ToF-SIMS Analysis of NHC-Modified Naturally Oxidized Cu and Au Surfaces Providing Evidence for Covalent Bonding

N-heterocyclic carbenes (NHCs) are surface functionalization agents that have been shown to bind to the surface of various metals via a strong covalent bond. Among various NHC subtypes, 1,3-diisopropyl-1H-benzoimidazolylidene (iPrNHC) has been extensively studied. Herein, we describe the use of time-of-flight secondary ion mass spectrometry (ToF-SIMS) to probe the interactions between iPrNHC and Cu and Au surfaces. This study provides unambiguous ToF-SIMS evidence of covalent bonding between the iPrNHC, deposited in solution, and the naturally oxidized copper surface by the detection of iPrNHC-Cu (C13H18N2Cu+), including the corresponding isotope pattern. Depending on the treatment after deposition, covalently bound mono- or biscarbene-Cu complexes were observed. The biscarbene-Cu complexes were readily removed by solvent rinsing, whereas the monocarbene-Cu complex was uniformly distributed in a surface-bound layer. A carbene layer that was vapor-deposited on Au was also examined by ToF-SIMS, which confirmed the presence of biscarbene-Au complexes (C26H36N4Au+) and much weaker monocarbene-Au complexes (C13H18N2Au+), corroborating results obtained by other analytical techniques. This study provides a robust methodological approach for analyzing interfacial bonds, NHC-metal complexes, and their distribution on metal surfaces using ToF-SIMS.