Reversible Single-Electron-Transfer to Oxygen in a Stable N-Heterocyclic Carbene Palladium(I) Metalloradical.

The chemical and electrochemical one-electron oxidation of [Pd(IPr) 2 ] ( 1 ) leads to the formation of mononuclear palladium(I) complex [Pd(IPr) 2 ][PF 6 ] ( 2 ). This bench-stable metalloradical has been fully characterized and its structure confirmed by X-ray diffraction analysis. EPR and DFT studies confirm the localization of the unpaired electron onto the metal center. Low temperature NMR and EPR measurements reveal the ability of complex 2 to reversibly coordinate and reduce the dioxygen molecule, leading to the formation of a three-coordinate complex, [Pd II (IPr) 2 (η 1 -O 2 )] + ( 4 ), in which the unpaired electron has been transferred to the superoxido ligand.