New benzothiazole based copper(II) hydrazone Schiff base complexes for selective and environmentally friendly oxidation of benzylic alcohols: The importance of the bimetallic species tuned by the choice of the counterion

摘要

Green and sustainable chemistry approaches necessitate an ongoing investigation towards new, environmentally benign and selective catalysts. We have thus prepared a new benzothiazole-scaffolded hydrazone Schiff base ligand L and coordinated it with copper(II) ions leading to five different complexes, the form of which in the solid state is counterion-dependent. Nitrate, chloride and trifiate anions lead to monometallic species, whereas utilization of sulfates and tetrafluoroborates is responsible for the formation of bimetallic assemblies. The catalytic efficiency of synthesized compounds was shown in Cu-TEMPO (2,2,6,6-tetramethyl-l-piperidinoxyl) aerobic oxidation of activated alcohols resulting in good to excellent conversions and 100% selectivity to form the corresponding aldehydes. Considering the experimental results and Density Functional Theory calculations we propose two most plausible reaction mechanisms that further corroborate the enhanced activity of bimetallic species. It appears that the tridentate character of the ligand sacrifices some of the overall catalytic efficiency for the selectivity of the process. Aqueous reaction medium, low catalyst loading, air as oxidant and exclusive oxidative selectivity render these Cu ll complexes promising candidates for further improvement. (C) 2020 Elsevier B.V. All rights reserved.