An Azoaromatic Ligand as Four Electron Four Proton Reservoir: Catalytic Dehydrogenation of Alcohols by Its Zinc(II) Complex

Electroprotic storage materials, though invaluable in energy-related research, are scanty among non-natural compounds. Herein, we report a zinc­(II) complex of the ligand 2,6-bis­(phenylazo)­pyridine (L), which acts as a multiple electron and proton reservoir during catalytic dehydrogenation of alcohols to aldehydes/ketones. The redox-inactive metal ion Zn­(II) serves as an oxophilic Lewis acid, while the ligand behaves as efficient storage of electron and proton. Synthesis, X-ray structure, and spectral characterizations of the catalyst, ZnLCl₂ (<b>1a</b>) along with the two hydrogenated complexes of <b>1a</b>, ZnH₂LCl₂ (<b>1b</b>), and ZnH₄LCl₂ (<b>1c</b>) are reported. It has been argued that the reversible azo-hydrazo redox couple of <b>1a</b> controls aerobic dehydrogenation of alcohols. Hydrogenated complexes are hyper-reactive and quantitatively reduce O₂ and <i>para</i>-benzoquinone to H₂O₂ and <i>para</i>-hydroquinone, respectively. Plausible mechanistic pathways for alcohol oxidation are discussed based on controlled experiments, isotope labeling, and spectral analysis of intermediates