Supramolecular Dimeric Mn Complexes: Synthesis, Structure, Magnetic Properties, and Catalytic Oxidation Studies.

In this study, a tetradentate Schiff-base ligand (HL), synthesized by the condensation of ethylenediamine with 2-hydroxy-3-methoxy-5-methylbenzaldehyde, was reacted with either manganese salts or manganese salts in the presence of various pseudohalides in methanol. This reaction resulted in the formation of five mononuclear Mn complexes: Mn(L)(HO)·1/2HO·1/2CHOH (), Mn(L)(HO)·HO (), [Mn(L)(N)(HO)]·1/3HO (), [Mn(L)(NCS)(HO)] (), and Mn(L)(HO) () (where dca is dicyanamide ion). X-ray crystallography revealed that the Mn centers adopt a hexa-coordinate pseudo-octahedral geometry, where the equatorial plane is constructed with phenoxo oxygen and imine nitrogen atoms from the Schiff base ligand, while the axial positions are occupied by water molecules or a combination of water and pseudohalides. Supramolecular interactions, primarily π-π stacking and hydrogen bonding, contribute to the formation of pseudodimeric structures in the solid state. Magnetic susceptibility measurements indicated antiferromagnetic coupling within quasi-dimers, primarily through hydrogen bonds. Catalytic studies showed that the complexes effectively catalyze the aerobic oxidation of substrates such as 2-aminophenol and 3,5-di--butylcatechol to yield 2-aminophenoxazin-3-one and 3,5-di--butylquinone, respectively. They also catalyze the oxidation of styrene to its corresponding oxirane, demonstrating their versatile catalytic proficiency. Mechanistic insights, supported by ESI mass spectrometry and EPR studies, suggest that catalysis involves the formation of a complex-substrate aggregate, followed by an intramolecular electron transfer.