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-α, cis-β, 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.