High-Density Self-Assembled Monolayers of NHCs on Au(111) – Observation of Normal and Mesoionic Bonding Modes

N-heterocyclic carbenes (NHCs) continue to attract interest as novel ligands for self–assembled monolayers on metals due to their propensity to form strong bonds to a variety of metals. However, understanding of the effect of NHC structural elements on adsorption geometry, NHC mobility, and on-surface intermolecular interactions remains limited. Herein we report the self-assembly of 1,3-diisopropyl imidazolylidene I-NHCiPr on Au(111) surfaces. This relatively simple NHC has never previously been examined on Au surfaces experimentally. We combine scanning tunnelling microscopy (STM) and density functional theory (DFT) calculations to provide detailed insight into this unique NHC-based SAM, which is the densest packed NHC-based SAM reported to date, utilizing approximately 14.5% of the available binding sites of the Au(111) surface. STM interrogations of the surface show the occasional formation of brighter adsorbates that we suggest can be attributed to metalation at the backbone C-4, rather than the expected C-2 site. This type of "abnormal" or mesoionic bonding mode is commonly observed in related organometallic compounds, suggesting that such complexes can provide a useful analogy for surface reactivity. The high density of the I-NHCiPr SAM and the generally high thermal and chemical robustness of NHC-based SAMs make this molecule ideally suited for a range of applications, providing an exciting platform for more complex surface functionalization.