Nonspectator Behavior of N-Heterocyclic Carbenes Directly Observed in High-Valent Copper Systems

Abstract

N-Heterocyclic carbenes (NHCs) have become highly popular ligands in modern copper chemistry, leading to the widespread application of Cu–NHCs in photoluminescent materials, catalysis, and biomedicine. In sharp contrast to copper(I) and copper(II) NHC complexes, examples of copper(III) NHCs are exceedingly rare. Synthetic methods for these complexes remain largely unexplored, and their reactivity is poorly understood. Additionally, although NHCs are typically considered spectator ligands, a longstanding debate persists regarding their potential nonspectator behavior at Cu(III), which is crucial for elucidating the true role of NHCs and for the rational design of Cu(III)/NHC systems. In this work, we present a rarely reported Cu(III) NHC system costabilized by CF₃ donors. By exploring synthetic strategies, photo- and thermo-induced reactivity studies, as well as experimental and computational investigations, this study enables, for the first time, the direct observation of NHC-involved reactivity at Cu(III), along with relevant mechanistic insights.