O-H bond oxidation by a monomeric Mn(III)-OMe complex

Area of Science:

• Inorganic Chemistry
• Bioinorganic Chemistry
• Reaction Mechanisms

Background:

• Manganese-containing mid-valent oxidants (Mn(III)-OR) are crucial for enzymatic proton-coupled electron-transfer (PCET) reactions.
• The Mn-dependent enzyme lipoxygenase utilizes a Mn(III)-OH unit for fatty acid peroxidation via PCET.

Purpose of the Study:

• To quantitatively generate and characterize the Mn(III)-OMe complex, [Mn(III)(OMe)(dpaq)](+).
• To investigate the O-H bond activation capabilities and reaction mechanism of [Mn(III)(OMe)(dpaq)](+).
• To compare the PCET reactivity of this mid-valent oxidant with other metal-oxo species.

Main Methods:

• Dioxygen activation of [Mn(II)(dpaq)](+) in methanol to generate [Mn(III)(OMe)(dpaq)](+).
• X-ray diffraction to determine the complex's structure.
• Kinetic studies including hydrogen/deuterium kinetic isotope effect (H/D KIE) measurements.
• Thermodynamic analysis of O-H bond activation (BDFEs).

Main Results:

• The stable [Mn(III)(OMe)(dpaq)](+) complex was successfully synthesized and structurally characterized.
• [Mn(III)(OMe)(dpaq)](+) activates phenolic O-H bonds (BDFEs < 79 kcal mol⁻¹) and TEMPOH.
• A H/D KIE of 1.8 suggests a concerted proton-electron transfer (CPET) mechanism for O-H bond oxidation.
• Compared to high-valent oxidants, [Mn(III)(OMe)(dpaq)](+) exhibits smaller H/D KIEs and weaker rate dependence on reaction driving force.

Conclusions:

• The study provides a stable Mn(III)-OMe complex capable of PCET reactions.
• The findings elucidate the CPET mechanism for O-H bond oxidation by mid-valent manganese oxidants.
• This work contributes to understanding the nuanced reactivity of mid-valent versus high-valent metal-oxo species in PCET processes.