N-Heterocyclic carbenes on copper: a dynamic and sensitive interplay between surface functionalization and etching

Within the expanding field of N-heterocyclic carbenes (NHCs) as surface ligands, this work investigates the etching of copper by (benz-)imidazol-2-ylidenes during surface functionalization. In order to control and distinguish between self-assembled monolayer (SAM) formation and NHC-mediated oxidative copper etching, experimental parameters were optimized. NHC design, concentration, solvent choice, temperature, visible light exposure, oxygen level, and deposition time dependencies were investigated, providing insight into the dynamic and sensitive interplay between functionalization and etching. Consistent with all experimental results, we present an etching mechanism driven by the formation of bis(NHC) Cu^I complexes. A previously unidentified, albeit frequently reported N 1s X-ray photoelectron spectroscopy (XPS) peak with a binding energy (BE) of 398.8(2) eV is cautiously interpreted as two NHCs bound to the same Cu⁰ adatom - the transient intermediate for metal etching. Its structure, its N 1s BE, and its high reduction potential (E _Cu⁰/Cu^I ) are supported by DFT. As this work sheds new light on N 1s XPS spectra interpretation and on NHC-mediated phase transition of copper, it helps to unify seemingly conflicting literature reports.