Effect of Ligand Topology on the Reactivity of Chiral Tetradentate Aminopyridine Manganese Complexes

In recent years, bioinspired manganese and iron complexes bearing linear tetradentate aminopyridine (N4) ligands have emerged as highly efficient catalysts in asymmetric oxidation reactions using hydrogen peroxide. In this type of octahedral metal complex, cis-alpha, cis-beta, and trans topologies may be formed, and many studies have reported the reactivity and topology of iron(II) complexes. However, the ligand topology effects of analogous manganese(II) complexes are generally still poorly understood. Herein, we report a systematic study on the ligand topology structure and catalytic activity in the asymmetric epoxidation of olefin with manganese complexes bearing different aryl-substituted mcp ligands (mcp = N,N'-dimethyl-N,N'-bis(2-pyridinylmethyl)-cyclohexane-1,2-diamine). In particular, a class of quasitrans topology manganese complexes bearing 4-tert-Bu-phenyl-substituted mcp ligands are prepared and characterized by single-crystal X-ray analysis. The quasitrans topology manganese complexes undergo efficient olefin epoxidation with H2O2 as the oxidant in the presence of carboxylic acid or sulfuric acid. In addition, the mechanism of activation of H2O2 by these complexes is discussed based on density functional theory calculations.