Influence of Hemilabile Arm and Amide Functionality in the Ligand Backbone on Chemical and Electrochemical Dioxygen Reduction Catalyzed by Mononuclear Copper(II) Complexes
Abhishek Das, Srijan Narayan Chowdhury, Sachidulal Biswas, Rajib Samanta, Achintesh Narayan Biswas, Tapan Kanti Paine
Abstract
Four mononuclear copper(II) complexes, [(DPA-OH)Cu(CH 3 OH)(ClO 4 )](ClO 4 ) ( 1 ), [(DPA-OMe)Cu(CH 3 OH)(ClO 4 )](ClO 4 ) ( 2 ), [(6-Amide-DPA-OMe)Cu](ClO 4 ) 2 ( 3 ) and [(6-Amide 2 -DPA-OMe)Cu](ClO 4 ) 2 ( 4 ), of flexidentate ligands bearing hemilabile (hydroxy)methoxyethyl and/or amide group on DPA (di(2-picolyl)amine) backbone were isolated to explore their potency in catalyzing chemical and electrochemical oxygen reduction reaction (ORR). The role of a hemilabile arm, as well as amide functionality on the ligand backbone in affecting the rate-determining step (RDS) of the overall catalytic cycle has been explored and compared with that of the analogous [(tmpa)Cu](ClO 4 ) 2 (tmpa = tris(2-pyridylmethyl)amine) and [(PV-tmpa)Cu](ClO 4 ) 2 (PV-tmpa = bis(pyrid-2-ylmethyl){[6-(pivalamido)pyrid-2-yl]methyl}-amine) complexes. The hemilabile arm in these complexes results in an overall third-order rate, with k cat values ranging from 10 3 to 10 4 M –2 s –1 during dioxygen reduction catalysis. All the complexes except complex 4 selectively reduce dioxygen via the 4e – /4H + reduction pathway to water (H 2 O) using decamethylferrocene (Fc*) as a sacrificial reductant in acidic acetone at 298 K. In contrast, altering the reaction conditions from a chemical to an electrochemical ambiance in phosphate buffer displays a reverse order of ORR activity of the complexes while maintaining the same product selectivity as observed in chemical ORR catalysis in acetone.