Synthesis, Structural and Physicochemical Properties of a Series of Manganese(II) Complexes with a Novel N5 Tripodal-Amidate Ligand and Their Potential Use as Water Oxidation Catalysts

Abstract

Water oxidation plays a crucial role in both natural and artificial photosynthesis, which is an attractive solution to the depletion of fossil fuels as energy sources due to the increasing consumption. Thus, the search for oxygen evolution reaction catalysts is a hot topic of research. Reaction of the N5-tripodal amidate ligand, N-{2-[(bis(pyridine-2-ylmethyl)amino)methyl]phenyl}picolinamide (Htrip) with MIIX2(X=Cl-, Br-, I-), in anhydrous ethyl alcohol, and C2H5ONa yields the complexes [MnII(trip)Cl] (1), [MnII(trip)Br] (2), and [MnII(trip)I] (3). Single crystal X-ray structure analysis of 1-3 revealed that the manganese(II) atom in the three manganese compounds occupies the center of a distorted octahedral coordination sphere consisting of two pyridine, one picoline and one amino nitrogen atoms on the equatorial plane, while the axial positions are occupied by one amido nitrogen atom and the halogen anion. The three manganese(II) complexes 1-3 constitute the first examples of mononuclear {MnII(N5trip)X} species to be reported. Magnetic susceptibility measurements showed that these complexes are high-spin d5 systems. Cyclic voltametric study of 1-3 revealed an unexpected two electron, electrochemically reversible redox process, assigned to the oxidation of MnII to MnIV. The electrochemical properties for oxygen evolution reaction of complexes 1-3 showed that the oxidized trip- ligand is responsible for the electrocatalytic oxidation of water to dioxygen.