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

Reversible O 2 -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 O 2 to form - :2 -peroxo-dicopper(II) complexes stable at room temperature, which release O 2 to attain reversible O 2 -binding. When a sterically hindered hexapyridine ligand methylated at the bridgehead positions is used, the O 2 -release becomes easier, and reversibility is greatly improved. Detailed structural studies of Cu I and Cu II 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 H 2 O 2 . A peroxo-diiron(III) complex is detected as an intermediate. Detailed isotopelabeling experiments showed that the peroxo intermediate is converted to dioxo--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 O 2 -binding and activation by the dicopper and diiron complexes.

Metal-catalyzed Hydrogen Peroxide Oxidation for Synthesis of Value-added Chemicals

“…From these results, we tentatively postulated that [Fe III (pic)(Me-pic)(Me-picH)OOH] was generated from 1 and H2O2, followed by abstraction of water to gener- 15) . These results indicate that a suitable combination of picolinate derivatives can control the reactivity of the iron(V) active species.…”

Practical Iron‐Catalyzed Hydrogen Peroxide Epoxidation of Aromatic Olefins using a Combination of Two Kinds of Simple Picolinate Ligands under Halide‐Free Reaction Conditions

“…MeCN plays a critical role in stabilizing the iron(III) and iron(V) species during the reaction (see the Supporting Information, Table S1). The 6‐methylpyridyl moiety of Me‐pic has steric repulsion between the iron ion and methyl group, accelerating the formation of [Fe(V)(O)(pic)(Me‐pic)(OOCC 5 H 3 N‐Me)] and tuning the spin states of the iron center to afford active iron(V) species that proceed with the efficient epoxidation 19a,22. These results indicate that a suitable combination of picolinate derivatives can control the reactivity of the active iron(V) species.…”

Versatile Coordination Mode of a New Pyridine-Based Ditopic Ligand with Transition Metals: From Regular Pyridine to Alkyne and Alkenyl Bindings and Indolizinium Formation

“…The light brown precipitate was washed 2−3 times with pentane and diethyl ether and was recrystallized from dichloromethane/hexane mixture to get white crystals of compound BPMPB with an 18.5% yield. 1 A yellow liquid was eluted as the second component from a column with a diethyl ether/acetone (1:1) mixture, to yield an oily compound which was identified as compound A of Figure 1 and characterized by comparison of its 1 H and 13 Preparation of the BPMPBM 2 Cl 4 Complexes. All the complexes were prepared by following a general procedure.…”

“…Within the family of pyridyl-containing ligands, modified compounds providing various coordination modes, or ligand field modulation, for instance, are still under current investigation . These include the family of bispidine, − N3-Py-R or N4-Py ligands, or hexapyridine dinucleating ligands, − for example. Considering capping ligands, we were interested in studying simple tripodal ligands in which the amine nitrogen of the tetraamine tripod would be replaced by a carbon atom as is the case for some tripyridine ligands, having a high flexibility at the coordination site.…”

“…These include the family of bispidine, − N3-Py-R or N4-Py ligands, or hexapyridine dinucleating ligands, − for example. Considering capping ligands, we were interested in studying simple tripodal ligands in which the amine nitrogen of the tetraamine tripod would be replaced by a carbon atom as is the case for some tripyridine ligands, having a high flexibility at the coordination site. In these systems, a pure pyridine-based tripod would be linked through the central carbon atom to a fourth arm, with this latter being potentially functionalized, and able to subsequently react with various substrates, or reactive groups.…”