An <i>o</i>-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 <i>o</i>-phenylene-bridged tetradented redox-noninnocent bis-azopyridyl ligand [<b>L</b>] and its copper complexes [<b>1</b>] and [<b>2</b>] were synthesized and characterized. The electron transfer events of [<b>L</b>], as well as [<b>1</b>] and [<b>2</b>], were characterized by single-crystal X-ray structure determination, various spectroscopic studies, and DFT calculations. While [<b>1</b>] [[<b>L</b>]Cu(II)Cl₂] has unreduced [<b>L</b>], [<b>2</b>], [[<b>L</b>]^•-Cu(I)Cl] contains a one-electron-reduced ligand [<b>L</b>]^•- , which is antiferromagnetically coupled with the unpaired spin on Cu(II). Reduction of [<b>2</b>] by one electron generated the complex [<b>2</b>]^•-, which remained in an electronically bistable situation in the form of valence tautomers: [<b>2A</b>]^•-, [[<b>L</b>]^•-Cu(I)Cl]^•- and [<b>2B</b>]^•-, [[<b>L</b>]^2-Cu(II)Cl]^•-. Further one-electron reduction of [<b>2</b>]^•- generated a mixture of Cu complexes: [<b>2A</b>]^-, [[<b>L</b>]^2-Cu(I)]^- and [<b>2B</b>]^-, [[<b>L</b>]^3•-Cu(II)]^-. Complexes [<b>1</b>] and [<b>2</b>] were examined for the catalytic oxidation of alcohols. The complex [<b>2</b>] was more efficient than [<b>1</b>]. The protocol was highly efficient and versatile with both primary as well as secondary aromatic and aliphatic alcohols. Mechanistic investigations showed that the complex [<b>2</b>] generated [[<b>L</b>]^•-Cu(I)OCH₂Ph]^•- (<b>A</b>) as the active catalyst, which subsequently, through its ligand-based redox events, acted as the catalyst over the course of the reaction.