Single-Electron Transfer in Palladium Complexes of 1,4-Naphthoquinone-Containing Bis(pyrazol-1-yl)methane Ligands

A 1,4-naphthoquinone-substituted bis(pyrazol-1-yl)methane ligand (N∧N) has been synthesized and transformed into its corresponding PdII chelate complex [(N∧N)PdCl2]. Both N∧N and [(N∧N)PdCl2] have been fully characterized by NMR spectroscopy, spectro-electrochemistry, and X-ray crystallography. After treatment of [(N∧N)PdCl2] with NEt3, the signature of a 1,4-naphthosemiquinonate radical is visible in the UV−vis- and electron paramagnetic resonance (EPR) spectrum of the reaction mixture; the free ligand N∧N does not react with NEt3 under the conditions applied. It is therefore concluded that NEt3 first reduces the PdII-ion of [(N∧N)PdCl2] to the zero-valent state and that this reaction is followed by a single-electron transfer from the metal atom to the 1,4-naphthoquinone moiety. The complex has been specifically designed to disfavor any direct Pd-to-naphthoquinone coordination. Electron transfer thus proceeds through space or, less likely, via σ-bonds of the ligand framework.