Reversible O2-Binding and Activation with Dicopper and Diiron Complexes Stabilized by Various Hexapyridine Ligands. Stability, Modulation, and Flexibility of the Dinuclear Structure as Key Aspects for the Dimetal/O2 Chemistry

Abstract

Abstract Reversible O2-binding and activation have been studied using dicopper and diiron complexes of various hexapyridine ligands as functional models of hemocyanin (Hc) and soluble methane monooxygenase (sMMO), respectively. Dicopper(I) complexes of sterically hindered hexapyridine ligands react with O2 to form μ-η2:η2-peroxo–dicopper(II) complexes stable at room temperature, which release O2 to attain reversible O2-binding. When a sterically hindered hexapyridine ligand methylated at the bridgehead positions is used, the O2-release becomes easier, and reversibility is greatly improved. Detailed structural studies of CuI and CuII complexes of sterically hindered tripyridine ligands showed that the bridgehead alkyl group causes a pyridine shift leading to structural modulation of the copper complexes. A hexapyridine ligand can be used to form a thermally stable peroxo–diiron(III) complex, which can oxygenate hydrocarbons upon addition of acid chloride/DMF. Diiron(III) complexes of a bis-tpa type hexapyridine ligand catalyze epoxidation of various alkenes with H2O2. A peroxo–diiron(III) complex is detected as an intermediate. Detailed isotope-labeling experiments showed that the peroxo intermediate is converted to dioxo–μ-oxo–diiron(IV) complex, and that three oxygen atoms of the active species scramble with each other. Stable dinuclear structure, structural modulation, and structural flexibility, which may be from the hexapyridine ligands, play key roles in reversible O2-binding and activation by the dicopper and diiron complexes.