Manganese Complexes of Pyrrole- and Indolecarboxamide Ligands: Synthesis, Structure, Electrochemistry, and Applications in Oxidative and Lewis-Acid-Assisted Catalysis

Abstract

This work shows the synthesis and structural, spectroscopic, and electrochemical properties of MnIII complexes supported with pyrrole- and indolecarboxamide ligands. In all cases, the respective ligand constitutes a N4 coordination sphere about the MnIII ion. The MnIII complexes of pyrrolecarboxamide ligands are square pyramidal with a fifth Cl atom, whereas analogous complexes of indolecarboxamide ligands are essentially square planar. Electrochemical studies reveal highly negative MnIII/II and moderately positive MnIV/III redox potentials. In situ generated Mn4+ species of the pyrrolecarboxamide ligands were characterized by absorption and electron paramagnetic resonance spectroscopy. All complexes functioned as catalysts in olefin epoxidation reactions by using PhIO as the oxo-transfer agent. All complexes also acted as Lewis acid catalysts for ring-opening reactions of assorted epoxides with various nucleophiles. We also show a one-pot, two-step epoxidation reaction followed by ring opening, which illustrates the catalytic significance of the present complexes. Importantly, MnIII complexes carrying electron-donating substituents on the ligand were found to be better catalysts.