Metal complexes involving indole rings: Structures and effects of metal–indole interactions

Indole is an electron-rich aromatic compound with characteristic properties and is widely distributed in natural products and in proteins as the important constituent of essential amino acid tryptophan. It is known to form a hydrophobic environment in proteins and to be involved in enzymatic reactions. In addition to the redox activities and various weak interactions, it shows versatile metal binding abilities through the nitrogen and carbon atoms. This review focuses on the properties of the indole ring in and around the coordination sphere and the structures and bonding modes of Cu(I), Cu(II), Pd(II), and Pt(II) complexes of indole-containing ligands. Reactivities of indole-containing Cu(I) complexes with dioxygen, indole-radical formation by oxidation of Pd(II) and Pt(II) complexes, and the effects of the proximal indole ring on the reactivity of the Cu(I) center are also reviewed.