Persistent radical effect-mediated Cu–O bond homolysis: EPR spectroscopic evidence in NHC-copper phenolate systems

Abstract

The spectroscopic observation of transient, low-valent copper intermediates under ambient conditions remains a formidable challenge due to their inherent high reactivity. Here, we report the spectroscopic observation and dynamic stabilization of copper-radical species, achieved through the persistent radical effect (PRE) in a series of N-heterocyclic carbene copper(I) phenolates. Crucially, while these molecules exist as routine Cu(I) complexes with intact Cu–O bonds in the solid state, this homolytic behavior is activated upon dissolution in specific non-polar solvents. Electron paramagnetic resonance spectroscopy confirms the homolysis of Cu–O bonds, generating NHC-copper radicals and relatively persistent phenoxy radicals. The coexistence and interaction of these radicals were unambiguously evidenced by hyperfine splitting analysis and the detection of characteristic half-field signals, indicative of spin-coupled diradicals. The stability of these radical pairs is shown to depend critically on the matched reactivity between the copper-centered and organic radicals. This work establishes a PRE-driven strategy for characterizing and mitigating highly reactive copper-radical intermediates without the need for rigid ligand frameworks, providing new insights into copper radical chemistry and offering a practical approach for taming transient metal centers.