

“…As summarized in Scheme 1 , top, regardless of their backbone structures, NHCs with primary substituents including Me, Et, and Bu give ordered overlayers composed of flat-lying M(NHC) 2 species resulting from the abstraction of a metal atom (M) from the surface. 6 , 13 , 25 − 28 …”

“…The tendency of NHC DBZ to adsorb on Cu(111) with the benzimidazole moiety perpendicular to the surface contrasts to the behavior of N,N- dialkylbenzimidazolylidenes (R = Me, Et) and the simpler N,N -dialkylimidazolylidenes (R = Me, Et, Bu), all of which form flat-lying M(NHC) 2 complexes ( Scheme 1 , top). 6 , 13 , 25 − 28 We compute that surface-bound N,N- dimethylbenzimidazolylidene is 0.45 eV more stable with the benzimidazole moiety parallel rather than perpendicular to the surface ( SI5 ). In the case of NHC DBZ , the energy cost associated with the benzimidazole moiety adopting a perpendicular geometry is more than compensated by the interaction of two aromatic N-substituents with the copper surface considering that one molecule of benzene, in an optimized geometry, has an adsorption energy of ∼0.8 eV on Cu(111).…”

“…N-heterocyclic carbenes (NHCs) are an emerging class of ligands for functionalizing extended metal surfaces, nanoparticles and nanoclusters. − The ability of NHCs to produce self-assembled monolayers on a range of metallic, non-metallic, and metalloid substrates has attracted considerable interest with potential applications in multiple fields including catalysis, microelectronics, biosensing, surface protection, and 3D MOF architectures. ,,,,,− In such applications, it is critical to control the orientation and packing of NHC monolayers to fine-tune surface density, metal accessibility, and ligand orientation. Although the factors dictating binding orientation and self-assembly are not fully elucidated, several studies have pointed toward the important effect of exocyclic nitrogen substituents (wingtips). ,,,− Sterically congested N-substituents, such as iPr, tBu, Mes, and Dipp (wingtip groups), allow access to geometries whereby the heterocycle is perpendicular to the surface. As summarized in Scheme , top, regardless of their backbone structures, NHCs with primary substituents including Me, Et, and Bu give ordered overlayers composed of flat-lying M­(NHC) 2 species resulting from the abstraction of a metal atom (M) from the surface. ,,− …”

“…Although the factors dictating binding orientation and self-assembly are not fully elucidated, several studies have pointed toward the important effect of exocyclic nitrogen substituents (wingtips). ,,,− Sterically congested N-substituents, such as iPr, tBu, Mes, and Dipp (wingtip groups), allow access to geometries whereby the heterocycle is perpendicular to the surface. As summarized in Scheme , top, regardless of their backbone structures, NHCs with primary substituents including Me, Et, and Bu give ordered overlayers composed of flat-lying M­(NHC) 2 species resulting from the abstraction of a metal atom (M) from the surface. ,,− …”