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

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.