Supramolecular\nDimeric Mn<sup>III</sup> Complexes:\nSynthesis, Structure, Magnetic Properties, and Catalytic Oxidation\nStudies

In this study, a tetradentate Schiff-base ligand (H<sub>2</sub>L), synthesized by the condensation of ethylenediamine with\n2-hydroxy-3-methoxy-5-methylbenzaldehyde,\nwas reacted with either manganese salts or manganese salts in the\npresence of various pseudohalides in methanol. This reaction resulted\nin the formation of five mononuclear Mn<sup>III</sup> complexes: [Mn(L)(H<sub>2</sub>O)<sub>2</sub>](NO<sub>3</sub>)·1/2H<sub>2</sub>O·1/2CH<sub>3</sub>OH (<b>1</b>), [Mn(L)(H<sub>2</sub>O)<sub>2</sub>](ClO<sub>4</sub>)·H<sub>2</sub>O (<b>2</b>), [Mn(L)(N<sub>3</sub>)(H<sub>2</sub>O)]·1/3H<sub>2</sub>O (<b>3</b>), [Mn(L)(NCS)(H<sub>2</sub>O)] (<b>4</b>), and [Mn(L)(H<sub>2</sub>O)<sub>2</sub>](dca) (<b>5</b>) (where dca is dicyanamide ion). X-ray crystallography\nrevealed that the Mn<sup>III</sup> centers adopt a hexa-coordinate\npseudo-octahedral geometry, where the equatorial plane is constructed\nwith phenoxo oxygen and imine nitrogen atoms from the Schiff base\nligand, while the axial positions are occupied by water molecules\nor a combination of water and pseudohalides. Supramolecular interactions,\nprimarily π–π stacking and hydrogen bonding, contribute\nto the formation of pseudodimeric structures in the solid state. Magnetic\nsusceptibility measurements indicated antiferromagnetic coupling within\nquasi-dimers, primarily through hydrogen bonds. Catalytic studies\nshowed that the complexes effectively catalyze the aerobic oxidation\nof substrates such as 2-aminophenol and 3,5-di-<i>tert</i>-butylcatechol to yield 2-aminophenoxazin-3-one and 3,5-di-<i>tert</i>-butylquinone, respectively. They also catalyze the\noxidation of styrene to its corresponding oxirane, demonstrating their\nversatile catalytic proficiency. Mechanistic insights, supported by\nESI mass spectrometry and EPR studies, suggest that catalysis involves\nthe formation of a complex–substrate aggregate, followed by\nan intramolecular electron transfer.