Theoretical Investigation on the Adsorption and Interface Bonding between N‑Heterocyclic Carbenes and Metal Surfaces

Compared with the well-established metal–thiolate interfaces, the study of the interface between N-heterocyclic carbenes (NHCs) and the metal substrate is much less explored, and the majority of work has been limited to the surfaces of Au, Ag, and Cu. The interface is closely related to the combination modes of NHCs on solid surfaces, which determines the morphologies of NHC-based self-assembly monolayers (SAMs). In this work, we performed theoretical investigations to take a fundamental look at how the methylated carbene (NHC^Me) ligands bind to the different solid surfaces (Au111, Ag111, Cu111, Ti0001, Pt111, Ru0001, and H-terminated Si111). On the clean surfaces without adatoms, the NHC^Me vertically binds with surface atoms by a single C–M bond. Compared with the experimentally characterized NHC–Au(111) interface, the stronger adsorption of NHC^Me on Pt (111) and Ti (0001) and the comparable binding strength on Ru (0001) indicate that they hold great promise to form highly strong and stable SAMs. Moreover, on the adatom surfaces, a dimer complex, NHC^Me–M_ad–NHC^Me, can be formed and is energetically more favored than the upright adsorption. Interestingly, the distance of adatoms from the surface, which is an intrinsic property of the transition metals, plays an important role in determining the molecular orientations of NHC^Me. The longer distance of coinage adatoms results in a nearly flat-lying “T-shaped” binding mode. By contrast, the shorter distance of adatoms on Pt (111) and Ru (0001) brings a winglike “V-shaped” configuration, which has thus far not been revealed in experiment. The results will invite the experimental design of stable NHC-based SAMs with attractive interface features beyond the coinage metals.