N′ -(4-Bromo-2-hydroxybenzylidene)-3-hydroxy-4-methoxybenzohydrazide and its novel hexanuclear manganese(III) metallamacrocyclic complex: synthesis, crystal structures and antimicrobial activity

A novel polydentate aroylhydrazone ligand, N′-(4-bromo-2-hydroxybenzylidene)-3-hydroxy-4-methoxybenzohydrazide (H2L), was successfully synthesized via a condensation reaction and characterized by IR, UV–Vis, and 1H NMR spectroscopic methods. Subsequent reaction of H2L with manganese bromide in methanol produced an unprecedented hexanuclear manganese(III) metallamacrocyclic complex, [Mn6L6(OH2)6]·12H2O. Both the free ligand and the complex were fully characterized by IR and UV–Vis spectroscopy, and their precise molecular and crystal packing structures were elucidated through single-crystal X-ray diffraction analysis. In the coordination assembly, each deprotonated aroylhydrazone ligand acts as a pentadentate bridging unit through phenolate oxygen, imine nitrogen, enolate oxygen, ether oxygen and methoxy oxygen donor atoms, linking adjacent Mn(III) ions to construct a large cyclic metallamacrocyclic skeleton. The complex exhibits remarkably improved antimicrobial activity compared with H2L: the complex has strong activity against S. aureus (MIC = 4.7 ± 0.2 μM) and B. subtilis (MIC = 9.4 ± 0.4 μM), and moderate activity against E. coli (MIC = 4.7 ± 0.2 μM) and mild suppression toward P. fluorescens (MIC = 4.7 ± 0.2 μM).