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
Shivali Hans, kamal kamal, Ambika Devi, Mohd Adham, Muskan Muskan, Sheetal Ranaut, Avtar Changotra, Shivnath Mazumder, Subhas Samanta
Abstract
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.