Elucidating the Influence of Anchoring Geometry on the Reactivity of NO₂-Functionalized N-Heterocyclic Carbene Monolayers

The development of chemically addressable N-heterocyclic carbene (NHC) based self-assembled monolayers (SAMs) requires in-depth understanding of the influence of NHC's anchoring geometry on its chemical functionality. Herein, it is demonstrated that the chemical reactivity of surface-anchored NO₂-functionalized NHCs (NO₂-NHCs) can be tuned by modifying the distance between the functional group and the reactive surface, which is governed by the deposition technique. Liquid deposition of NO₂-NHCs on Pt(111) induced a SAM in which the NO₂-aryl groups were flat-lying on the surface. The high proximity between the NO₂ groups and the Pt surface led to high reactivity, and 85% of the NO₂ groups were reduced at room temperature. Lower reactivity was obtained with vapor-deposited NO₂-NHCs that assumed a preferred upright geometry. The separation between the NO₂ groups in the vapor-deposited NO₂-NHCs and the reactive surface circumvented their surface-induced reduction, which was facilitated only after exposure to harsher reducing conditions.