An o-Phenylene Bridged Noninnocent Bis-Azopyridyl Ligand and Its Copper Complexes: Synthesis, Characterization of Electron Transfer Events, and Use of the Cu Complexes for Oxidation of Alcohols

In this report, an o -phenylene-bridged tetradented redox-noninnocent bis-azopyridyl ligand L and its copper complexes 1 and 2 were synthesized and characterized. The electron transfer events of L , as well as 1 and 2 , were characterized by single-crystal X-ray structure determination, various spectroscopic studies, and DFT calculations. While 1 [ L Cu(II)Cl 2 ] has unreduced L , 2 , [ L •– Cu(I)Cl] contains a one-electron-reduced ligand L •–, which is antiferromagnetically coupled with the unpaired spin on Cu(II). Reduction of 2 by one electron generated the complex 2 •–, which remained in an electronically bistable situation in the form of valence tautomers: 2A •–, [ L •– Cu(I)Cl] •– and 2B •–, [ L 2– Cu(II)Cl] •– . Further one-electron reduction of 2 •– generated a mixture of Cu complexes: 2A −, [ L 2– Cu(I)] − and 2B −, [ L 3•– Cu(II)] − . Complexes 1 and 2 were examined for the catalytic oxidation of alcohols. The complex 2 was more efficient than 1 . The protocol was highly efficient and versatile with both primary as well as secondary aromatic and aliphatic alcohols. Mechanistic investigations showed that the complex 2 generated [ L •– Cu(I)OCH 2 Ph] •– ( A ) as the active catalyst, which subsequently, through its ligand-based redox events, acted as the catalyst over the course of the reaction.