Self-assembly of N-heterocyclic carbenes on Au(111)

Abstract

Although the self-assembly of organic ligands on gold has been dominated by sulfur-based ligands for decades, a new ligand class, N-heterocyclic carbenes (NHCs), has appeared as an interesting alternative. However, fundamental questions surrounding self-assembly of this new ligand remain unanswered. Herein, we describe the effect of NHC structure, surface coverage, and substrate temperature on mobility, thermal stability, NHC surface geometry, and self-assembly. Analysis of NHC adsorption and self-assembly by scanning tunneling microscopy and density functional theory have revealed the importance of NHC-surface interactions and attractive NHC-NHC interactions on NHC monolayer structures. A remarkable way these interactions manifest is the need for a threshold NHC surface coverage to produce upright, adatom-mediated adsorption motifs with low surface diffusion. NHC wingtip structure is also critical, with primary substituents leading to the formation of flat-lying NHC2Au complexes, which have high mobility when isolated, but self-assemble into stable ordered lattices at higher surface concentrations. These and other studies of NHC surface chemistry will be crucial for the success of these next-generation monolayers.

Reversible Self‐Assembly of an N‐Heterocyclic Carbene on Metal Surfaces

“…Besides, fluctional motion or rotation at 78 K could cause the structureless features as well. To determine the detail molecular isomers in the trimer/dimer phases and expose their underlying influence in self‐assembly transitions, [13] we have adopted DFT calculations to prove that conformational switching is the direct reason.…”

“…Since the seminal report by Arduengo et al., [1] N‐heterocyclic carbenes (NHCs) have fostered considerable interest in catalysis, [2–4] as ligands for p‐block elements [5] and transition metals, [6] thanks to their unique properties [7, 8] . Owing to the strong σ‐donating ability and the high tunability of the side‐groups, manifold applications dealing with the functionalization of surfaces [9–13] and nanoparticles [14–21] have been reported [22–24] . The even stronger propensity towards σ‐donation of cyclic (alkyl)(amino) carbenes (CAAC) [25, 26] has been highlighted both in catalytic applications [27] and in pioneering on‐surface reactivity studies by Johnson and co‐workers, [28] but little is known about the fundamental behavior of such entities on metal surfaces [29] .…”

N-Heterocyclic Carbene Ligand Stability on Gold Nanoparticles in Biological Media

“…This change may be due to a difference in the binding of ligands to the substrate surface between roughened solid Au and AuNPs, which could affect the average configuration of isopropyl side groups. 29 …”

Reversible Selbstorganisation eines N‐heterocyclischen Carbens auf Metalloberflächen

“…haben N‐heterocyclische Carbene (NHC) dank ihrer einzigartigen Eigenschaften [2, 3] als Liganden für p‐Block‐Elemente [4] und Übergangsmetalle [5] erhebliches Interesse in der Katalyse geweckt [6–8] . Aufgrund der starken σ‐Donor Fähigkeit und der flexiblen Ausgestaltung der Seitengruppen wurde über vielfältige Anwendungen im Bereich der Funktionalisierung von Oberflächen [9–13] und Nanopartikeln [14–21] berichtet [22–24] . Die noch stärkere Neigung zur σ‐Donation cyclischer (Alkyl)(Amino)‐carbene (CAAC) [25, 26] wurde sowohl in katalytischen Anwendungen [27] als auch in innovativen Studien zur Reaktivität auf Oberflächen von Johnson und Mitarbeitern [28] hervorgehoben, doch ist über das grundlegende Verhalten solcher Einheiten auf Metalloberflächen wenig bekannt [29] .…”

“…Um die molekularen Isomere in den Trimer‐/Dimer‐Phasen im Detail zu bestimmen und ihren Einfluss auf die Übergänge in der Selbstorganisation aufzudecken, [13] haben wir DFT‐Berechnungen durchgeführt, um zu beweisen, dass Konformationswechsel der direkte Grund sind. Aufgrund der Konformationsisomerie des Cyclohexyl‐Fragments werden nur die äquatorialen und axialen Sesselkonformere entsprechend der Orientierung des Cyclohexyls in Bezug auf die Dipp‐Ebene erwartet.…”