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

Abstract

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 C₂ 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-CO₂ adducts. These CO₂-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 C₂ 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 H₂, while the C₂-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.