O-H bond oxidation by a monomeric MnIII-OMe complex

Manganese-containing, mid-valent oxidants (Mn-III-OR) that mediate proton-coupled electron-transfer (PCET) reactions are central to a variety of crucial enzymatic processes. The Mn-dependent enzyme lipoxygenase is such an example, where a Mn-III-OH unit activates fatty acid substrates for peroxidation by an initial PCET. This present work describes the quantitative generation of the Mn-III-OMe complex, [Mn-III(OMe)(dpaq)](+) (dpaq = 2-[bis(pyridin-2-ylmethyl)] amino-N-quinolin-8-yl-acetamidate) via dioxygen activation by [Mn-II(dpaq)](+) in methanol at 25 degrees C. The X-ray diffraction structure of [Mn-III(OMe)(dpaq)](+) exhibits a Mn-OMe group, with a Mn-O distance of 1.825(4), that is trans to the amide functionality of the dpaq ligand. The [Mn-III(OMe)(dpaq)](+) complex is quite stable in solution, with a half-life of 26 days in MeCN at 25 degrees C. [Mn-III(OMe)(dpaq)](+) can activate phenolic O-H bonds with bond dissociation free energies (BDFEs) of less than 79 kcal mol-1 and reacts with the weak O-H bond of TEMPOH (TEMPOH = 2,2'6,6'- tetramethylpiperidine-1-ol) with a hydrogen/deuterium kinetic isotope effect (H/D KIE) of 1.8 in MeCN at 25 degrees C. This isotope effect, together with other experimental evidence, is suggestive of a concerted proton-electron transfer (CPET) mechanism for O-H bond oxidation by [Mn-III(OMe)(dpaq)](+). A kinetic and thermodynamic comparison of the O-H bond oxidation reactivity of [Mn-III(OMe)(dpaq)](+) to other M-III-OR oxidants is presented as an aid to gain more insight into the PCET reactivity of mid-valent oxidants. In contrast to high-valent counterparts, the limited examples of M-III-OR oxidants exhibit smaller H/D KIEs and show weaker dependence of their oxidation rates on the driving force of the PCET reaction with O-H bonds.