Reactivity variance between stereoisomers of saturated N-heterocyclic carbenes on gold surfaces

Controlling the chirality of molecule-surface systems is essential for applications ranging from heterogeneous catalysis to biosensing. N-heterocyclic carbenes (NHCs) are quickly becoming a dominant ligand for noble metal surface passivation, but the potential of chiral NHC scaffolds remains untapped. In this work, two stereoisomers, one C 2 symmetric and one C s symmetric, of saturated N-heterocyclic carbenes (NHCs) and a structurally related unsaturated NHC ( C 2v symmetric) were synthesized as NHC-CO 2 adducts. These CO 2 -protected NHCs were deposited on gold films and their presence on the films was confirmed by laser desorption ionization mass spectrometry (LDI-MS) and surface enhanced Raman spectroscopy (SERS). Surprisingly, the C s NHC, but not the chiral C 2 NHC, partially degrades to the independently synthesized unsaturated NHC upon binding to the gold. Theoretical calculations assist in explaining this phenomenon by showing that the NHCs primarily lie flat on the gold surfaces, which exposes the backbone protons on the C s -symmetric NHC to a formal elimination of H 2 , while the C 2 -symmetric NHC remains protected from this elimination reaction. These results raise critical questions as to how the structure of NHC ligands may be tuned to influence binding and reactivity on gold surfaces. Two stereoisomers, one C 2 symmetric and one C s symmetric, of saturated N-heterocyclic carbenes (NHCs) were placed on gold films and they demonstrate different reactivity.